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dhcpd.conf(5)                 File Formats Manual                dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration file

       The  dhcpd.conf  file contains configuration information for dhcpd, the
       Internet Systems Consortium DHCP Server.

       The dhcpd.conf file is a free-form ASCII text file.  It  is  parsed  by
       the  recursive-descent  parser  built into dhcpd.  The file may contain
       extra tabs and newlines for formatting purposes.  Keywords in the  file
       are  case-insensitive.  Comments may be placed anywhere within the file
       (except within quotes).  Comments begin with the # character and end at
       the end of the line.

       The file essentially consists of a list of statements.  Statements fall
       into two broad categories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how  long  a
       lease  to  offer),  whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters  to  provide  to  the
       client (e.g., use gateway

       Declarations  are  used  to  describe  the  topology of the network, to
       describe clients on the network,  to  provide  addresses  that  can  be
       assigned  to  clients,  or to apply a group of parameters to a group of
       declarations.  In any group of parameters and declarations, all parame‐
       ters  must  be  specified before any declarations which depend on those
       parameters may be specified.

       Declarations about network topology include the shared-network and  the
       subnet  declarations.   If  clients  on  a  subnet  are  to be assigned
       addresses dynamically, a range declaration must appear within the  sub‐
       net  declaration.   For  clients with statically assigned addresses, or
       for installations where only known clients will be  served,  each  such
       client  must  have a host declaration.  If parameters are to be applied
       to a group of declarations which are not related strictly on a per-sub‐
       net basis, the group declaration can be used.

       For  every  subnet  which will be served, and for every subnet to which
       the dhcp server is connected, there must  be  one  subnet  declaration,
       which  tells  dhcpd how to recognize that an address is on that subnet.
       A subnet declaration is required for each subnet even if  no  addresses
       will be dynamically allocated on that subnet.

       Some  installations  have  physical  networks on which more than one IP
       subnet operates.  For example, if there is a site-wide requirement that
       8-bit  subnet  masks  be  used, but a department with a single physical
       ethernet network expands to the point where it has more than 254 nodes,
       it may be necessary to run two 8-bit subnets on the same ethernet until
       such time as a new physical network can be added.  In  this  case,  the
       subnet  declarations  for  these  two  networks  must  be enclosed in a
       shared-network declaration.

       Note that even when the shared-network declaration is absent, an  empty
       one  is  created  by  the  server to contain the subnet (and any scoped
       parameters included in the subnet).  For practical purposes, this means
       that  "stateless"  DHCP  clients,  which are not tied to addresses (and
       therefore subnets) will receive  the  same  configuration  as  stateful

       Some  sites  may  have  departments which have clients on more than one
       subnet, but it may be desirable to offer those clients a uniform set of
       parameters  which  are  different than what would be offered to clients
       from other departments on the same subnet.  For clients which  will  be
       declared  explicitly  with host declarations, these declarations can be
       enclosed in a group declaration along with  the  parameters  which  are
       common to that department.  For clients whose addresses will be dynami‐
       cally assigned, class declarations and conditional declarations may  be
       used  to  group  parameter  assignments based on information the client

       When a client is to be booted, its boot parameters  are  determined  by
       consulting that client's host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,  sub‐
       net  and shared-network declarations for the IP address assigned to the
       client.  Each of these declarations itself  appears  within  a  lexical
       scope,  and  all  declarations at less specific lexical scopes are also
       consulted for client option declarations.  Scopes are never  considered
       twice,  and  if  parameters  are  declared  in more than one scope, the
       parameter declared in the most specific scope is the one that is used.

       When dhcpd tries to find a host declaration  for  a  client,  it  first
       looks for a host declaration which has a fixed-address declaration that
       lists an IP address that is valid for the subnet or shared  network  on
       which  the  client  is  booting.  If it doesn't find any such entry, it
       tries to find an entry which has no fixed-address declaration.

       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       group {
         group-specific parameters...
         host zappo.test.isc.org {
           host-specific parameters...
         host beppo.test.isc.org {
           host-specific parameters...
         host harpo.test.isc.org {
           host-specific parameters...

                                      Figure 1

       Notice that at the beginning of the file, there's a  place  for  global
       parameters.  These might be things like the organization's domain name,
       the addresses of the name servers (if they are  common  to  the  entire
       organization), and so on.  So, for example:

            option domain-name "isc.org";
            option domain-name-servers ns1.isc.org, ns2.isc.org;

                                      Figure 2

       As  you  can see in Figure 2, you can specify host addresses in parame‐
       ters using their domain names rather than their numeric  IP  addresses.
       If  a given hostname resolves to more than one IP address (for example,
       if that host has two ethernet interfaces), then  where  possible,  both
       addresses are supplied to the client.

       The  most obvious reason for having subnet-specific parameters as shown
       in Figure 1 is that each subnet, of necessity, has its own router.   So
       for the first subnet, for example, there should be something like:

            option routers;

       Note  that  the  address  here  is  specified numerically.  This is not
       required - if you have a different domain name for  each  interface  on
       your  router, it's perfectly legitimate to use the domain name for that
       interface instead of the numeric address.  However, in many cases there
       may  be only one domain name for all of a router's IP addresses, and it
       would not be appropriate to use that name here.

       In Figure 1 there is also a  group  statement,  which  provides  common
       parameters  for  a set of three hosts - zappo, beppo and harpo.  As you
       can see, these hosts are all in the test.isc.org domain,  so  it  might
       make  sense  for a group-specific parameter to override the domain name
       supplied to these hosts:

            option domain-name "test.isc.org";

       Also, given the domain they're in, these are  probably  test  machines.
       If we wanted to test the DHCP leasing mechanism, we might set the lease
       timeout somewhat shorter than the default:

            max-lease-time 120;
            default-lease-time 120;

       You may have noticed that while some parameters start with  the  option
       keyword, some do not.  Parameters starting with the option keyword cor‐
       respond to actual DHCP options, while parameters that do not start with
       the  option  keyword  either  control  the  behavior of the DHCP server
       (e.g., how long a lease dhcpd will give out), or specify client parame‐
       ters  that  are not optional in the DHCP protocol (for example, server-
       name and filename).

       In Figure 1, each  host  had  host-specific  parameters.   These  could
       include  such  things  as  the  hostname  option, the name of a file to
       upload (the filename parameter) and the  address  of  the  server  from
       which  to upload the file (the next-server parameter).  In general, any
       parameter can appear anywhere that parameters are allowed, and will  be
       applied according to the scope in which the parameter appears.

       Imagine that you have a site with a lot of NCD X-Terminals.  These ter‐
       minals come in a variety of models, and you want to  specify  the  boot
       files  for each model.  One way to do this would be to have host decla‐
       rations for each server and group them by model:

       group {
         filename "Xncd19r";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
         host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
         host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }

       group {
         filename "Xncd19c";
         next-server ncd-booter;

         host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
         host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }

       group {
         filename "XncdHMX";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
         host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
         host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }

       The pool declaration can be used to specify a pool  of  addresses  that
       will be treated differently than another pool of addresses, even on the
       same network segment or subnet.  For example, you may want to provide a
       large  set  of  addresses that can be assigned to DHCP clients that are
       registered to your DHCP  server,  while  providing  a  smaller  set  of
       addresses,  possibly  with  short  lease  times, that are available for
       unknown clients.  If you have a firewall, you may be  able  to  arrange
       for addresses from one pool to be allowed access to the Internet, while
       addresses in another pool are not, thus encouraging users  to  register
       their DHCP clients.  To do this, you would set up a pair of pool decla‐

       subnet netmask {
         option routers;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           allow unknown-clients;

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           deny unknown-clients;

       It is also possible to set up entirely different subnets for known  and
       unknown  clients - address pools exist at the level of shared networks,
       so address ranges within pool declarations can be on different subnets.

       As you can see in the preceding example, pools can  have  permit  lists
       that  control  which  clients  are allowed access to the pool and which
       aren't.  Each entry in a pool's permit  list  is  introduced  with  the
       allow  or  deny  keyword.  If a pool has a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to  be  assigned  addresses  from the pool.  If a pool has a deny list,
       then only those clients that do not match any entries on the deny  list
       will  be  eligible.    If  both permit and deny lists exist for a pool,
       then only clients that match the permit list and do not match the  deny
       list will be allowed access.

       Address  allocation  is actually only done when a client is in the INIT
       state and has sent a DHCPDISCOVER message.  If the client thinks it has
       a  valid lease and sends a DHCPREQUEST to initiate or renew that lease,
       the server has only three choices - it can ignore the DHCPREQUEST, send
       a  DHCPNAK to tell the client it should stop using the address, or send
       a DHCPACK, telling the client to go ahead and use  the  address  for  a

       If  the  server  finds  the  address the client is requesting, and that
       address is available to the client, the server will send a DHCPACK.  If
       the  address  is  no longer available, or the client isn't permitted to
       have it, the server will send a DHCPNAK.  If the server  knows  nothing
       about  the address, it will remain silent, unless the address is incor‐
       rect for the network segment to which the client has been attached  and
       the server is authoritative for that network segment, in which case the
       server will send a DHCPNAK  even  though  it  doesn't  know  about  the

       There  may  be a host declaration matching the client's identification.
       If that host declaration  contains  a  fixed-address  declaration  that
       lists  an IP address that is valid for the network segment to which the
       client is connected.  In this case,  the  DHCP  server  will  never  do
       dynamic  address  allocation.   In this case, the client is required to
       take the address specified in the  host  declaration.   If  the  client
       sends  a  DHCPREQUEST  for  some other address, the server will respond
       with a DHCPNAK.

       When the DHCP server allocates a new address for  a  client  (remember,
       this  only  happens  if  the  client has sent a DHCPDISCOVER), it first
       looks to see if the client already has a valid lease on an IP  address,
       or  if there is an old IP address the client had before that hasn't yet
       been reassigned.  In that case, the server will take that  address  and
       check  it  to  see  if the client is still permitted to use it.  If the
       client is no longer permitted to use it, the  lease  is  freed  if  the
       server  thought it was still in use - the fact that the client has sent
       a DHCPDISCOVER proves to the server that the client is no longer  using
       the lease.

       If no existing lease is found, or if the client is forbidden to receive
       the existing lease, then the server will look in the  list  of  address
       pools  for  the  network  segment to which the client is attached for a
       lease that is not in use and that the client is permitted to have.   It
       looks through each pool declaration in sequence (all range declarations
       that appear outside of pool declarations are grouped into a single pool
       with  no  permit  list).   If  the  permit list for the pool allows the
       client to be allocated an address from that pool, the pool is  examined
       to  see  if  there  is an address available.  If so, then the client is
       tentatively assigned that address.  Otherwise, the next pool is tested.
       If  no  addresses  are  found  that  can  be assigned to the client, no
       response is sent to the client.

       If an address is found that the client is permitted to have,  and  that
       has  never  been  assigned to any client before, the address is immedi‐
       ately allocated to the client.  If the address is available for alloca‐
       tion but has been previously assigned to a different client, the server
       will keep looking in hopes of finding an address that has never  before
       been assigned to a client.

       The  DHCP  server  generates  the list of available IP addresses from a
       hash table.  This means that the addresses are not sorted in  any  par‐
       ticular  order, and so it is not possible to predict the order in which
       the DHCP server will allocate IP addresses.  Users of previous versions
       of  the  ISC  DHCP server may have become accustomed to the DHCP server
       allocating IP addresses in ascending order, but this is no longer  pos‐
       sible, and there is no way to configure this behavior with version 3 of
       the ISC DHCP server.

       The DHCP server checks IP addresses to see if they are  in  use  before
       allocating  them  to  clients.   It  does  this by sending an ICMP Echo
       request message to the IP address being allocated.   If  no  ICMP  Echo
       reply  is  received within a second, the address is assumed to be free.
       This is only done for leases that have been specified in  range  state‐
       ments, and only when the lease is thought by the DHCP server to be free
       - i.e., the DHCP server or its failover peer has not listed  the  lease
       as in use.

       If  a  response  is  received  to an ICMP Echo request, the DHCP server
       assumes that there is a configuration error - the IP address is in  use
       by  some  host  on the network that is not a DHCP client.  It marks the
       address as abandoned, and will not assign it  to  clients.   The  lease
       will +remain abandoned for a minimum of abandon-lease-time seconds.

       If  a  DHCP  client tries to get an IP address, but none are available,
       but there are abandoned IP addresses, then the DHCP server will attempt
       to  reclaim  an abandoned IP address.  It marks one IP address as free,
       and then does the same ICMP Echo request  check  described  previously.
       If there is no answer to the ICMP Echo request, the address is assigned
       to the client.

       The DHCP server does not cycle through abandoned IP  addresses  if  the
       first  IP  address  it tries to reclaim is free.  Rather, when the next
       DHCPDISCOVER comes in from the client, it will attempt a new allocation
       using  the  same method described here, and will typically try a new IP

       This version of the ISC DHCP server supports the DHCP failover protocol
       as  documented  in draft-ietf-dhc-failover-12.txt.  This is not a final
       protocol document, and we have not done interoperability  testing  with
       other vendors' implementations of this protocol, so you must not assume
       that this implementation conforms to the standard.  If you wish to  use
       the  failover  protocol, make sure that both failover peers are running
       the same version of the ISC DHCP server.

       The failover protocol allows two DHCP servers (and no more than two) to
       share  a  common address pool.  Each server will have about half of the
       available IP addresses in the pool at any given  time  for  allocation.
       If one server fails, the other server will continue to renew leases out
       of the pool, and will allocate new addresses out of the roughly half of
       available  addresses  that  it  had  when communications with the other
       server were lost.

       It is possible during a prolonged failure to tell the remaining  server
       that  the other server is down, in which case the remaining server will
       (over time) reclaim all the addresses the other  server  had  available
       for  allocation,  and  begin to reuse them.  This is called putting the
       server into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using  the
       omshell  (1)  command  or  by  stopping  the  server,  editing the last
       failover state declaration  in  the  lease  file,  and  restarting  the
       server.  If you use this last method, change the "my state" line to:

       failover peer name state {
       my state partner-down;
       peer state state at date;

       It is only required to change "my state" as shown above.

       When the other server comes back online, it should automatically detect
       that it has been offline and request a complete update from the  server
       that  was running in the PARTNER-DOWN state, and then both servers will
       resume processing together.

       It is possible to get into a dangerous situation: if you put one server
       into  the PARTNER-DOWN state, and then *that* server goes down, and the
       other server comes back up, the other server will  not  know  that  the
       first  server  was  in  the PARTNER-DOWN state, and may issue addresses
       previously issued by the other server to different  clients,  resulting
       in  IP  address  conflicts.   Before putting a server into PARTNER-DOWN
       state, therefore, make sure that the  other  server  will  not  restart

       The  failover  protocol  defines  a primary server role and a secondary
       server role.  There are some differences in how  primaries  and  secon‐
       daries  act, but most of the differences simply have to do with provid‐
       ing a way for each peer to behave in the opposite way from  the  other.
       So one server must be configured as primary, and the other must be con‐
       figured as secondary, and it doesn't  matter  too  much  which  one  is

       When  a  server  starts  that  has not previously communicated with its
       failover peer, it must establish communications with its failover  peer
       and  synchronize  with it before it can serve clients.  This can happen
       either because you have just configured your DHCP  servers  to  perform
       failover  for  the  first time, or because one of your failover servers
       has failed catastrophically and lost its database.

       The initial recovery process  is  designed  to  ensure  that  when  one
       failover  peer  loses  its database and then resynchronizes, any leases
       that the failed server gave out before it failed will be honored.  When
       the  failed  server starts up, it notices that it has no saved failover
       state, and attempts to contact its peer.

       When it has established contact, it asks the peer for a  complete  copy
       its  peer's lease database.  The peer then sends its complete database,
       and sends a message indicating that it is done.  The failed server then
       waits until MCLT has passed, and once MCLT has passed both servers make
       the transition back into normal operation.  This waiting period ensures
       that  any leases the failed server may have given out while out of con‐
       tact with its partner will have expired.

       While the failed server is recovering, its partner remains in the part‐
       ner-down state, which means that it is serving all clients.  The failed
       server provides no service at all to DHCP clients until it has made the
       transition into normal operation.

       In  the case where both servers detect that they have never before com‐
       municated with their partner, they both come up in this recovery  state
       and follow the procedure we have just described.  In this case, no ser‐
       vice will be provided to DHCP clients until MCLT has expired.

       In order to configure failover, you need to write  a  peer  declaration
       that  configures the failover protocol, and you need to write peer ref‐
       erences in each pool declaration for which you  want  to  do  failover.
       You  do  not  have to do failover for all pools on a given network seg‐
       ment.   You must not tell one server it's doing failover on a  particu‐
       lar  address  pool and tell the other it is not.  You must not have any
       common address pools on which you are not doing failover.  A pool  dec‐
       laration that utilizes failover would look like this:

       pool {
            failover peer "foo";
            pool specific parameters

       The   server currently  does very  little  sanity checking,  so if  you
       configure it wrong, it will just  fail in odd ways.  I would  recommend
       therefore  that you either do  failover or don't do failover, but don't
       do any mixed pools.  Also,  use the same master configuration file  for
       both   servers,  and  have  a  separate file  that  contains  the  peer
       declaration and includes the master file.  This will help you to  avoid
       configuration   mismatches.  As our  implementation evolves,  this will
       become  less of  a  problem.  A  basic  sample dhcpd.conf  file for   a
       primary server might look like this:

       failover peer "foo" {
         address anthrax.rc.example.com;
         port 519;
         peer address trantor.rc.example.com;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

         This  determines  whether  the  server  is  primary  or secondary, as
         described earlier under DHCP FAILOVER.

       The address statement

         address address;

         The address statement declares the IP address or DNS  name  on  which
         the  server should listen for connections from its failover peer, and
         also the value to use for the DHCP Failover Protocol  server  identi‐
         fier.   Because  this  value  is used as an identifier, it may not be

       The peer address statement

         peer address address;

         The peer address statement declares the IP address  or  DNS  name  to
         which  the  server  should  connect  to  reach  its failover peer for
         failover messages.

       The port statement

         port port-number;

         The port statement declares the TCP port on which the  server  should
         listen  for  connections from its failover peer.   This statement may
         not currently be omitted, because the failover protocol does not  yet
         have a reserved TCP port number.

       The peer port statement

         peer port port-number;

         The  peer  port  statement  declares the TCP port to which the server
         should connect to reach its  failover  peer  for  failover  messages.
         This  statement may not be omitted because the failover protocol does
         not yet have a reserved TCP port number.   The port  number  declared
         in  the  peer  port  statement  may  be  the  same as the port number
         declared in the port statement.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server how many  sec‐
         onds  may  pass  without  receiving  a message from its failover peer
         before it assumes that connection has failed.  This number should  be
         small enough that a transient network failure that breaks the connec‐
         tion will not result in the servers being out of communication for  a
         long  time,  but large enough that the server isn't constantly making
         and breaking connections.  This parameter must be specified.

       The max-unacked-updates statement

         max-unacked-updates count;

         The max-unacked-updates statement tells the remote  DHCP  server  how
         many BNDUPD messages it can send before it receives a BNDACK from the
         local system.  We don't have enough  operational  experience  to  say
         what  a good value for this is, but 10 seems to work.  This parameter
         must be specified.

       The mclt statement

         mclt seconds;

         The mclt statement defines the Maximum Client Lead Time.  It must  be
         specified  on the primary, and may not be specified on the secondary.
         This is the length of time for which a lease may be renewed by either
         failover peer without contacting the other.  The longer you set this,
         the longer it  will  take  for  the  running  server  to  recover  IP
         addresses  after moving into PARTNER-DOWN state.  The shorter you set
         it, the more load your servers will experience when they are not com‐
         municating.   A  value of something like 3600 is probably reasonable,
         but again bear in mind that we have no  real  operational  experience
         with this.

       The split statement

         split bits;

         The  split statement specifies the split between the primary and sec‐
         ondary for the purposes of load balancing.  Whenever a client makes a
         DHCP  request,  the DHCP server runs a hash on the client identifica‐
         tion, resulting in value from 0 to 255.  This is  used  as  an  index
         into  a  256 bit field.  If the bit at that index is set, the primary
         is responsible.  If the bit at that index is not set,  the  secondary
         is  responsible.   The split value determines how many of the leading
         bits are set to one.  So, in practice, higher split values will cause
         the  primary  to  serve more clients than the secondary.  Lower split
         values, the converse.  Legal values are between 0 and 256  inclusive,
         of  which  the  most reasonable is 128.  Note that a value of 0 makes
         the secondary responsible for all clients and a value  of  256  makes
         the primary responsible for all clients.

       The hba statement

         hba colon-separated-hex-list;

         The  hba  statement  specifies the split between the primary and sec‐
         ondary as a bitmap rather than a cutoff, which  theoretically  allows
         for  finer-grained  control.   In practice, there is probably no need
         for such fine-grained control, however.  An example hba statement:

           hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

         This is equivalent to a split 128;  statement,  and  identical.   The
         following two examples are also equivalent to a split of 128, but are
         not identical:

           hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

           hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

         They are equivalent, because half the bits are set to 0, half are set
         to  1  (0xa and 0x5 are 1010 and 0101 binary respectively) and conse‐
         quently this would roughly divide the  clients  equally  between  the
         servers.  They are not identical, because the actual peers this would
         load balance to each server are different for each example.

         You must only have split or hba defined, never both.  For most cases,
         the  fine-grained  control that hba offers isn't necessary, and split
         should be used.

       The load balance max seconds statement

         load balance max seconds seconds;

         This statement allows you to configure a cutoff after which load bal‐
         ancing  is  disabled.   The  cutoff is based on the number of seconds
         since the client sent its first DHCPDISCOVER or DHCPREQUEST  message,
         and only works with clients that correctly implement the secs field -
         fortunately most clients do.  We recommend setting this to  something
         like 3 or 5.  The effect of this is that if one of the failover peers
         gets into a state where it is responding to failover messages but not
         responding to some client requests, the other failover peer will take
         over its client load automatically as the clients retry.

       The Failover pool balance statements.

          max-lease-misbalance percentage;
          max-lease-ownership percentage;
          min-balance seconds;
          max-balance seconds;

         This version of the DHCP Server evaluates pool balance on a schedule,
         rather  than  on demand as leases are allocated.  The latter approach
         proved to be slightly klunky when pool misbalanced reach total  satu‐
         ration  —  when  any server ran out of leases to assign, it also lost
         its ability to notice it had run dry.

         In order to understand pool balance, some elements of  its  operation
         first  need  to  be  defined.   First,  there are ´free´ and ´backup´
         leases.  Both of these  are  referred  to  as  ´free  state  leases´.
         ´free´  and  ´backup´  are  ´the free states´ for the purpose of this
         document.  The difference is that only the primary may allocate  from
         ´free´  leases  unless under special circumstances, and only the sec‐
         ondary may allocate ´backup´ leases.

         When pool balance is performed, the only plausible expectation is  to
         provide  a  50/50  split  of  the  free  state leases between the two
         servers.  This is because no one can predict which server will  fail,
         regardless  of the relative load placed upon the two servers, so giv‐
         ing each server half the leases gives both servers the same amount of
         ´failure  endurance´.   Therefore,  there  is no way to configure any
         different behaviour, outside of  some  very  small  windows  we  will
         describe shortly.

         The  first  thing  calculated  on  any  pool  balance  run is a value
         referred to as ´lts´, or "Leases To Send".  This, simply, is the dif‐
         ference  in the count of free and backup leases, divided by two.  For
         the secondary, it is the difference in the backup  and  free  leases,
         divided  by  two.   The resulting value is signed: if it is positive,
         the local server is expected to hand out leases  to  retain  a  50/50
         balance.   If  it  is  negative, the remote server would need to send
         leases to balance the pool.  Once the lts  value  reaches  zero,  the
         pool  is perfectly balanced (give or take one lease in the case of an
         odd number of total free state leases).

         The current approach is still  something  of  a  hybrid  of  the  old
         approach,  marked  by the presence of the max-lease-misbalance state‐
         ment.  This parameter configures what used to be a 10% fixed value in
         previous  versions:  if lts is less than free+backup * max-lease-mis‐
         balance percent, then the server will skip balancing a given pool (it
         won't  bother  moving  any  leases,  even  if some leases "should" be
         moved).  The meaning of this value is also somewhat overloaded,  how‐
         ever,  in  that  it also governs the estimation of when to attempt to
         balance the pool (which may then also be skipped over).   The  oldest
         leases  in  the  free  and backup states are examined.  The time they
         have resided in their respective queues is used  as  an  estimate  to
         indicate how much time it is probable it would take before the leases
         at the top of the list would be consumed (and thus, how long it would
         take  to  use all leases in that state).  This percentage is directly
         multiplied by this time, and fit into the schedule if it falls within
         the  min-balance  and  max-balance  configured values.  The scheduled
         pool check time is only moved in a downwards direction, it  is  never
         increased.  Lastly, if the lts is more than double this number in the
         negative direction, the local server  will  ´panic´  and  transmit  a
         Failover  protocol POOLREQ message, in the hopes that the remote sys‐
         tem will be woken up into action.

         Once the lts value exceeds  the  max-lease-misbalance  percentage  of
         total  free  state leases as described above, leases are moved to the
         remote server.  This is done in two passes.

         In the first pass, only leases whose most recent bound  client  would
         have been served by the remote server - according to the Load Balance
         Algorithm (see above split and hba configuration  statements)  -  are
         given  away  to  the  peer.  This first pass will happily continue to
         give away leases, decrementing the lts value by one for  each,  until
         the  lts value has reached the negative of the total number of leases
         multiplied by the max-lease-ownership percentage.  So it  is  through
         this  value that you can permit a small misbalance of the lease pools
         - for the purpose of giving the peer  more  than  a  50/50  share  of
         leases  in  the hopes that their clients might some day return and be
         allocated by the peer (operating normally).  This process is referred
         to  as  ´MAC  Address  Affinity´,  but  this is somewhat misnamed: it
         applies equally to DHCP Client Identifier options.   Note  also  that
         affinity  is  applied to leases when they enter the state ´free´ from
         ´expired´ or ´released´.  In this case also, leases will not be moved
         from free to backup if the secondary already has more than its share.

         The  second  pass  is  only  entered  into if the first pass fails to
         reduce the lts underneath the total number of free state leases  mul‐
         tiplied  by  the  max-lease-ownership  percentage.  In this pass, the
         oldest leases are given over to the peer without second thought about
         the  Load  Balance  Algorithm, and this continues until the lts falls
         under this value.  In this way, the local server  will  also  happily
         keep  a  small percentage of the leases that would normally load bal‐
         ance to itself.

         So, the  max-lease-misbalance  value  acts  as  a  behavioural  gate.
         Smaller values will cause more leases to transition states to balance
         the pools over time, higher values will decrease the amount of change
         (but may lead to pool starvation if there's a run on leases).

         The  max-lease-ownership  value  permits a small (percentage) skew in
         the lease balance of a percentage of the total number of  free  state

         Finally,  the  min-balance and max-balance make certain that a sched‐
         uled rebalance event happens within a reasonable timeframe (not to be
         thrown off by, for example, a 7 year old free lease).

         Plausible  values  for  the percentages lie between 0 and 100, inclu‐
         sive, but values over 50 are indistinguishable from one another (once
         lts  exceeds  50% of the free state leases, one server must therefore
         have 100% of the leases in its respective free state).  It is  recom‐
         mended  to  select a max-lease-ownership value that is lower than the
         value selected for the max-lease-misbalance value.   max-lease-owner‐
         ship defaults to 10, and max-lease-misbalance defaults to 15.

         Plausible values for the min-balance and max-balance times also range
         from 0 to (2^32)-1 (or the limit of your  local  time_t  value),  but
         default  to  values 60 and 3600 respectively (to place balance events
         between 1 minute and 1 hour).

       Clients can be separated into classes, and treated differently  depend‐
       ing on what class they are in.  This separation can be done either with
       a conditional statement, or with a match  statement  within  the  class
       declaration.   It is possible to specify a limit on the total number of
       clients within a particular class or subclass that may hold  leases  at
       one  time, and it is possible to specify automatic subclassing based on
       the contents of the client packet.

       To add clients to classes based  on  conditional  evaluation,  you  can
       specify a matching expression in the class statement:

       class "ras-clients" {
         match if substring (option dhcp-client-identifier, 1, 3) = "RAS";

       Note  that  whether  you use matching expressions or add statements (or
       both) to classify clients, you must always write  a  class  declaration
       for any class that you use.  If there will be no match statement and no
       in-scope statements for a class, the declaration should look like this:

       class "ras-clients" {

       In addition to classes, it is possible to declare subclasses.   A  sub‐
       class is a class with the same name as a regular class, but with a spe‐
       cific submatch expression which is hashed for quick matching.  This  is
       essentially  a  speed  hack  - the main difference between five classes
       with match expressions and one class with five subclasses  is  that  it
       will be quicker to find the subclasses.  Subclasses work as follows:

       class "allocation-class-1" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       class "allocation-class-2" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet netmask {
         pool {
           allow members of "allocation-class-1";
         pool {
           allow members of "allocation-class-2";

       The data following the class name in the subclass declaration is a con‐
       stant value to use in matching the  match  expression  for  the  class.
       When class matching is done, the server will evaluate the match expres‐
       sion and then look the result up in the hash  table.   If  it  finds  a
       match, the client is considered a member of both the class and the sub‐

       Subclasses can be declared with or without scope.  In the  above  exam‐
       ple,  the  sole purpose of the subclass is to allow some clients access
       to one address pool, while other clients are given access to the  other
       pool,  so these subclasses are declared without scopes.  If part of the
       purpose of the subclass were to define different parameter  values  for
       some clients, you might want to declare some subclasses with scopes.

       In  the above example, if you had a single client that needed some con‐
       figuration parameters, while most didn't, you might write the following
       subclass declaration for that client:

       subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
         option root-path "samsara:/var/diskless/alphapc";
         filename "/tftpboot/netbsd.alphapc-diskless";

       In  this  example,  we've  used subclassing as a way to control address
       allocation on a per-client basis.  However, it's also possible  to  use
       subclassing  in ways that are not specific to clients - for example, to
       use the value of the vendor-class-identifier option to  determine  what
       values  to  send in the vendor-encapsulated-options option.  An example
       of this is shown under the VENDOR  ENCAPSULATED  OPTIONS  head  in  the
       dhcp-options(5) manual page.

       You may specify a limit to the number of clients in a class that can be
       assigned leases.  The effect of this will be to make it difficult for a
       new  client  in  a  class  to get an address.  Once a class with such a
       limit has reached its limit, the only way a new client  in  that  class
       can  get  a  lease  is  for an existing client to relinquish its lease,
       either by letting it  expire,  or  by  sending  a  DHCPRELEASE  packet.
       Classes with lease limits are specified as follows:

       class "limited-1" {
         lease limit 4;

       This will produce a class in which a maximum of four members may hold a
       lease at one time.

       It is possible to declare a spawning class.   A  spawning  class  is  a
       class  that  automatically produces subclasses based on what the client
       sends.  The reason that spawning classes were created was  to  make  it
       possible  to  create  lease-limited classes on the fly.  The envisioned
       application is a cable-modem environment where the ISP wishes  to  pro‐
       vide  clients  at  a particular site with more than one IP address, but
       does not wish to provide such clients with their own subnet,  nor  give
       them  an  unlimited  number of IP addresses from the network segment to
       which they are connected.

       Many cable modem head-end systems can be  configured  to  add  a  Relay
       Agent Information option to DHCP packets when relaying them to the DHCP
       server.  These systems typically add a circuit ID or remote  ID  option
       that uniquely identifies the customer site.  To take advantage of this,
       you can write a class declaration as follows:

       class "customer" {
         spawn with option agent.circuit-id;
         lease limit 4;

       Now whenever a request comes in from a customer site,  the  circuit  ID
       option  will  be checked against the class's hash table.  If a subclass
       is found that matches the circuit ID, the client will be classified  in
       that  subclass and treated accordingly.  If no subclass is found match‐
       ing the circuit ID, a new  one  will  be  created  and  logged  in  the
       dhcpd.leases file, and the client will be classified in this new class.
       Once the client has been classified, it will be  treated  according  to
       the  rules  of the class, including, in this case, being subject to the
       per-site limit of four leases.

       The use of the subclass spawning mechanism is not restricted  to  relay
       agent  options  - this particular example is given only because it is a
       fairly straightforward one.

       In some cases, it may be useful to  use  one  expression  to  assign  a
       client  to a particular class, and a second expression to put it into a
       subclass of that class.  This can be done by combining the match if and
       spawn with statements, or the match if and match statements.  For exam‐

       class "jr-cable-modems" {
         match if option dhcp-vendor-identifier = "jrcm";
         spawn with option agent.circuit-id;
         lease limit 4;

       class "dv-dsl-modems" {
         match if option dhcp-vendor-identifier = "dvdsl";
         spawn with option agent.circuit-id;
         lease limit 16;

       This allows you to have two classes that both have the same spawn  with
       expression without getting the clients in the two classes confused with
       each other.

       The DHCP server has the ability to dynamically update the  Domain  Name
       System.   Within  the  configuration files, you can define how you want
       the Domain Name System to be updated.  These updates are RFC 2136  com‐
       pliant  so  any DNS server supporting RFC 2136 should be able to accept
       updates from the DHCP server.

       Two DNS update  schemes  are  currently  implemented,  and  another  is
       planned.   The  two  that  are currently implemented are the ad-hoc DNS
       update mode and the interim DHCP-DNS interaction draft update mode.  In
       the  future  we plan to add a third mode which will be the standard DNS
       update method based on the RFCS for DHCP-DNS interaction and DHCID  The
       DHCP  server  must  be  configured to use one of the two currently-sup‐
       ported methods, or not to do dns updates.  This can be  done  with  the
       ddns-update-style configuration parameter.

       The  ad-hoc  Dynamic  DNS  update scheme is now deprecated and does not
       work.  In future releases of the ISC DHCP server, this scheme will  not
       likely  be  available.   The interim scheme works, allows for failover,
       and should now be used.  The following description  is  left  here  for
       informational purposes only.

       The ad-hoc Dynamic DNS update scheme implemented in this version of the
       ISC DHCP server is a prototype design, which does not have much  to  do
       with  the standard update method that is being standardized in the IETF
       DHC working group, but rather implements some very basic,  yet  useful,
       update  capabilities.  This mode does not work with the failover proto‐
       col because it does not account for the possibility  of  two  different
       DHCP servers updating the same set of DNS records.

       For  the  ad-hoc DNS update method, the client's FQDN is derived in two
       parts.  First, the hostname is determined.  Then, the  domain  name  is
       determined, and appended to the hostname.

       The DHCP server determines the client's hostname by first looking for a
       ddns-hostname configuration option, and using that if  it  is  present.
       If  no such option is present, the server looks for a valid hostname in
       the FQDN option sent by the client.  If one is found, it is used;  oth‐
       erwise,  if  the  client sent a host-name option, that is used.  Other‐
       wise, if there is a host declaration that applies to  the  client,  the
       name from that declaration will be used.  If none of these applies, the
       server will not have a hostname for the client, and will not be able to
       do a DNS update.

       The  domain  name  is determined from the ddns-domainname configuration
       option.  The default configuration for this option is:

         option server.ddns-domainname = config-option domain-name;

       So if this configuration option is not configured to a different  value
       (over-riding  the  above  default),  or if a domain-name option has not
       been configured for the  client's  scope,  then  the  server  will  not
       attempt to perform a DNS update.

       The client's fully-qualified domain name, derived as we have described,
       is used as the name on which an "A"  record  will  be  stored.   The  A
       record  will contain the IP address that the client was assigned in its
       lease.  If there is already an A record with the same name in  the  DNS
       server, no update of either the A or PTR records will occur - this pre‐
       vents a client from claiming that its hostname is the name of some net‐
       work   server.    For   example,   if  you  have  a  fileserver  called
       "fs.sneedville.edu", and the client claims its hostname is "fs", no DNS
       update  will  be  done  for  that  client, and an error message will be

       If the A record update succeeds, a PTR record update for  the  assigned
       IP  address  will  be  done,  pointing to the A record.  This update is
       unconditional - it will be done even if another PTR record of the  same
       name  exists.   Since  the  IP  address  has  been assigned to the DHCP
       server, this should be safe.

       Please note that the current implementation assumes clients only have a
       single  network  interface.   A client with two network interfaces will
       see unpredictable behavior.  This is considered  a  bug,  and  will  be
       fixed  in  a later release.  It may be helpful to enable the one-lease-
       per-client parameter so that roaming clients do not trigger  this  same

       The  DHCP protocol normally involves a four-packet exchange - first the
       client sends a DHCPDISCOVER message, then the server sends a DHCPOFFER,
       then  the  client sends a DHCPREQUEST, then the server sends a DHCPACK.
       In the current version of the server, the server will do a  DNS  update
       after  it has received the DHCPREQUEST, and before it has sent the DHC‐
       PACK.  It only sends the DNS update if it has  not  sent  one  for  the
       client's  address  before,  in order to minimize the impact on the DHCP

       When the client's lease expires, the DHCP server (if it is operating at
       the  time, or when next it operates) will remove the client's A and PTR
       records from the DNS database.  If the client  releases  its  lease  by
       sending  a  DHCPRELEASE  message, the server will likewise remove the A
       and PTR records.

       The interim DNS update scheme  operates  mostly  according  to  several
       drafts considered by the IETF.  While the drafts have since become RFCs
       the code was written before they were finalized and there are some dif‐
       ferences  between  our  code and the final RFCs.  We plan to update our
       code, probably adding a standard DNS update option, at some time.   The
       basic framework is similar with the main material difference being that
       a DHCID RR was assigned in the RFCs whereas our code continues  to  use
       an  experimental  TXT  record.   The  format  of the TXT record bears a
       resemblance to the DHCID RR but it is  not  equivalent  (MD5  vs  SHA1,
       field length differences etc).  The standard RFCs are:

                            RFC 4701 (updated by RF5494)
                                      RFC 4702
                                      RFC 4703

       And the corresponding drafts were:


       Because  our implementation is slightly different than the standard, we
       will briefly document the operation of this update style here.

       The first point to understand about this style of DNS  update  is  that
       unlike  the  ad-hoc  style, the DHCP server does not necessarily always
       update both the A and the PTR records.  The FQDN option includes a flag
       which,  when  sent  by  the client, indicates that the client wishes to
       update its own A record.  In that case, the server  can  be  configured
       either  to  honor the client's intentions or ignore them.  This is done
       with the  statement  allow  client-updates;  or  the  statement  ignore
       client-updates;.  By default, client updates are allowed.

       If the server is configured to allow client updates, then if the client
       sends a fully-qualified domain name in the FQDN option, the server will
       use  that  name  the  client  sent in the FQDN option to update the PTR
       record.  For example, let us say that the client is a visitor from  the
       "radish.org"  domain,  whose  hostname is "jschmoe".  The server is for
       the "example.org" domain.  The DHCP client indicates in the FQDN option
       that  its  FQDN  is  "jschmoe.radish.org.".   It also indicates that it
       wants to update its own A record.  The DHCP server therefore  does  not
       attempt  to  set  up  an A record for the client, but does set up a PTR
       record for the IP address that  it  assigns  the  client,  pointing  at
       jschmoe.radish.org.   Once  the  DHCP  client has an IP address, it can
       update its own A record, assuming that the "radish.org" DNS server will
       allow it to do so.

       If  the  server  is  configured  not to allow client updates, or if the
       client doesn´t want to do its own update, the server will simply choose
       a name for the client. By default, the server will choose from the fol‐
       lowing three values:

            1. fqdn option (if present)
            2. hostname option (if present)
            3. Configured hostname option (if defined).

       If these defaults for choosing the host name are  not  appropriate  you
       can  write  your own statement to set the ddns-hostname variable as you
       wish.  If none of the above are found the server will use the host dec‐
       laration name (if one) and use-host-decl-names is on.

       It  will  use  its own domain name for the client.  It will then update
       both the A and PTR record, using the name that it chose for the client.
       If  the  client sends a fully-qualified domain name in the fqdn option,
       the server uses only the leftmost part of the  domain  name  -  in  the
       example above, "jschmoe" instead of "jschmoe.radish.org".

       Further,  if  the  ignore  client-updates;  directive is used, then the
       server will in addition send a response in the DHCP packet,  using  the
       FQDN  Option, that implies to the client that it should perform its own
       updates if it chooses to do so.  With deny client-updates;, a  response
       is sent which indicates the client may not perform updates.

       The  other  difference between the ad-hoc scheme and the interim scheme
       is that with the interim scheme, a method is used that allows more than
       one  DHCP server to update the DNS database without accidentally delet‐
       ing A records that shouldn't be deleted nor failing to  add  A  records
       that should be added.  The scheme works as follows:

       When  the  DHCP  server  issues a client a new lease, it creates a text
       string that is an MD5 hash over the DHCP client's  identification  (see
       draft-ietf-dnsext-dhcid-rr-??.txt  for  details).  The update adds an A
       record with the name the server chose and a TXT record  containing  the
       hashed identifier string (hashid).  If this update succeeds, the server
       is done.

       If the update fails because the A record already exists, then the  DHCP
       server  attempts  to  add the A record with the prerequisite that there
       must be a TXT record in the same name as the new A record, and that TXT
       record's  contents  must  be equal to hashid.  If this update succeeds,
       then the client has its A record and PTR record.  If it fails, then the
       name  the  client has been assigned (or requested) is in use, and can't
       be used by the client.  At this point the DHCP server gives  up  trying
       to do a DNS update for the client until the client chooses a new name.

       The  interim  DNS  update  scheme  is  called  interim for two reasons.
       First, it does not quite follow the RFCs.  The  RFCs  call  for  a  new
       DHCID RRtype while he interim DNS update scheme uses a TXT record.  The
       ddns-resolution draft called for the DHCP server to put a DHCID  RR  on
       the PTR record, but the interim update method does not do this.  In the
       final RFC this requirement was relaxed such that a  server  may  add  a
       DHCID RR to the PTR record.

       In  addition to these differences, the server also does not update very
       aggressively.  Because each DNS update involves a round trip to the DNS
       server,  there  is a cost associated with doing updates even if they do
       not actually modify the  DNS  database.   So  the  DHCP  server  tracks
       whether  or not it has updated the record in the past (this information
       is stored on the lease) and does not attempt to update records that  it
       thinks it has already updated.

       This  can  lead  to cases where the DHCP server adds a record, and then
       the record is deleted through some  other  mechanism,  but  the  server
       never  again  updates  the  DNS  because  it thinks the data is already
       there.  In this case the data can be removed  from  the  lease  through
       operator  intervention,  and  once  this has been done, the DNS will be
       updated the next time the client renews.

       When you set your DNS server up to allow updates from the DHCP  server,
       you  may  be  exposing  it to unauthorized updates.  To avoid this, you
       should use TSIG signatures -  a  method  of  cryptographically  signing
       updates  using a shared secret key.  As long as you protect the secrecy
       of this key, your updates should also be secure.  Note,  however,  that
       the  DHCP  protocol  itself  provides no security, and that clients can
       therefore provide information to the DHCP server which the DHCP  server
       will  then  use  in  its updates, with the constraints described previ‐

       The DNS server must be configured to allow updates for  any  zone  that
       the DHCP server will be updating.  For example, let us say that clients
       in  the  sneedville.edu  domain  will  be  assigned  addresses  on  the  subnet.   In  that case, you will need a key declaration
       for the TSIG key you will be using, and also two  zone  declarations  -
       one  for the zone containing A records that will be updates and one for
       the zone containing PTR records - for ISC BIND, something like this:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone "example.org" {
            type master;
            file "example.org.db";
            allow-update { key DHCP_UPDATER; };

       zone "17.10.10.in-addr.arpa" {
            type master;
            file "10.10.17.db";
            allow-update { key DHCP_UPDATER; };

       You will also have to configure your DHCP server to do updates to these
       zones.   To  do  so,  you  need  to  add  something  like  this to your
       dhcpd.conf file:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone EXAMPLE.ORG. {
         key DHCP_UPDATER;

       zone 17.127.10.in-addr.arpa. {
         key DHCP_UPDATER;

       The primary statement specifies the IP address of the name server whose
       zone information is to be updated.

       Note that the zone declarations have to correspond to authority records
       in your name server - in the above example, there must be an SOA record
       for  "example.org."  and for "17.10.10.in-addr.arpa.".  For example, if
       there were a subdomain "foo.example.org"  with  no  separate  SOA,  you
       could not write a zone declaration for "foo.example.org."  Also keep in
       mind that zone names in your DHCP configuration should end  in  a  ".";
       this  is  the  preferred syntax.  If you do not end your zone name in a
       ".", the DHCP server will figure it out.  Also note that  in  the  DHCP
       configuration,  zone  names  are not encapsulated in quotes where there
       are in the DNS configuration.

       You should choose your own secret key, of course.  The ISC BIND 8 and 9
       distributions  come  with  a  program for generating secret keys called
       dnssec-keygen.  The version that comes with BIND 9 is likely to produce
       a  substantially more random key, so we recommend you use that one even
       if you are not using BIND 9 as your DNS server.  If you are using  BIND
       9's dnssec-keygen, the above key would be created as follows:

            dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

       If  you  are  using the BIND 8 dnskeygen program, the following command
       will generate a key as seen above:

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

       You may wish to enable logging of DNS updates on your DNS  server.   To
       do so, you might write a logging statement like the following:

       logging {
            channel update_debug {
                 file "/var/log/update-debug.log";
                 severity  debug 3;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            channel security_info    {
                 file "/var/log/named-auth.info";
                 severity  info;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;

            category update { update_debug; };
            category security { security_info; };

       You  must  create  the  /var/log/named-auth.info  and  /var/log/update-
       debug.log files before starting the name server.  For more  information
       on configuring ISC BIND, consult the documentation that accompanies it.

       There  are three kinds of events that can happen regarding a lease, and
       it is possible to declare statements  that  occur  when  any  of  these
       events  happen.  These events are the commit event, when the server has
       made a commitment of a certain lease to a client,  the  release  event,
       when  the  client  has released the server from its commitment, and the
       expiry event, when the commitment expires.

       To declare a set of statements to execute when an  event  happens,  you
       must  use the on statement, followed by the name of the event, followed
       by a series of statements to execute when the event  happens,  enclosed
       in braces.  Events are used to implement DNS updates, so you should not
       define your own event handlers if you are using the built-in DNS update

       The  built-in  version  of the DNS update mechanism is in a text string
       towards the top of server/dhcpd.c.  If  you  want  to  use  events  for
       things  other than DNS updates, and you also want DNS updates, you will
       have to start out by copying this code into your  dhcpd.conf  file  and
       modifying it.

       The include statement

        include "filename";

       The  include statement is used to read in a named file, and process the
       contents of that file as though it were entered in place of the include

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]

       The  shared-network  statement  is  used to inform the DHCP server that
       some IP subnets actually share the same physical network.  Any  subnets
       in  a  shared network should be declared within a shared-network state‐
       ment.  Parameters specified in the  shared-network  statement  will  be
       used  when  booting clients on those subnets unless parameters provided
       at the subnet or host level override them.  If any subnet in  a  shared
       network has addresses available for dynamic allocation, those addresses
       are collected into a common pool for that shared network  and  assigned
       to  clients  as needed.  There is no way to distinguish on which subnet
       of a shared network a client should boot.

       Name should be the name of the shared network.  This name is used  when
       printing debugging messages, so it should be descriptive for the shared
       network.  The name may have the syntax of a valid domain name (although
       it  will  never  be  used  as  such),  or it may be any arbitrary name,
       enclosed in quotes.

       The subnet statement

        subnet subnet-number netmask netmask {
          [ parameters ]
          [ declarations ]

       The subnet statement is used to provide dhcpd with  enough  information
       to tell whether or not an IP address is on that subnet.  It may also be
       used  to  provide  subnet-specific  parameters  and  to  specify   what
       addresses  may be dynamically allocated to clients booting on that sub‐
       net.  Such addresses are specified using the range declaration.

       The subnet-number should be an IP address or domain name which resolves
       to the subnet number of the subnet being described.  The netmask should
       be an IP address or domain name which resolves to the  subnet  mask  of
       the  subnet being described.  The subnet number, together with the net‐
       mask, are sufficient to determine whether any given IP  address  is  on
       the specified subnet.

       Although  a  netmask must be given with every subnet declaration, it is
       recommended that if there is any variance in subnet masks at a site,  a
       subnet-mask  option statement be used in each subnet declaration to set
       the desired subnet mask, since any subnet-mask  option  statement  will
       override the subnet mask declared in the subnet statement.

       The subnet6 statement

        subnet6 subnet6-number {
          [ parameters ]
          [ declarations ]

       The  subnet6 statement is used to provide dhcpd with enough information
       to tell whether or not an IPv6 address is on that subnet6.  It may also
       be  used  to  provide  subnet-specific  parameters  and to specify what
       addresses may be dynamically allocated to clients booting on that  sub‐

       The  subnet6-number  should be an IPv6 network identifier, specified as

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For any subnet on which addresses will be assigned  dynamically,  there
       must  be  at  least one range statement.  The range statement gives the
       lowest and highest IP addresses in a range.  All IP  addresses  in  the
       range should be in the subnet in which the range statement is declared.
       The dynamic-bootp flag may be specified if addresses in  the  specified
       range  may  be  dynamically  assigned  to BOOTP clients as well as DHCP
       clients.  When specifying a single address, high-address can  be  omit‐

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For  any  IPv6 subnet6 on which addresses will be assigned dynamically,
       there must be at least one range6 statement. The range6  statement  can
       either  be  the  lowest  and highest IPv6 addresses in a range6, or use
       CIDR notation, specified as ip6-address/bits. All IP addresses  in  the
       range6  should  be  in  the  subnet6  in  which the range6 statement is

       The temporay variant makes the prefix (by default on 64 bits) available
       for  temporary  (RFC  4941)  addresses. A new address per prefix in the
       shared network is computed  at  each  request  with  an  IA_TA  option.
       Release and Confirm ignores temporary addresses.

       Any IPv6 addresses given to hosts with fixed-address6 are excluded from
       the range6, as are IPv6 addresses on the server itself.

       The prefix6 statement

       prefix6 low-address high-address / bits;

       The prefix6 is the range6 equivalent for Prefix Delegation (RFC  3633).
       Prefixes  of  bits  length  are  assigned between low-address and high-

       Any IPv6 prefixes given to static entries  (hosts)  with  fixed-prefix6
       are excluded from the prefix6.

       This  statement is currently global but it should have a shared-network

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

       The host declaration provides a way for the DHCP server to  identify  a
       DHCP  or BOOTP client.  This allows the server to provide configuration
       information including fixed addresses or, in DHCPv6, fixed prefixes for
       a specific client.

       If  it  is  desirable to be able to boot a DHCP or BOOTP client on more
       than one subnet with fixed v4 addresses, more than one address  may  be
       specified  in  the  fixed-address  declaration,  or  more than one host
       statement may be specified matching the same client.

       The fixed-address6 delcaration is used for v6 addresses.  At this  time
       it  only  works  with a single address.  For multiple addresses specify
       multiple host statements.

       If client-specific boot parameters must change based on the network  to
       which the client is attached, then multiple host declarations should be
       used.  The host declarations will only match a client if one  of  their
       fixed-address  statements  is  viable on the subnet (or shared network)
       where the client is attached.  Conversely, for a  host  declaration  to
       match  a client being allocated a dynamic address, it must not have any
       fixed-address statements.  You may therefore need  a  mixture  of  host
       declarations  for  any  given client...some having fixed-address state‐
       ments, others without.

       hostname should be a name identifying the host.  If a  hostname  option
       is not specified for the host, hostname is used.

       Host declarations are matched to actual DHCP or BOOTP clients by match‐
       ing the dhcp-client-identifier option specified in the host declaration
       to  the  one supplied by the client, or, if the host declaration or the
       client does not provide a dhcp-client-identifier  option,  by  matching
       the  hardware parameter in the host declaration to the network hardware
       address supplied by the client.  BOOTP clients do not normally  provide
       a  dhcp-client-identifier, so the hardware address must be used for all
       clients that may boot using the BOOTP protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host
       declaration,  and  specify  any  option  with a fixed value to identify

       Please be aware that only the  dhcp-client-identifier  option  and  the
       hardware  address can be used to match a host declaration, or the host-
       identifier option parameter for DHCPv6 servers.  For example, it is not
       possible  to  match  a host declaration to a host-name option.  This is
       because the host-name option cannot be guaranteed to be unique for  any
       given client, whereas both the hardware address and dhcp-client-identi‐
       fier option are at least theoretically guaranteed to  be  unique  to  a
       given client.

       The group statement

        group {
          [ parameters ]
          [ declarations ]

       The group statement is used simply to apply one or more parameters to a
       group of declarations.  It can be used to group hosts, shared networks,
       subnets, or even other groups.

       The  allow  and  deny statements can be used to control the response of
       the DHCP server to various sorts of requests.  The allow and deny  key‐
       words  actually have different meanings depending on the context.  In a
       pool context, these keywords can be used to set  up  access  lists  for
       address  allocation pools.  In other contexts, the keywords simply con‐
       trol general server behavior with respect to clients  based  on  scope.
       In  a  non-pool context, the ignore keyword can be used in place of the
       deny keyword to prevent logging of denied requests.

       The following usages of allow and deny will work in any scope, although
       it is not recommended that they be used in pool declarations.

       The unknown-clients keyword

        allow unknown-clients;
        deny unknown-clients;
        ignore unknown-clients;

       The unknown-clients flag is used to tell dhcpd whether or not to dynam‐
       ically assign addresses to unknown clients.  Dynamic address assignment
       to  unknown clients is allowed by default.  An unknown client is simply
       a client that has no host declaration.

       The use of this option  is  now  deprecated.   If  you  are  trying  to
       restrict  access  on your network to known clients, you should use deny
       unknown-clients; inside of your address pool, as  described  under  the

       The bootp keyword

        allow bootp;
        deny bootp;
        ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to bootp
       queries.  Bootp queries are allowed by default.

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

       The booting flag is used to tell dhcpd whether or  not  to  respond  to
       queries  from  a particular client.  This keyword only has meaning when
       it appears in a host declaration.  By default, booting is allowed,  but
       if it is disabled for a particular client, then that client will not be
       able to get an address from the DHCP server.

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host declarations can match client messages based on  the  DHCP  Client
       Identifier  option  or  based on the client's network hardware type and
       MAC address.  If the MAC address is used,  the  host  declaration  will
       match  any  client  with that MAC address - even clients with different
       client identifiers.  This doesn't normally happen, but is possible when
       one  computer  has more than one operating system installed on it - for
       example, Microsoft Windows and NetBSD or Linux.

       The duplicates flag tells the DHCP server that if a request is received
       from  a  client that matches the MAC address of a host declaration, any
       other leases matching that MAC  address  should  be  discarded  by  the
       server,  even  if  the UID is not the same.  This is a violation of the
       DHCP protocol, but can prevent clients whose client identifiers  change
       regularly  from  holding  many  leases  at  the same time.  By default,
       duplicates are allowed.

       The declines keyword

        allow declines;
        deny declines;
        ignore declines;

       The DHCPDECLINE message is used by DHCP clients to  indicate  that  the
       lease  the server has offered is not valid.  When the server receives a
       DHCPDECLINE  for  a  particular  address,  it  normally  abandons  that
       address,  assuming that some unauthorized system is using it.  Unfortu‐
       nately, a malicious or buggy client can,  using  DHCPDECLINE  messages,
       completely  exhaust the DHCP server's allocation pool.  The server will
       eventually reclaim these leases, but not while the  client  is  running
       through  the  pool. This may cause serious thrashing in the DNS, and it
       will also cause the DHCP server to forget old DHCP client address allo‐

       The declines flag tells the DHCP server whether or not to honor DHCPDE‐
       CLINE messages.  If it is set to deny or ignore in a particular  scope,
       the DHCP server will not respond to DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large
       IPv6 address space and the internal  limits  imposed  by  the  server's
       address  generation mechanism we don't think it is necessary for DHCPv6
       servers at this time.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

       The client-updates flag tells the DHCP server whether or not  to  honor
       the  client's  intention to do its own update of its A record.  This is
       only relevant when doing interim DNS updates.   See  the  documentation
       under the heading THE INTERIM DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer DHC‐
       PLEASEQUERY packets. The answer to  a  DHCPLEASEQUERY  packet  includes
       information about a specific lease, such as when it was issued and when
       it will expire. By default, the server will not respond to these  pack‐

       The  uses  of the allow and deny keywords shown in the previous section
       work pretty much the same way whether the client is sending a  DHCPDIS‐
       COVER  or  a  DHCPREQUEST message - an address will be allocated to the
       client (either the old address it's requesting, or a new  address)  and
       then  that address will be tested to see if it's okay to let the client
       have it.  If the client requested it, and it's  not  okay,  the  server
       will  send  a  DHCPNAK  message.  Otherwise, the server will simply not
       respond to the client.  If it is  okay  to  give  the  address  to  the
       client, the server will send a DHCPACK message.

       The  primary  motivation  behind  pool  declarations is to have address
       allocation pools whose allocation policies are different.  A client may
       be denied access to one pool, but allowed access to another pool on the
       same network segment.  In order for this to work, access control has to
       be  done  during  address  allocation,  not after address allocation is

       When a DHCPREQUEST message is processed, address allocation simply con‐
       sists  of looking up the address the client is requesting and seeing if
       it's still available for the client.  If it is, then  the  DHCP  server
       checks  both  the  address  pool permit lists and the relevant in-scope
       allow and deny statements to see if it's okay to give the lease to  the
       client.   In the case of a DHCPDISCOVER message, the allocation process
       is done as described previously in the ADDRESS ALLOCATION section.

       When declaring permit lists for address allocation pools, the following
       syntaxes are recognized following the allow or deny keywords:


       If  specified, this statement either allows or prevents allocation from
       this pool to any client that has a host declaration (i.e.,  is  known).
       A  client  is known if it has a host declaration in any scope, not just
       the current scope.


       If specified, this statement either allows or prevents allocation  from
       this  pool  to  any  client  that has no host declaration (i.e., is not

        members of "class";

       If specified, this statement either allows or prevents allocation  from
       this pool to any client that is a member of the named class.

        dynamic bootp clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any bootp client.

        authenticated clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool  to  any  client  that has been authenticated using the DHCP
       authentication protocol.  This is not yet supported.

        unauthenticated clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool to any client that has not been authenticated using the DHCP
       authentication protocol.  This is not yet supported.

        all clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool  to  all clients.  This can be used when you want to write a
       pool declaration for some reason, but hold it in reserve, or  when  you
       want  to  renumber  your  network  quickly, and thus want the server to
       force all clients that have been allocated addresses from this pool  to
       obtain new addresses immediately when they next renew.

        after time;

       If  specified, this statement either allows or prevents allocation from
       this pool after a given date. This can be used when you  want  to  move
       clients  from one pool to another. The server adjusts the regular lease
       time so that the latest expiry time is  at  the  given  time+min-lease-
       time.   A short min-lease-time enforces a step change, whereas a longer
       min-lease-time allows for a gradual  change.   time  is  either  second
       since  epoch,  or  a  UTC  time string e.g.  4 2007/08/24 09:14:32 or a
       string with time zone offset in  seconds  e.g.  4  2007/08/24  11:14:32

       The abandon-lease-time statement

         adandon-lease-time time;

         Time  should be the maximum amount of time (in seconds) that an aban‐
         doned IPv4 lease remains unavailable  for  assignment  to  a  client.
         Abandoned leases will only be offered to clients if there are no free
         leases.  If not defined, the default abandon lease time is 86400 sec‐
         onds  (24 hours).  Note the abandoned lease time for a given lease is
         preserved across server restarts.  The parameter may only be  set  at
         the global scope and is evaluated only once during server startup.

         Values  less  than sixty seconds are not recommended as this is below
         the ping check threshold and can  cause  leases  once  abandoned  but
         since  returned  to  the  free  state  to  not be pinged before being
         offered.  If the requested time is larger than 0x7FFFFFFF - 1 or  the
         sum  of  the  current  time  plus  the  abandoned time isgreater than
         0x7FFFFFFF it is treated as infinite.

       The adaptive-lease-time-threshold statement

         adaptive-lease-time-threshold percentage;

         When the number of allocated leases within a  pool  rises  above  the
         percentage  given  in  this  statement, the DHCP server decreases the
         lease length for new clients within this pool to min-lease-time  sec‐
         onds.  Clients  renewing  an already valid (long) leases get at least
         the remaining time from the current lease. Since  the  leases  expire
         faster,  the  server  may  either  recover more quickly or avoid pool
         exhaustion entirely.  Once the number of allocated leases drop  below
         the  threshold, the server reverts back to normal lease times.  Valid
         percentages are between 1 and 99.

       The always-broadcast statement

         always-broadcast flag;

         The DHCP and BOOTP protocols both require DHCP and BOOTP  clients  to
         set the broadcast bit in the flags field of the BOOTP message header.
         Unfortunately, some DHCP and BOOTP clients do not do this, and there‐
         fore may not receive responses from the DHCP server.  The DHCP server
         can be made to always broadcast its responses to clients  by  setting
         this  flag  to  ´on´ for the relevant scope; relevant scopes would be
         inside a conditional statement, as a parameter for a class, or  as  a
         parameter for a host declaration.  To avoid creating excess broadcast
         traffic on your network, we recommend that you restrict  the  use  of
         this  option  to as few clients as possible.  For example, the Micro‐
         soft DHCP client is known not to have this problem, as are the  Open‐
         Transport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some  BOOTP clients expect RFC1048-style responses, but do not follow
         RFC1048 when sending their requests.  You can tell that a  client  is
         having this problem if it is not getting the options you have config‐
         ured for it and if you see in  the  server  log  the  message  "(non-
         rfc1048)" printed with each BOOTREQUEST that is logged.

         If you want to send rfc1048 options to such a client, you can set the
         always-reply-rfc1048 option in that client's  host  declaration,  and
         the  DHCP  server  will respond with an RFC-1048-style vendor options
         field.  This flag can be set  in  any  scope,  and  will  affect  all
         clients covered by that scope.

       The authoritative statement


         not authoritative;

         The  DHCP server will normally assume that the configuration informa‐
         tion about a given network segment is not known to be correct and  is
         not  authoritative.   This is so that if a naive user installs a DHCP
         server not fully understanding how to configure it, it does not  send
         spurious  DHCPNAK  messages  to  clients that have obtained addresses
         from a legitimate DHCP server on the network.

         Network administrators setting  up  authoritative  DHCP  servers  for
         their networks should always write authoritative; at the top of their
         configuration file to indicate that the DHCP server should send DHCP‐
         NAK  messages to misconfigured clients.  If this is not done, clients
         will be unable to get a correct IP  address  after  changing  subnets
         until  their  old  lease  has  expired, which could take quite a long

         Usually, writing authoritative; at the top level of the  file  should
         be  sufficient.  However, if a DHCP server is to be set up so that it
         is aware of some networks for which it is authoritative and some net‐
         works  for  which  it  is  not, it may be more appropriate to declare
         authority on a per-network-segment basis.

         Note that the most specific scope for which the concept of  authority
         makes  any  sense  is the physical network segment - either a shared-
         network statement or a subnet statement that is not contained  within
         a shared-network statement.  It is not meaningful to specify that the
         server is authoritative for some subnets within a shared network, but
         not  authoritative  for  others, nor is it meaningful to specify that
         the server is authoritative for some host declarations and  not  oth‐

       The boot-unknown-clients statement

         boot-unknown-clients flag;

         If  the  boot-unknown-clients statement is present and has a value of
         false or off, then clients for which there  is  no  host  declaration
         will not be allowed to obtain IP addresses.  If this statement is not
         present or has a value of true or on, then clients without host  dec‐
         larations  will  be  allowed to obtain IP addresses, as long as those
         addresses are not restricted by  allow  and  deny  statements  within
         their pool declarations.

       The db-time-format statement

         db-time-format [ default | local ] ;

         The  DHCP  server  software  outputs  several timestamps when writing
         leases to persistent storage.  This configuration  parameter  selects
         one  of two output formats.  The default format prints the day, date,
         and time in UTC, while the local format prints  the  system  seconds-
         since-epoch,  and  helpfully  provides the day and time in the system
         timezone in a comment.  The time formats are described in  detail  in
         the dhcpd.leases(5) manpage.

       The ddns-hostname statement

         ddns-hostname name;

         The  name  parameter should be the hostname that will be used in set‐
         ting up the client's A and PTR records.  If no ddns-hostname is spec‐
         ified  in  scope,  then the server will derive the hostname automati‐
         cally, using an algorithm that  varies  for  each  of  the  different
         update methods.

       The ddns-domainname statement

         ddns-domainname name;

         The name parameter should be the domain name that will be appended to
         the client's hostname to form a fully-qualified domain-name (FQDN).

       The ddns-rev-domainname statement

         ddns-rev-domainname name; The name parameter  should  be  the  domain
         name  that  will  be  appended to the client's reversed IP address to
         produce a name for use in the client's PTR record.  By default,  this
         is "in-addr.arpa.", but the default can be overridden here.

         The  reversed  IP  address  to  which this domain name is appended is
         always the IP  address  of  the  client,  in  dotted  quad  notation,
         reversed  -  for example, if the IP address assigned to the client is, then the reversed  IP  address  is   So  a
         client  with that IP address would, by default, be given a PTR record

       The ddns-update-style parameter

         ddns-update-style style;

         The style parameter must be one of  ad-hoc,  interim  or  none.   The
         ddns-update-style  statement  is only meaningful in the outer scope -
         it is evaluated once after reading the dhcpd.conf file,  rather  than
         each  time  a client is assigned an IP address, so there is no way to
         use different DNS update styles for different clients. The default is

       The ddns-updates statement

          ddns-updates flag;

         The  ddns-updates  parameter  controls whether or not the server will
         attempt to do a DNS update when a lease is confirmed.   Set  this  to
         off  if  the server should not attempt to do updates within a certain
         scope.  The ddns-updates parameter is on by default.  To disable  DNS
         updates  in all scopes, it is preferable to use the ddns-update-style
         statement, setting the style to none.

       The default-lease-time statement

         default-lease-time time;

         Time should be the length in seconds that will be assigned to a lease
         if  the client requesting the lease does not ask for a specific expi‐
         ration time.  This is used for both DHCPv4 and DHCPv6 leases  (it  is
         also  known as the "valid lifetime" in DHCPv6).  The default is 43200

       The delayed-ack and max-ack-delay statements

         delayed-ack count; max-ack-delay microseconds;

         Count should be an integer value from zero to 2^16-1, and defaults to
         28.   The  count  represents  how many DHCPv4 replies maximum will be
         queued pending transmission until after a database commit event.   If
         this  number  is reached, a database commit event (commonly resulting
         in fsync() and representing a performance penalty) will be made,  and
         the  reply  packets  will be transmitted in a batch afterwards.  This
         preserves the RFC2131 direction  that  "stable  storage"  be  updated
         prior  to  replying  to  clients.  Should the DHCPv4 sockets "go dry"
         (select() returns immediately with no read sockets),  the  commit  is
         made and any queued packets are transmitted.

         Similarly, microseconds indicates how many microseconds are permitted
         to pass inbetween queuing a packet pending an fsync,  and  performing
         the  fsync.   Valid  values  range  from 0 to 2^32-1, and defaults to
         250,000 (1/4 of a second).

         The delayed-ack feature is not compiled in by default,  but  must  be
         enabled  at  compile  time  with  ´./configure --enable-delayed-ack´.
         While we no longer consider it experimental and we don't know of  any
         issues  with it, in order to minimize problems with existing configu‐
         ration files we have left it disabled by default.

       The do-forward-updates statement

         do-forward-updates flag;

         The do-forward-updates statement instructs  the  DHCP  server  as  to
         whether it should attempt to update a DHCP client's A record when the
         client acquires or renews a lease.   This  statement  has  no  effect
         unless  DNS  updates  are  enabled  and  ddns-update-style  is set to
         interim.  Forward updates are enabled by default.  If this  statement
         is  used  to  disable  forward  updates,  the  DHCP server will never
         attempt to update the client's A record, and will only  ever  attempt
         to update the client's PTR record if the client supplies an FQDN that
         should be placed in the PTR record using the fqdn option.  If forward
         updates  are enabled, the DHCP server will still honor the setting of
         the client-updates flag.

       The dynamic-bootp-lease-cutoff statement

         dynamic-bootp-lease-cutoff date;

         The dynamic-bootp-lease-cutoff statement sets the ending time for all
         leases  assigned dynamically to BOOTP clients.  Because BOOTP clients
         do not have any way of renewing leases, and  don't  know  that  their
         leases  could expire, by default dhcpd assigns infinite leases to all
         BOOTP clients.  However, it may make sense in some situations to  set
         a cutoff date for all BOOTP leases - for example, the end of a school
         term, or the time at night when a facility is closed and all machines
         are required to be powered off.

         Date  should be the date on which all assigned BOOTP leases will end.
         The date is specified in the form:

                                 W YYYY/MM/DD HH:MM:SS

         W is the day of the week expressed as a number from zero (Sunday)  to
         six  (Saturday).  YYYY is the year, including the century.  MM is the
         month expressed as a number from 1 to 12.   DD  is  the  day  of  the
         month,  counting from 1.  HH is the hour, from zero to 23.  MM is the
         minute and SS is the second.  The time is always in Coordinated  Uni‐
         versal Time (UTC), not local time.

       The dynamic-bootp-lease-length statement

         dynamic-bootp-lease-length length;

         The dynamic-bootp-lease-length statement is used to set the length of
         leases dynamically assigned to BOOTP clients.  At some sites, it  may
         be  possible to assume that a lease is no longer in use if its holder
         has not used BOOTP or DHCP to get its address within a  certain  time
         period.   The  period  is specified in length as a number of seconds.
         If a client reboots using BOOTP during the timeout period, the  lease
         duration  is reset to length, so a BOOTP client that boots frequently
         enough will never lose its lease.  Needless to  say,  this  parameter
         should be adjusted with extreme caution.

       The filename statement

         filename "filename";

         The filename statement can be used to specify the name of the initial
         boot file which is to be loaded by a client.  The filename should  be
         a filename recognizable to whatever file transfer protocol the client
         can be expected to use to load the file.

       The fixed-address declaration

         fixed-address address [, address ... ];

         The fixed-address declaration is used to assign one or more fixed  IP
         addresses  to a client.  It should only appear in a host declaration.
         If more than one address is supplied, then when the client boots,  it
         will be assigned the address that corresponds to the network on which
         it is booting.  If none of the addresses in the fixed-address  state‐
         ment are valid for the network to which the client is connected, that
         client will not match the host  declaration  containing  that  fixed-
         address  declaration.   Each address in the fixed-address declaration
         should be either an IP address or a domain name that resolves to  one
         or more IP addresses.

       The fixed-address6 declaration

         fixed-address6 ip6-address ;

         The  fixed-address6  declaration  is  used  to  assign  a  fixed IPv6
         addresses to a client.  It should only appear in a host declaration.

       The fixed-prefix6 declaration

         fixed-prefix6 low-address / bits;

         The fixed-prefix6 declaration is used to assign a fixed  IPv6  prefix
         to a client.  It should only appear in a host declaration, but multi‐
         ple fixed-prefix6 statements may appear in a single host declaration.

         The low-address specifies the start of the prefix and the bits speci‐
         fies the size of the prefix in bits.

         If there are multiple prefixes for a given host entry the server will
         choose one that matches the requested prefix size or, if none  match,
         the first one.

       The get-lease-hostnames statement

         get-lease-hostnames flag;

         The  get-lease-hostnames  statement  is used to tell dhcpd whether or
         not to look up the domain name corresponding to  the  IP  address  of
         each  address  in  the  lease  pool and use that address for the DHCP
         hostname option.  If flag is true, then this lookup is done  for  all
         addresses  in the current scope.  By default, or if flag is false, no
         lookups are done.

       The hardware statement

         hardware hardware-type hardware-address;

         In order for a BOOTP client to be recognized,  its  network  hardware
         address  must  be declared using a hardware clause in the host state‐
         ment.  hardware-type must be the name of a physical  hardware  inter‐
         face  type.   Currently,  only  the ethernet and token-ring types are
         recognized, although support for a fddi hardware  type  (and  others)
         would  also  be  desirable.   The hardware-address should be a set of
         hexadecimal octets (numbers from 0 through ff) separated  by  colons.
         The hardware statement may also be used for DHCP clients.

       The host-identifier option statement

         host-identifier option option-name option-data;

         This  identifies a DHCPv6 client in a host statement.  option-name is
         any option, and option-data is the value  for  the  option  that  the
         client will send. The option-data must be a constant value.

       The infinite-is-reserved statement

         infinite-is-reserved flag;

         ISC DHCP now supports ´reserved´ leases.  See the section on RESERVED
         LEASES below.  If this flag is  on,  the  server  will  automatically
         reserve  leases  allocated  to  clients  which  requested an infinite
         (0xffffffff) lease-time.

         The default is off.

       The lease-file-name statement

         lease-file-name name;

         Name should be the name of the DHCP server's lease file.  By default,
         this  is DBDIR/dhcpd.leases.  This statement must appear in the outer
         scope of the configuration file - if it appears in some other  scope,
         it  will have no effect.  Furthermore, it has no effect if overridden
         by the -lf flag or the PATH_DHCPD_DB environment variable.

       The limit-addrs-per-ia statement

         limit-addrs-per-ia number;

         By default, the DHCPv6 server will limit clients to one IAADDR per IA
         option,  meaning  one address.  If you wish to permit clients to hang
         onto multiple addresses at a time, configure a larger number here.

         Note that there is no present  method  to  configure  the  server  to
         forcibly  configure the client with one IP address per each subnet on
         a shared network.  This is left to future work.

       The dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name is the name of the lease file to use if and only if  the  server
         is  running in DHCPv6 mode.  By default, this is DBDIR/dhcpd6.leases.
         This statement, like lease-file-name, must appear in the outer  scope
         of the configuration file.  It has no effect if overridden by the -lf
         flag or the PATH_DHCPD6_DB environment  variable.   If  dhcpv6-lease-
         file-name  is not specified, but lease-file-name is, the latter value
         will be used.

       The local-port statement

         local-port port;

         This statement causes the DHCP server to listen for DHCP requests  on
         the UDP port specified in port, rather than on port 67.

       The local-address statement

         local-address address;

         This  statement  causes  the  DHCP server to listen for DHCP requests
         sent to the specified address,  rather  than  requests  sent  to  all
         addresses.  Since serving directly attached DHCP clients implies that
         the server must respond to requests sent to the all-ones IP  address,
         this  option  cannot be used if clients are on directly attached net‐
         works; it is only  realistically  useful  for  a  server  whose  only
         clients are reached via unicasts, such as via DHCP relay agents.

         Note:   This  statement  is only effective if the server was compiled
         using the USE_SOCKETS #define statement, which is default on a  small
         number  of  operating  systems, and must be explicitly chosen at com‐
         pile-time for all others.  You can be sure if your server is compiled
         with USE_SOCKETS if you see lines of this format at startup:

          Listening on Socket/eth0

         Note  also  that since this bind()s all DHCP sockets to the specified
         address, that only one address may be supported  in  a  daemon  at  a
         given time.

       The log-facility statement

         log-facility facility;

         This statement causes the DHCP server to do all of its logging on the
         specified log facility once the dhcpd.conf file has  been  read.   By
         default  the  DHCP  server logs to the daemon facility.  Possible log
         facilities include auth, authpriv,  cron,  daemon,  ftp,  kern,  lpr,
         mail,  mark,  news,  ntp,  security,  syslog,  user, uucp, and local0
         through local7.  Not all of these facilities  are  available  on  all
         systems,  and  there  may be other facilities available on other sys‐

         In addition to setting this value, you may need to modify  your  sys‐
         log.conf  file to configure logging of the DHCP server.  For example,
         you might add a line like this:

              local7.debug /var/log/dhcpd.log

         The syntax of the syslog.conf file may be different on some operating
         systems  -  consult  the  syslog.conf manual page to be sure.  To get
         syslog to start logging to the new file, you must  first  create  the
         file  with correct ownership and permissions (usually, the same owner
         and permissions of your /var/log/messages or  /usr/adm/messages  file
         should  be  fine) and send a SIGHUP to syslogd.  Some systems support
         log rollover using a shell script  or  program  called  newsyslog  or
         logrotate, and you may be able to configure this as well so that your
         log file doesn't grow uncontrollably.

         Because the log-facility setting  is  controlled  by  the  dhcpd.conf
         file,  log  messages  printed  while  parsing  the dhcpd.conf file or
         before parsing it are logged to the default log facility.  To prevent
         this,  see  the  README  file  included with this distribution, which
         describes BUG: where is that mentioned in README?  how to change  the
         default  log  facility.  When this parameter is used, the DHCP server
         prints its startup message a second time after parsing the configura‐
         tion file, so that the log will be as complete as possible.

       The max-lease-time statement

         max-lease-time time;

         Time should be the maximum length in seconds that will be assigned to
         a lease.  If not defined, the default maximum lease  time  is  86400.
         The only exception to this is that Dynamic BOOTP lease lengths, which
         are not specified by the client, are not limited by this maximum.

       The min-lease-time statement

         min-lease-time time;

         Time should be the minimum length in seconds that will be assigned to
         a  lease.   The  default  is the minimum of 300 seconds or max-lease-

       The min-secs statement

         min-secs seconds;

         Seconds should be the minimum number of seconds since a client  began
         trying  to acquire a new lease before the DHCP server will respond to
         its request.  The number of seconds  is  based  on  what  the  client
         reports, and the maximum value that the client can report is 255 sec‐
         onds.  Generally, setting this to one will result in the DHCP  server
         not  responding  to the client's first request, but always responding
         to its second request.

         This can be used to set up a secondary DHCP server which never offers
         an  address  to  a  client  until the primary server has been given a
         chance to do so.  If the primary server is down, the client will bind
         to  the secondary server, but otherwise clients should always bind to
         the primary.  Note that this does not, by itself,  permit  a  primary
         server and a secondary server to share a pool of dynamically-allocat‐
         able addresses.

       The next-server statement

         next-server server-name;

         The next-server statement is used to specify the host address of  the
         server  from  which  the initial boot file (specified in the filename
         statement) is to be loaded.   Server-name  should  be  a  numeric  IP
         address or a domain name.

       The omapi-port statement

         omapi-port port;

         The  omapi-port  statement causes the DHCP server to listen for OMAPI
         connections on the specified port.  This  statement  is  required  to
         enable  the  OMAPI  protocol, which is used to examine and modify the
         state of the DHCP server as it is running.

       The one-lease-per-client statement

         one-lease-per-client flag;

         If this flag is enabled, whenever a client sends a DHCPREQUEST for  a
         particular lease, the server will automatically free any other leases
         the client holds.  This presumes that when the client sends a DHCPRE‐
         QUEST,  it has forgotten any lease not mentioned in the DHCPREQUEST -
         i.e., the client has only a single network interface and it does  not
         remember leases it's holding on networks to which it is not currently
         attached.  Neither of these assumptions are guaranteed  or  provable,
         so we urge caution in the use of this statement.

       The pid-file-name statement

         pid-file-name name;

         Name  should  be the name of the DHCP server's process ID file.  This
         is the file in which the DHCP server's process ID is stored when  the
         server  starts.   By  default,  this  is  RUNDIR/dhcpd.pid.  Like the
         lease-file-name statement, this statement must appear  in  the  outer
         scope  of  the configuration file.  It has no effect if overridden by
         the -pf flag or the PATH_DHCPD_PID environment variable.

         The dhcpv6-pid-file-name statement

            dhcpv6-pid-file-name name;

            Name is the name of the pid file to use if and only if the  server
            is  running in DHCPv6 mode.  By default, this is DBDIR/dhcpd6.pid.
            This statement, like pid-file-name, must appear in the outer scope
            of  the configuration file.  It has no effect if overridden by the
            -pf  flag  or  the  PATH_DHCPD6_PID  environment   variable.    If
            dhcpv6-pid-file-name  is  not specified, but pid-file-name is, the
            latter value will be used.

         The ping-check statement

            ping-check flag;

            When the DHCP server is considering dynamically allocating  an  IP
            address  to a client, it first sends an ICMP Echo request (a ping)
            to the address being assigned.  It waits for a second, and  if  no
            ICMP  Echo  response has been heard, it assigns the address.  If a
            response is heard, the lease is abandoned, and the server does not
            respond to the client.  The lease will remain abandoned for a min‐
            imum of abandon-lease-time seconds.

            If a there are no free  addressses  but  there  are  abandoned  IP
            addresses, the DHCP server will attempt to reclaim an abandoned IP
            address regardless of the value of abandon-lease-time.

            This ping check introduces a default one-second delay in  respond‐
            ing  to  DHCPDISCOVER  messages,  which  can be a problem for some
            clients.  The default delay of one second may be configured  using
            the  ping-timeout parameter.  The ping-check configuration parame‐
            ter can be used to control checking - if its value  is  false,  no
            ping check is done.

         The ping-timeout statement

            ping-timeout seconds;

            If  the DHCP server determined it should send an ICMP echo request
            (a ping) because the ping-check statement  is  true,  ping-timeout
            allows  you  to  configure how many seconds the DHCP server should
            wait for an ICMP Echo response  to  be  heard,  if  no  ICMP  Echo
            response  has been received before the timeout expires, it assigns
            the address.  If a response is heard, the lease is abandoned,  and
            the  server  does  not respond to the client.  If no value is set,
            ping-timeout defaults to 1 second.

         The preferred-lifetime statement

            preferred-lifetime seconds;

            IPv6 addresses have ´valid´ and ´preferred´ lifetimes.  The  valid
            lifetime  determines  at what point at lease might be said to have
            expired, and is no longer useable.  A  preferred  lifetime  is  an
            advisory  condition  to  help applications move off of the address
            and onto currently valid addresses (should there still be any open
            TCP sockets or similar).

            The preferred lifetime defaults to 5/8 the default lease time.

         The remote-port statement

            remote-port port;

            This  statement  causes the DHCP server to transmit DHCP responses
            to DHCP clients upon the UDP port specified in port,  rather  than
            on  port 68.  In the event that the UDP response is transmitted to
            a DHCP Relay, the server generally uses the local-port  configura‐
            tion  value.   Should  the  DHCP  Relay  happen to be addressed as
  , however, the DHCP Server transmits its response to  the
            remote-port  configuration  value.   This is generally only useful
            for testing purposes, and this configuration value  should  gener‐
            ally not be used.

         The server-identifier statement

            server-identifier hostname;

            The  server-identifier  statement  can be used to define the value
            that is sent in the DHCP Server  Identifier  option  for  a  given
            scope.   The  value  specified  must be an IP address for the DHCP
            server, and must be reachable by all clients served by a  particu‐
            lar scope.

            The  use  of  the server-identifier statement is not recommended -
            the only reason to use it is to  force  a  value  other  than  the
            default  value  to  be  sent  on occasions where the default value
            would be incorrect.  The default value is  the  first  IP  address
            associated  with  the  physical  network  interface  on  which the
            request arrived.

            The usual case where the server-identifier statement needs  to  be
            sent  is  when  a physical interface has more than one IP address,
            and the one being sent by default isn't appropriate  for  some  or
            all clients served by that interface.  Another common case is when
            an alias is defined for the purpose  of  having  a  consistent  IP
            address  for  the  DHCP server, and it is desired that the clients
            use this IP address when contacting the server.

            Supplying a value for the dhcp-server-identifier option is equiva‐
            lent to using the server-identifier statement.

         The server-duid statement

            server-duid LLT [ hardware-type timestamp hardware-address ] ;

            server-duid EN enterprise-number enterprise-identifier ;

            server-duid LL [ hardware-type hardware-address ] ;

            The server-duid statement configures the server DUID. You may pick
            either LLT (link local address plus time), EN (enterprise), or  LL
            (link local).

            If you choose LLT or LL, you may specify the exact contents of the
            DUID.  Otherwise the server will generate a DUID of the  specified

            If  you  choose EN, you must include the enterprise number and the

            If there is a server-duid statement in the lease file it will take
            precedence over the server-duid statement from the config file and
            a dhcp6.server-id option in the config file will override both.

            The default server-duid type is LLT.

         The server-name statement

            server-name name ;

            The server-name statement can be used to inform the client of  the
            name  of  the server from which it is booting.  Name should be the
            name that will be provided to the client.

         The dhcpv6-set-tee-times statement

            dhcpv6-set-tee-times flag;

            The dhcpv6-set-tee-times statement enables setting T1  and  T2  to
            the  values  recommended in RFC 3315 (Section 22.4).  When setting
            T1 and T2, the server will use dhcp-renewal-time and  dhcp-rebind‐
            ing-time,  respectively.   A value of zero tells the client it may
            choose its own value.

            When those options are not defined then values will be set to zero
            unless  the  global  dhcpv6-set-tee-timesis  enabled.   When  this
            option is enabled the times are calculated as recommended  by  RFC
            3315, Section 22.4:

                  T1 will be set to 0.5 times the shortest preferred lifetime
                  in the reply.  If the "shortest" preferred lifetime is
                  0xFFFFFFFF,  T1 will set to 0xFFFFFFFF.

                  T2 will be set to 0.8 times the shortest preferred lifetime
                  in the reply.  If the "shortest" preferred lifetime is
                  0xFFFFFFFF,  T2 will set to 0xFFFFFFFF.

            Keep  in  mind  that given sufficiently small lease lifetimes, the
            above calculations will result in the two values being equal.  For
            example,  a  9  second lease lifetime would yield T1 = T2 = 4 sec‐
            onds, which would cause clients to issue rebinds only.  In such  a
            case it would likely be better to explicitly define the values.

            Note  that dhcpv6-set-tee-times is intended to be transitional and
            will likely be removed in  a  future  release.  Once  removed  the
            behavior will be to use the configured values when present or cal‐
            culate them per the RFC. If you want zeros, define them as zeros.

         The site-option-space statement

            site-option-space name ;

            The site-option-space statement can be used to determine from what
            option  space  site-local options will be taken.  This can be used
            in much the same way as the vendor-option-space statement.   Site-
            local  options  in  DHCP are those options whose numeric codes are
            greater than 224.  These options are  intended  for  site-specific
            uses, but are frequently used by vendors of embedded hardware that
            contains DHCP clients.  Because site-specific  options  are  allo‐
            cated  on  an ad hoc basis, it is quite possible that one vendor's
            DHCP client might use the same option code that  another  vendor's
            client uses, for different purposes.  The site-option-space option
            can be used to assign a different set of site-specific options for
            each  such vendor, using conditional evaluation (see dhcp-eval (5)
            for details).

         The stash-agent-options statement

            stash-agent-options flag;

            If the stash-agent-options parameter is true for a  given  client,
            the  server  will  record the relay agent information options sent
            during the client's initial DHCPREQUEST message  when  the  client
            was  in  the  SELECTING  state  and behave as if those options are
            included in all subsequent DHCPREQUEST messages sent in the RENEW‐
            ING  state.  This works around a problem with relay agent informa‐
            tion options, which is that they usually not appear in DHCPREQUEST
            messages  sent  by  the client in the RENEWING state, because such
            messages are unicast directly to the server and not sent through a
            relay agent.

         The update-conflict-detection statement

            update-conflict-detection flag;

            If  the  update-conflict-detection  parameter  is true, the server
            will perform standard  DHCID  multiple-client,  one-name  conflict
            detection.   If  the parameter has been set false, the server will
            skip this check and instead simply tear down any previous bindings
            to install the new binding without question.  The default is true.

         The update-optimization statement

            update-optimization flag;

            If  the update-optimization parameter is false for a given client,
            the server will attempt a DNS update for that client each time the
            client  renews  its  lease,  rather than only attempting an update
            when it appears to be necessary.  This will allow the DNS to  heal
            from  database  inconsistencies  more easily, but the cost is that
            the DHCP server must do many more DNS updates.  We recommend leav‐
            ing  this  option enabled, which is the default.  This option only
            affects the behavior of the interim DNS update scheme, and has  no
            effect  on the ad-hoc DNS update scheme.  If this parameter is not
            specified, or is true, the DHCP server will only update  when  the
            client  information changes, the client gets a different lease, or
            the client's lease expires.

         The update-static-leases statement

            update-static-leases flag;

            The update-static-leases flag, if enabled, causes the DHCP  server
            to  do  DNS  updates  for  clients even if those clients are being
            assigned their IP address using a fixed-address statement  -  that
            is,  the client is being given a static assignment.  This can only
            work with the interim DNS update scheme.  It  is  not  recommended
            because  the  DHCP  server  has no way to tell that the update has
            been done, and therefore will not delete the record when it is not
            in  use.   Also,  the server must attempt the update each time the
            client renews its lease, which could have  a  significant  perfor‐
            mance  impact in environments that place heavy demands on the DHCP

         The use-host-decl-names statement

            use-host-decl-names flag;

            If the use-host-decl-names parameter is true  in  a  given  scope,
            then  for  every host declaration within that scope, the name pro‐
            vided for the host declaration will be supplied to the  client  as
            its hostname.  So, for example,

                group {
                  use-host-decl-names on;

                  host joe {
                    hardware ethernet 08:00:2b:4c:29:32;
                    fixed-address joe.example.com;

            is equivalent to

                  host joe {
                    hardware ethernet 08:00:2b:4c:29:32;
                    fixed-address joe.example.com;
                    option host-name "joe";

            Additionally, enabling use-host-decl-names instructs the server to
            use the host declaration name in the the forward DNS name,  if  no
            other  values are available.  This value selection process is dis‐
            cussed in more detail under DNS updates.

            An option host-name statement within a host declaration will over‐
            ride the use of the name in the host declaration.

            It  should  be noted here that most DHCP clients completely ignore
            the host-name option sent by the DHCP server, and there is no  way
            to  configure them not to do this.  So you generally have a choice
            of either not having any hostname to  client  IP  address  mapping
            that  the  client  will  recognize,  or  doing DNS updates.  It is
            beyond the scope of this document to describe  how  to  make  this

         The use-lease-addr-for-default-route statement

            use-lease-addr-for-default-route flag;

            If  the  use-lease-addr-for-default-route  parameter  is true in a
            given scope, then instead of sending the value  specified  in  the
            routers option (or sending no value at all), the IP address of the
            lease being assigned is  sent  to  the  client.   This  supposedly
            causes  Win95  machines  to ARP for all IP addresses, which can be
            helpful if your router is configured for proxy ARP.   The  use  of
            this  feature  is  not recommended, because it won't work for many
            DHCP clients.

         The vendor-option-space statement

            vendor-option-space string;

            The vendor-option-space  parameter  determines  from  what  option
            space  vendor  options  are  taken.  The use of this configuration
            parameter is illustrated in the dhcp-options(5)  manual  page,  in
            the VENDOR ENCAPSULATED OPTIONS section.

       Sometimes  it's  helpful  to  be able to set the value of a DHCP server
       parameter based on some value that the client has sent.   To  do  this,
       you  can  use  expression  evaluation.   The  dhcp-eval(5)  manual page
       describes how to write expressions.  To assign the result of an evalua‐
       tion to an option, define the option as follows:

         my-parameter = expression ;

       For example:

         ddns-hostname = binary-to-ascii (16, 8, "-",
                                          substring (hardware, 1, 6));

       It's  often  useful to allocate a single address to a single client, in
       approximate perpetuity.  Host  statements  with  fixed-address  clauses
       exist  to  a  certain  extent  to  serve this purpose, but because host
       statements are intended to  approximate  ´static  configuration´,  they
       suffer from not being referenced in a littany of other Server Services,
       such as dynamic DNS, failover, ´on events´ and so forth.

       If a standard dynamic lease, as from any  range  statement,  is  marked
       ´reserved´, then the server will only allocate this lease to the client
       it is identified by (be that by client identifier or hardware address).

       In practice, this means that the lease follows the normal state engine,
       enters  ACTIVE  state  when  the  client is bound to it, expires, or is
       released, and any events or services that would  normally  be  supplied
       during  these  events are processed normally, as with any other dynamic
       lease.  The only difference is that  failover  servers  treat  reserved
       leases  as  special  when  they  enter the FREE or BACKUP states - each
       server applies the lease into the state it may allocate from - and  the
       leases  are  not  placed  on the queue for allocation to other clients.
       Instead they may only be ´found´ by client  identity.   The  result  is
       that the lease is only offered to the returning client.

       Care  should  probably  be taken to ensure that the client only has one
       lease within a given subnet that it is identified by.

       Leases may be set ´reserved´  either  through  OMAPI,  or  through  the
       ´infinite-is-reserved´  configuration  option (if this is applicable to
       your environment and mixture of clients).

       It should also be noted that leases marked ´reserved´  are  effectively
       treated the same as leases marked ´bootp´.

       DHCP  option  statements  are  documented in the dhcp-options(5) manual

       Expressions used in DHCP option statements and elsewhere are documented
       in the dhcp-eval(5) manual page.

       dhcpd(8),   dhcpd.leases(5),  dhcp-options(5),  dhcp-eval(5),  RFC2132,

       dhcpd.conf(5) is maintained by ISC.  Information about Internet Systems
       Consortium can be found at https://www.isc.org.

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