Provided by: isc-dhcp-server_4.1.ESV-R4-0ubuntu5_amd64 bug


       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
       parameters 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  subnet  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-subnet 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 ones.

       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 dynamically 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,  subnet  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 {
           host-specific parameters...
         host {
           host-specific parameters...
         host {
           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 "";
            option domain-name-servers,;

                                                Figure 2

       As  you  can  see  in  Figure  2, you can specify host addresses in parameters 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 domain, so it might make sense for a group-specific parameter to override the
       domain name supplied to these hosts:

            option domain-name "";

       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

            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  correspond  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 parameters
       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 terminals 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 declarations 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 declarations:

       subnet netmask {
         option routers;

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

         # Known clients get this pool.
         pool {
           option domain-name-servers,;
           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 while.

       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 incorrect 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 address.

       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 immediately allocated to the client.   If
       the address is available for allocation 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 particular 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 possible, 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  statements,  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.

       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 address.


       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 automatically.

       The failover protocol defines a primary server role and a secondary server  role.    There
       are  some  differences  in  how primaries and secondaries act, but most of the differences
       simply have to do with providing 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 configured
       as secondary, and it doesn't matter too much which one is which.


       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 contact with its partner
       will have expired.

       While the failed server is recovering, its partner  remains  in  the  partner-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 communicated with  their
       partner,  they  both  come up in this recovery state and follow the procedure we have just
       described.   In this case, no service will be provided to  DHCP  clients  until  MCLT  has


       In  order  to configure failover, you need to write a peer declaration that configures the
       failover protocol, and you need to write peer references  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 segment.    You must not tell one server  it's  doing  failover  on  a  particular
       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 declaration 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" {
         port 519;
         peer address;
         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

       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 identifier.  Because this value is used as  an  identifier,  it
         may not be omitted.

       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 seconds 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
         connection  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 communicating.   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 index;

         The  split  statement  specifies  the  split  between  the primary and secondary for the
         purposes of load balancing.   Whenever a client makes a DHCP request,  the  DHCP  server
         runs  a  hash  on  the client identification, 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 255, of which the most
         reasonable is 128.

       The hba statement

         hba colon-separated-hex-list;

         The  hba  statement  specifies  the  split between the primary and secondary 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 consequently 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 balancing 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 saturation...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 secondary 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 giving 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 difference 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 statement.  This parameter configures what used  to
         be  a 10% fixed value in previous versions: if lts is less than free+backup * max-lease-
         misbalance 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, however, 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 system 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

         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 multiplied 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 balance 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

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

         Finally,  the  min-balance and max-balance make certain that a scheduled 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, inclusive, 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 recommended to select a max-lease-ownership value that is  lower  than  the  value
         selected  for  the  max-lease-misbalance value.  max-lease-ownership 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  depending  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 subclass is a class with
       the same name as a regular class, but with a specific 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 constant value to use
       in matching the match expression for the class.  When class matching is done,  the  server
       will  evaluate the match expression 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 subclass.

       Subclasses can be declared with or without scope.   In the above example, 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  configuration
       parameters, while most didn't, you might write the following subclass declaration for that

       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


       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
       provide 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 matching 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 example:

       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 compliant 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-supported 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  protocol
       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

       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; otherwise, if the client sent a host-name option, that is used.  Otherwise, 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  prevents  a
       client  from claiming that its hostname is the name of some network server.   For example,
       if you have a fileserver called "", and the client claims its hostname is
       "fs", no DNS update will be done for that client, and an error message will be logged.

       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 DHCPACK.   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 server.

       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 differences 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

       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 "" domain, whose hostname is "jschmoe".   The server is for the
       "" domain.   The DHCP client indicates in the FQDN  option  that  its  FQDN  is
       "".   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   Once the DHCP client has an IP address, it  can  update  its  own  A
       record, assuming that the "" 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  from  either  the
       fqdn  option (if present) or the hostname option (if present).  It will use its own domain
       name for the client, just as in the ad-hoc update scheme.  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 "".

       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.

       Also,  if  the  use-host-decl-names  configuration  option  is  enabled,  then  the   host
       declaration's  hostname  will  be used in place of the hostname option, and the same rules
       will apply as described above.

       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 deleting 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

       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 previously.

       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 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 "" {
            type master;
            file "";
            allow-update { key DHCP_UPDATER; };

       zone "" {
            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 {
         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  ""  and  for
       "".    For example, if there were a subdomain "" with
       no separate SOA, you could not write a zone declaration for ""  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/";
                 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/ 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 mechanism.

       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 statement.

       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 statement.  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 subnet.   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 netmask, 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 subnet.

       The subnet6-number should be an IPv6 network identifier, specified as ip6-address/bits.

       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 omitted.

       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-address.

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

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

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

       The  host declaration provides a scope in which to provide configuration information about
       a specific client, and also provides a way to assign a client a fixed address.   The  host
       declaration  provides  a  way  for the DHCP server to identify a DHCP or BOOTP client, and
       also a way to assign the client a static IP address.

       If it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet  with
       fixed  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.

       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 statements, 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 matching 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

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

       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-identifier 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


       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 keywords 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  control
       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


       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  dynamically  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 heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

       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.

       This  option  does not satisfy the requirement of failover peers for denying dynamic bootp
       clients.  The deny dynamic bootp clients; option should be used instead. See the ALLOW AND
       DENY WITHIN POOL DECLARATIONS section of this man page for more details.

       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.
       Unfortunately, a malicious or buggy client can,  using  DHCPDECLINE  messages,  completely
       exhaust  the  DHCP  server's  allocation pool.   The server will reclaim these leases, but
       while the client is running through the pool, it may cause serious thrashing in  the  DNS,
       and it will also cause the DHCP server to forget old DHCP client address allocations.

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

       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

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer DHCPLEASEQUERY 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 packets.


       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  DHCPDISCOVER  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 consists 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 known).

        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 -7200


       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 seconds. 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 therefore 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 Microsoft DHCP
         client is known not to have this problem, as are the OpenTransport 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 configured 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 information  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 DHCPNAK 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 time.

         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 networks 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 others.

       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  declarations  will  be  allowed  to obtain IP addresses, as long as those
         addresses  are  not  restricted  by  allow  and  deny  statements  within   their   pool

       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 setting up the client's A
         and PTR records.   If no ddns-hostname is specified  in  scope,  then  the  server  will
         derive  the  hostname  automatically,  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 "", but the default can be overridden

         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  of

       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 none.

       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 expiration time.  This is used for both
         DHCPv4  and  DHCPv6  leases  (it  is also known as the "valid lifetime" in DHCPv6).  The
         default is 43200 seconds.

       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).

         Please  note  that  as delayed-ack is currently experimental, the delayed-ack feature is
         not compiled in by default, but must  be  enabled  at  compile  time  with  ´./configure

       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 Universal 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
         statement  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 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 statement.  hardware-type must be the  name
         of  a  physical  hardware  interface 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

       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

       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 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 compile-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 systems.

         In  addition  to  setting  this  value,  you may need to modify your syslog.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  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 configuration
         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-time.

       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 seconds.   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-allocatable

       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 DHCPREQUEST, 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/   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/  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

         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.

            This  ping  check introduces a default one-second delay in responding 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
            parameter can be used to control checking - if its value is false, no ping  check  is

         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 the renew+rebind timers, or 3/4 the default  lease
            time if none were specified.

         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
            configuration 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 generally 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
            particular 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 equivalent  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 type.

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

            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

         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 allocated 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  RENEWING
            state.    This  works around a problem with relay agent information 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 leaving 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

         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  performance
            impact in environments that place heavy demands on the DHCP server.

         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 provided 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;

            is equivalent to

                  host joe {
                    hardware ethernet 08:00:2b:4c:29:32;
                    option host-name "joe";

            An  option host-name statement within a host declaration will override 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 determination.

         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 evaluation 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 page.


       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, RFC2131.


       dhcpd.conf(5)  was  written  by  Ted Lemon under a contract with Vixie Labs.   Funding for
       this project was provided by Internet  Systems  Consortium.   Information  about  Internet
       Systems Consortium can be found at