Provided by: isc-dhcp-server_4.3.3-5ubuntu12_amd64 bug

NAME

       dhcpd.conf - dhcpd configuration file

DESCRIPTION

       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 220.177.244.7).

       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
       sends.

       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.

EXAMPLES

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

       global parameters...

       subnet 204.254.239.0 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.10 204.254.239.30;
       }

       subnet 204.254.239.32 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.42 204.254.239.62;
       }

       subnet 204.254.239.64 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.74 204.254.239.94;
       }

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

                                                Figure 1

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

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

                                                Figure 2

       As  you  can  see  in  Figure  2, you can specify host addresses in 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 204.254.239.1;

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

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

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

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

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

       You  may  have  noticed  that while some parameters start with the option keyword, some do
       not.  Parameters starting with the option keyword 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; }
       }

ADDRESS POOLS

       The  pool  and  pool6 declarations 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 10.0.0.0 netmask 255.255.255.0 {
         option routers 10.0.0.254;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           range 10.0.0.200 10.0.0.253;
           allow unknown-clients;
         }

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           range 10.0.0.5 10.0.0.199;
           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.

       The pool6 declaration is similar to the pool declaration.  Currently it  is  only  allowed
       within  a  subnet6  declaration,  and  may  not  be  included directly in a shared network
       declaration.  In addition to the range6 statement it allows the prefix6  statement  to  be
       included.  You may include range6 statements for both NA and TA and prefixy6 statements in
       a single pool6 statement.

DYNAMIC ADDRESS ALLOCATION

       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.

IP ADDRESS CONFLICT PREVENTION

       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.

DHCP FAILOVER

       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.

FAILOVER STARTUP

       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
       expired.

CONFIGURING FAILOVER

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

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

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

       The address statement

         address address;

         The  address  statement  declares  the IP address or DNS name on which the server should
         listen for connections from its failover peer, and also the value to use  for  the  DHCP
         Failover  Protocol  server  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 be omitted, in  which  case  the
         IANA assigned port number 647 will be used by default.

       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 be omitted, in  which
         case the IANA assigned port number 647 will be used by default.

       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 bits;

         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 256 inclusive, of
         which the most reasonable  is  128.   Note  that  a  value  of  0  makes  the  secondary
         responsible  for  all  clients  and a value of 256 makes the primary responsible for all
         clients.

       The hba statement

         hba colon-separated-hex-list;

         The hba statement specifies the split between the primary  and  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:
               00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;

         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:
               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:
               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 auto-partner-down statement

         auto-partner-down seconds;

         This  statement  instructs  the  server  to  initiate  a  timed  delay upon entering the
         communications-interrupted state (any situation of being out-of-contact with the  remote
         failover peer).  At the conclusion of the timer, the server will automatically enter the
         partner-down state.  This permits the server to allocate leases from the partner's  free
         lease pool after an STOS+MCLT timer expires, which can be dangerous if the partner is in
         fact operating at the time (the two servers will give conflicting bindings).

         Think very carefully before enabling this feature.  The partner-down and communications-
         interrupted  states are intentionally segregated because there do exist situations where
         a failover server can fail to communicate with its peer, but still has  the  ability  to
         receive  and  reply to requests from DHCP clients.  In general, this feature should only
         be used in those deployments where the failover servers are directly  connected  to  one
         another, such as by a dedicated hardwired link ("a heartbeat cable").

         A zero value disables the auto-partner-down feature (also the default), and any positive
         value indicates the time in seconds to wait before automatically entering partner-down.

       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
         passes.

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

         The second pass is only entered  into  if  the  first  pass  fails  to  reduce  the  lts
         underneath  the  total number of free state leases 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
         leases).

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

CLIENT CLASSING

       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.

       Classing support for DHCPv6 clients was addded in 4.3.0.  It follows the same rules as for
       DHCPv4 except that support for billing classes has not been added yet.

       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" {
       }

SUBCLASSES

       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 10.0.0.0 netmask 255.255.255.0 {
         pool {
           allow members of "allocation-class-1";
           range 10.0.0.11 10.0.0.50;
         }
         pool {
           allow members of "allocation-class-2";
           range 10.0.0.51 10.0.0.100;
         }
       }

       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
       client:

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

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

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION

       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.

SPAWNING CLASSES

       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.

COMBINING MATCH, MATCH IF AND SPAWN WITH

       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.

DYNAMIC DNS UPDATES

       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.

       There are two DNS schemes implemented.  The interim option is based on draft revisions  of
       the DDNS documents while the standard option is based on the RFCs for DHCP-DNS interaction
       and DHCIDs.  A third option, ad-hoc, was deprecated and has now been removed from the code
       base.   The  DHCP  server  must  be  configured  to use one of the two currently-supported
       methods, or not to do DNS updates.

       New installations should use the standard option. Older installations may want to continue
       using  the  interim  option  for  backwards  compatibility with the DNS database until the
       database can be updated.  This  can  be  done  with  the  ddns-update-style  configuration
       parameter.

THE DNS UPDATE SCHEME

       the  interim  and  standard  DNS  update schemes operate mostly according to work from the
       IETF.  The interim version was based on the drafts in  progress  at  the  time  while  the
       standard is based on the completed RFCs.  The standard RFCs are:

                                      RFC 4701 (updated by RF5494)
                                                RFC 4702
                                                RFC 4703

       And the corresponding drafts were:

                                   draft-ietf-dnsext-dhcid-rr-??.txt
                                   draft-ietf-dhc-fqdn-option-??.txt
                                 draft-ietf-dhc-ddns-resolution-??.txt

       The basic framework for the two schemes is similar with the main material difference being
       that a DHCID RR is used in the standard version while the interim versions uses a TXT  RR.
       The  format of the TXT record bears a resemblance to the DHCID RR but it is not equivalent
       (MD5 vs SHA2, field length differences etc).

       In these two schemes the DHCP server does not necessarily always update both the A and the
       PTR  records.   The  FQDN option includes a flag which, when sent by the client, indicates
       that the client wishes to update its own A record.   In  that  case,  the  server  can  be
       configured  either to honor the client´s intentions or ignore them.  This is done with the
       statement allow client-updates; or the  statement  ignore  client-updates;.   By  default,
       client updates are allowed.

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

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

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

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

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

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

       Both the standard and interim options also include a method to allow 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.  For the standard option the
       method works as follows:

       When  the DHCP server issues a client a new lease, it creates a text string that is an SHA
       hash over the DHCP client´s identification (see RFCs 4701 & 4702 for details).  The update
       attempts  to  add an A record with the name the server chose and a DHCID record containing
       the hashed identifier string (hashid).  If this update succeeds, the server is done.

       If the update fails because the A record already exists, then the DHCP server attempts  to
       add  the A record with the prerequisite that there must be a DHCID record in the same name
       as the new A record, and that DHCID 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 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.

       The interim DNS update scheme was written before the RFCs  were  finalized  and  does  not
       quite  follow  them.   The  RFCs  call  for a new DHCID RRtype while he interim DNS update
       scheme uses a TXT record.  In addition 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.

DYNAMIC DNS UPDATE SECURITY

       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 sneedville.edu  domain  will  be
       assigned  addresses  on  the  10.10.17.0/24  subnet.   In  that  case, you will need a key
       declaration for the TSIG key you will be using, and also two zone declarations -  one  for
       the  zone  containing  A  records that will be updates and one for the zone containing PTR
       records - for ISC BIND, something like this:

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

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

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

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

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

       zone EXAMPLE.ORG. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       zone 17.127.10.in-addr.arpa. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       The  primary  statement specifies the IP address of the name server whose zone information
       is to be updated.  In addition to the primary statement there  are  also  the  primary6  ,
       secondary and secondary6 statements.  The primary6 statement specifies an IPv6 address for
       the name server.  The secondaries provide for additional addresses for name servers to  be
       used  if  the  primary  does  not  respond.  The number of name servers the DDNS code will
       attempt to use before giving up is limited and is currently set to three.

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

       You  should choose your own secret key, of course.  The ISC BIND 9 distribution comes with
       a program for generating secret keys called dnssec-keygen.  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

       The key name, algorithm, and secret must match that being used by the DNS server. The DHCP
       server currently supports the following algorithms:

               HMAC-MD5
               HMAC-SHA1
               HMAC-SHA224
               HMAC-SHA256
               HMAC-SHA384
               HMAC-SHA512

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

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

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

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

REFERENCE: EVENTS

       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.

REFERENCE: DECLARATIONS

       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
       declared.

       The temporary 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
       prefix6.

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

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

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

       If client-specific boot parameters must change based on the network to which the client is
       attached, then multiple host declarations should be used.  The host declarations will only
       match a client if one of their fixed-address statements is viable on the subnet (or shared
       network)  where  the  client  is  attached.  Conversely, for a host declaration to match a
       client being allocated a dynamic address, it must not have any  fixed-address  statements.
       You  may  therefore need a mixture of host declarations for any given client...some having
       fixed-address 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
       protocol.

       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
       groups.

REFERENCE: ALLOW AND DENY

       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
       requests.

ALLOW DENY AND IGNORE IN SCOPE

       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.

       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 eventually reclaim these
       leases, but not while the client is running through  the  pool.  This  may  cause  serious
       thrashing  in  the  DNS,  and it will also cause the DHCP server to forget old DHCP client
       address 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 declines flag is only supported by DHCPv4 servers.  Given the large IPv6 address space
       and  the  internal  limits  imposed  by the server's address generation mechanism we don't
       think it is necessary for DHCPv6 servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
           a) Client renews a specific address:
           If a client using a given DUID submits a DHCP REQUEST containing
           the last address abandoned by that DUID, the address will be
           reassigned to that client.

           b) Upon the second restart following an address abandonment.  When
           an address is abandoned it is both recorded as such in the lease
           file and retained as abandoned in server memory until the server
           is restarted. Upon restart, the server will process the lease file
           and all addresses whose last known state is abandoned will be
           retained as such in memory but not rewritten to the lease file.
           This means that a subsequent restart of the server will not see the
           abandoned addresses in the lease file and therefore have no record
           of them as abandoned in memory and as such perceive them as free
           for assignment.

       The total number addresses in a pool, available for a  given  DUID  value,  is  internally
       limited  by the server's address generation mechanism.  If through mistaken configuration,
       multiple clients are using the same DUID  they  will  competing  for  the  same  addresses
       causing  the  server  to  reach  this  internal  limit rather quickly.  The internal limit
       isolates this type of activity such that address range is not  exhausted  for  other  DUID
       values.   The  appearance  of  the  following  error  log,  can  be  an indication of this
       condition:

           "Best match for DUID <XX> is an abandoned address, This may be a
            result of multiple clients attempting to use this DUID"

           where <XX> is an actual DUID value depicted as colon separated
           string of bytes in hexadecimal values.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

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

       The leasequery keyword

        allow leasequery;
        deny leasequery;

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

ALLOW AND DENY WITHIN POOL DECLARATIONS

       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 done.

       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:

        known-clients;

       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.

        unknown-clients;

       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

REFERENCE: PARAMETERS

       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

         authoritative;

         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
         declarations.

       The db-time-format statement

         db-time-format [ default | local ] ;

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

       The ddns-hostname statement

         ddns-hostname name;

         The name parameter should be the hostname that will be used in 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 dns-local-address4 and dns-local-address6 statements

         ddns-local-address4 address;

         ddns-local-address6 address;

         The  address parameter should be the local IPv4 or IPv6 address the server should use as
         the from address when sending DDNS update requests.

       The ddns-rev-domainname statement

         ddns-rev-domainname name;

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

         The reversed IP address to which this domain name is appended is always the  IP  address
         of  the  client,  in  dotted  quad  notation,  reversed - for example, if the IP address
         assigned to the client is 10.17.92.74, then the reversed IP address is 74.92.17.10.   So
         a   client  with  that  IP  address  would,  by  default,  be  given  a  PTR  record  of
         10.17.92.74.in-addr.arpa.

       The ddns-update-style parameter

         ddns-update-style style;

         The style parameter must be one of standard, 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
         --enable-delayed-ack´.

       The dhcp-cache-threshold statement

         dhcp-cache-threshold percentage;

         The  dhcp-cache-threshold  statement  takes  one  integer  parameter with allowed values
         between 0 and 100. The default value is 25 (25%  of  the  lease  time).  This  parameter
         expresses  the  percentage  of the total lease time, measured from the beginning, during
         which a client's attempt to renew its lease will result in getting the already  assigned
         lease, rather than an extended lease.

         Clients  that  attempt  renewal  frequently can cause the server to update and write the
         database frequently resulting in a performance impact on the  server.   The  dhcp-cache-
         threshold  statement  instructs  the DHCP server to avoid updating leases too frequently
         thus avoiding this behavior.  Instead the server assigns the same lease (i.e. reuses it)
         with  no  modifications  except  for CLTT (Client Last Transmission Time) which does not
         require disk operations. This feature applies to IPv4 only.

         When an existing lease is matched to a renewing client, it will be reused if all of  the
         following conditions are true:
             1. The dhcp-cache-threshold is larger than zero
             2. The current lease is active
             3. The percentage of the lease time that has elapsed is less than
             dhcp-cache-threshold
             4. The client information provided in the renewal does not alter
             any of the following:
                a. DNS information and DNS updates are enabled
                b. Billing class to which the lease is associated

       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.  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 dont-use-fsync statement

         dont-use-fsync flag;

         The dont-use-fsync statement instructs the DHCP server if it should  call  fsync()  when
         writing leases to the lease file.  By default and if the flag is set to false the server
         will call fsync().  Suppressing the call to fsync() may increase the performance of  the
         server  but  it  also  adds a risk that a lease will not be properly written to the disk
         after it has been issued to a client and before the server  stops.   This  can  lead  to
         duplicate  leases  being  issued  to  different  clients.   Using  this  option  is  not
         recommended.

       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 echo-client-id statement

         echo-client-id flag;

         The echo-client-id statement is used to enable or disable RFC 6842  compliant  behavior.
         If  the  echo-client-id  statement  is present and has a value of true or on, and a DHCP
         DISCOVER or REQUEST is received which contains the client identifier option (Option code
         61),  the  server will copy the option into its response (DHCP ACK or NAK) per RFC 6842.
         In other words if the client sends the option it will receive it back. By default,  this
         flag is off and client identifiers will not echoed back to the client.

       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 fixed-prefix6 declaration

         fixed-prefix6 low-address / bits;

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

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

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

         If  there  are  multiple  host  delcarations the server will try to choose a declaration
         where the fixed-address6 matches the client's subnet.  If none match it will choose  one
         that doesn't have a fixed-address6 statement.

         Note  Well: Unlike the fixed address the fixed prefix does not need to match a subnet in
         order to be served.  This allows you to provide a prefix to a client that is outside  of
         the subnet on which the client makes the request to the the server.

       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
         clients.

       The host-identifier option statement

         host-identifier option option-name option-data;

         or

         host-identifier v6relopt number 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.   In  the  v6relopts  case the additional number is the relay to
         examine for the specified option name and value.  The values are the  same  as  for  the
         v6relay  option.   0 is a no-op, 1 is the relay closest to the client, 2 the next one in
         and so on.  Values that are larger than the maximum  number  of  relays  (currently  32)
         indicate the relay closest to the server independent of number.

       The ignore-client-uids statement

         ignore-client-uids flag;

         If  the  ignore-client-uids  statement is present and has a value of true or on, the UID
         for clients will not be recorded.  If this statement is not present or has  a  value  of
         false or off, then client UIDs will be recorded.

       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
         /var/lib/dhcp/dhcpd.leases.  This statement must  appear  in  the  outer  scope  of  the
         configuration  file  -  if  it  appears  in  some  other  scope, it will have no effect.
         Furthermore, it has no effect if  overridden  by  the  -lf  flag  or  the  PATH_DHCPD_DB
         environment variable.

       The limit-addrs-per-ia statement

         limit-addrs-per-ia number;

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

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

       The dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name is the name of the lease file to use if and only if the server is running in DHCPv6
         mode.  By default, this is /var/lib/dhcp/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 networks; it  is
         only realistically useful for a server whose only clients are reached via unicasts, such
         as via DHCP relay agents.

         Note:  This statement is only effective if the server was compiled using the USE_SOCKETS
         #define  statement, which is default on a small number of operating systems, and must be
         explicitly chosen at 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 file should be fine) and send a SIGHUP
         to syslogd.  Some systems support log rollover using a shell script  or  program  called
         newsyslog  or  logrotate, and you may be able to configure this as well so that your log
         file doesn't grow uncontrollably.

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

       The log-threshold-high and log-threshold-low statements

         log-threshold-high percentage;

         log-threshold-low percentage;

         The log-threshold-low and log-threshold-high statements  are  used  to  control  when  a
         message is output about pool usage.  The value for both of them is the percentage of the
         pool in use.  If the high threshold is 0 or has not been specified, no messages will  be
         produced.   If a high threshold is given, a message is output once the pool usage passes
         that level.  After that, no more messages will be output  until  the  pool  usage  falls
         below  the  low  threshold.  If the low threshold is not given, it default to a value of
         zero.

         A special case occurs when the low threshold is set to be higer than the high threshold.
         In  this  case,  a  message will be generated each time a lease is acknowledged when the
         pool usage is above the high threshold.

         Note that threshold logging will be automatically  disabled  for  shared  subnets  whose
         total number of addresses is larger than (2^64)-1.  The server will emit a log statement
         at startup when threshold logging is disabled as shown below:

             "Threshold logging disabled for shared subnet of ranges: <addresses>"

         This is likely to have no practical runtime effect as CPUs are  unlikely  to  support  a
         server actually reaching such a large number of leases.

       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 addresses.

       The next-server statement

         next-server server-name;

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

       The omapi-port statement

         omapi-port port;

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

       The one-lease-per-client statement

         one-lease-per-client flag;

         If  this  flag is enabled, whenever a client sends a DHCPREQUEST for a particular lease,
         the server will automatically free any other leases the  client  holds.   This  presumes
         that  when  the  client sends a 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
         /var/run/dhcpd.pid.   Like  the lease-file-name statement, this statement must appear in
         the outer scope of the configuration file.  It has no effect if overridden  by  the  -pf
         flag or the PATH_DHCPD_PID environment variable.

         The dhcpv6-pid-file-name statement

            dhcpv6-pid-file-name name;

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

         The ping-check statement

            ping-check flag;

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

            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
            done.

         The ping-timeout statement

            ping-timeout seconds;

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

         The preferred-lifetime statement

            preferred-lifetime seconds;

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

            The  preferred lifetime defaults to the renew+rebind timers, or 3/4 the default lease
            time if none were specified.

         The prefix-length-mode statement

            prefix-length-mode mode;

            According to RFC  3633,  DHCPv6  clients  may  specify  preferences  when  soliciting
            prefixes  by  including  an  IA_PD  Prefix  option within the IA_PD option. Among the
            preferences that may be conveyed is the "prefix-length". When non-zero it indicates a
            client's  desired  length  for  offered  prefixes.   The RFC states that servers "MAY
            choose to use the information...to  select  prefix(es)"  but  does  not  specify  any
            particular  rules  for  doing so. The prefix-length-mode statement can be used to set
            the prefix selection rules employed by the  server,  when  clients  send  a  non-zero
            prefix-length  value.  The  mode  parameter  must  be  one  of ignore, prefer, exact,
            minimum, or maximum where:

            1. ignore - The requested  length  is  ignored.  The  server  will  offer  the  first
            available prefix.

            2.  prefer - The server will offer the first available prefix with the same length as
            the requested length.  If none are found then  it  will  offer  the  first  available
            prefix of any length.

            3.  exact  - The server will offer the first available prefix with the same length as
            the requested length.  If none are found, it  will  return  a  status  indicating  no
            prefixes available.  This is the default behavior.

            4. minimum - The server will offer the first available prefix with the same length as
            the requested length.  If none are found, it will return the first  available  prefix
            whose  length  is  greater  than (e.g. longer than), the requested value.  If none of
            those are found, it will return a  status  indicating  no  prefixes  available.   For
            example, if client requests a length of /60, and the server has available prefixes of
            lengths /56 and /64, it will offer prefix of length /64.

            5. maximum - The server will offer the first available prefix with the same length as
            the  requested  length.  If none are found, it will return the first available prefix
            whose length is less than (e.g. shorter than), the requested value.  If none of those
            are found, it will return a status indicating no prefixes available.  For example, if
            client requests a length of /60, and the server has available prefixes of lengths /56
            and /64, it will offer a prefix of length /56.

            In general "first available" is determined by the order in which pools are defined in
            the server's configuration.  For example, if a subnet is defined  with  three  prefix
            pools A,B, and C:

            subnet 3000::/64 {
                 # pool A
                 pool6 {
                      :
                 }
                 # pool B
                 pool6 {
                      :
                 }
                 # pool C
                 pool6 {
                      :
                 }
            }

            then  the  pools will be checked in the order A, B, C. For modes prefer, minimum, and
            maximum this may mean checking the pools in that order twice.  A first  pass  through
            is made looking for an available prefix of exactly the preferred length.  If none are
            found, then a second pass is performed starting with pool A  but  with  appropriately
            adjusted length criteria.

         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  127.0.0.1,
            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-id-check statement

            server-id-check flag;

            The   server-id-check  statement  is  used  to  control  whether  or  not  a  server,
            participating in failover, verifies that  the  value  of  the  dhcp-server-identifier
            option in received DHCP REQUESTs match the server's id before processing the request.
            Server id checking is disabled by default.  Setting this flag enables id checking and
            thereafter  the  server will only process requests that match.  Note the flag setting
            should be consistent between failover partners.

            Unless overridden by use of the server-identifier statement,  the  value  the  server
            uses  as  its  id  will  be the first IP address associated with the physical network
            interface on which the request arrived.

            In order to reduce runtime overhead the server only checks for a server id option  in
            the  global  and  subnet  scopes.  Complicated configurations may result in different
            server ids for this check and when the server id for a reply  packet  is  determined,
            which would prohibit the server from responding.

            The  primary  use  for this option is when a client broadcasts a request but requires
            that the response come from a specific failover peer.  An example of  this  would  be
            when  a  client  reboots  while its lease is still active - in this case both servers
            will normally respond.  Most of the time the client won't check the server id and can
            use  either  of the responses.  However if the client does check the server id it may
            reject the response if it came from the wrong peer.  If the timing is such  that  the
            "wrong"  peer  responds  first most of the time the client may not get an address for
            some time.

            Care should be taken before enabling this option.

         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-
            identifier.

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

            The default server-duid type is LLT.

         The server-name statement

            server-name name ;

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

         The 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
            expires.

         The update-static-leases statement

            update-static-leases flag;

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

            is equivalent to

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

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

            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.

SETTING PARAMETER VALUES USING EXPRESSIONS

       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));

RESERVED LEASES

       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´.

REFERENCE: OPTION STATEMENTS

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

REFERENCE: EXPRESSIONS

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

SEE ALSO

       dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.

AUTHOR

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

                                                                                    dhcpd.conf(5)