Provided by: isc-dhcp-server_4.3.3-5ubuntu12.10_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)