dhcpd.leases - DHCP client lease database


   The Internet Systems Consortium DHCP Server keeps a persistent database
   of leases that it has assigned.  This database  is  a  free-form  ASCII
   file  containing a series of lease declarations.  Every time a lease is
   acquired, renewed or released, its new value is recorded at the end  of
   the  lease  file.   So if more than one declaration appears for a given
   lease, the last one in the file is the current one.

   When dhcpd is first installed, there is no lease  database.    However,
   dhcpd  requires  that a lease database be present before it will start.
   To make the initial lease database, just create an  empty  file  called
   /var/lib/dhcp/dhcpd.leases.   You can do this with:

        touch /var/lib/dhcp/dhcpd.leases

   In  order to prevent the lease database from growing without bound, the
   file is rewritten  from  time  to  time.    First,  a  temporary  lease
   database  is created and all known leases are dumped to it.   Then, the
   old lease database is renamed  /var/lib/dhcp/dhcpd.leases~.    Finally,
   the newly written lease database is moved into place.

   In  order  to  process both DHCPv4 and DHCPv6 messages you will need to
   run two separate  instances  of  the  dhcpd  process.   Each  of  these
   instances will need it's own lease file.  You can use the -lf option on
   the server's command line to specify a different lease  file  name  for
   one or both servers.


   Lease  descriptions  are  stored in a format that is parsed by the same
   recursive  descent  parser  used  to   read   the   dhcpd.conf(5)   and
   dhclient.conf(5)  files.   Lease  files can contain lease declarations,
   and  also  group  and  subgroup  declarations,  host  declarations  and
   failover state declarations.  Group, subgroup and host declarations are
   used to record objects created using the OMAPI protocol.

   The lease file is a log-structured file - whenever a lease changes, the
   contents of that lease are written to the end of the file.   This means
   that it is entirely possible and quite reasonable for there to  be  two
   or  more  declarations  of the same lease in the lease file at the same
   time.   In that case,  the  instance  of  that  particular  lease  that
   appears last in the file is the one that is in effect.

   Group,  subgroup and host declarations in the lease file are handled in
   the same manner, except that if any of these  objects  are  deleted,  a
   rubout  is  written  to  the  lease  file.    This  is  just  the  same
   declaration, with { deleted; } in the scope of the declaration.    When
   the  lease  file  is rewritten, any such rubouts that can be eliminated
   are eliminated.   It  is  possible  to  delete  a  declaration  in  the
   dhcpd.conf  file; in this case, the rubout can never be eliminated from
   the dhcpd.leases file.


   While the lease file formats for DHCPv4 and DHCPv6 are  different  they
   share  many  common  statements and structures.  This section describes
   the common  statements  while  the  succeeding  sections  describe  the
   protocol specific statements.


   A  date  is specified in two ways, depending on the configuration value
   for the db-time-format parameter.  If it was set to default,  then  the
   date fields appear as follows:

   weekday year/month/day hour:minute:second

   The weekday is present to make it easy for a human to tell when a lease
   expires - it's specified as a number from zero to six, with zero  being
   Sunday.   The  day  of week is ignored on input.  The year is specified
   with the century, so it should generally  be  four  digits  except  for
   really long leases.  The month is specified as a number starting with 1
   for January.  The day of the month is likewise specified starting  with
   1.   The hour is a number between 0 and 23, the minute a number between
   0 and 59, and the second also a number between 0 and 59.

   Lease times are specified in Universal Coordinated Time (UTC),  not  in
   the  local time zone.  There is probably nowhere in the world where the
   times recorded on a lease are always the same as wall clock times.   On
   most  unix  machines, you can display the current time in UTC by typing
   date -u.

   If the db-time-format was configured to local,  then  the  date  fields
   appear as follows:

    epoch  <seconds-since-epoch>;  #  <day-name> <month-name> <day-number>
   <hours>:<minutes>:<seconds> <year>

   The seconds-since-epoch is as according to  the  system's  local  clock
   (often  referred  to  as "unix time").  The # symbol supplies a comment
   that describes what actual time this is as according  to  the  system's
   configured timezone, at the time the value was written.  It is provided
   only for human inspection.

   If a lease will never expire, date is never instead of an actual date.

   General Variables

   As part of the processing of a lease information may be attached to the
   lease  structure,  for example the DDNS information or if you specify a
   variable in your configuration file.  Some  of  these,  like  the  DDNS
   information, have specific descriptions below.  For others, such as any
   you might define, a generic line of the following will be included.

   set variable = value;

   The set statement sets the value of  a  variable  on  the  lease.   For
   general information on variables, see the dhcp-eval(5) manual page.

   DDNS Variables

   The ddns-text and ddns-dhcid variables

   These variables are used to record the value of the client's identification
   record when the server has updated DNS for a particular lease.  The text
   record is used with the interim DDNS update style while the dhcid record
   is used for the standard DDNS update style.

   The ddns-fwd-name variable

   This variable records the value of the name used in
   updating the client's A record if a DDNS update has been successfully
   done by the server.   The server may also have used this name to
   update the client's PTR record.

   The ddns-client-fqdn variable

   If the server is configured both to use the interim or standard DDNS update
   style, and to allow clients to update their own FQDNs, then if the
   client did in fact update its own FQDN, the
   ddns-client-fqdn variable records the name that the client has
   indicated it is using.   This is the name that the server will have
   used to update the client's PTR record in this case.

   The ddns-rev-name variable

   If the server successfully updates the client's PTR record, this
   variable will record the name that the DHCP server used for the PTR
   record.   The name to which the PTR record points will be either the
   ddns-fwd-name or the ddns-client-fqdn.

   Executable Statements

   on events { statements... }
   The on statement records a list of statements to execute if a
   certain event occurs.   The possible events that can occur for an
   active lease are release and expiry.   More than one event
   can be specified - if so, the events are separated by '|' characters.


   lease ip-address { statements... }

   Each  lease  declaration  includes  the single IP address that has been
   leased to the client.   The statements within  the  braces  define  the
   duration of the lease and to whom it is assigned.

   starts date;
   ends date;
   tstp date;
   tsfp date;
   atsfp date;
   cltt date;

   The  start  and  end  time of a lease are recorded using the starts and
   ends statements.   The  tstp  statement  is  present  if  the  failover
   protocol  is being used, and indicates what time the peer has been told
   the lease expires.   The tsfp statement is also present if the failover
   protocol  is  being  used, and indicates the lease expiry time that the
   peer has acknowledged.  The atsfp statement is  the  actual  time  sent
   from  the  failover  partner.   The cltt statement is the client's last
   transaction time.

   See the description of dates in the section on common structures.

   hardware hardware-type mac-address;

   The hardware statement records the MAC address of the network interface
   on  which  the  lease  will  be  used.   It is specified as a series of
   hexadecimal octets, separated by colons.

   uid client-identifier;

   The uid statement records the client identifier used by the  client  to
   acquire   the   lease.    Clients  are  not  required  to  send  client
   identifiers, and this statement only appears if the client did in  fact
   send  one.    Client  identifiers  are  normally  an  ARP  type  (1 for
   ethernet) followed by the  MAC  address,  just  like  in  the  hardware
   statement, but this is not required.

   The client identifier is recorded as a colon-separated hexadecimal list
   or as a quoted string.   If it is recorded as a quoted  string  and  it
   contains  one  or  more  non-printable characters, those characters are
   represented as octal escapes - a backslash character followed by  three
   octal digits.

   client-hostname hostname ;

   Most DHCP clients will send their hostname in the host-name option.  If
   a client sends its hostname in this way, the hostname  is  recorded  on
   the  lease  with a client-hostname statement.   This is not required by
   the protocol, however, so many specialized DHCP clients do not  send  a
   host-name option.

   binding state state;
   next binding state state;

   The  binding  state statement declares the lease's binding state.  When
   the DHCP server is not configured  to  use  the  failover  protocol,  a
   lease's  binding  state may be active, free or abandoned.  The failover
   protocol adds some additional  transitional  states,  as  well  as  the
   backup   state,  which  indicates  that  the  lease  is  available  for
   allocation by the failover  secondary.  Please  see  the  dhcpd.conf(5)
   manual page for more information about abandoned leases.

   The  next  binding  state statement indicates what state the lease will
   move to when the current state expires.   The  time  when  the  current
   state expires is specified in the ends statement.

   rewind binding state state;

   This  statement is part of an optimization for use with failover.  This
   helps a server rewind a lease to the state most recently transmitted to
   its peer.

   option agent.circuit-id string;
   option agent.remote-id string;

   These  statements  are  used  to  record  the  circuit ID and remote ID
   options sent by the relay agent, if the  relay  agent  uses  the  relay
   agent  information  option.    This  allows  these  options  to be used
   consistently  in  conditional  evaluations  even  when  the  client  is
   contacting the server directly rather than through its relay agent.

   The vendor-class-identifier variable

   The  server  retains the client-supplied Vendor Class Identifier option
   for informational  purposes,  and  to  render  them  in  DHCPLEASEQUERY


   If  present,  they  indicate that the BOOTP and RESERVED failover flags
   (respectively) should be set.  BOOTP and RESERVED  dynamic  leases  are
   treated  differently  than  normal  dynamic leases, as they may only be
   used by the client to which they are currently allocated.

   Other Additional options or executable statements may be included,  see
   the description of them in the section on common structures.


   ia_ta  IAID_DUID { statements... }
   ia_na  IAID_DUID { statements... }
   ia_pd  IAID_DUID { statements... }

   Each  lease  declaration  starts  with a tag indicating the type of the
   lease.  ia_ta is for temporary addresses, ia_na  is  for  non-temporary
   addresses  and  ia_pd  is for prefix delegation.  Following this tag is
   the combined IAID and DUID from the client for this lease.

   The IAID_DUID value is recorded as a colon-separated  hexadecimal  list
   or  as  a  quoted string.   If it is recorded as a quoted string and it
   contains one or more non-printable  characters,  those  characters  are
   represented  as octal escapes - a backslash character followed by three
   octal digits.

   cltt date;

   The cltt statement is the client's last transaction time.

   See the description of dates in the section on common structures.

   iaaddr ipv6-address { statements... }
   iaprefix ipv6-address/prefix-length { statements... }

   Within a given lease there can be multiple iaaddr and iaprefix statements.
   Each will have either an IPv6 address or an IPv6 prefix (an address and
   a prefix length indicating a CIDR style block of addresses).  The following
   statements may occur Within each iaaddr or iaprefix.

   binding state state;

   The binding state statement declares the lease's binding state.
   In DHCPv6 you will normally see this as active or expired.

   preferred-life lifetime;

   The IPv6 preferred lifetime associated with this address, in seconds.

   max-life lifetime;

   The valid lifetime associated with this address, in seconds.

   ends date;

   The end time of the lease.  See the description of dates in the section on
   common structures.

   Additional options or executable statements may be included.  See the description
   of them in the section on common structures.


   The state of any failover peering arrangements is also recorded in  the
   lease file, using the failover peer statement:

   failover peer name state {
   my state state at date;
   peer state state at date;

   The  states  of the peer named name is being recorded.   Both the state
   of the running server (my state) and the other failover  partner  (peer
   state)  are  recorded.    The  following  states are possible: unknown-
   state, partner-down,  normal,  communications-interrupted,  resolution-
   interrupted,   potential-conflict,   recover,  recover-done,  shutdown,
   paused, and startup.


   /var/lib/dhcp/dhcpd.leases /var/lib/dhcp/dhcpd.leases~


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


   dhcpd(8)  is  maintained  by  ISC.   Information about Internet Systems
   Consortium can be found at: https://www.isc.org/


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