Ubuntu Manpage: dhcpd.leases - DHCP client lease database

Provided by: isc-dhcp-server_4.2.4-7ubuntu12.13_amd64

NAME

       dhcpd.leases - DHCP client lease database

DESCRIPTION

       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 DBDIR/dhcpd.leases.   You can do this with:

            touch DBDIR/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 DBDIR/dhcpd.leases~.   Finally, the newly written lease database is moved
       into place.

FORMAT

       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.

THE LEASE DECLARATION

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 specified if the failover protocol is being used, and indicates what time the peer has been told the
lease expires. The tsfp statement is also specified 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.

The 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.

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.

abandoned;

The abandoned statement indicates that the DHCP server has abandoned the lease. In that case, the
abandoned statement will be used to indicate that the lease should not be reassigned. Please see the
dhcpd.conf(5) manual page for information about abandoned leases.

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 will be either active or free. 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.

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.

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

The option agent.circuit-id and option agent.remote-id statements are used to record the circuit ID and
remote ID options send 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.

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.

The ddns-text variable

The ddns-text variable is used to record the value of the client's TXT identification record when the
interim ddns update style has been used to update the DNS for a particular lease.

The ddns-fwd-name variable

The ddns-fwd-name 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 to use the interim ddns update style, and is also configured to allow clients
to update their own fqdns, and the client did in fact update its own fqdn, then 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.

The vendor-class-identifier variable

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

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.

bootp; reserved; These two statements are effectively flags. 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.

THE FAILOVER PEER STATE DECLARATION

       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.

FILES

       DBDIR/dhcpd.leases DBDIR/dhcpd.leases~

AUTHOR

       dhcpd(8) was written by Ted Lemon under a contract with  Vixie  Labs.    Funding  for  this  project  was
       provided  by Internet Systems Consortium.  Information about Internet Systems Consortium can be found at:
       https://www.isc.org/

                                                                                                 dhcpd.leases(5)