dhcpd.leases, dhcpd6.leases

DHCP server database of assigned leases

Name:

  • /var/db/dhcpd.leases for DHCPv4
  • /var/db/dhcpd6.leases for DHCPv6

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 /var/db/dhcpd.leases for DHCPv4, or /var/db/dhcpd6.leases for DHCPv6. You can do this with:

touch /var/db/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/db/dhcpd.leases~ (/var/db/dhcpd6.leases~ for DHCPv6). 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 and dhclient.conf 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's 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

A lease declaration takes this form:

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, according to the system's configured time zone, 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.

The other statements include the following:

hardware hardware-type mac-address;
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;
Records the client identifier used by the client to acquire the lease. Clients are not required to send client identifiers, and this statement appears only 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;
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. For information about abandoned leases, see the documentation for dhcpd.conf .
binding state state; next binding state state;
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;
Sets the value of a variable on the lease. For general information on variables, see the DHCP Conditional Evaluation entry.

The variables include:

  • ddns-text — 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.
  • ddns-fwd-name — 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.
  • ddns-client-fqdn — 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.
  • ddns-rev-name — 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.
on events { statements... }
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
  • startup

Contributing author:

dhcpd 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: http://www.isc.org/ .

See also:

RFC2132, RFC2131

Last modified: 2013-12-21



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