DHCP server database of assigned leases
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:
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. You can display the current time in UTC by typing date -u.
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.
The other statements include the following:
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.
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.
The variables include:
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:
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