Manpages

dhcpd.leases

NAME

DESCRIPTION

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.

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.

There is a window of vulnerability where if the dhcpd process is killed or the system crashes after the old lease database has been renamed but before the new one has been moved into place, there will be no /var/lib/dhcp/dhcpd.leases. In this case, dhcpd will refuse to start, and will require manual intervention. DO NOT simply create a new lease file when this happens - if you do, you will lose all your old bindings, and chaos will ensue. Instead, rename /var/lib/dhcp/dhcpd.leases~ to /var/lib/dhcp/dhcpd.leases, restoring the old, valid lease file, and then start dhcpd. This guarantees that a valid lease file will be restored.  

FORMAT

 lease ip-address { statements... }

Each lease declaration include 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.

The start and end time of a lease are recorded using the ``starts'' and ``ends'' statements:

  starts date;
  ends date;

Dates are specified 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 Greenwich Mean Time (GMT), not in the local time zone. Since Greenwich is actually on Daylight Savings Time part of the year, there is probably nowhere in the world where the times recorded on a lease are always the same as wall clock times. On a unix machine, one can often figure out the current time in GMT by typing date -u.

The MAC address of the network interface that was used to acquire the lease is recorded with the hardware statement:

 hardware hardware-type mac-address;

The MAC address is specified as a series of hexadecimal octets, seperated by colons.

If the client used a client identifier to acquire its address, the client identifier is recorded using the uid statement:

 uid client-identifier;

The client identifier is recorded as a series of hexadecimal octets, regardless of whether the client specifies an ASCII string or uses the newer hardware type/MAC address format.

If the client sends a hostname using the Client Hostname option, as specified in some versions of the DHCP-DNS Interaction draft, that hostname is recorded using the client-hostname statement.

 client-hostname "hostname";

If the client sends its hostname using the Hostname option, as Windows 95 does, it is recorded using the hostname statement.

 hostname "hostname";

The DHCP server may determine that a lease has been misused in some way, either because a client that has been assigned a lease NAKs it, or because the server's own attempt to see if an address is in use prior to reusing it reveals that the address is in fact already in use. In that case, the abandoned statement will be used to indicate that the lease should not be reassigned.

 abandoned;

Abandoned leases are reclaimed automatically. When a client asks for a new address, and the server finds that there are no new addresses, it checks to see if there are any abandoned leases, and allocates the least recently abandoned lease. The standard mechanisms for checking for lease address conflicts are still followed, so if the abandoned lease's IP address is still in use, it will be reabandoned.

If a client requests an abandoned address, the server assumes that the reason the address was abandoned was that the lease file was corrupted, and that the client is the machine that responded when the lease was probed, causing it to be abandoned. In that case, the address is immediately assigned to the client.  

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