Provided by:
dhcp_2.0pl5-19.4_i386 
NAME
dhcpd.leases - DHCP client lease database
DESCRIPTION
The Internet Software 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.
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 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. Currently, the only declaration that is used
in the dhcpd.leases file is the lease declaration.
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,
separated 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.
FILES
/var/lib/dhcp/dhcpd.leases
SEE ALSO
dhcpd(8), dhcp-options(5), dhcpd.conf(5), RFC2132, RFC2131.
AUTHOR
dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a contract
with Vixie Labs. Funding for this project was provided by the
Internet Software Corporation. Information about the Internet Software
Consortium can be found at http://www.isc.org/isc.
dhcpd.leases(5)