dhcpd.leases - DHCP client lease database
DESCRIPTION
The Internet Software Consortium DHCP Server keeps a per
sistent 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 with
out 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 pro
cess 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.
``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 sec
ond 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 proba
bly 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 hexadeci
mal 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";
option, as Windows 95 does, it is recorded using the host
name statement.
hostname "hostname";
The DHCP server may determine that a lease has been mis
used 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 indi
cate 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 immedi
ately 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. Informa
tion about the Internet Software Consortium can be found
at http://www.isc.org/isc.
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