Provided by: isc-dhcp-server_4.2.4-7ubuntu12.13_amd64 
NAME
dhcpd.leases - DHCP client lease database
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 DBDIR/dhcpd.leases. You can do this with:
touch DBDIR/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 DBDIR/dhcpd.leases~. 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(5) and dhclient.conf(5) 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 is 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
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 as
according to the system's configured timezone, 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.
hardware hardware-type mac-address;
The hardware statement 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;
The uid statement records the client identifier used by the client to acquire the lease.
Clients are not required to send client identifiers, and this statement only appears 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;
The abandoned statement 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. Please see the dhcpd.conf(5) manual page for information about abandoned
leases.
binding state state; next binding state state;
The binding state statement declares 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;
The set statement sets the value of a variable on the lease. For general information on
variables, see the dhcp-eval(5) manual page.
The ddns-text variable
The ddns-text variable is 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.
The ddns-fwd-name variable
The ddns-fwd-name variable 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.
The ddns-client-fqdn variable
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.
The ddns-rev-name variable
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.
The vendor-class-identifier variable
The server retains the client-supplied Vendor Class Identifier option for informational
purposes, and to render them in DHCPLEASEQUERY responses.
on events { statements... } The on statement records 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, and startup.
FILES
DBDIR/dhcpd.leases DBDIR/dhcpd.leases~
SEE ALSO
dhcpd(8), dhcp-options(5), dhcp-eval(5), dhcpd.conf(5), RFC2132, RFC2131.
AUTHOR
dhcpd(8) 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: https://www.isc.org/
dhcpd.leases(5)