dhcpd.leases (5) - Linux Manuals
dhcpd.leases: DHCP client lease database
NAME
dhcpd.leases - DHCP client lease databaseDESCRIPTION
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/lib/dhcpd/dhcpd.leases. You can do this with:
touch /var/lib/dhcpd/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/dhcpd/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:
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 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.
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;
}
FILES
/var/lib/dhcpd/dhcpd.leases /var/lib/dhcpd/dhcpd.leases~
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/