isc-dhcp/server/dhcpd.leases.5

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.\" $Id: dhcpd.leases.5,v 1.17 2011/09/19 00:24:50 sar Exp $
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.TH dhcpd.leases 5
.SH NAME
dhcpd.leases - DHCP client lease database
.SH 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.
.PP
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:
.PP
.nf
	touch DBDIR/dhcpd.leases
.fi
.PP
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.
.PP
In order to process both DHCPv4 and DHCPv6 messages you will need to
run two separate instances of the dhcpd process.  Each of these
instances will need it's own lease file.  You can use the \fI-lf\fR
option on the server's command line to specify a different lease file
name for one or both servers.
.SH FORMAT
Lease descriptions are stored in a format that is parsed by the same
recursive descent parser used to read the
.B dhcpd.conf(5)
and
.B 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.
.PP
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.
.PP
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
\fIrubout\fR is written to the lease file.   This is just the same
declaration, with \fB{ deleted; }\fR 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 \fBdhcpd.conf\fR file; in this case, the rubout
can never be eliminated from the \fBdhcpd.leases\fR file.
.SH COMMON STATEMENTS FOR LEASE DECLARATIONS
While the lease file formats for DHCPv4 and DHCPv6 are different
they share many common statements and structures.  This section
describes the common statements while the succeeding sections
describe the protocol specific statements.
.PP
.B Dates
.PP
A \fIdate\fR is specified in two ways, depending on the configuration
value for the \fBdb-time-format\fR parameter.  If it was set to \fIdefault\fR,
then the \fIdate\fR fields appear as follows:
.PP
.I weekday year\fB/\fImonth\fB/\fIday hour\fB:\fIminute\fB:\fIsecond\fR
.PP
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.
.PP
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
\fBdate -u\fR.
.PP
If the \fBdb-time-format\fR was configured to \fIlocal\fR, then
the \fIdate\fR fields appear as follows:
.PP
 \fBepoch\fR \fI<seconds-since-epoch>\fR\fB; #\fR \fI<day-name> <month-name>
<day-number> <hours>\fR\fB:\fR\fI<minutes>\fR\fB:\fR\fI<seconds> <year>\fR
.PP
The \fIseconds-since-epoch\fR is as according to the system's local clock (often
referred to as "unix time").  The \fB#\fR 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.
.PP
If a lease will never expire, \fIdate\fR is \fBnever\fR instead of an
actual date.
.PP
.B General Variables
.PP
As part of the processing of a lease information may be attached to the
lease structure, for example the DDNS information or if you specify a
variable in your configuration file.  Some of these, like the DDNS
information, have specific descriptions below.  For others, such as
any you might define, a generic line of the following will be included.
.PP
.B set \fIvariable\fB = \fIvalue\fB;
.PP
The \fBset\fR statement sets the value of a variable on the lease.
For general information on variables, see the \fBdhcp-eval(5)\fR
manual page.
.PP
.B DDNS Variables
.PP
.nf
.B The \fIddns-text\fB and \fIddns-dhcid\fB variables
.PP
These variables are used to record the value of the client's identification
record when the server has updated DNS for a particular lease.  The text
record is used with the interim DDNS update style while the dhcid record
is used for the standard DDNS update style.
.PP
.B The \fIddns-fwd-name\fB variable
.PP
This 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.
.PP
.B The \fIddns-client-fqdn\fB variable
.PP
If the server is configured both to use the interim or standard DDNS update
style, and to allow clients to update their own FQDNs, then if the
client did in fact update its own FQDN, the
\fIddns-client-fqdn\fR 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.
.PP
.B The \fIddns-rev-name\fB variable
.PP
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
\fIddns-fwd-name\fR or the \fIddns-client-fqdn\fR.
.PP
.B Executable Statements
.PP
.B on \fIevents\fB { \fIstatements...\fB }
The \fBon\fR statement records a list of statements to execute if a
certain event occurs.   The possible events that can occur for an
active lease are \fBrelease\fR and \fBexpiry\fR.   More than one event
can be specified - if so, the events are separated by '|' characters.
.PP
The \fIauthoring-byte-order\fR statement
.RS 0.25i
.PP
.B authoring-byte-order \fR[ \fIbig-endian\fR | \fIlittle-endian\fR ] \fB;\fR
.PP
This statement is automatically added to the top of new lease files by
the server. It indicates the internal byte order of the server.  This
permits lease files generated on a server with one form of byte order
to be read by a server with a different form.  Lease files which do not
contain this entry are simply treated as having the same byte order as
the server reading them.  If you are migrating lease files generated
by a server that predates this statement and is of a different byte
order than the your destination server, you can manually add this
statement.  It must proceed any lease entries.  Valid values for this
parameter are \fIlittle-endian\fR and \fIbig-endian\fR.
.RE
.PP
.SH THE DHCPv4 LEASE DECLARATION
.PP
.B lease \fIip-address\fB { \fIstatements...\fB }
.PP
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.
.PP
.nf
.B starts \fIdate\fB;\fR
.B ends \fIdate\fB;\fR
.B tstp \fIdate\fB;\fR
.B tsfp \fIdate\fB;\fR
.B atsfp \fIdate\fB;\fR
.B cltt \fIdate\fB;\fR
.fi
.PP
The start and end time of a lease are recorded using the \fBstarts\fR
and \fBends\fR statements.   The \fBtstp\fR statement is present if
the failover protocol is being used, and indicates what time the peer
has been told the lease expires.   The \fBtsfp\fR statement is
also present if the failover protocol is being used, and indicates
the lease expiry time that the peer has acknowledged.
The \fBatsfp\fR statement is the actual time sent from the failover
partner.
The \fBcltt\fR statement is the client's last transaction time.
.PP
See the description of dates in the section on common structures.
.PP
.B hardware \fIhardware-type mac-address\fB;\fR
.PP
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.
.PP
.B uid \fIclient-identifier\fB;\fR
.PP
The \fBuid\fR 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 \fBhardware\fI
statement, but this is not required.
.PP
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.  The format used is determined by the lease-id-format
parameter, which defaults to octal.
.PP
.B client-hostname "\fIhostname\fB";\fR
.PP
Most DHCP clients will send their hostname in the \fIhost-name\fR
option.  If a client sends its hostname in this way, the hostname is
recorded on the lease with a \fBclient-hostname\fR statement.   This
is not required by the protocol, however, so many specialized DHCP
clients do not send a host-name option.
.PP
.nf
.B binding state \fIstate\fB;
.B next binding state \fIstate\fB;
.fi
.PP
The \fBbinding state\fR statement declares the lease's binding state.
When the DHCP server is not configured to use the failover protocol, a
lease's binding state may be \fBactive\fR, \fBfree\fR or \fBabandoned\fR.
The failover protocol adds some additional transitional states, as well as
the \fBbackup\fR state, which indicates that the lease is available
for allocation by the failover secondary. Please see the \fBdhcpd.conf(5)\fR
manual page for more information about abandoned leases.
.PP
The \fBnext binding state\fR 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 \fIends\fR statement.
.PP
.B rewind binding state \fIstate\fB;
.PP
This statement is part of an optimization for
use with failover.  This helps a server rewind a lease to the state most
recently transmitted to its peer.
.PP
.nf
.B option agent.circuit-id \fIstring\fR;
.B option agent.remote-id \fIstring\fR;
.fi
.PP
These statements are used to record the circuit ID and remote ID options
sent by the relay agent, if the relay agent uses the \fIrelay agent
information option\fR.   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.
.PP
.B The \fIvendor-class-identifier\fB variable
.PP
The server retains the client-supplied Vendor Class Identifier option
for informational purposes, and to render them in DHCPLEASEQUERY responses.
.PP
.nf
.B bootp;
.B reserved;
.fi
.PP
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.
.PP
.B Other
Additional options or executable statements may be included, see the description
of them in the section on common structures.
.RE
.PP
.SH THE DHCPv6 LEASE (IA) DECLARATION
.PP
.nf
.B ia_ta \fI IAID_DUID\fB { \fIstatements...\fB }
.B ia_na \fI IAID_DUID\fB { \fIstatements...\fB }
.B ia_pd \fI IAID_DUID\fB { \fIstatements...\fB }
.fi
.PP
Each lease declaration starts with a tag indicating the type of the lease.
ia_ta is for temporary addresses, ia_na is for non-temporary addresses and
ia_pd is for prefix delegation.  Following this tag is the combined IAID
and DUID from the client for this lease.
.PP
The IAID_DUID value 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.  The format used is governed by the lease-id-format parameter,
which defaults to octal.
.PP
.B cltt \fIdate\fB;\fR
.PP
The \fBcltt\fR statement is the client's last transaction time.
.PP
See the description of dates in the section on common structures.
.PP
.nf
.B iaaddr \fIipv6-address\fB { \fIstatements...\fB }
.B iaprefix \fIipv6-address/prefix-length\fB { \fIstatements...\fB }
.PP
Within a given lease there can be multiple iaaddr and iaprefix statements.
Each will have either an IPv6 address or an IPv6 prefix (an address and
a prefix length indicating a CIDR style block of addresses).  The following
statements may occur Within each iaaddr or iaprefix.
.PP
.B binding state \fIstate\fB;
.PP
The \fBbinding state\fR statement declares the lease's binding state.
In DHCPv6 you will normally see this as \fIactive\fR or \fIexpired\fR.
.PP
.B  preferred-life \fIlifetime\fB;
.PP
The IPv6 preferred lifetime associated with this address, in seconds.
.PP
.B max-life \fIlifetime\fB;
.PP
The valid lifetime associated with this address, in seconds.
.PP
.B ends \fIdate\fB;\fR
.PP
The end time of the lease.  See the description of dates in the section on
common structures.
.PP
Additional options or executable statements may be included.  See the description
of them in the section on common structures.
.PP
.RE
.SH THE FAILOVER PEER STATE DECLARATION
The state of any failover peering arrangements is also recorded in the
lease file, using the \fBfailover peer\fR statement:
.PP
.nf
.B failover peer "\fIname\fB" state {
.B   my   state \fIstate\fB at \fIdate\fB;
.B   peer state \fIstate\fB at \fIdate\fB;
.B }
.fi
.PP
The states of the peer named \fIname\fR is being recorded.   Both the
state of the running server (\fBmy state\fR) and the other failover
partner (\fIpeer state\fR) are recorded.   The following states are
possible: \fBunknown-state\fR, \fBpartner-down\fR, \fBnormal\fR,
\fBcommunications-interrupted\fR, \fBresolution-interrupted\fR,
\fBpotential-conflict\fR, \fBrecover\fR, \fBrecover-done\fR,
\fBshutdown\fR, \fBpaused\fR, and \fBstartup\fR.
.RE
.SH FILES
.B DBDIR/dhcpd.leases DBDIR/dhcpd.leases~
.SH SEE ALSO
dhcpd(8), dhcp-options(5), dhcp-eval(5), dhcpd.conf(5), RFC2132, RFC2131.
.SH AUTHOR
.B dhcpd(8)
is maintained by ISC.
Information about Internet Systems Consortium can be found at:
.B https://www.isc.org/