MAN page from Mandrake 9.X dhcp-server-3.0-1.rc12.2mdk.i586.rpm


Section: Maintenance Commands (8)


dhcpd - Dynamic Host Configuration Protocol Server 




The Internet Software Consortium DHCP Server, dhcpd, implements theDynamic Host Configuration Protocol (DHCP) and the Internet BootstrapProtocol (BOOTP). DHCP allows hosts on a TCP/IP network to requestand be assigned IP addresses, and also to discover information aboutthe network to which they are attached. BOOTP provides similarfunctionality, with certain restrictions. 


This software is free software. At various times its development hasbeen underwritten by various organizations, including the ISC andVixie Enterprises. The development of 3.0 has been funded almostentirely by Nominum, Inc.

At this point development is being shepherded by Ted Lemon, and hostedby the ISC, but the future of this project depends on you. If youhave features you want, please consider implementing them. 


The DHCP protocol allows a host which is unknown to the networkadministrator to be automatically assigned a new IP address out of apool of IP addresses for its network. In order for this to work, thenetwork administrator allocates address pools in each subnet andenters them into the dhcpd.conf(5) file.

On startup, dhcpd reads thedhcpd.conffile and stores a list of available addresses on each subnet inmemory. When a client requests an address using the DHCP protocol,dhcpd allocates an address for it. Each client is assigned a lease,which expires after an amount of time chosen by the administrator (bydefault, one day). Before leases expire, the clients to which leasesare assigned are expected to renew them in order to continue to usethe addresses. Once a lease has expired, the client to which thatlease was assigned is no longer permitted to use the leased IPaddress.

In order to keep track of leases across system reboots and serverrestarts, dhcpd keeps a list of leases it has assigned in thedhcpd.leases(5) file. Before dhcpd grants a lease to a host, itrecords the lease in this file and makes sure that the contents of thefile are flushed to disk. This ensures that even in the event of asystem crash, dhcpd will not forget about a lease that it hasassigned. On startup, after reading the dhcpd.conf file, dhcpdreads the dhcpd.leases file to refresh its memory about what leaseshave been assigned.

New leases are appended to the end of the dhcpd.leasesfile. In order to prevent the file from becoming arbitrarily large,from time to time dhcpd creates a new dhcpd.leases file from itsin-core lease database. Once this file has been written to disk, theold file is renameddhcpd.leases~,and the new file is renamed dhcpd.leases. If the system crashes inthe middle of this process, whichever dhcpd.leases file remains willcontain all the lease information, so there is no need for a specialcrash recovery process.

BOOTP support is also provided by this server. Unlike DHCP, the BOOTPprotocol does not provide a protocol for recoveringdynamically-assigned addresses once they are no longer needed. It isstill possible to dynamically assign addresses to BOOTP clients, butsome administrative process for reclaiming addresses is required. Bydefault, leases are granted to BOOTP clients in perpetuity, althoughthe network administrator may set an earlier cutoff date or a shorterlease length for BOOTP leases if that makes sense.

BOOTP clients may also be served in the old standard way, which is tosimply provide a declaration in the dhcpd.conf file for eachBOOTP client, permanently assigning an address to each client.

Whenever changes are made to the dhcpd.conf file, dhcpd must berestarted. To restart dhcpd, send a SIGTERM (signal 15) to theprocess ID contained in/var/run/,and then re-invoke dhcpd. Because the DHCP server database is not aslightweight as a BOOTP database, dhcpd does not automatically restartitself when it sees a change to the dhcpd.conf file.

Note: We get a lot of complaints about this. We realize that it wouldbe nice if one could send a SIGHUP to the server and have it reloadthe database. This is not technically impossible, but it wouldrequire a great deal of work, our resources are extremely limited, andthey can be better spent elsewhere. So please don't complain aboutthis on the mailing list unless you're prepared to fund a project toimplement this feature, or prepared to do it yourself. 


The names of the network interfaces on which dhcpd should listen forbroadcasts may be specified on the command line. This should be doneon systems where dhcpd is unable to identify non-broadcast interfaces,but should not be required on other systems. If no interface namesare specified on the command line dhcpd will identify all networkinterfaces which are up, elimininating non-broadcast interfaces ifpossible, and listen for DHCP broadcasts on each interface.

If dhcpd should listen on a port other than the standard (port 67),the-pflag may used. It should be followed by the udp port number on whichdhcpd should listen. This is mostly useful for debugging purposes.

To run dhcpd as a foreground process, rather than allowing it to runas a daemon in the background, the-fflag should be specified. This is useful when running dhcpd under adebugger, or when running it out of inittab on System V systems.

To have dhcpd log to the standard error descriptor, specify the-dflag. This can be useful for debugging, and also at sites where acomplete log of all dhcp activity must be kept but syslogd is notreliable or otherwise cannot be used. Normally, dhcpd will log alloutput using the syslog(3) function with the log facility set toLOG_DAEMON.

Dhcpd can be made to use an alternate configuration file with the-cfflag, or an alternate lease file with the-lfflag. Because of the importance of using the same lease database atall times when running dhcpd in production, these options should beused only for testing lease files or database files in anon-production environment.

When starting dhcpd up from a system startup script (e.g., /etc/rc),it may not be desirable to print out the entire copyright message onstartup. To avoid printing this message, the-qflag may be specified.

The DHCP server reads two files on startup: a configuration file, anda lease database. If the-tflag is specified, the server will simply test the configuration filefor correct syntax, but will not attempt to perform any networkoperations. This can be used to test the a new configuration fileautomatically before installing it.

The-Tflag can be used to test the lease database file in a similar way.

The -tf and -play options allow you to specify a file intowhich the entire startup state of the server and all the transactionsit processes are either logged or played back from. This can beuseful in submitting bug reports - if you are getting a core dumpevery so often, you can start the server with the -tf option andthen, when the server dumps core, the trace file will contain all thetransactions that led up to it dumping core, so that the problem canbe easily debugged with -play.

The -play option must be specified with an alternate lease file,using the -lf switch, so that the DHCP server doesn't wipe outyour existing lease file with its test data. The DHCP server willrefuse to operate in playback mode unless you specify an alternatelease file. 


The syntax of the dhcpd.conf(5) file is discussed separately. Thissection should be used as an overview of the configuration process,and the dhcpd.conf(5) documentation should be consulted for detailedreference information.



dhcpd needs to know the subnet numbers and netmasks of all subnets forwhich it will be providing service. In addition, in order todynamically allocate addresses, it must be assigned one or more rangesof addresses on each subnet which it can in turn assign to clienthosts as they boot. Thus, a very simple configuration providing DHCPsupport might look like this:
        subnet netmask {          range;        }

Multiple address ranges may be specified like this:

        subnet netmask {          range;          range;        }

If a subnet will only be provided with BOOTP service and no dynamicaddress assignment, the range clause can be left out entirely, but thesubnet statement must appear.


Lease Lengths

DHCP leases can be assigned almost any length from zero seconds toinfinity. What lease length makes sense for any given subnet, or forany given installation, will vary depending on the kinds of hostsbeing served.

For example, in an office environment where systems are added fromtime to time and removed from time to time, but move relativelyinfrequently, it might make sense to allow lease times of a month ofmore. In a final test environment on a manufacturing floor, it maymake more sense to assign a maximum lease length of 30 minutes -enough time to go through a simple test procedure on a networkappliance before packaging it up for delivery.

It is possible to specify two lease lengths: the default length thatwill be assigned if a client doesn't ask for any particular leaselength, and a maximum lease length. These are specified as clausesto the subnet command:

        subnet netmask {          range;          default-lease-time 600;          max-lease-time 7200;        }

This particular subnet declaration specifies a default lease time of600 seconds (ten minutes), and a maximum lease time of 7200 seconds(two hours). Other common values would be 86400 (one day), 604800(one week) and 2592000 (30 days).

Each subnet need not have the same lease---in the case of an officeenvironment and a manufacturing environment served by the same DHCPserver, it might make sense to have widely disparate values fordefault and maximum lease times on each subnet. 

BOOTP Support

Each BOOTP client must be explicitly declared in the dhcpd.conffile. A very basic client declaration will specify the clientnetwork interface's hardware address and the IP address to assign tothat client. If the client needs to be able to load a boot file fromthe server, that file's name must be specified. A simple bootpclient declaration might look like this:
        host haagen {          hardware ethernet 08:00:2b:4c:59:23;          fixed-address;          filename "/tftpboot/haagen.boot";        }


DHCP (and also BOOTP with Vendor Extensions) provide a mechanismwhereby the server can provide the client with information about howto configure its network interface (e.g., subnet mask), and also howthe client can access various network services (e.g., DNS, IP routers,and so on).

These options can be specified on a per-subnet basis, and, for BOOTPclients, also on a per-client basis. In the event that a BOOTPclient declaration specifies options that are also specified in itssubnet declaration, the options specified in the client declarationtake precedence. A reasonably complete DHCP configuration mightlook something like this:

        subnet netmask {          range;          default-lease-time 600 max-lease-time 7200;          option subnet-mask;          option broadcast-address;          option routers;          option domain-name-servers,;          option domain-name "";        }

A bootp host on that subnet that needs to be in a different domain anduse a different name server might be declared as follows:

        host haagen {          hardware ethernet 08:00:2b:4c:59:23;          fixed-address;          filename "/tftpboot/haagen.boot";          option domain-name-servers;          option domain-name "";        }

A more complete description of the dhcpd.conf file syntax is providedin dhcpd.conf(5). 


The DHCP server provides the capability to modify some of itsconfiguration while it is running, without stopping it, modifying itsdatabase files, and restarting it. This capability is currentlyprovided using OMAPI - an API for manipulating remote objects. OMAPIclients connect to the server using TCP/IP, authenticate, and can thenexamine the server's current status and make changes to it.

Rather than implementing the underlying OMAPI protocol directly, userprograms should use the dhcpctl API or OMAPI itself. Dhcpctl is awrapper that handles some of the housekeeping chores that OMAPI doesnot do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)and omapi(3).

OMAPI exports objects, which can then be examined and modified. TheDHCP server exports the following objects: lease, host,failover-state and group. Each object has a number of methods thatare provided: lookup, create, and destroy. In addition, it ispossible to look at attributes that are stored on objects, and in somecases to modify those attributes. 


Leases can't currently be created or destroyed, but they can be lookedup to examine and modify their state.

Leases have the following attributes:

state integer lookup, examine

1 = free2 = active3 = expired4 = released5 = abandoned6 = reset7 = backup8 = reserved9 = bootp

ip-address data lookup, examine

The IP address of the lease.

dhcp-client-identifier data lookup, examine, update

Theclient identifier that the client used when it acquired the lease.Not all clients send client identifiers, so this may be empty.

client-hostname data examine, update

The value the client sent in the host-name option.

host handle examine

the host declaration associated with this lease, if any.

subnet handle examine

the subnet object associated with this lease (the subnet object is notcurrently supported).

pool handle examine

the pool object associted with this lease (the pool object is notcurrently supported).

billing-class handle examine

the handle to the class to which this lease is currently billed, ifany (the class object is not currently supported).

hardware-address data examine, update

the hardware address (chaddr) field sent by the client when itacquired its lease.

hardware-type integer examine, update

the type of the network interface that the client reported when itacquired its lease.

ends time examine

the time when the lease's current state ends, as understood by theclient.

tstp time examine

the time when the lease's current state ends, as understood by theserver.
tsfp time examine
the time when the lease's current state ends, as understood by thefailover peer (if there is no failover peer, this value isundefined).

cltt time examine

The time of the last transaction with the client on this lease.


Hosts can be created, destroyed, looked up, examined and modified.If a host declaration is created or deleted using OMAPI, thatinformation will be recorded in the dhcpd.leases file. It ispermissible to delete host declarations that are declared in thedhcpd.conf file.

Hosts have the following attributes:

name data lookup, examine, modify

the name of the host declaration. This name must be unique among allhost declarations.

group handle examine, modify

the named group associated with the host declaration, if there is one.

hardware-address data lookup, examine, modify

the link-layer address that will be used to match the client, if any.Only valid if hardware-type is also present.

hardware-type integer lookup, examine, modify

the type of the network interface that will be used to match theclient, if any. Only valid if hardware-address is also present.

dhcp-client-identifier data lookup, examine, modify

the dhcp-client-identifier option that will be used to match theclient, if any.

ip-address data examine, modify

a fixed IP address which is reserved for a DHCP client that matchesthis host declaration. The IP address will only be assigned to theclient if it is valid for the network segment to which the client isconnected.

statements data modify

a list of statements in the format of the dhcpd.conf file that will beexecuted whenever a message from the client is being processed.

known integer examine, modify

if nonzero, indicates that a client matching this host declarationwill be treated as known in pool permit lists. If zero, theclient will not be treated as known.


Named groups can be created, destroyed, looked up, examined andmodified. If a group declaration is created or deleted using OMAPI,that information will be recorded in the dhcpd.leases file. It ispermissible to delete group declarations that are declared in thedhcpd.conf file.

Named groups currently can only be associated withhosts - this allows one set of statements to be efficiently attachedto more than one host declaration.

Groups have the following attributes:

name data

the name of the group. All groups that are created using OMAPI musthave names, and the names must be unique among all groups.

statements data

a list of statements in the format of the dhcpd.conf file that will beexecuted whenever a message from a client whose host declarationreferences this group is processed.


The control object allows you to shut the server down. If the serveris doing failover with another peer, it will make a clean transitioninto the shutdown state and notify its peer, so that the peer can gointo partner down, and then record the "recover" state in the leasefile so that when the server is restarted, it will automaticallyresynchronize with its peer.

On shutdown the server will also attempt to cleanly shut down allOMAPI connections. If these connections do not go down cleanly afterfive seconds, they are shut down pre-emptively. It can take as muchas 25 seconds from the beginning of the shutdown process to the timethat the server actually exits.

To shut the server down, open its control object and set the stateattribute to 2. 


The failover-state object is the object that tracks the state of thefailover protocol as it is being managed for a given failover peer.The failover object has the following attributes (please seedhcpd.conf (5)for explanations about what these attributes mean):

name data examine

Indicates the name of the failover peer relationship, as described inthe server's dhcpd.conf file.

partner-address data examine

Indicates the failover partner's IP address.

local-address data examine

Indicates the IP address that is being used by the DHCP server forthis failover pair.

partner-port data examine

Indicates the TCP port on which the failover partner is listening forfailover protocol connections.

local-port data examine

Indicates the TCP port on which the DHCP server is listening forfailover protocol connections for this failover pair.

max-outstanding-updates integer examine

Indicates the number of updates that can be outstanding andunacknowledged at any given time, in this failover relationship.

mclt integer examine

Indicates the maximum client lead time in this failover relationship.

load-balance-max-secs integer examine

Indicates the maximum value for the secs field in a client requestbefore load balancing is bypassed.

load-balance-hba data examine

Indicates the load balancing hash bucket array for this failoverrelationship.

local-state integer examine, modify

Indicates the present state of the DHCP server in this failoverrelationship. Possible values for state are:

1  - partner down2  - normal3  - communications interrupted4  - resolution interrupted5  - potential conflict6  - recover7  - recover done8  - shutdown9  - paused10 - startup11 - recover wait

In general it is not a good idea to make changes to this state.However, in the case that the failover partner is known to be down, itcan be useful to set the DHCP server's failover state to partnerdown. At this point the DHCP server will take over service of thefailover partner's leases as soon as possible, and will give outnormal leases, not leases that are restricted by MCLT. If you do putthe DHCP server into the partner-down when the other DHCP server isnot in the partner-down state, but is not reachable, IP addressassignment conflicts are possible, even likely. Once a server hasbeen put into partner-down mode, its failover partner must not bebrought back online until communication is possible between the twoservers.

partner-state integer examine

Indicates the present state of the failover partner.

local-stos integer examine

Indicates the time at which the DHCP server entered its present statein this failover relationship.

partner-stos integer examine

Indicates the time at which the failover partner entered its present state.

hierarchy integer examine

Indicates whether the DHCP server is primary (0) or secondary (1) inthis failover relationship.

last-packet-sent integer examine

Indicates the time at which the most recent failover packet was sentby this DHCP server to its failover partner.

last-timestamp-received integer examine

Indicates the timestamp that was on the failover message most recentlyreceived from the failover partner.

skew integer examine

Indicates the skew between the failover partner's clock and this DHCPserver's clock

max-response-delay integer examine

Indicates the time in seconds after which, if no message is receivedfrom the failover partner, the partner is assumed to be out ofcommunication.

cur-unacked-updates integer examine

Indicates the number of update messages that have been received fromthe failover partner but not yet processed.


/etc/dhcpd.conf, /var/lib/dhcp/dhcpd.leases, /var/run/,/var/lib/dhcp/dhcpd.leases~. 


dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5) 


dhcpd(8)was originally written by Ted Lemon under a contract with Vixie Labs.Funding for this project was provided by the Internet SoftwareConsortium. Version 3 of the DHCP server was funded by Nominum, Inc.Information about the Internet Software Consortium is available at about Nominum and support contracts for DHCP and BIND canbe found at



Lease Lengths
BOOTP Support

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