dhcpd(8)

📦
version 4.4.3 · 6 versions
, in fedora 43, fedora 44

💖 dhcpd — Dynamic Host Configuration Protocol Server 💖

Okay bestie, this is the dhcpd(8) page from the System Manager's Manual, and we're gonna make it make sense. The main character is dhcpd, aka the Dynamic Host Configuration Protocol Server — fr fr, it's giving network-core energy. We're on part 1, so buckle up. 🦋

✨ NAME ✨

dhcpd — Dynamic Host Configuration Protocol Server. That's the whole name, no cap. 💅

🎀 SYNOPSIS 🎀

dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -4o6
       port ] [ -s server ] [ -cf config-file ] [ -lf lease-file ] [ -pf pid-
       file ] [ --no-pid ] [ -user user ] [ -group group ] [ -chroot dir ] [
       -tf trace-output-file ] [ -play trace-playback-file ] [ if0 [ ...ifN ] ]

       dhcpd --version

That second bit is the version-flex shortcut: dhcpd --version. ✨

🌈 DESCRIPTION 🌈

The Internet Systems Consortium DHCP Server, dhcpd, implements the Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap Protocol (BOOTP).

  • DHCP lets hosts on a TCP/IP network request and be assigned IP addresses, and also discover information about the network to which they are attached.
  • BOOTP provides similar functionality, with certain restrictions.

🔥 OPERATION 🔥

Okay so here's how the whole address-and-lease situation works, bestie. 👑

  • The DHCP protocol allows a host which is unknown to the network administrator to be automatically assigned a new IP address out of a pool of IP addresses for its network. For this to work, the network administrator allocates address pools in each subnet and enters them into the dhcpd.conf(5) file.
  • There are two versions of the DHCP protocol: DHCPv4 and DHCPv6. At startup the server may be started for one or the other via the -4 or -6 arguments.
  • On startup, dhcpd reads the dhcpd.conf file and stores a list of available addresses on each subnet in memory. 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 (by default, one day). Before leases expire, the clients to which leases are assigned are expected to renew them in order to continue to use the addresses. Once a lease has expired, the client to which that lease was assigned is no longer permitted to use the leased IP address.
  • In order to keep track of leases across system reboots and server restarts, dhcpd keeps a list of leases it has assigned in the dhcpd.leases(5) file. Before dhcpd grants a lease to a host, it records the lease in this file and makes sure that the contents of the file are flushed to disk. This ensures that even in the event of a system crash, dhcpd will not forget about a lease that it has assigned. On startup, after reading the dhcpd.conf file, dhcpd reads the dhcpd.leases file to refresh its memory about what leases have been assigned.
  • New leases are appended to the end of the dhcpd.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhcpd creates a new dhcpd.leases file from its in-core lease database. Once this file has been written to disk, the old file is renamed dhcpd.leases~, and the new file is renamed dhcpd.leases. If the system crashes in the middle of this process, whichever dhcpd.leases file remains will contain all the lease information, so there is no need for a special crash recovery process.
  • BOOTP support is also provided by this server. Unlike DHCP, the BOOTP protocol does not provide a protocol for recovering dynamically-assigned addresses once they are no longer needed. It is still possible to dynamically assign addresses to BOOTP clients, but some administrative process for reclaiming addresses is required. By default, leases are granted to BOOTP clients in perpetuity, although the network administrator may set an earlier cutoff date or a shorter lease length for BOOTP leases if that makes sense.
  • BOOTP clients may also be served in the old standard way, which is to simply provide a declaration in the dhcpd.conf file for each BOOTP client, permanently assigning an address to each client.
  • Whenever changes are made to the dhcpd.conf file, dhcpd must be restarted. To restart dhcpd, send a SIGTERM (signal 15) to the process ID contained in /var/run/dhcpd.pid, and then re-invoke dhcpd. Because the DHCP server database is not as lightweight as a BOOTP database, dhcpd does not automatically restart itself when it sees a change to the dhcpd.conf file.
Note: We get a lot of complaints about this. We realize that it would be nice if one could send a SIGHUP to the server and have it reload the database. This is not technically impossible, but it would require a great deal of work, our resources are extremely limited, and they can be better spent elsewhere. So please don't complain about this on the mailing list unless you're prepared to fund a project to implement this feature, or prepared to do it yourself.

👑 COMMAND LINE 👑

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

💅 COMMAND LINE OPTIONS 💅

  • -4 — Run as a DHCP server. This is the default and cannot be combined with -6.
  • -6 — Run as a DHCPv6 server. This cannot be combined with -4.
  • -4o6 port — Participate in the DHCPv4 over DHCPv6 protocol specified by RFC 7341. This associates a DHCPv4 and a DHCPv6 server to allow the v4 server to receive v4 requests that were encapsulated in a v6 packet. Communication between the two servers is done on a pair of UDP sockets bound to ::1 port and port + 1. Both servers must be launched using the same port argument.
  • -p port — The UDP port number on which dhcpd should listen. If unspecified, dhcpd uses the default port of 67. This is mostly useful for debugging purposes.
  • -s address — Specify an address or host name to which dhcpd should send replies rather than the broadcast address (255.255.255.255). This option is only supported in IPv4.
  • -f — Force dhcpd to run as a foreground process instead of as a daemon in the background. This is useful when running dhcpd under a debugger, or when running it out of inittab on System V systems.
  • -d — Send log messages to the standard error descriptor. This can be useful for debugging, and also at sites where a complete log of all dhcp activity must be kept but syslogd is not reliable or otherwise cannot be used. Normally, dhcpd will log all output using the syslog(3) function with the log facility set to LOG_DAEMON. Note that -d implies -f (the daemon will not fork itself into the background).
  • -q — Be quiet at startup. This suppresses the printing of the entire copyright message during startup. This might be desirable when starting dhcpd from a system startup script (e.g., /etc/rc).
  • -t — Test the configuration file. The server tests the configuration file for correct syntax, but will not attempt to perform any network operations. This can be used to test a new configuration file automatically before installing it.
  • -T — Test the lease file. The server tests the lease file for correct syntax, but will not attempt to perform any network operations. In addition to reading the lease file, it will also write the leases to a temporary lease file. The current lease file will not be modified and the temporary lease file will be removed upon completion of the test. This can be used to test a new lease file automatically before installing it.
  • -user user — Setuid to user after completing privileged operations, such as creating sockets that listen on privileged ports. This also causes the lease file to be owned by user. This option is only available if the code was compiled with the PARANOIA patch (./configure --enable-paranoia).
  • -group group — Setgid to group after completing privileged operations, such as creating sockets that listen on privileged ports. This also causes the lease file to use group. This option is only available if the code was compiled with the PARANOIA patch (./configure --enable-paranoia).
  • -chroot dir — Chroot to dir. This may occur before or after reading the configuration files depending on whether the code was compiled with the EARLY_CHROOT option enabled (./configure --enable-early-chroot). This option is only available if the code was compiled with the PARANOIA patch (./configure --enable-paranoia).
  • -tf tracefile — Specify a file into which the entire startup state of the server and all the transactions it processes are logged. This can be useful in submitting bug reports — if you are getting a core dump every so often, you can start the server with the -tf option and then, when the server dumps core, the trace file will contain all the transactions that led up to it dumping core, so that the problem can be easily debugged with -play.
  • -play playfile — Specify a file from which the entire startup state of the server and all the transactions it processed are read. The -play option must be specified with an alternate lease file, using the -lf switch, so that the DHCP server doesn't wipe out your existing lease file with its test data. The DHCP server will refuse to operate in playback mode unless you specify an alternate lease file.
  • --version — Print version number and exit.

Modifying default file locations: The following options can be used to modify the locations dhcpd uses for its files. Because of the importance of using the same lease database at all times when running dhcpd in production, these options should be used only for testing lease files or database files in a non-production environment.

  • -cf config-file — Path to alternate configuration file.
  • -lf lease-file — Path to alternate lease file.
  • -pf pid-file — Path to alternate pid file.
  • --no-pid — Option to disable writing pid files. By default the program will write a pid file. If the program is invoked with this option it will not check for an existing server process.

🌈 PORTS 🌈

okay so this is the part where the server is just, like, opening sockets everywhere — and honestly? it's giving multitasking main character energy 💅. the server may use multiple UDP and TCP ports to do its different jobs, and which ports get opened depends on both how you compiled your code and the configuration you supply. so there's no single vibe check here — but here's the general idea of what might be in use:

  • normally a DHCPv4 server opens a raw UDP socket to receive and send most DHCPv4 packets. it also opens a fallback UDP socket for sending unicast packets. and normally these both use the well known port number for BOOTPS. 🎀
  • for each DHCPv4 failover peer you list in the configuration file, there's a TCP socket listening for connections on the ports specified in the configuration file. when the peer actually connects, there's another socket for the established connection — and for that established connection, the side (primary or secondary) opening the connection will use a random port. lowkey chaotic but that's how it is 🦋
  • for DHCPv6, the server opens a UDP socket on the well known dhcpv6-server port.
  • the server opens an icmp socket for doing ping requests to check if addresses are in use. yes bestie, it pings to make sure nobody's already squatting on the address 👀
  • if you've included an omapi-port statement in your configuration file, the server will open a TCP socket on that port to listen for OMAPI connections. when something connects, another port gets used for the established connection.
  • when DDNS is enabled at compile time (see includes/site.h), the server opens both a v4 and a v6 UDP socket on random ports — unless DDNS updates are globally disabled by setting ddns-update-style to none in the configuration file. 🫶

🌸 CONFIGURATION 🌸

the syntax of the dhcpd.conf(5) file is discussed separately — like, in its own whole manpage. this section is just the overview of the configuration process, so if you want the detailed reference info you gotta go consult dhcpd.conf(5). iykyk 💖

✨ Subnets ✨

dhcpd needs to know the subnet numbers and netmasks of all subnets for which it will be providing service. and like, on top of that — to actually dynamically allocate addresses — it must be assigned one or more ranges of addresses on each subnet which it can then assign to client hosts as they boot. so a super simple config providing DHCP support might look like this:

subnet 239.252.197.0 netmask 255.255.255.0 {
  range 239.252.197.10 239.252.197.250;
}

multiple address ranges may be specified like this:

subnet 239.252.197.0 netmask 255.255.255.0 {
  range 239.252.197.10 239.252.197.107;
  range 239.252.197.113 239.252.197.250;
}

and if a subnet will only be provided with BOOTP service and no dynamic address assignment, the range clause can be left out entirely — but the subnet statement must still appear. no skipping that one, bestie 🙈

🎀 Lease Lengths 🎀

DHCP leases can be assigned almost any length from zero seconds to infinity. what lease length actually makes sense for a given subnet, or for a given installation, totally varies depending on the kinds of hosts being served. it's contextual 💅

for example: in an office environment where systems are added from time to time and removed from time to time, but move relatively infrequently, it might make sense to allow lease times of a month or more. but in a final test environment on a manufacturing floor, it may make more sense to assign a maximum lease length of 30 minutes — enough time to go through a simple test procedure on a network appliance before packaging it up for delivery. slay, ship it 📦

so it's possible to specify two lease lengths:

  1. the default length that will be assigned if a client doesn't ask for any particular lease length
  2. a maximum lease length

these are specified as clauses to the subnet command:

subnet 239.252.197.0 netmask 255.255.255.0 {
  range 239.252.197.10 239.252.197.107;
  default-lease-time 600;
  max-lease-time 7200;
}

this particular subnet declaration specifies a default lease time of 600 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 does not need to have the same lease — like, in the case of an office environment and a manufacturing environment served by the same DHCP server, it might make sense to have widely disparate values for default and maximum lease times on each subnet. no cap 🧚

🦋 BOOTP Support 🦋

every BOOTP client must be explicitly declared in the dhcpd.conf file. no vibes-based auto-detection here, sorry 😭

a very basic client declaration will specify:

  • the client network interface's hardware address
  • the IP address to assign to that client

and if the client needs to be able to load a boot file from the server, that file's name must be specified too.

a simple bootp client declaration might look like this:

host haagen {
  hardware ethernet 08:00:2b:4c:59:23;
  fixed-address 239.252.197.9;
  filename "/tftpboot/haagen.boot";
}

💜 Options 💜

DHCP (and also BOOTP with Vendor Extensions) provide a mechanism whereby the server can provide the client with information about how to configure its network interface (e.g., subnet mask), and also how the client can access various network services (e.g., DNS, IP routers, and so on). it's literally the server sliding the client a little cheat sheet 📝

these options can be specified on a per-subnet basis, and, for BOOTP clients, also on a per-client basis. and here's the important bit: in the event that a BOOTP client declaration specifies options that are also specified in its subnet declaration, the options specified in the client declaration take precedence. client wins, periodt 👑

a reasonably complete DHCP configuration might look something like this:

subnet 239.252.197.0 netmask 255.255.255.0 {
  range 239.252.197.10 239.252.197.250;
  default-lease-time 600;
  max-lease-time 7200;
  option subnet-mask 255.255.255.0;
  option broadcast-address 239.252.197.255;
  option routers 239.252.197.1;
  option domain-name-servers 239.252.197.2, 239.252.197.3;
  option domain-name "isc.org";
}

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

host haagen {
  hardware ethernet 08:00:2b:4c:59:23;
  fixed-address 239.252.197.9;
  filename "/tftpboot/haagen.boot";
  option domain-name-servers 192.5.5.1;
  option domain-name "example.com";
}

a more complete description of the dhcpd.conf file syntax is provided in dhcpd.conf(5). go read it, bestie, it's a whole thing 🌸

👑 OMAPI 👑

the DHCP server provides the capability to modify some of its configuration while it is running — without stopping it, without modifying its database files, and without restarting it. like, live edits?? iconic ✨

this capability is currently provided using OMAPI — an API for manipulating remote objects. OMAPI clients connect to the server using TCP/IP, authenticate, and can then examine the server's current status and make changes to it.

rather than implementing the underlying OMAPI protocol directly, user programs should use the dhcpctl API or OMAPI itself. dhcpctl is a wrapper that handles some of the housekeeping chores that OMAPI does not do automatically. dhcpctl and OMAPI are documented in dhcpctl(3) and omapi(3). 🫶

OMAPI exports objects, which can then be examined and modified. the DHCP server exports the following objects:

  • lease
  • host
  • failover-state
  • group

each object has a number of methods that are provided:

  • lookup
  • create
  • destroy

in addition, it's possible to look at attributes that are stored on objects, and in some cases to modify those attributes. 🔥

💅 THE LEASE OBJECT 💅

lowkey a tragedy but: leases can't currently be created or destroyed. they can, however, be looked up to examine and modify their state. so you can look, you just can't touch 😭

leases have the following attributes:

  • state integer — lookup, examine. and here's the state lore:
    • 1 = free
    • 2 = active
    • 3 = expired
    • 4 = released
    • 5 = abandoned
    • 6 = reset
    • 7 = backup
    • 8 = reserved
    • 9 = bootp
  • ip-address data — lookup, examine. the IP address of the lease. 🎀
  • dhcp-client-identifier data — lookup, examine, update. the client 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 not currently supported).
  • pool handle — examine. the pool object associated with this lease (the pool object is not currently supported).
  • billing-class handle — examine. the handle to the class to which this lease is currently billed, if any (the class object is not currently supported).
  • hardware-address data — examine, update. the hardware address (chaddr) field sent by the client when it acquired its lease.
  • hardware-type integer — examine, update. the type of the network interface that the client reported when it acquired its lease.
  • ends time — examine. the time when the lease's current state ends, as understood by the client.
  • tstp time — examine. the time when the lease's current state ends, as understood by the server.
  • tsfp time — examine. the adjusted time when the lease's current state ends, as understood by the failover peer (if there is no failover peer, this value is undefined). generally this value is only adjusted for expired, released, or reset leases while the server is operating in partner-down state, and otherwise is simply the value supplied by the peer.
  • atsfp time — examine. the actual tsfp value sent from the peer. this value is forgotten when a lease binding state change is made, to facilitate retransmission logic. 🧠
  • cltt time — examine. the time of the last transaction with the client on this lease. ⏰

🦋 THE HOST OBJECT 🦋

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

Hosts have the following attributes:

  • name — data; lookup, examine, modify — the name of the host declaration. This name must be unique among all host 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 the client, 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 the client, if any.
  • ip-address — data; examine, modify — a fixed IP address which is reserved for a DHCP client that matches this host declaration. The IP address will only be assigned to the client if it is valid for the network segment to which the client is connected.
  • statements — data; modify — a list of statements in the format of the dhcpd.conf file that will be executed whenever a message from the client is being processed.
  • known — integer; examine, modify — if nonzero, indicates that a client matching this host declaration will be treated as known in pool permit lists. If zero, the client will not be treated as known.

🎀 THE GROUP OBJECT 🎀

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

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

Groups have the following attributes:

  • name — data — the name of the group. All groups that are created using OMAPI must have 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 be executed whenever a message from a client whose host declaration references this group is processed.

💅 THE CONTROL OBJECT 💅

The control object allows you to shut the server down. If the server is doing failover with another peer, it will make a clean transition into the shutdown state and notify its peer, so that the peer can go into partner down, and then record the "recover" state in the lease file so that when the server is restarted, it will automatically resynchronize with its peer.

On shutdown the server will also attempt to cleanly shut down all OMAPI connections. If these connections do not go down cleanly after five seconds, they are shut down preemptively. It can take as much as 25 seconds from the beginning of the shutdown process to the time that the server actually exits.

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

🌈 THE FAILOVER-STATE OBJECT 🌈

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

  • name — data; examine — Indicates the name of the failover peer relationship, as described in the 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 for this failover pair.
  • partner-port — data; examine — Indicates the TCP port on which the failover partner is listening for failover protocol connections.
  • local-port — data; examine — Indicates the TCP port on which the DHCP server is listening for failover protocol connections for this failover pair.
  • max-outstanding-updates — integer; examine — Indicates the number of updates that can be outstanding and unacknowledged 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 request before load balancing is bypassed.
  • load-balance-hba — data; examine — Indicates the load balancing hash bucket array for this failover relationship.
  • local-state — integer; examine, modify — Indicates the present state of the DHCP server in this failover relationship. Possible values for state are:
1   - startup
2   - normal
3   - communications interrupted
4   - partner down
5   - potential conflict
6   - recover
7   - paused
8   - shutdown
9   - recover done
10  - resolution interrupted
11  - conflict done
254 - recover wait

(Note that some of the above values have changed since DHCP 3.0.x.)

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, it can be useful to set the DHCP server's failover state to partner down. At this point the DHCP server will take over service of the failover partner's leases as soon as possible, and will give out normal leases, not leases that are restricted by MCLT. If you do put the DHCP server into the partner-down when the other DHCP server is not in the partner-down state, but is not reachable, IP address assignment conflicts are possible, even likely. Once a server has been put into partner-down mode, its failover partner must not be brought back online until communication is possible between the two servers.
  • 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 state in 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) in this failover relationship.
  • last-packet-sent — integer; examine — Indicates the time at which the most recent failover packet was sent by this DHCP server to its failover partner.
  • last-timestamp-received — integer; examine — Indicates the timestamp that was on the failover message most recently received from the failover partner.
  • skew — integer; examine — Indicates the skew between the failover partner's clock and this DHCP server's clock
  • max-response-delay — integer; examine — Indicates the time in seconds after which, if no message is received from the failover partner, the partner is assumed to be out of communication.
  • cur-unacked-updates — integer; examine — Indicates the number of update messages that have been received from the failover partner but not yet processed.

🍓 FILES 🍓

  • /etc/dhcp/dhcpd.conf
  • /var/lib/dhcpd/dhcpd.leases
  • /var/run/dhcpd.pid
  • /var/lib/dhcpd/dhcpd.leases~

🌸 SEE ALSO 🌸

👑 AUTHOR 👑

dhcpd(8) was originally written by Ted Lemon under a contract with Vixie Labs. Funding for this project was provided by Internet Systems Consortium. Version 3 of the DHCP server was funded by Nominum, Inc. Information about Internet Systems Consortium is available at https://www.isc.org/.

💖 And that's the whole page, bestie — the footer's source/version and date would normally sit right here, but this copy only gives us dhcpd(8), so I'm not inventing a version or a date. No cap, periodt. 🎀✨

📦 where this page comes from