1dhcpd(8) System Manager's Manual dhcpd(8)
2
6 dhcpd - Dynamic Host Configuration Protocol Server
7
9 dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -4o6
10 port ] [ -s server ] [ -cf config-file ] [ -lf lease-file ] [ -pf pid-
11 file ] [ --no-pid ] [ -user user ] [ -group group ] [ -chroot dir ] [
12 -tf trace-output-file ] [ -play trace-playback-file ] [ if0 [ ...ifN ]
13 ]
14 dhcpd --version
16
18 The Internet Systems Consortium DHCP Server, dhcpd, implements the
19 Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap
20 Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request and
21 be assigned IP addresses, and also to discover information about the
22 network to which they are attached. BOOTP provides similar functional‐
23 ity, with certain restrictions.
24
26 The DHCP protocol allows a host which is unknown to the network admin‐
27 istrator to be automatically assigned a new IP address out of a pool of
28 IP addresses for its network. In order for this to work, the network
29 administrator allocates address pools in each subnet and enters them
30 into the dhcpd.conf(5) file.
31 There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
33 startup the server may be started for one or the other via the -4 or
34 -6 arguments.
35 On startup, dhcpd reads the dhcpd.conf file and stores a list of avail‐
37 able addresses on each subnet in memory. When a client requests an
38 address using the DHCP protocol, dhcpd allocates an address for it.
39 Each client is assigned a lease, which expires after an amount of time
40 chosen by the administrator (by default, one day). Before leases
41 expire, the clients to which leases are assigned are expected to renew
42 them in order to continue to use the addresses. Once a lease has
43 expired, the client to which that lease was assigned is no longer per‐
44 mitted to use the leased IP address.
45 In order to keep track of leases across system reboots and server
47 restarts, dhcpd keeps a list of leases it has assigned in the
48 dhcpd.leases(5) file. Before dhcpd grants a lease to a host, it
49 records the lease in this file and makes sure that the contents of the
50 file are flushed to disk. This ensures that even in the event of a
51 system crash, dhcpd will not forget about a lease that it has assigned.
52 On startup, after reading the dhcpd.conf file, dhcpd reads the
53 dhcpd.leases file to refresh its memory about what leases have been
54 assigned.
55 New leases are appended to the end of the dhcpd.leases file. In order
57 to prevent the file from becoming arbitrarily large, from time to time
58 dhcpd creates a new dhcpd.leases file from its in-core lease database.
59 Once this file has been written to disk, the old file is renamed
60 dhcpd.leases~, and the new file is renamed dhcpd.leases. If the system
61 crashes in the middle of this process, whichever dhcpd.leases file
62 remains will contain all the lease information, so there is no need for
63 a special crash recovery process.
64 BOOTP support is also provided by this server. Unlike DHCP, the BOOTP
66 protocol does not provide a protocol for recovering dynamically-
67 assigned addresses once they are no longer needed. It is still possi‐
68 ble to dynamically assign addresses to BOOTP clients, but some adminis‐
69 trative process for reclaiming addresses is required. By default,
70 leases are granted to BOOTP clients in perpetuity, although the network
71 administrator may set an earlier cutoff date or a shorter lease length
72 for BOOTP leases if that makes sense.
73 BOOTP clients may also be served in the old standard way, which is to
75 simply provide a declaration in the dhcpd.conf file for each BOOTP
76 client, permanently assigning an address to each client.
77 Whenever changes are made to the dhcpd.conf file, dhcpd must be
79 restarted. To restart dhcpd, send a SIGTERM (signal 15) to the process
80 ID contained in /var/run/dhcpd.pid, and then re-invoke dhcpd. Because
81 the DHCP server database is not as lightweight as a BOOTP database,
82 dhcpd does not automatically restart itself when it sees a change to
83 the dhcpd.conf file.
84 Note: We get a lot of complaints about this. We realize that it would
86 be nice if one could send a SIGHUP to the server and have it reload the
87 database. This is not technically impossible, but it would require a
88 great deal of work, our resources are extremely limited, and they can
89 be better spent elsewhere. So please don't complain about this on the
90 mailing list unless you're prepared to fund a project to implement this
91 feature, or prepared to do it yourself.
92
94 The names of the network interfaces on which dhcpd should listen for
95 broadcasts may be specified on the command line. This should be done
96 on systems where dhcpd is unable to identify non-broadcast interfaces,
97 but should not be required on other systems. If no interface names are
98 specified on the command line dhcpd will identify all network inter‐
99 faces which are up, eliminating non-broadcast interfaces if possible,
100 and listen for DHCP broadcasts on each interface.
101
103 -4 Run as a DHCP server. This is the default and cannot be combined
104 with -6.
105 -6 Run as a DHCPv6 server. This cannot be combined with -4.
107 -4o6 port
109 Participate in the DHCPv4 over DHCPv6 protocol specified by RFC
110 7341. This associates a DHCPv4 and a DHCPv6 server to allow the
111 v4 server to receive v4 requests that were encapsulated in a v6
112 packet. Communication between the two servers is done on a pair
113 of UDP sockets bound to ::1 port and port + 1. Both servers must
114 be launched using the same port argument.
115 -p port
117 The UDP port number on which dhcpd should listen. If unspeci‐
118 fied dhcpd uses the default port of 67. This is mostly useful
119 for debugging purposes.
120 -s address
122 Specify an address or host name to which dhcpd should send
123 replies rather than the broadcast address (255.255.255.255).
124 This option is only supported in IPv4.
125 -f Force dhcpd to run as a foreground process instead of as a dae‐
127 mon in the background. This is useful when running dhcpd under
128 a debugger, or when running it out of inittab on System V sys‐
129 tems.
130 -d Send log messages to the standard error descriptor. This can be
132 useful for debugging, and also at sites where a complete log of
133 all dhcp activity must be kept but syslogd is not reliable or
134 otherwise cannot be used. Normally, dhcpd will log all output
135 using the syslog(3) function with the log facility set to
136 LOG_DAEMON. Note that -d implies -f (the daemon will not fork
137 itself into the background).
138 -q Be quiet at startup. This suppresses the printing of the entire
140 copyright message during startup. This might be desirable when
141 starting dhcpd from a system startup script (e.g., /etc/rc).
142 -t Test the configuration file. The server tests the configuration
144 file for correct syntax, but will not attempt to perform any
145 network operations. This can be used to test a new configura‐
146 tion file automatically before installing it.
147 -T Test the lease file. The server tests the lease file for cor‐
149 rect syntax, but will not attempt to perform any network opera‐
150 tions. This can be used to test a new lease file automatically
151 before installing it.
152 -user user
154 Setuid to user after completing privileged operations, such as
155 creating sockets that listen on privileged ports. This also
156 causes the lease file to be owned by user. This option is only
157 available if the code was compiled with the PARANOIA patch
158 (./configure --enable-paranoia).
159 -group group
161 Setgid to group after completing privileged operations, such as
162 creating sockets that listen on privileged ports. This also
163 causes the lease file to use group. This option is only avail‐
164 able if the code was compiled with the PARANOIA patch (./config‐
165 ure --enable-paranoia).
166 -chroot dir
168 Chroot to directory. This may occur before or after reading the
169 configuration files depending on whether the code was compiled
170 with the EARLY_CHROOT option enabled (./configure --enable-
171 early-chroot). This option is only available if the code was
172 compiled with the PARANOIA patch (./configure --enable-para‐
173 noia).
174 -tf tracefile
176 Specify a file into which the entire startup state of the server
177 and all the transactions it processes are logged. This can be
178 useful in submitting bug reports - if you are getting a core
179 dump every so often, you can start the server with the -tf
180 option and then, when the server dumps core, the trace file will
181 contain all the transactions that led up to it dumping core, so
182 that the problem can be easily debugged with -play.
183 -play playfile
185 Specify a file from which the entire startup state of the server
186 and all the transactions it processed are read. The -play
187 option must be specified with an alternate lease file, using the
188 -lf switch, so that the DHCP server doesn't wipe out your exist‐
189 ing lease file with its test data. The DHCP server will refuse
190 to operate in playback mode unless you specify an alternate
191 lease file.
192 --version
194 Print version number and exit.
195 Modifying default file locations: The following options can be used to
197 modify the locations dhcpd uses for its files. Because of the impor‐
198 tance of using the same lease database at all times when running dhcpd
199 in production, these options should be used only for testing lease
200 files or database files in a non-production environment.
201 -cf config-file
203 Path to alternate configuration file.
204 -lf lease-file
206 Path to alternate lease file.
207 -pf pid-file
209 Path to alternate pid file.
210 --no-pid
212 Option to disable writing pid files. By default the program
213 will write a pid file. If the program is invoked with this
214 option it will not check for an existing server process.
215
217 During operations the server may use multiple UDP and TCP ports to pro‐
218 vide different functions. Which ports are opened depends on both the
219 way you compiled your code and the configuration you supply. The fol‐
220 lowing should provide you an idea of what ports may be in use.
221 Normally a DHCPv4 server will open a raw UDP socket to receive and send
223 most DHCPv4 packets. It also opens a fallback UDP socket for use in
224 sending unicast packets. Normally these will both use the well known
225 port number for BOOTPS.
226 For each DHCPv4 failover peer you list in the configuration file there
228 will be a TCP socket listening for connections on the ports specified
229 in the configuration file. When the peer connects there will be
230 another socket for the established connection. For the established
231 connection the side (primary or secondary) opening the connection will
232 use a random port.
233 For DHCPv6 the server opens a UDP socket on the well known
235 dhcpv6-server port.
236 The server opens an icmp socket for doing ping requests to check if
238 addresses are in use.
239 If you have included an omapi-port statement in your configuration file
241 then the server will open a TCP socket on that port to listen for OMPAI
242 connections. When something connects another port will be used for the
243 established connection.
244 When DDNS is enabled at compile time (see includes/site.h) the server
246 will open both a v4 and a v6 UDP socket on random ports, unless DDNS
247 updates are globally disabled by setting ddns-update-style to none in
248 the configuration file.
249
251 The syntax of the dhcpd.conf(5) file is discussed separately. This
252 section should be used as an overview of the configuration process, and
253 the dhcpd.conf(5) documentation should be consulted for detailed refer‐
254 ence information.
255
257 dhcpd needs to know the subnet numbers and netmasks of all subnets for
258 which it will be providing service. In addition, in order to dynami‐
259 cally allocate addresses, it must be assigned one or more ranges of
260 addresses on each subnet which it can in turn assign to client hosts as
261 they boot. Thus, a very simple configuration providing DHCP support
262 might look like this:
263 subnet 239.252.197.0 netmask 255.255.255.0 {
265 range 239.252.197.10 239.252.197.250;
266 }
267 Multiple address ranges may be specified like this:
269 subnet 239.252.197.0 netmask 255.255.255.0 {
271 range 239.252.197.10 239.252.197.107;
272 range 239.252.197.113 239.252.197.250;
273 }
274 If a subnet will only be provided with BOOTP service and no dynamic
276 address assignment, the range clause can be left out entirely, but the
277 subnet statement must appear.
278
280 DHCP leases can be assigned almost any length from zero seconds to
281 infinity. What lease length makes sense for any given subnet, or for
282 any given installation, will vary depending on the kinds of hosts being
283 served.
284 For example, in an office environment where systems are added from time
286 to time and removed from time to time, but move relatively infre‐
287 quently, it might make sense to allow lease times of a month or more.
288 In a final test environment on a manufacturing floor, it may make more
289 sense to assign a maximum lease length of 30 minutes - enough time to
290 go through a simple test procedure on a network appliance before pack‐
291 aging it up for delivery.
292 It is possible to specify two lease lengths: the default length that
294 will be assigned if a client doesn't ask for any particular lease
295 length, and a maximum lease length. These are specified as clauses to
296 the subnet command:
297 subnet 239.252.197.0 netmask 255.255.255.0 {
299 range 239.252.197.10 239.252.197.107;
300 default-lease-time 600;
301 max-lease-time 7200;
302 }
303 This particular subnet declaration specifies a default lease time of
305 600 seconds (ten minutes), and a maximum lease time of 7200 seconds
306 (two hours). Other common values would be 86400 (one day), 604800 (one
307 week) and 2592000 (30 days).
308 Each subnet need not have the same lease—in the case of an office envi‐
310 ronment and a manufacturing environment served by the same DHCP server,
311 it might make sense to have widely disparate values for default and
312 maximum lease times on each subnet.
313
315 Each BOOTP client must be explicitly declared in the dhcpd.conf file.
316 A very basic client declaration will specify the client network inter‐
317 face's hardware address and the IP address to assign to that client.
318 If the client needs to be able to load a boot file from the server,
319 that file's name must be specified. A simple bootp client declaration
320 might look like this:
321 host haagen {
323 hardware ethernet 08:00:2b:4c:59:23;
324 fixed-address 239.252.197.9;
325 filename "/tftpboot/haagen.boot";
326 }
327
329 DHCP (and also BOOTP with Vendor Extensions) provide a mechanism
330 whereby the server can provide the client with information about how to
331 configure its network interface (e.g., subnet mask), and also how the
332 client can access various network services (e.g., DNS, IP routers, and
333 so on).
334 These options can be specified on a per-subnet basis, and, for BOOTP
336 clients, also on a per-client basis. In the event that a BOOTP client
337 declaration specifies options that are also specified in its subnet
338 declaration, the options specified in the client declaration take
339 precedence. A reasonably complete DHCP configuration might look some‐
340 thing like this:
341 subnet 239.252.197.0 netmask 255.255.255.0 {
343 range 239.252.197.10 239.252.197.250;
344 default-lease-time 600 max-lease-time 7200;
345 option subnet-mask 255.255.255.0;
346 option broadcast-address 239.252.197.255;
347 option routers 239.252.197.1;
348 option domain-name-servers 239.252.197.2, 239.252.197.3;
349 option domain-name "isc.org";
350 }
351 A bootp host on that subnet that needs to be in a different domain and
353 use a different name server might be declared as follows:
354 host haagen {
356 hardware ethernet 08:00:2b:4c:59:23;
357 fixed-address 239.252.197.9;
358 filename "/tftpboot/haagen.boot";
359 option domain-name-servers 192.5.5.1;
360 option domain-name "example.com";
361 }
362 A more complete description of the dhcpd.conf file syntax is provided
364 in dhcpd.conf(5).
365
367 The DHCP server provides the capability to modify some of its configu‐
368 ration while it is running, without stopping it, modifying its database
369 files, and restarting it. This capability is currently provided using
370 OMAPI - an API for manipulating remote objects. OMAPI clients connect
371 to the server using TCP/IP, authenticate, and can then examine the
372 server's current status and make changes to it.
373 Rather than implementing the underlying OMAPI protocol directly, user
375 programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
376 wrapper that handles some of the housekeeping chores that OMAPI does
377 not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
378 and omapi(3).
379 OMAPI exports objects, which can then be examined and modified. The
381 DHCP server exports the following objects: lease, host, failover-state
382 and group. Each object has a number of methods that are provided:
383 lookup, create, and destroy. In addition, it is possible to look at
384 attributes that are stored on objects, and in some cases to modify
385 those attributes.
386
388 Leases can't currently be created or destroyed, but they can be looked
389 up to examine and modify their state.
390 Leases have the following attributes:
392 state integer lookup, examine
394 1 = free
395 2 = active
396 3 = expired
397 4 = released
398 5 = abandoned
399 6 = reset
400 7 = backup
401 8 = reserved
402 9 = bootp
403 ip-address data lookup, examine
405 The IP address of the lease.
406 dhcp-client-identifier data lookup, examine, update
408 The client identifier that the client used when it acquired the
409 lease. Not all clients send client identifiers, so this may be
410 empty.
411 client-hostname data examine, update
413 The value the client sent in the host-name option.
414 host handle examine
416 the host declaration associated with this lease, if any.
417 subnet handle examine
419 the subnet object associated with this lease (the subnet object is
420 not currently supported).
421 pool handle examine
423 the pool object associated with this lease (the pool object is not
424 currently supported).
425 billing-class handle examine
427 the handle to the class to which this lease is currently billed,
428 if any (the class object is not currently supported).
429 hardware-address data examine, update
431 the hardware address (chaddr) field sent by the client when it
432 acquired its lease.
433 hardware-type integer examine, update
435 the type of the network interface that the client reported when it
436 acquired its lease.
437 ends time examine
439 the time when the lease's current state ends, as understood by the
440 client.
441 tstp time examine
443 the time when the lease's current state ends, as understood by the
444 server.
445 tsfp time examine
446 the adjusted time when the lease's current state ends, as under‐
447 stood by the failover peer (if there is no failover peer, this
448 value is undefined). Generally this value is only adjusted for
449 expired, released, or reset leases while the server is operating
450 in partner-down state, and otherwise is simply the value supplied
451 by the peer.
452 atsfp time examine
453 the actual tsfp value sent from the peer. This value is forgotten
454 when a lease binding state change is made, to facilitate retrans‐
455 mission logic.
456 cltt time examine
458 The time of the last transaction with the client on this lease.
459
461 Hosts can be created, destroyed, looked up, examined and modified. If
462 a host declaration is created or deleted using OMAPI, that information
463 will be recorded in the dhcpd.leases file. It is permissible to delete
464 host declarations that are declared in the dhcpd.conf file.
465 Hosts have the following attributes:
467 name data lookup, examine, modify
469 the name of the host declaration. This name must be unique among
470 all host declarations.
471 group handle examine, modify
473 the named group associated with the host declaration, if there is
474 one.
475 hardware-address data lookup, examine, modify
477 the link-layer address that will be used to match the client, if
478 any. Only valid if hardware-type is also present.
479 hardware-type integer lookup, examine, modify
481 the type of the network interface that will be used to match the
482 client, if any. Only valid if hardware-address is also present.
483 dhcp-client-identifier data lookup, examine, modify
485 the dhcp-client-identifier option that will be used to match the
486 client, if any.
487 ip-address data examine, modify
489 a fixed IP address which is reserved for a DHCP client that
490 matches this host declaration. The IP address will only be
491 assigned to the client if it is valid for the network segment to
492 which the client is connected.
493 statements data modify
495 a list of statements in the format of the dhcpd.conf file that
496 will be executed whenever a message from the client is being pro‐
497 cessed.
498 known integer examine, modify
500 if nonzero, indicates that a client matching this host declaration
501 will be treated as known in pool permit lists. If zero, the
502 client will not be treated as known.
503
505 Named groups can be created, destroyed, looked up, examined and modi‐
506 fied. If a group declaration is created or deleted using OMAPI, that
507 information will be recorded in the dhcpd.leases file. It is permissi‐
508 ble to delete group declarations that are declared in the dhcpd.conf
509 file.
510 Named groups currently can only be associated with hosts - this allows
512 one set of statements to be efficiently attached to more than one host
513 declaration.
514 Groups have the following attributes:
516 name data
518 the name of the group. All groups that are created using OMAPI
519 must have names, and the names must be unique among all groups.
520 statements data
522 a list of statements in the format of the dhcpd.conf file that
523 will be executed whenever a message from a client whose host dec‐
524 laration references this group is processed.
525
527 The control object allows you to shut the server down. If the server
528 is doing failover with another peer, it will make a clean transition
529 into the shutdown state and notify its peer, so that the peer can go
530 into partner down, and then record the "recover" state in the lease
531 file so that when the server is restarted, it will automatically resyn‐
532 chronize with its peer.
533 On shutdown the server will also attempt to cleanly shut down all OMAPI
535 connections. If these connections do not go down cleanly after five
536 seconds, they are shut down preemptively. It can take as much as 25
537 seconds from the beginning of the shutdown process to the time that the
538 server actually exits.
539 To shut the server down, open its control object and set the state
541 attribute to 2.
542
544 The failover-state object is the object that tracks the state of the
545 failover protocol as it is being managed for a given failover peer.
546 The failover object has the following attributes (please see dhcpd.conf
547 [1m(5) for explanations about what these attributes mean):
548 name data examine
550 Indicates the name of the failover peer relationship, as described
551 in the server's dhcpd.conf file.
552 partner-address data examine
554 Indicates the failover partner's IP address.
555 local-address data examine
557 Indicates the IP address that is being used by the DHCP server for
558 this failover pair.
559 partner-port data examine
561 Indicates the TCP port on which the failover partner is listening
562 for failover protocol connections.
563 local-port data examine
565 Indicates the TCP port on which the DHCP server is listening for
566 failover protocol connections for this failover pair.
567 max-outstanding-updates integer examine
569 Indicates the number of updates that can be outstanding and unac‐
570 knowledged at any given time, in this failover relationship.
571 mclt integer examine
573 Indicates the maximum client lead time in this failover relation‐
574 ship.
575 load-balance-max-secs integer examine
577 Indicates the maximum value for the secs field in a client request
578 before load balancing is bypassed.
579 load-balance-hba data examine
581 Indicates the load balancing hash bucket array for this failover
582 relationship.
583 local-state integer examine, modify
585 Indicates the present state of the DHCP server in this failover
586 relationship. Possible values for state are:
587 1 - startup
589 2 - normal
590 3 - communications interrupted
591 4 - partner down
592 5 - potential conflict
593 6 - recover
594 7 - paused
595 8 - shutdown
596 9 - recover done
597 10 - resolution interrupted
598 11 - conflict done
599 254 - recover wait
600 (Note that some of the above values have changed since DHCP
602 3.0.x.)
603 In general it is not a good idea to make changes to this state.
605 However, in the case that the failover partner is known to be
606 down, it can be useful to set the DHCP server's failover state to
607 partner down. At this point the DHCP server will take over ser‐
608 vice of the failover partner's leases as soon as possible, and
609 will give out normal leases, not leases that are restricted by
610 MCLT. If you do put the DHCP server into the partner-down when
611 the other DHCP server is not in the partner-down state, but is not
612 reachable, IP address assignment conflicts are possible, even
613 likely. Once a server has been put into partner-down mode, its
614 failover partner must not be brought back online until communica‐
615 tion is possible between the two servers.
616 partner-state integer examine
618 Indicates the present state of the failover partner.
619 local-stos integer examine
621 Indicates the time at which the DHCP server entered its present
622 state in this failover relationship.
623 partner-stos integer examine
625 Indicates the time at which the failover partner entered its
626 present state.
627 hierarchy integer examine
629 Indicates whether the DHCP server is primary (0) or secondary (1)
630 in this failover relationship.
631 last-packet-sent integer examine
633 Indicates the time at which the most recent failover packet was
634 sent by this DHCP server to its failover partner.
635 last-timestamp-received integer examine
637 Indicates the timestamp that was on the failover message most
638 recently received from the failover partner.
639 skew integer examine
641 Indicates the skew between the failover partner's clock and this
642 DHCP server's clock
643 max-response-delay integer examine
645 Indicates the time in seconds after which, if no message is
646 received from the failover partner, the partner is assumed to be
647 out of communication.
648 cur-unacked-updates integer examine
650 Indicates the number of update messages that have been received
651 from the failover partner but not yet processed.
652
654 /etc/dhcp/dhcpd.conf, /var/lib/dhcpd/dhcpd.leases, /var/run/dhcpd.pid,
655 /var/lib/dhcpd/dhcpd.leases~.
656
658 dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
659
661 dhcpd(8) was originally written by Ted Lemon under a contract with
662 Vixie Labs. Funding for this project was provided by Internet Systems
663 Consortium. Version 3 of the DHCP server was funded by Nominum, Inc.
664 Information about Internet Systems Consortium is available at
665 https://www.isc.org/.
666 dhcpd(8)