1dhcpd(8) System Manager's Manual dhcpd(8)
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6 dhcpd - Dynamic Host Configuration Protocol Server
7
9 dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -s
10 server ] [ -cf config-file ] [ -lf lease-file ] [ -pf pid-file ] [
11 --no-pid ] [ -tf trace-output-file ] [ -user user ] [ -group group ] [
12 -chroot dir ] [ -play trace-playback-file ] [ if0 [ ...ifN ] ]
13 dhcpd --version
15
17 The Internet Systems Consortium DHCP Server, dhcpd, implements the
18 Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap
19 Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request and
20 be assigned IP addresses, and also to discover information about the
21 network to which they are attached. BOOTP provides similar functional‐
22 ity, with certain restrictions.
23
25 The DHCP protocol allows a host which is unknown to the network admin‐
26 istrator to be automatically assigned a new IP address out of a pool of
27 IP addresses for its network. In order for this to work, the network
28 administrator allocates address pools in each subnet and enters them
29 into the dhcpd.conf(5) file.
30 There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
32 startup the server may be started for one or the other via the -4 or
33 -6 arguments.
34 On startup, dhcpd reads the dhcpd.conf file and stores a list of avail‐
36 able addresses on each subnet in memory. When a client requests an
37 address using the DHCP protocol, dhcpd allocates an address for it.
38 Each client is assigned a lease, which expires after an amount of time
39 chosen by the administrator (by default, one day). Before leases
40 expire, the clients to which leases are assigned are expected to renew
41 them in order to continue to use the addresses. Once a lease has
42 expired, the client to which that lease was assigned is no longer per‐
43 mitted to use the leased IP address.
44 In order to keep track of leases across system reboots and server
46 restarts, dhcpd keeps a list of leases it has assigned in the
47 dhcpd.leases(5) file. Before dhcpd grants a lease to a host, it
48 records the lease in this file and makes sure that the contents of the
49 file are flushed to disk. This ensures that even in the event of a
50 system crash, dhcpd will not forget about a lease that it has assigned.
51 On startup, after reading the dhcpd.conf file, dhcpd reads the
52 dhcpd.leases file to refresh its memory about what leases have been
53 assigned.
54 New leases are appended to the end of the dhcpd.leases file. In order
56 to prevent the file from becoming arbitrarily large, from time to time
57 dhcpd creates a new dhcpd.leases file from its in-core lease database.
58 Once this file has been written to disk, the old file is renamed
59 dhcpd.leases~, and the new file is renamed dhcpd.leases. If the system
60 crashes in the middle of this process, whichever dhcpd.leases file
61 remains will contain all the lease information, so there is no need for
62 a special crash recovery process.
63 BOOTP support is also provided by this server. Unlike DHCP, the BOOTP
65 protocol does not provide a protocol for recovering dynamically-
66 assigned addresses once they are no longer needed. It is still possi‐
67 ble to dynamically assign addresses to BOOTP clients, but some adminis‐
68 trative process for reclaiming addresses is required. By default,
69 leases are granted to BOOTP clients in perpetuity, although the network
70 administrator may set an earlier cutoff date or a shorter lease length
71 for BOOTP leases if that makes sense.
72 BOOTP clients may also be served in the old standard way, which is to
74 simply provide a declaration in the dhcpd.conf file for each BOOTP
75 client, permanently assigning an address to each client.
76 Whenever changes are made to the dhcpd.conf file, dhcpd must be
78 restarted. To restart dhcpd, send a SIGTERM (signal 15) to the process
79 ID contained in /var/run/dhcpd.pid, and then re-invoke dhcpd. Because
80 the DHCP server database is not as lightweight as a BOOTP database,
81 dhcpd does not automatically restart itself when it sees a change to
82 the dhcpd.conf file.
83 Note: We get a lot of complaints about this. We realize that it would
85 be nice if one could send a SIGHUP to the server and have it reload the
86 database. This is not technically impossible, but it would require a
87 great deal of work, our resources are extremely limited, and they can
88 be better spent elsewhere. So please don't complain about this on the
89 mailing list unless you're prepared to fund a project to implement this
90 feature, or prepared to do it yourself.
91
93 The names of the network interfaces on which dhcpd should listen for
94 broadcasts may be specified on the command line. This should be done
95 on systems where dhcpd is unable to identify non-broadcast interfaces,
96 but should not be required on other systems. If no interface names are
97 specified on the command line dhcpd will identify all network inter‐
98 faces which are up, eliminating non-broadcast interfaces if possible,
99 and listen for DHCP broadcasts on each interface.
100
102 -4 Run as a DHCP server. This is the default and cannot be combined
103 with -6.
104 -6 Run as a DHCPv6 server. This cannot be combined with -4.
106 -p port
108 The udp port number on which dhcpd should listen. If unspeci‐
109 fied dhcpd uses the default port of 67. This is mostly useful
110 for debugging purposes.
111 -s address
113 Specify an address or host name to which dhcpd should send
114 replies rather than the broadcast address (255.255.255.255).
115 This option is only supported in IPv4.
116 -f Force dhcpd to run as a foreground process instead of as a dae‐
118 mon in the background. This is useful when running dhcpd under
119 a debugger, or when running it out of inittab on System V sys‐
120 tems.
121 -d Send log messages to the standard error descriptor. This can be
123 useful for debugging, and also at sites where a complete log of
124 all dhcp activity must be kept but syslogd is not reliable or
125 otherwise cannot be used. Normally, dhcpd will log all output
126 using the syslog(3) function with the log facility set to
127 LOG_DAEMON. Note that -d implies -f (the daemon will not fork
128 itself into the background).
129 -q Be quiet at startup. This suppresses the printing of the entire
131 copyright message during startup. This might be desirable when
132 starting dhcpd from a system startup script (e.g., /etc/rc).
133 -t Test the configuration file. The server tests the configuration
135 file for correct syntax, but will not attempt to perform any
136 network operations. This can be used to test a new configura‐
137 tion file automatically before installing it.
138 -T Test the lease file. The server tests the lease file for cor‐
140 rect syntax, but will not attempt to perform any network opera‐
141 tions. This can be used to test a new leaes file automatically
142 before installing it.
143 -tf tracefile
145 Specify a file into which the entire startup state of the server
146 and all the transactions it processes are logged. This can be
147 useful in submitting bug reports - if you are getting a core
148 dump every so often, you can start the server with the -tf
149 option and then, when the server dumps core, the trace file will
150 contain all the transactions that led up to it dumping core, so
151 that the problem can be easily debugged with -play.
152 -play playfile
154 Specify a file from which the entire startup state of the server
155 and all the transactions it processed are read. The -play
156 option must be specified with an alternate lease file, using the
157 -lf switch, so that the DHCP server doesn't wipe out your exist‐
158 ing lease file with its test data. The DHCP server will refuse
159 to operate in playback mode unless you specify an alternate
160 lease file.
161 --version
163 Print version number and exit.
164 -user user
166 Setuid to user after completing privileged operations, such as
167 creating sockets that listen on privileged ports.
168 -group group
170 Setgid to group after completing privileged operations, such as
171 creating sockets that listen on privileged ports.
172 -chroot dir
174 Chroot to directory after processing the command line arguments,
175 but before reading the configuration file.
176 Modifying default file locations: The following options can be used to
178 modify the locations dhcpd uses for it's files. Because of the impor‐
179 tance of using the same lease database at all times when running dhcpd
180 in production, these options should be used only for testing lease
181 files or database files in a non-production environment.
182 -cf config-file
184 Path to alternate configuration file.
185 -lf lease-file
187 Path to alternate lease file.
188 -pf pid-file
190 Path to alternate pid file.
191 --no-pid
193 Option to disable writing pid files. By default the program
194 will write a pid file. If the program is invoked with this
195 option it will not check for an existing server process.
196
198 The syntax of the dhcpd.conf(5) file is discussed separately. This
199 section should be used as an overview of the configuration process, and
200 the dhcpd.conf(5) documentation should be consulted for detailed refer‐
201 ence information.
202
204 dhcpd needs to know the subnet numbers and netmasks of all subnets for
205 which it will be providing service. In addition, in order to dynami‐
206 cally allocate addresses, it must be assigned one or more ranges of
207 addresses on each subnet which it can in turn assign to client hosts as
208 they boot. Thus, a very simple configuration providing DHCP support
209 might look like this:
210 subnet 239.252.197.0 netmask 255.255.255.0 {
212 range 239.252.197.10 239.252.197.250;
213 }
214 Multiple address ranges may be specified like this:
216 subnet 239.252.197.0 netmask 255.255.255.0 {
218 range 239.252.197.10 239.252.197.107;
219 range 239.252.197.113 239.252.197.250;
220 }
221 If a subnet will only be provided with BOOTP service and no dynamic
223 address assignment, the range clause can be left out entirely, but the
224 subnet statement must appear.
225
227 DHCP leases can be assigned almost any length from zero seconds to
228 infinity. What lease length makes sense for any given subnet, or for
229 any given installation, will vary depending on the kinds of hosts being
230 served.
231 For example, in an office environment where systems are added from time
233 to time and removed from time to time, but move relatively infre‐
234 quently, it might make sense to allow lease times of a month or more.
235 In a final test environment on a manufacturing floor, it may make more
236 sense to assign a maximum lease length of 30 minutes - enough time to
237 go through a simple test procedure on a network appliance before pack‐
238 aging it up for delivery.
239 It is possible to specify two lease lengths: the default length that
241 will be assigned if a client doesn't ask for any particular lease
242 length, and a maximum lease length. These are specified as clauses to
243 the subnet command:
244 subnet 239.252.197.0 netmask 255.255.255.0 {
246 range 239.252.197.10 239.252.197.107;
247 default-lease-time 600;
248 max-lease-time 7200;
249 }
250 This particular subnet declaration specifies a default lease time of
252 600 seconds (ten minutes), and a maximum lease time of 7200 seconds
253 (two hours). Other common values would be 86400 (one day), 604800 (one
254 week) and 2592000 (30 days).
255 Each subnet need not have the same lease—in the case of an office envi‐
257 ronment and a manufacturing environment served by the same DHCP server,
258 it might make sense to have widely disparate values for default and
259 maximum lease times on each subnet.
260
262 Each BOOTP client must be explicitly declared in the dhcpd.conf file.
263 A very basic client declaration will specify the client network inter‐
264 face's hardware address and the IP address to assign to that client.
265 If the client needs to be able to load a boot file from the server,
266 that file's name must be specified. A simple bootp client declaration
267 might look like this:
268 host haagen {
270 hardware ethernet 08:00:2b:4c:59:23;
271 fixed-address 239.252.197.9;
272 filename "/tftpboot/haagen.boot";
273 }
274
276 DHCP (and also BOOTP with Vendor Extensions) provide a mechanism
277 whereby the server can provide the client with information about how to
278 configure its network interface (e.g., subnet mask), and also how the
279 client can access various network services (e.g., DNS, IP routers, and
280 so on).
281 These options can be specified on a per-subnet basis, and, for BOOTP
283 clients, also on a per-client basis. In the event that a BOOTP client
284 declaration specifies options that are also specified in its subnet
285 declaration, the options specified in the client declaration take
286 precedence. A reasonably complete DHCP configuration might look some‐
287 thing like this:
288 subnet 239.252.197.0 netmask 255.255.255.0 {
290 range 239.252.197.10 239.252.197.250;
291 default-lease-time 600 max-lease-time 7200;
292 option subnet-mask 255.255.255.0;
293 option broadcast-address 239.252.197.255;
294 option routers 239.252.197.1;
295 option domain-name-servers 239.252.197.2, 239.252.197.3;
296 option domain-name "isc.org";
297 }
298 A bootp host on that subnet that needs to be in a different domain and
300 use a different name server might be declared as follows:
301 host haagen {
303 hardware ethernet 08:00:2b:4c:59:23;
304 fixed-address 239.252.197.9;
305 filename "/tftpboot/haagen.boot";
306 option domain-name-servers 192.5.5.1;
307 option domain-name "vix.com";
308 }
309 A more complete description of the dhcpd.conf file syntax is provided
311 in dhcpd.conf(5).
312
314 The DHCP server provides the capability to modify some of its configu‐
315 ration while it is running, without stopping it, modifying its database
316 files, and restarting it. This capability is currently provided using
317 OMAPI - an API for manipulating remote objects. OMAPI clients connect
318 to the server using TCP/IP, authenticate, and can then examine the
319 server's current status and make changes to it.
320 Rather than implementing the underlying OMAPI protocol directly, user
322 programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
323 wrapper that handles some of the housekeeping chores that OMAPI does
324 not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
325 and omapi(3).
326 OMAPI exports objects, which can then be examined and modified. The
328 DHCP server exports the following objects: lease, host, failover-state
329 and group. Each object has a number of methods that are provided:
330 lookup, create, and destroy. In addition, it is possible to look at
331 attributes that are stored on objects, and in some cases to modify
332 those attributes.
333
335 Leases can't currently be created or destroyed, but they can be looked
336 up to examine and modify their state.
337 Leases have the following attributes:
339 state integer lookup, examine
341 1 = free
342 2 = active
343 3 = expired
344 4 = released
345 5 = abandoned
346 6 = reset
347 7 = backup
348 8 = reserved
349 9 = bootp
350 ip-address data lookup, examine
352 The IP address of the lease.
353 dhcp-client-identifier data lookup, examine, update
355 The client identifier that the client used when it acquired the
356 lease. Not all clients send client identifiers, so this may be
357 empty.
358 client-hostname data examine, update
360 The value the client sent in the host-name option.
361 host handle examine
363 the host declaration associated with this lease, if any.
364 subnet handle examine
366 the subnet object associated with this lease (the subnet object is
367 not currently supported).
368 pool handle examine
370 the pool object associated with this lease (the pool object is not
371 currently supported).
372 billing-class handle examine
374 the handle to the class to which this lease is currently billed,
375 if any (the class object is not currently supported).
376 hardware-address data examine, update
378 the hardware address (chaddr) field sent by the client when it
379 acquired its lease.
380 hardware-type integer examine, update
382 the type of the network interface that the client reported when it
383 acquired its lease.
384 ends time examine
386 the time when the lease's current state ends, as understood by the
387 client.
388 tstp time examine
390 the time when the lease's current state ends, as understood by the
391 server.
392 tsfp time examine
393 the adjusted time when the lease's current state ends, as under‐
394 stood by the failover peer (if there is no failover peer, this
395 value is undefined). Generally this value is only adjusted for
396 expired, released, or reset leases while the server is operating
397 in partner-down state, and otherwise is simply the value supplied
398 by the peer.
399 atsfp time examine
400 the actual tsfp value sent from the peer. This value is forgotten
401 when a lease binding state change is made, to facilitate retrans‐
402 mission logic.
403 cltt time examine
405 The time of the last transaction with the client on this lease.
406
408 Hosts can be created, destroyed, looked up, examined and modified. If
409 a host declaration is created or deleted using OMAPI, that information
410 will be recorded in the dhcpd.leases file. It is permissible to delete
411 host declarations that are declared in the dhcpd.conf file.
412 Hosts have the following attributes:
414 name data lookup, examine, modify
416 the name of the host declaration. This name must be unique among
417 all host declarations.
418 group handle examine, modify
420 the named group associated with the host declaration, if there is
421 one.
422 hardware-address data lookup, examine, modify
424 the link-layer address that will be used to match the client, if
425 any. Only valid if hardware-type is also present.
426 hardware-type integer lookup, examine, modify
428 the type of the network interface that will be used to match the
429 client, if any. Only valid if hardware-address is also present.
430 dhcp-client-identifier data lookup, examine, modify
432 the dhcp-client-identifier option that will be used to match the
433 client, if any.
434 ip-address data examine, modify
436 a fixed IP address which is reserved for a DHCP client that
437 matches this host declaration. The IP address will only be
438 assigned to the client if it is valid for the network segment to
439 which the client is connected.
440 statements data modify
442 a list of statements in the format of the dhcpd.conf file that
443 will be executed whenever a message from the client is being pro‐
444 cessed.
445 known integer examine, modify
447 if nonzero, indicates that a client matching this host declaration
448 will be treated as known in pool permit lists. If zero, the
449 client will not be treated as known.
450
452 Named groups can be created, destroyed, looked up, examined and modi‐
453 fied. If a group declaration is created or deleted using OMAPI, that
454 information will be recorded in the dhcpd.leases file. It is permissi‐
455 ble to delete group declarations that are declared in the dhcpd.conf
456 file.
457 Named groups currently can only be associated with hosts - this allows
459 one set of statements to be efficiently attached to more than one host
460 declaration.
461 Groups have the following attributes:
463 name data
465 the name of the group. All groups that are created using OMAPI
466 must have names, and the names must be unique among all groups.
467 statements data
469 a list of statements in the format of the dhcpd.conf file that
470 will be executed whenever a message from a client whose host dec‐
471 laration references this group is processed.
472
474 The control object allows you to shut the server down. If the server
475 is doing failover with another peer, it will make a clean transition
476 into the shutdown state and notify its peer, so that the peer can go
477 into partner down, and then record the "recover" state in the lease
478 file so that when the server is restarted, it will automatically resyn‐
479 chronize with its peer.
480 On shutdown the server will also attempt to cleanly shut down all OMAPI
482 connections. If these connections do not go down cleanly after five
483 seconds, they are shut down preemptively. It can take as much as 25
484 seconds from the beginning of the shutdown process to the time that the
485 server actually exits.
486 To shut the server down, open its control object and set the state
488 attribute to 2.
489
491 The failover-state object is the object that tracks the state of the
492 failover protocol as it is being managed for a given failover peer.
493 The failover object has the following attributes (please see dhcpd.conf
494 [1m(5) for explanations about what these attributes mean):
495 name data examine
497 Indicates the name of the failover peer relationship, as described
498 in the server's dhcpd.conf file.
499 partner-address data examine
501 Indicates the failover partner's IP address.
502 local-address data examine
504 Indicates the IP address that is being used by the DHCP server for
505 this failover pair.
506 partner-port data examine
508 Indicates the TCP port on which the failover partner is listening
509 for failover protocol connections.
510 local-port data examine
512 Indicates the TCP port on which the DHCP server is listening for
513 failover protocol connections for this failover pair.
514 max-outstanding-updates integer examine
516 Indicates the number of updates that can be outstanding and unac‐
517 knowledged at any given time, in this failover relationship.
518 mclt integer examine
520 Indicates the maximum client lead time in this failover relation‐
521 ship.
522 load-balance-max-secs integer examine
524 Indicates the maximum value for the secs field in a client request
525 before load balancing is bypassed.
526 load-balance-hba data examine
528 Indicates the load balancing hash bucket array for this failover
529 relationship.
530 local-state integer examine, modify
532 Indicates the present state of the DHCP server in this failover
533 relationship. Possible values for state are:
534 1 - startup
536 2 - normal
537 3 - communications interrupted
538 4 - partner down
539 5 - potential conflict
540 6 - recover
541 7 - paused
542 8 - shutdown
543 9 - recover done
544 10 - resolution interrupted
545 11 - conflict done
546 254 - recover wait
547 (Note that some of the above values have changed since DHCP
549 3.0.x.)
550 In general it is not a good idea to make changes to this state.
552 However, in the case that the failover partner is known to be
553 down, it can be useful to set the DHCP server's failover state to
554 partner down. At this point the DHCP server will take over ser‐
555 vice of the failover partner's leases as soon as possible, and
556 will give out normal leases, not leases that are restricted by
557 MCLT. If you do put the DHCP server into the partner-down when
558 the other DHCP server is not in the partner-down state, but is not
559 reachable, IP address assignment conflicts are possible, even
560 likely. Once a server has been put into partner-down mode, its
561 failover partner must not be brought back online until communica‐
562 tion is possible between the two servers.
563 partner-state integer examine
565 Indicates the present state of the failover partner.
566 local-stos integer examine
568 Indicates the time at which the DHCP server entered its present
569 state in this failover relationship.
570 partner-stos integer examine
572 Indicates the time at which the failover partner entered its
573 present state.
574 hierarchy integer examine
576 Indicates whether the DHCP server is primary (0) or secondary (1)
577 in this failover relationship.
578 last-packet-sent integer examine
580 Indicates the time at which the most recent failover packet was
581 sent by this DHCP server to its failover partner.
582 last-timestamp-received integer examine
584 Indicates the timestamp that was on the failover message most
585 recently received from the failover partner.
586 skew integer examine
588 Indicates the skew between the failover partner's clock and this
589 DHCP server's clock
590 max-response-delay integer examine
592 Indicates the time in seconds after which, if no message is
593 received from the failover partner, the partner is assumed to be
594 out of communication.
595 cur-unacked-updates integer examine
597 Indicates the number of update messages that have been received
598 from the failover partner but not yet processed.
599
601 /etc/dhcp/dhcpd.conf, /var/lib/dhcpd/dhcpd.leases, /var/run/dhcpd.pid,
602 /var/lib/dhcpd/dhcpd.leases~.
603
605 dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
606
608 dhcpd(8) was originally written by Ted Lemon under a contract with
609 Vixie Labs. Funding for this project was provided by Internet Systems
610 Consortium. Version 3 of the DHCP server was funded by Nominum, Inc.
611 Information about Internet Systems Consortium is available at
612 https://www.isc.org/.
613 dhcpd(8)