1.\" $OpenBSD: dhcpd.conf.5,v 1.26 2020/05/16 16:58:11 jmc Exp $ 2.\" 3.\" Copyright (c) 1995, 1996, 1997, 1998, 1998, 1999 4.\" The Internet Software Consortium. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of The Internet Software Consortium nor the names 16.\" of its contributors may be used to endorse or promote products derived 17.\" from this software without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND 20.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, 21.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 22.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 23.\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR 24.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 25.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 26.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF 27.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 28.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 29.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 30.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" This software has been written for the Internet Software Consortium 34.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie 35.\" Enterprises. To learn more about the Internet Software Consortium, 36.\" see ``http://www.isc.org/isc''. To learn more about Vixie 37.\" Enterprises, see ``http://www.vix.com''. 38.\" 39.Dd $Mdocdate: May 16 2020 $ 40.Dt DHCPD.CONF 5 41.Os 42.Sh NAME 43.Nm dhcpd.conf 44.Nd DHCP server configuration file 45.Sh DESCRIPTION 46The 47.Nm 48file contains configuration information for 49.Xr dhcpd 8 , 50the Internet Software Consortium DHCP Server. 51.Pp 52The 53.Nm 54file is a free-form ASCII text file. 55It is parsed by the recursive-descent parser built into 56.Xr dhcpd 8 . 57The file may contain extra tabs and newlines for formatting purposes. 58Keywords in the file are case-insensitive. 59Comments may be placed anywhere within the file (except within quotes). 60Comments begin with the 61.Sq # 62character and end at the end of the line. 63.Pp 64The file essentially consists of a list of statements. 65Statements fall into two broad categories \- parameters and declarations. 66.Pp 67Parameter statements say how to do something (e.g., how long a 68lease to offer), whether to do something (e.g., should 69.Xr dhcpd 8 70provide addresses to unknown clients), or what parameters to provide to the 71client (e.g., use gateway 220.177.244.7). 72.Pp 73Declarations are used to describe the topology of the 74network, to describe clients on the network, to provide addresses that 75can be assigned to clients, or to apply a group of parameters to a 76group of declarations. 77In any group of parameters and declarations, all parameters must be specified 78before any declarations which depend on those parameters may be specified. 79.Pp 80Declarations about network topology include the 81.Ic shared-network 82and the 83.Ic subnet 84declarations. 85If clients on a subnet are to be assigned addresses dynamically, a 86.Ic range 87declaration must appear within the 88.Ic subnet 89declaration. 90For clients with statically assigned addresses, or for installations where 91only known clients will be served, each such client must have a 92.Ic host 93declaration. 94If parameters are to be applied to a group of declarations which are not 95related strictly on a per-subnet basis, the 96.Ic group 97declaration can be used. 98.Pp 99For every subnet which will be served, and for every subnet 100to which the dhcp server is connected, there must be one 101.Ic subnet 102declaration, which tells 103.Xr dhcpd 8 104how to recognize that an address is on that subnet. 105A 106.Ic subnet 107declaration is required for each subnet even if no addresses will be 108dynamically allocated on that subnet. 109.Pp 110Some installations have physical networks on which more than one IP 111subnet operates. 112For example, if there is a site-wide requirement that 8-bit subnet masks 113be used, but a department with a single physical Ethernet network expands 114to the point where it has more than 254 nodes, it may be necessary to run 115two 8-bit subnets on the same Ethernet until such time as a new physical 116network can be added. 117In this case, the 118.Ic subnet 119declarations for these two networks may be enclosed in a 120.Ic shared-network 121declaration. 122.Pp 123Some sites may have departments which have clients on more than one 124subnet, but it may be desirable to offer those clients a uniform set 125of parameters which are different than what would be offered to 126clients from other departments on the same subnet. 127For clients which will be declared explicitly with 128.Ic host 129declarations, these declarations can be enclosed in a 130.Ic group 131declaration along with the parameters which are common to that department. 132For clients whose addresses will be dynamically assigned, there is currently no 133way to group parameter assignments other than by network topology. 134.Pp 135When a client is to be booted, its boot parameters are determined by 136first consulting that client's 137.Ic host 138declaration (if any), then consulting the 139.Ic group 140declaration (if any) which enclosed that 141.Ic host 142declaration, then consulting the 143.Ic subnet 144declaration for the subnet on which the client is booting, then consulting the 145.Ic shared-network 146declaration (if any) containing that subnet, and finally consulting the 147top-level parameters which may be specified outside of any declaration. 148.Pp 149When 150.Xr dhcpd 8 151tries to find a 152.Ic host 153declaration for a client, it first looks for a 154.Ic host 155declaration which has a 156.Ar fixed-address 157parameter which matches the subnet or shared network on which the client 158is booting. 159If it doesn't find any such entry, it then tries to find an entry which has no 160.Ar fixed-address 161parameter. 162If no such entry is found, then 163.Xr dhcpd 8 164acts as if there is no entry in the 165.Nm 166file for that client, even if there is an entry for that client on a 167different subnet or shared network. 168.Sh EXAMPLES 169A typical 170.Nm 171file will look something like this: 172.Pp 173Example 1 174.Bd -unfilled -offset indent 175.Ar global parameters... 176 177shared-network ISC-BIGGIE { 178.Pf " " Ar shared-network-specific parameters ... 179 subnet 204.254.239.0 netmask 255.255.255.224 { 180.Pf " " Ar subnet-specific parameters ... 181 range 204.254.239.10 204.254.239.30; 182 } 183 subnet 204.254.239.32 netmask 255.255.255.224 { 184.Pf " " Ar subnet-specific parameters ... 185 range 204.254.239.42 204.254.239.62; 186 } 187} 188 189subnet 204.254.239.64 netmask 255.255.255.224 { 190.Pf " " Ar subnet-specific parameters ... 191 range 204.254.239.74 204.254.239.94; 192} 193 194group { 195.Pf " " Ar group-specific parameters ... 196 host zappo.test.isc.org { 197.Pf " " Ar host-specific parameters ... 198 } 199 host beppo.test.isc.org { 200.Pf " " Ar host-specific parameters ... 201 } 202 host harpo.test.isc.org { 203.Pf " " Ar host-specific parameters ... 204 } 205} 206.Ed 207.Pp 208Notice that at the beginning of the file, there's a place 209for global parameters. 210These might be things like the organization's domain name, 211the addresses of the name servers 212(if they are common to the entire organization), and so on. 213So, for example: 214.Pp 215Example 2 216.Bd -literal -offset indent 217option domain-name \&"isc.org\&"; 218option domain-name-servers ns1.isc.org, ns2.isc.org; 219.Ed 220.Pp 221As you can see in Example 2, it's legal to specify host addresses in 222parameters as hostnames rather than as numeric IP addresses. 223.Pp 224In Example 1, you can see that both the shared-network statement and 225the subnet statements can have parameters. 226Let us say that the shared network ISC-BIGGIE supports an entire department \- 227perhaps the accounting department. 228If accounting has its own domain, then a shared-network-specific parameter 229might be: 230.Pp 231.Dl option domain-name \&"accounting.isc.org\&"; 232.Pp 233All subnet declarations appearing in the shared-network declaration 234would then have the domain-name option set to 235.Dq accounting.isc.org 236instead of just 237.Dq isc.org . 238.Pp 239The most obvious reason for having subnet-specific parameters as 240shown in Example 1 is that each subnet, of necessity, has its own router. 241So for the first subnet, for example, there should be something like: 242.Pp 243.Dl option routers 204.254.239.1; 244.Pp 245Note that the address here is specified numerically. 246This is not required \- if you have a different hostname for each 247interface on your router, it's perfectly legitimate to use the hostname 248for that interface instead of the numeric address. 249However, in many cases there may be only one hostname for all of a router's 250IP addresses, and it would not be appropriate to use that name here. 251.Pp 252In Example 1 there is also a 253.Ic group 254statement, which provides common parameters for a set of three hosts \- zappo, 255beppo and harpo. 256As you can see, these hosts are all in the test.isc.org domain, so it 257might make sense for a group-specific parameter to override the domain 258name supplied to these hosts: 259.Pp 260.Dl option domain-name \&"test.isc.org\&"; 261.Pp 262Also, given the domain they're in, these are probably test machines. 263If we wanted to test the DHCP leasing mechanism, we might set the 264lease timeout somewhat shorter than the default: 265.Bd -literal -offset indent 266max-lease-time 120; 267default-lease-time 120; 268.Ed 269.Pp 270You may have noticed that while some parameters start with the 271.Ic option 272keyword, some do not. 273Parameters starting with the 274.Ic option 275keyword correspond to actual DHCP options, while parameters that do not start 276with the option keyword either control the behaviour of the DHCP server 277(e.g., how long a lease 278.Xr dhcpd 8 279will give out), or specify client parameters that are not optional in the 280DHCP protocol (for example, server-name and filename). 281.Pp 282In Example 1, each host had 283.Ar host-specific parameters . 284These could include such things as the 285.Ic hostname 286option, the name of a file to download (the 287.Ar filename 288parameter) and the address of the server from which to download the file (the 289.Ar next-server 290parameter). 291In general, any parameter can appear anywhere that parameters are allowed, 292and will be applied according to the scope in which the parameter appears. 293.Pp 294Imagine that you have a site with a lot of NCD X-Terminals. 295These terminals come in a variety of models, and you want to specify the 296boot files for each model. 297One way to do this would be to have host declarations for each server 298and group them by model: 299.Bd -literal -offset indent 300group { 301 filename "Xncd19r"; 302 next-server ncd-booter; 303 304 host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; } 305 host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; } 306 host ncd8 { hardware ethernet 0:c0:c3:22:46:81; } 307} 308 309group { 310 filename "Xncd19c"; 311 next-server ncd-booter; 312 313 host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; } 314 host ncd3 { hardware ethernet 0:c0:c3:00:14:11; } 315} 316 317group { 318 filename "XncdHMX"; 319 next-server ncd-booter; 320 321 host ncd5 { hardware ethernet 0:c0:c3:11:90:23; } 322 host ncd6 { hardware ethernet 0:c0:c3:91:a7:8; } 323 host ncd7 { hardware ethernet 0:c0:c3:cc:a:8f; } 324} 325.Ed 326.Sh REFERENCE: DECLARATIONS 327The 328.Ic shared-network 329statement informs the DHCP server that some IP subnets actually 330share the same physical network: 331.Bd -unfilled -offset indent 332.Ic shared-network Ar name No { 333.Pf " " Op Ar parameters 334.Pf " " Op Ar declarations 335} 336.Ed 337.Pp 338Any subnets in a shared network should be declared within a 339.Ic shared-network 340statement. 341Parameters specified in the 342.Ic shared-network 343statement will be used when booting clients on those subnets unless 344parameters provided at the subnet or host level override them. 345If any subnet in a shared network has addresses available for dynamic 346allocation, those addresses are collected into a common pool for that 347shared network and assigned to clients as needed. 348There is no way to distinguish on which subnet of a shared network a 349client should boot. 350.Pp 351.Ar name 352should be the name of the shared network. 353This name is used when printing debugging messages, so it should be 354descriptive for the shared network. 355The name may have the syntax of a valid hostname 356(although it will never be used as such), or it may be any arbitrary 357name, enclosed in quotes. 358.Pp 359The 360.Ic subnet 361statement provides 362.Xr dhcpd 8 363with enough information to tell whether or not an IP address is on that subnet: 364.Bd -unfilled -offset indent 365.Ic subnet Ar subnet-number Ic netmask Ar netmask No { 366.Pf " " Op Ar parameters 367.Pf " " Op Ar declarations 368} 369.Ed 370.Pp 371It may also be used to provide subnet-specific parameters and to 372specify what addresses may be dynamically allocated to clients booting 373on that subnet. 374Such addresses are specified using the 375.Ic range 376declaration. 377.Pp 378The 379.Ar subnet-number 380and 381.Ar netmask 382should be specified as numeric IP addresses. 383The subnet number, together with the netmask, are sufficient to determine 384whether any given IP address is on the specified subnet. 385.Pp 386Although a netmask must be given with every subnet declaration, it is 387recommended that if there is any variance in subnet masks at a site, a 388subnet-mask option statement be used in each subnet declaration to set 389the desired subnet mask, since any subnet-mask option statement will 390override the subnet mask declared in the subnet statement. 391.Pp 392The 393.Ic range 394statement gives the lowest and highest IP addresses in a range: 395.Bd -filled -offset indent 396.Ic range Op Ic dynamic-bootp 397.Ar low-address Oo Ar high-address Oc ; 398.Ed 399.Pp 400For any subnet on which addresses will be assigned dynamically, there 401must be at least one 402.Ic range 403statement. 404All IP addresses in the range should be in the subnet in which the 405.Ic range 406statement is declared. 407The 408.Ic dynamic-bootp 409flag may be specified if addresses in the specified range may be dynamically 410assigned to BOOTP clients as well as DHCP clients. 411When specifying a single address, 412.Ar high-address 413can be omitted. 414.Pp 415There must be at least one 416.Ic host 417statement for every BOOTP client that is to be served: 418.Bd -unfilled -offset indent 419.Ic host Ar hostname No { 420.Pf " " Op Ar parameters 421.Pf " " Op Ar declarations 422} 423.Ed 424.Pp 425.Ic host 426statements may also be specified for DHCP clients, although this is 427not required unless booting is only enabled for known hosts. 428.Pp 429If it is desirable to be able to boot a DHCP or BOOTP 430client on more than one subnet with fixed addresses, more than one 431address may be specified in the 432.Ar fixed-address 433parameter, or more than one 434.Ic host 435statement may be specified. 436.Pp 437If client-specific boot parameters must change based on the network 438to which the client is attached, then multiple 439.Ic host 440statements should be used. 441.Pp 442If a client is to be booted using a fixed address if it's 443possible, but should be allocated a dynamic address otherwise, then a 444.Ic host 445statement must be specified without a 446.Ar fixed-address 447clause. 448.Ar hostname 449should be a name identifying the host. 450If a 451.Ar hostname 452option is not specified for the host, 453.Ar hostname 454is used. 455.Pp 456.Ic host 457declarations are matched to actual DHCP or BOOTP clients by matching the 458.Ic dhcp-client-identifier 459option specified in the 460.Ic host 461declaration to the one supplied by the client or, if the 462.Ic host 463declaration or the client does not provide a 464.Ic dhcp-client-identifier 465option, by matching the 466.Ar hardware 467parameter in the 468.Ic host 469declaration to the network hardware address supplied by the client. 470BOOTP clients do not normally provide a 471.Ar dhcp-client-identifier , 472so the hardware address must be used for all clients that may boot using 473the BOOTP protocol. 474.Pp 475The 476.Ic group 477statement is used simply to apply one or more parameters to a group of 478declarations: 479.Bd -unfilled -offset indent 480.Ic group No { 481.Pf " " Op Ar parameters 482.Pf " " Op Ar declarations 483} 484.Ed 485.Pp 486It can be used to group hosts, shared networks, subnets, or even other groups. 487.Sh REFERENCE: ALLOW and DENY 488The 489.Ic allow 490and 491.Ic deny 492statements can be used to control the behaviour of 493.Xr dhcpd 8 494to various sorts of requests. 495.Pp 496The 497.Ar unknown-clients 498flag tells 499.Xr dhcpd 8 500whether or not to dynamically assign addresses to unknown clients: 501.Bd -literal -offset indent 502allow unknown-clients; 503deny unknown-clients; 504.Ed 505.Pp 506Dynamic address assignment to unknown clients is allowed by default. 507.Pp 508The 509.Ar bootp 510flag tells 511.Xr dhcpd 8 512whether or not to respond to bootp queries: 513.Bd -literal -offset indent 514allow bootp; 515deny bootp; 516.Ed 517.Pp 518Bootp queries are allowed by default. 519.Pp 520The 521.Ar booting 522flag tells 523.Xr dhcpd 8 524whether or not to respond to queries from a particular client: 525.Bd -literal -offset indent 526allow booting; 527deny booting; 528.Ed 529.Pp 530This keyword only has meaning when it appears in a host declaration. 531By default, booting is allowed, but if it is disabled for a particular client, 532then that client will not be able to get an address from the DHCP server. 533.Sh REFERENCE: PARAMETERS 534The 535.Ic default-lease-time 536statement specifies the 537.Ar time 538in seconds that will be assigned to a lease 539if the client requesting the lease does not ask for a specific expiration time: 540.Pp 541.D1 Ic default-lease-time Ar time ; 542.Pp 543The 544.Ic max-lease-time 545statement specifies the maximum 546.Ar time 547in seconds that will be assigned to a lease 548if the client requesting the lease asks for a specific expiration time: 549.Pp 550.D1 Ic max-lease-time Ar time ; 551.Pp 552The 553.Ic hardware 554statement allows a 555BOOTP client to be recognized in a 556.Ic host 557statement: 558.Pp 559.D1 Ic hardware Ar hardware-type hardware-address ; 560.Pp 561.Ar hardware-type 562must be the name of a hardware interface type. 563Currently, the 564.Cm ethernet , 565.Cm token-ring 566and 567.Cm fddi 568physical interface types are recognized, 569although support for DHCP-over-IPsec virtual interface type 570.Cm ipsec-tunnel 571is provided. 572The 573.Ar hardware-address 574should be a set of colon-separated hexadecimal octets (0-ff) 575or a hostname that can be looked up in 576.Xr ethers 5 577when the configuration is read. 578The 579.Ic hardware 580statement may also be used for DHCP clients. 581.Pp 582The 583.Ic filename 584statement can be used to specify the name of the initial boot file which 585is to be loaded by a client: 586.Pp 587.D1 Ic filename Qq Ar filename ; 588.Pp 589The 590.Ar filename 591should be a filename recognizable to whatever file transfer protocol 592the client can be expected to use to load the file. 593.Pp 594The 595.Ic server-name 596statement can be used to inform the client of the name of the server 597from which it is booting: 598.Pp 599.D1 Ic server-name Qq Ar name ; 600.Pp 601.Ar name 602should be the name that will be provided to the client. 603.Pp 604The 605.Ic next-server 606statement specifies the host address of 607the server from which the initial boot file (specified in the 608.Ic filename 609statement) is to be loaded: 610.Pp 611.D1 Ic next-server Ar server-name ; 612.Pp 613.Ar server-name 614should be a numeric IP address or a hostname. 615If no 616.Ic next-server 617parameter applies to a given client, the DHCP server's IP address is used. 618.Pp 619The 620.Ic fixed-address 621statement assigns one or more fixed IP addresses to a client: 622.Pp 623.D1 Ic fixed-address Ar address Op , Ar address ... ; 624.Pp 625It should only appear in a 626.Ic host 627declaration. 628If more than one address is supplied, then when the client boots, it will be 629assigned the address which corresponds to the network on which it is booting. 630If none of the addresses in the 631.Ic fixed-address 632statement are on the network on which the client is booting, that client will 633not match the 634.Ic host 635declaration containing that 636.Ic fixed-address 637statement. 638Each 639.Ar address 640should be either an IP address or a hostname which resolves to one 641or more IP addresses. 642.Pp 643Clients with fixed addresses are not assigned DHCP leases, 644and may therefore not be used with the 645.Fl ACL 646table options of 647.Xr dhcpd 8 . 648.Pp 649The 650.Ic dynamic-bootp-lease-cutoff 651statement sets the ending time for all leases assigned dynamically to 652BOOTP clients: 653.Pp 654.D1 Ic dynamic-bootp-lease-cutoff Ar date ; 655.Pp 656Because BOOTP clients do not have any way of renewing leases, 657and don't know that their leases could expire, by default 658.Xr dhcpd 8 659assigns infinite leases to all BOOTP clients. 660However, it may make sense in some situations to set a cutoff date for all 661BOOTP leases \- for example, the end of a school term, 662or the time at night when a facility is closed and all 663machines are required to be powered off. 664.Pp 665.Ar date 666should be the date on which all assigned BOOTP leases will end. 667The date is specified in the form: 668.Pp 669.Dl W YYYY/MM/DD HH:MM:SS 670.Pp 671W is the day of the week expressed as a number from zero (Sunday) 672to six (Saturday). 673YYYY is the year, including the century. 674MM is the month expressed as a number from 1 to 12. 675DD is the day of the month, counting from 1. 676HH is the hour, from zero to 23. 677MM is the minute and SS is the second. 678The time is always in Coordinated Universal Time (UTC), not local time. 679.Pp 680The 681.Ic dynamic-bootp-lease-length 682statement sets the length of leases dynamically assigned to BOOTP clients: 683.Pp 684.D1 Ic dynamic-bootp-lease-length Ar length ; 685.Pp 686At some sites, it may be possible to assume that a lease is no longer in 687use if its holder has not used BOOTP or DHCP to get its address within 688a certain time period. 689The period is specified in 690.Ar length 691as a number of seconds. 692If a client reboots using BOOTP during the timeout period, the lease 693duration is reset to 694.Ar length , 695so a BOOTP client that boots frequently enough will never lose its lease. 696Needless to say, this parameter should be adjusted with extreme caution. 697.Pp 698The 699.Ic get-lease-hostnames 700statement tells 701.Xr dhcpd 8 702whether or not to look up the hostname corresponding to the IP address of 703each address in the lease pool and use that address for the DHCP 704.Ic hostname 705option: 706.Pp 707.D1 Ic get-lease-hostnames Ar flag ; 708.Pp 709If 710.Ar flag 711is true, then this lookup is done for all addresses in the current scope. 712By default, or if 713.Ar flag 714is false, no lookups are done. 715.Pp 716If the 717.Ic use-host-decl-names 718parameter is true in a given scope, then for every host declaration within 719that scope, the name provided for the host declaration will be supplied to 720the client as its hostname: 721.Pp 722.D1 Ic use-host-decl-names Ar flag ; 723.Pp 724So, for example: 725.Bd -literal -offset indent 726group { 727 use-host-decl-names on; 728 729 host joe { 730 hardware ethernet 08:00:2b:4c:29:32; 731 fixed-address joe.fugue.com; 732 } 733} 734.Ed 735.Pp 736is equivalent to: 737.Bd -literal -offset indent 738host joe { 739 hardware ethernet 08:00:2b:4c:29:32; 740 fixed-address joe.fugue.com; 741 option host-name "joe"; 742} 743.Ed 744.Pp 745An 746.Ic option host-name 747statement within a host declaration will override the use of the name 748in the host declaration. 749.Pp 750The 751.Ic authoritative 752statement: 753.Pp 754.D1 Ic authoritative ; 755.D1 Ic not authoritative ; 756.Pp 757The DHCP server will normally assume that the configuration 758information about a given network segment is known to be correct and 759is authoritative. 760So if a client requests an IP address on a given network segment that the 761server knows is not valid for that segment, the server will respond with a 762DHCPNAK message, causing the client to forget its IP address and try to get 763a new one. 764.Pp 765If a DHCP server is being configured by somebody who is not the 766network administrator and who therefore does not wish to assert this 767level of authority, then the statement 768.Dq not authoritative 769should be written in the appropriate scope in the configuration file. 770.Pp 771Usually, writing 772.Ic not authoritative; 773at the top level of the file should be sufficient. 774However, if a DHCP server is to be set up so that it is aware of some 775networks for which it is authoritative and some networks for which it is not, 776it may be more appropriate to declare authority on a per-network-segment basis. 777.Pp 778Note that the most specific scope for which the concept of authority 779makes any sense is the physical network segment \- either a 780shared-network statement or a subnet statement that is not contained 781within a shared-network statement. 782It is not meaningful to specify that the server is authoritative for some 783subnets within a shared network, but not authoritative for others, 784nor is it meaningful to specify that the server is authoritative for some 785host declarations and not others. 786.Pp 787The 788.Ic use-lease-addr-for-default-route 789statement: 790.Pp 791.D1 Ic use-lease-addr-for-default-route Ar flag ; 792.Pp 793If the 794.Ic use-lease-addr-for-default-route 795parameter is true in a given scope, then instead of sending the value 796specified in the routers option (or sending no value at all), 797the IP address of the lease being assigned is sent to the client. 798This supposedly causes Win95 machines to ARP for all IP addresses, 799which can be helpful if your router is configured for proxy ARP. 800.Pp 801If 802.Ic use-lease-addr-for-default-route 803is enabled and an option routers statement are both in scope, 804the routers option will be preferred. 805The rationale for this is that in situations where you want to use 806this feature, you probably want it enabled for a whole bunch of 807Windows 95 machines, and you want to override it for a few other machines. 808Unfortunately, if the opposite happens to be true for your 809site, you are probably better off not trying to use this flag. 810.Pp 811The 812.Ic always-reply-rfc1048 813statement: 814.Pp 815.D1 Ic always-reply-rfc1048 Ar flag ; 816.Pp 817Some BOOTP clients expect RFC 1048-style responses, but do not follow 818RFC 1048 when sending their requests. 819You can tell that a client is having this problem if it is not getting 820the options you have configured for it and if you see in the server log 821the message 822.Dq (non-rfc1048) 823printed with each BOOTREQUEST that is logged. 824.Pp 825If you want to send RFC 1048 options to such a client, you can set the 826.Ic always-reply-rfc1048 827option in that client's host declaration, and the DHCP server will 828respond with an RFC 1048-style vendor options field. 829This flag can be set in any scope, and will affect all clients covered 830by that scope. 831.Pp 832The 833.Ic server-identifier 834statement can be used to define the value that is sent in the 835DHCP Server Identifier option for a given scope: 836.Pp 837.D1 Ic server-identifier Ar hostname ; 838.Pp 839The value specified 840.Em must 841be an IP address for the DHCP server, and must be reachable by all 842clients served by a particular scope. 843.Pp 844The use of the server-identifier statement is not recommended \- the only 845reason to use it is to force a value other than the default value to be 846sent on occasions where the default value would be incorrect. 847The default value is the first IP address associated with the physical 848network interface on which the request arrived. 849.Pp 850The usual case where the 851.Ic server-identifier 852statement needs to be sent is when a physical interface has more than one 853IP address, and the one being sent by default isn't appropriate for some 854or all clients served by that interface. 855Another common case is when an alias is defined for the purpose of 856having a consistent IP address for the DHCP server, and it is desired 857that the clients use this IP address when contacting the server. 858.Pp 859Supplying a value for the 860.Ic dhcp-server-identifier 861option is equivalent to using the 862.Ic server-identifier 863statement. 864.Pp 865The 866.Ic echo-client-id 867statement controls RFC 6842 compliant behavior and has a default 868value of true: 869.Pp 870.D1 Ic echo-client-id Ar flag ; 871.Pp 872Some devices, especially old printers, require 873.Ic echo-client-id 874to be false. 875.Pp 876When 877.Ic echo-client-id 878is true 879.Xr dhcpd 8 880copies option dhcp-client-identifier (code 61) 881from 882DHCP DISCOVER or REQUEST messages into 883the DHCP ACK or NAK reply sent to the client. 884.Sh REFERENCE: OPTION STATEMENTS 885DHCP option statements are documented in the 886.Xr dhcp-options 5 887manual page. 888.Sh FILES 889.Bl -tag -width /etc/examples/dhcpd.conf -compact 890.It Pa /etc/examples/dhcpd.conf 891Example configuration file. 892.El 893.Sh SEE ALSO 894.Xr dhcp-options 5 , 895.Xr dhcpd.leases 5 , 896.Xr dhcpd 8 897.Sh STANDARDS 898.Rs 899.%A R. Droms 900.%D March 1997 901.%R RFC 2131 902.%T Dynamic Host Configuration Protocol 903.Re 904.Pp 905.Rs 906.%A S. Alexander 907.%A R. Droms 908.%D March 1997 909.%R RFC 2132 910.%T DHCP Options and BOOTP Vendor Extensions 911.Re 912.Pp 913.Rs 914.%A M. Patrick 915.%D January 2001 916.%R RFC 3046 917.%T DHCP Relay Agent Information Option 918.Re 919.Pp 920.Rs 921.%A B. Patel 922.%A B. Aboba 923.%A S. Kelly 924.%A V. Gupta 925.%D January 2003 926.%R RFC 3456 927.%T Dynamic Host Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode 928.Re 929.Sh AUTHORS 930.An -nosplit 931.Xr dhcpd 8 932was written by 933.An Ted Lemon Aq Mt mellon@vix.com 934under a contract with Vixie Labs. 935.Pp 936The current implementation was reworked by 937.An Henning Brauer Aq Mt henning@openbsd.org . 938