dhcpd(8) dhcpd(8) NNAAMMEE dhcpd - Dynamic Host Configuration Protocol Server SSYYNNOOPPSSIISS ddhhccppdd [ --pp _p_o_r_t ] [ --ff ] [ _i_f_0 [ _._._._i_f_N ] ] DDEESSCCRRIIPPTTIIOONN The Internet Software Consortium DHCP Server, dhcpd, implements the Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request and be assigned IP addresses, and also to discover information about the net- work to which they are attached. BOOTP provides similar functionality, with certain restrictions. OOPPEERRAATTIIOONN 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. In order for this to work, the network administrator allo- cates address pools in each subnet and enters them into the dhcpd.conf(5) file. On startup, dhcpd reads the _d_h_c_p_d_._c_o_n_f 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 arbi- trarily 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 _d_h_c_p_d_._l_e_a_s_e_s_~, and the new file is renamed dhcpd.leases. If the system crashes in the middle of this process, whichever dhcpd.leases file remains will 1 dhcpd(8) dhcpd(8) 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 perpetu- ity, 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 _/_v_a_r_/_r_u_n_/_d_h_c_p_d_._p_i_d, 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. CCOOMMMMAANNDD LLIINNEE Dhcpd normally identifies all interfaces on the system which are up, and listens on each interface. If possi- ble, point-to-point interfaces and the loopback interface are eliminated, but on some systems this is not possible. For this reason, the interfaces on which dhcpd should lis- ten may be explicitly specified on the command line. Dhcpd normally listens on port 67, which is the BOOTP Server Port (the DHCP and BOOTP protocols both use this port). If desired, dhcpd may be invoked with the --pp flag, followed by a port number, so as to provide DHCP service on a different port. This is mostly useful for debugging purposes. On some System-V systems, it may be desirable to run dhcp from /etc/inittab. If so, dhcpd should be invoked with the --ff flag, which causes dhcpd to run in the foreground; otherwise, dhcpd automatically detaches itself from the process group that started it and runs in the background. This is also useful when running dhcpd under a debugger. Normally dhcpd logs its status using syslog(3). If the --dd flag is specified, dhcpd will also log its status to its 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 2 dhcpd(8) dhcpd(8) otherwise cannot be used. CCOONNFFIIGGUURRAATTIIOONN The syntax of the dhcpd.conf(8) file is discussed seper- ately. This section should be used as an overview of the configuration process, and the dhcpd.conf(8) documentation should be consulted for detailed reference information. SSuubbnneettss dhcpd needs to know the subnet numbers and netmasks of all subnets for which it will be providing service. In addi- tion, in order to dynamically allocate addresses, it must be assigned one or more ranges of addresses on each subnet which it can in turn assign to client hosts as they boot. Thus, a very simple configuration 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; } 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 appear. LLeeaassee LLeennggtthhss DHCP leases can be assigned almost any length from zero seconds to infinity. What lease length makes sense for any given subnet, or for any given installation, will vary depending on the kinds of hosts being served. 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 of more. In a final test environ- ment 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 appli- ance before packaging it up for delivery. It is possible to specify two lease lengths: the default length that will be assigned if a client doesn't ask for any particular lease length, and a maximum lease length. These are specified as clauses to the subnet command: 3 dhcpd(8) dhcpd(8) 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 need not have the same lease--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. BBOOOOTTPP SSuuppppoorrtt Each BOOTP client must be explicitly declared in the dhcpd.conf file. A very basic client declaration will specify the client network interface's hardware address and the IP address to assign to that client. If the client needs to be able to load a boot file from the server, that file's name must be specified. 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"; } OOppttiioonnss 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). These options can be specified on a per-subnet basis, and, for BOOTP clients, also on a per-client basis. 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 prece- dence. An 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; 4 dhcpd(8) dhcpd(8) option domain-name-servers 239.252.197.2, 239.252.197.3; option domain-name "isc.org"; } A bootp host on that subnet that needs to be in a differ- ent 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 "vix.com"; } A more complete description of the dhcpd.conf file syntax is provided in dhcpd.conf(5). FFIILLEESS //eettcc//ddhhccppdd..ccoonnff,, //vvaarr//ddbb//ddhhccppdd..lleeaasseess,, //vvaarr//rruunn//ddhhccppdd..ppiidd,, //vvaarr//ddbb//ddhhccppdd..lleeaasseess~~.. SSEEEE AALLSSOO dhcpd.conf(5), dhcpd.leases(5) AAUUTTHHOORR ddhhccppdd((88)) was written by Ted Lemon under a contract with Vixie Labs. Funding for this project was provided by the Internet Software Corporation. Informa- tion about the Internet Software Consortium can be found at hhttttpp::////wwwwww..iisscc..oorrgg//iisscc.. 5