dhcpd.conf(5) dhcpd.conf(5) NNAAMMEE dhcpd.conf - dhcpd configuration file DDEESSCCRRIIPPTTIIOONN The dhcpd.conf file contains configuration information for _d_h_c_p_d_, the Internet Software Consortium DHCP Server. The dhcpd.conf file is a free-form ASCII text file. It is parsed by the recursive-descent parser built into dhcpd. The file may contain extra tabs and newlines for formatting purposes. Keywords in the file are case-insen­ sitive. Comments may be placed anywhere within the file (except within quotes). Comments begin with the # char­ acter and end at the end of the line. The file essentially consists of a list of statements. Statements fall into two broad categories - parameters and declarations. Parameter statements either say how to do something (e.g., how long a lease to offer), whether to do something (e.g., should dhcpd provide addresses to unknown clients), or what parameters to provide to the client (e.g., use gate­ way 220.177.244.7). Declarations are used to describe the topology of the net­ work, to describe clients on the network, to provide addresses that can be assigned to clients, or to apply a group of parameters to a group of declarations. In any group of parameters and declarations, all parameters must be specified before any declarations which depend on those parameters may be specified. Declarations about network topology include the _s_h_a_r_e_d_-_n_e_t_w_o_r_k and the _s_u_b_n_e_t declarations. If clients on a subnet are to be assigned addresses dynamically, a _r_a_n_g_e declaration must appear within the _s_u_b_n_e_t declara­ tion. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a _h_o_s_t declaration. If param­ eters are to be applied to a group of declarations which are not related strictly on a per-subnet basis, the _g_r_o_u_p declaration can be used. For every subnet which will be served, and for every sub­ net to which the dhcp server is connected, there must be one _s_u_b_n_e_t declaration, which tells dhcpd how to recognize that an address is on that subnet. A _s_u_b_n_e_t declaration is required for each subnet even if no addresses will be dynamically allocated on that subnet. Some installations have physical networks on which more than one IP subnet operates. For example, if there is a site-wide requirement that 8-bit subnet masks be used, but a department with a single physical ethernet network expands to the point where it has more than 254 nodes, it may be necessary to run two 8-bit subnets on the same eth­ ernet until such time as a new physical network can be added. In this case, the _s_u_b_n_e_t declarations for these two networks may be enclosed in a _s_h_a_r_e_d_-_n_e_t_w_o_r_k declaration. Some sites may have departments which have clients on more than one subnet, but it may be desirable to offer those clients a uniform set of parameters which are different than what would be offered to clients from other depart­ ments on the same subnet. For clients which will be declared explicitly with _h_o_s_t declarations, these declara­ tions can be enclosed in a _g_r_o_u_p declaration along with the parameters which are common to that department. For clients whose addresses will be dynamically assigned, there is currently no way to group parameter assignments other than by network topology. When a client is to be booted, its boot parameters are determined by first consulting that client's _h_o_s_t declara­ tion (if any), then consulting the _g_r_o_u_p declaration (if any) which enclosed that _h_o_s_t declaration, then consulting the _s_u_b_n_e_t declaration for the subnet on which the client is booting, then consulting the _s_h_a_r_e_d_-_n_e_t_w_o_r_k declaration (if any) containing that subnet, and finally consulting the top-level parameters which may be specified outside of any declaration. When dhcpd tries to find a _h_o_s_t declaration for a client, it first looks for a _h_o_s_t declaration which has a _f_i_x_e_d_- _a_d_d_r_e_s_s parameter which matches the subnet or shared net­ work on which the client is booting. If it doesn't find any such entry, it then tries to find an entry which has no _f_i_x_e_d_-_a_d_d_r_e_s_s parameter. If no such entry is found, then dhcpd acts as if there is no entry in the dhcpd.conf file for that client, even if there is an entry for that client on a different subnet or shared network. EEXXAAMMPPLLEESS A typical dhcpd.conf file will look something like this: _g_l_o_b_a_l _p_a_r_a_m_e_t_e_r_s_._._. shared-network ISC-BIGGIE { _s_h_a_r_e_d_-_n_e_t_w_o_r_k_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. subnet 204.254.239.0 netmask 255.255.255.224 { _s_u_b_n_e_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. range 204.254.239.10 204.254.239.30; } subnet 204.254.239.32 netmask 255.255.255.224 { _s_u_b_n_e_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. range 204.254.239.42 204.254.239.62; } } subnet 204.254.239.64 netmask 255.255.255.224 { _s_u_b_n_e_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. range 204.254.239.74 204.254.239.94; } group { _g_r_o_u_p_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. host zappo.test.isc.org { _h_o_s_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. } host beppo.test.isc.org { _h_o_s_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. } host harpo.test.isc.org { _h_o_s_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s_._._. } } Figure 1 Notice that at the beginning of the file, there's a place for global parameters. These might be things like the organization's domain name, the addresses of the name servers (if they are common to the entire organization), and so on. So, for example: option domain-name "isc.org"; option domain-name-servers ns1.isc.org, ns2.isc.org; Figure 2 As you can see in Figure 2, it's legal to specify host addresses in parameters as domain names rather than as numeric IP addresses. If a given hostname resolves to more than one IP address (for example, if that host has two ethernet interfaces), both addresses are supplied to the client. In Figure 1, you can see that both the shared-network statement and the subnet statements can have parameters. Let us say that the shared network _I_S_C_-_B_I_G_G_I_E supports an entire department - perhaps the accounting department. If accounting has its own domain, then a shared-network- specific parameter might be: option domain-name "accounting.isc.org"; All subnet declarations appearing in the shared-network declaration would then have the domain-name option set to "accounting.isc.org" instead of just "isc.org". The most obvious reason for having subnet-specific parame­ ters as shown in Figure 1 is that each subnet, of neces­ sity, has its own router. So for the first subnet, for example, there should be something like: option routers 204.254.239.1; Note that the address here is specified numerically. This is not required - if you have a different domain name for each interface on your router, it's perfectly legiti­ mate to use the domain name for that interface instead of the numeric address. However, in many cases there may be only one domain name for all of a router's IP addresses, and it would not be appropriate to use that name here. In Figure 1 there is also a _g_r_o_u_p statement, which pro­ vides common parameters for a set of three hosts - zappo, beppo and harpo. As you can see, these hosts are all in the test.isc.org domain, so it might make sense for a group-specific parameter to override the domain name sup­ plied to these hosts: option domain-name "test.isc.org"; Also, given the domain they're in, these are probably test machines. If we wanted to test the DHCP leasing mecha­ nism, we might set the lease timeout somewhat shorter than the default: max-lease-time 120; default-lease-time 120; You may have noticed that while some parameters start with the _o_p_t_i_o_n keyword, some do not. Parameters starting with the _o_p_t_i_o_n keyword correspond to actual DHCP options, while parameters that do not start with the option keyword either control the behaviour of the DHCP server (e.g., how long a lease dhcpd will give out), or specify client parameters that are not optional in the DHCP protocol (for example, server-name and filename). In Figure 1, each host had _h_o_s_t_-_s_p_e_c_i_f_i_c _p_a_r_a_m_e_t_e_r_s. These could include such things as the _h_o_s_t_n_a_m_e option, the name of a file to upload (the _f_i_l_e_n_a_m_e _p_a_r_a_m_e_t_e_r_) _a_n_d _t_h_e _a_d_d_r_e_s_s _o_f _t_h_e _s_e_r_v_e_r _f_r_o_m _w_h_i_c_h _t_o _u_p_l_o_a_d _t_h_e _f_i_l_e _(_t_h_e _n_e_x_t_-_s_e_r_v_e_r parameter). In general, any parameter can appear anywhere that parameters are allowed, and will be applied according to the scope in which the parameter appears. Imagine that you have a site with a lot of NCD X-Termi­ nals. These terminals come in a variety of models, and you want to specify the boot files for each models. One way to do this would be to have host declarations for each server and group them by model: group { filename "Xncd19r"; next-server ncd-booter; host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; } host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; } host ncd8 { hardware ethernet 0:c0:c3:22:46:81; } } group { filename "Xncd19c"; next-server ncd-booter; host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; } host ncd3 { hardware ethernet 0:c0:c3:00:14:11; } } group { filename "XncdHMX"; next-server ncd-booter; host ncd1 { hardware ethernet 0:c0:c3:11:90:23; } host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; } host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; } } RREEFFEERREENNCCEE:: DDEECCLLAARRAATTIIOONNSS TThhee _s_h_a_r_e_d_-_n_e_t_w_o_r_k ssttaatteemmeenntt sshhaarreedd--nneettwwoorrkk _n_a_m_e {{ [ _p_a_r_a_m_e_t_e_r_s ] [ _d_e_c_l_a_r_a_t_i_o_n_s ] }} The _s_h_a_r_e_d_-_n_e_t_w_o_r_k statement is used to inform the DHCP server that some IP subnets actually share the same physi­ cal network. Any subnets in a shared network should be declared within a _s_h_a_r_e_d_-_n_e_t_w_o_r_k statement. Parameters specified in the _s_h_a_r_e_d_-_n_e_t_w_o_r_k statement will be used when booting clients on those subnets unless parameters provided at the subnet or host level override them. If any subnet in a shared network has addresses available for dynamic allocation, those addresses are collected into a common pool for that shared network and assigned to clients as needed. There is no way to distinguish on which subnet of a shared network a client should boot. _N_a_m_e should be the name of the shared network. This name is used when printing debugging messages, so it should be descriptive for the shared network. The name may have the syntax of a valid domain name (although it will never be used as such), or it may be any arbitrary name, enclosed in quotes. TThhee _s_u_b_n_e_t ssttaatteemmeenntt ssuubbnneett _s_u_b_n_e_t_-_n_u_m_b_e_r nneettmmaasskk _n_e_t_m_a_s_k {{ [ _p_a_r_a_m_e_t_e_r_s ] [ _d_e_c_l_a_r_a_t_i_o_n_s ] }} The _s_u_b_n_e_t statement is used to provide dhcpd with enough information to tell whether or not an IP address is on that subnet. It may also be used to provide subnet-spe­ cific parameters and to specify what addresses may be dynamically allocated to clients booting on that subnet. Such addresses are specified using the _r_a_n_g_e declaration. The _s_u_b_n_e_t_-_n_u_m_b_e_r should be an IP address or domain name which resolves to the subnet number of the subnet being described. The _n_e_t_m_a_s_k should be an IP address or domain name which resolves to the subnet mask of the subnet being described. The subnet number, together with the netmask, are sufficient to determine whether any given IP address is on the specified subnet. Although a netmask must be given with every subnet decla­ ration, it is recommended that if there is any variance in subnet masks at a site, a subnet-mask option statement be used in each subnet declaration to set the desired subnet mask, since any subnet-mask option statement will override the subnet mask declared in the subnet statement. TThhee _r_a_n_g_e ssttaatteemmeenntt rraannggee [ ddyynnaammiicc--bboooottpp ] _l_o_w_-_a_d_d_r_e_s_s [ _h_i_g_h_-_a_d_d_r_e_s_s];; For any subnet on which addresses will be assigned dynami­ cally, there must be at least one _r_a_n_g_e statement. The range statement gives the lowest and highest IP addresses in a range. All IP addresses in the range should be in the subnet in which the _r_a_n_g_e statement is declared. The _d_y_n_a_m_i_c_-_b_o_o_t_p flag may be specified if addresses in the specified range may be dynamically assigned to BOOTP clients as well as DHCP clients. When specifying a sin­ gle address, _h_i_g_h_-_a_d_d_r_e_s_s can be omitted. TThhee _h_o_s_t ssttaatteemmeenntt hhoosstt _h_o_s_t_n_a_m_e { [ _p_a_r_a_m_e_t_e_r_s ] [ _d_e_c_l_a_r_a_t_i_o_n_s ] }} There must be at least one hhoosstt statement for every BOOTP client that is to be served. hhoosstt statements may also be specified for DHCP clients, although this is not required unless booting is only enabled for known hosts. If it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet with fixed addresses, more than one address may be specified in the _f_i_x_e_d_-_a_d_d_r_e_s_s parameter, or more than one hhoosstt statement may be speci­ fied. If client-specific boot parameters must change based on the network to which the client is attached, then multiple hhoosstt statements should be used. If a client is to be booted using a fixed address if it's possible, but should be allocated a dynamic address other­ wise, then a hhoosstt statement must be specified without a ffiixxeedd--aaddddrreessss clause. _h_o_s_t_n_a_m_e should be a name identify­ ing the host. If a _h_o_s_t_n_a_m_e option is not specified for the host, _h_o_s_t_n_a_m_e is used. _H_o_s_t declarations are matched to actual DHCP or BOOTP clients by matching the dhcp-client-identifier option specified in the _h_o_s_t declaration to the one supplied by the client, or, if the _h_o_s_t declaration or the client does not provide a dhcp-client-identifier option, by matching the _h_a_r_d_w_a_r_e parameter in the _h_o_s_t declaration to the net­ work hardware address supplied by the client. BOOTP clients do not normally provide a _d_h_c_p_-_c_l_i_e_n_t_-_i_d_e_n_t_i_f_i_e_r, so the hardware address must be used for all clients that may boot using the BOOTP protocol. TThhee _g_r_o_u_p ssttaatteemmeenntt ggrroouupp { [ _p_a_r_a_m_e_t_e_r_s ] [ _d_e_c_l_a_r_a_t_i_o_n_s ] }} The group statement is used simply to apply one or more parameters to a group of declarations. It can be used to group hosts, shared networks, subnets, or even other groups. RREEFFEERREENNCCEE:: AALLLLOOWW aanndd DDEENNYY The _a_l_l_o_w and _d_e_n_y statements can be used to control the behaviour of dhcpd to various sorts of requests. TThhee _u_n_k_n_o_w_n_-_c_l_i_e_n_t_s kkeeyywwoorrdd aallllooww uunnkknnoowwnn--cclliieennttss;; ddeennyy uunnkknnoowwnn--cclliieennttss;; The uunnkknnoowwnn--cclliieennttss flag is used to tell dhcpd whether or not to dynamically assign addresses to unknown clients. Dynamic address assignment to unknown clients is aalllloowwed by default. TThhee _b_o_o_t_p kkeeyywwoorrdd aallllooww bboooottpp;; ddeennyy bboooottpp;; The bboooottpp flag is used to tell dhcpd whether or not to respond to bootp queries. Bootp queries are aalllloowwed by default. TThhee _b_o_o_t_i_n_g kkeeyywwoorrdd aallllooww bboooottiinngg;; ddeennyy bboooottiinngg;; The bboooottiinngg flag is used to tell dhcpd whether or not to respond to queries from a particular client. This keyword only has meaning when it appears in a host declaration. By default, booting is aalllloowwed, but if it is disabled for a particular client, then that client will not be able to get and address from the DHCP server. RREEFFEERREENNCCEE:: PPAARRAAMMEETTEERRSS TThhee _d_e_f_a_u_l_t_-_l_e_a_s_e_-_t_i_m_e ssttaatteemmeenntt ddeeffaauulltt--lleeaassee--ttiimmee _t_i_m_e;; _T_i_m_e should be the length in seconds that will be assigned to a lease if the client requesting the lease does not ask for a specific expiration time. TThhee _m_a_x_-_l_e_a_s_e_-_t_i_m_e ssttaatteemmeenntt mmaaxx--lleeaassee--ttiimmee _t_i_m_e;; _T_i_m_e should be the maximum length in seconds that will be assigned to a lease if the client requesting the lease asks for a specific expiration time. TThhee _h_a_r_d_w_a_r_e ssttaatteemmeenntt hhaarrddwwaarree _h_a_r_d_w_a_r_e_-_t_y_p_e _h_a_r_d_w_a_r_e_-_a_d_d_r_e_s_s;; In order for a BOOTP client to be recognized, its network hardware address must be declared using a _h_a_r_d_w_a_r_e clause in the _h_o_s_t statement. _h_a_r_d_w_a_r_e_-_t_y_p_e must be the name of a physical hardware interface type. Currently, only the eetthheerrnneett and ttookkeenn--rriinngg types are recognized, although support for a ffddddii hardware type (and others) would also be desirable. The _h_a_r_d_w_a_r_e_-_a_d_d_r_e_s_s should be a set of hexadecimal octets (numbers from 0 through ff) seperated by colons. The _h_a_r_d_w_a_r_e statement may also be used for DHCP clients. TThhee _f_i_l_e_n_a_m_e ssttaatteemmeenntt ffiilleennaammee ""_f_i_l_e_n_a_m_e"";; The _f_i_l_e_n_a_m_e statement can be used to specify the name of the initial boot file which is to be loaded by a client. The _f_i_l_e_n_a_m_e should be a filename recognizable to whatever file transfer protocol the client can be expected to use to load the file. TThhee _s_e_r_v_e_r_-_n_a_m_e ssttaatteemmeenntt sseerrvveerr--nnaammee ""_n_a_m_e"";; The _s_e_r_v_e_r_-_n_a_m_e statement can be used to inform the client of the name of the server from which it is booting. _N_a_m_e should be the name that will be provided to the client. TThhee _n_e_x_t_-_s_e_r_v_e_r ssttaatteemmeenntt nneexxtt--sseerrvveerr _s_e_r_v_e_r_-_n_a_m_e;; The _n_e_x_t_-_s_e_r_v_e_r statement is used to specify the host address of the server from which the initial boot file (specified in the _f_i_l_e_n_a_m_e statement) is to be loaded. _S_e_r_v_e_r_-_n_a_m_e should be a numeric IP address or a domain name. If no _n_e_x_t_-_s_e_r_v_e_r parameter applies to a given client, the DHCP server's IP address is used. TThhee _f_i_x_e_d_-_a_d_d_r_e_s_s ssttaatteemmeenntt ffiixxeedd--aaddddrreessss _a_d_d_r_e_s_s [,, _a_d_d_r_e_s_s ... ];; The _f_i_x_e_d_-_a_d_d_r_e_s_s statement is used to assign one or more fixed IP addresses to a client. It should only appear in a _h_o_s_t declaration. If more than one address is supplied, then when the client boots, it will be assigned the address which corresponds to the network on which it is booting. If none of the addresses in the _f_i_x_e_d_-_a_d_d_r_e_s_s statement are on the network on which the client is boot­ ing, that client will not match the _h_o_s_t declaration con­ taining that _f_i_x_e_d_-_a_d_d_r_e_s_s statement. Each _a_d_d_r_e_s_s should be either an IP address or a domain name which resolves to one or more IP addresses. TThhee _d_y_n_a_m_i_c_-_b_o_o_t_p_-_l_e_a_s_e_-_c_u_t_o_f_f ssttaatteemmeenntt ddyynnaammiicc--bboooottpp--lleeaassee--ccuuttooffff _d_a_t_e;; The _d_y_n_a_m_i_c_-_b_o_o_t_p_-_l_e_a_s_e_-_c_u_t_o_f_f statement sets the ending time for all leases assigned dynamically to BOOTP clients. Because BOOTP clients do not have any way of renewing leases, and don't know that their leases could expire, by default dhcpd assignes infinite leases to all BOOTP clients. However, it may make sense in some situations to set a cutoff date for all BOOTP leases - for example, the end of a school term, or the time at night when a facility is closed and all machines are required to be powered off. _D_a_t_e should be the date on which all assigned BOOTP leases will end. The date is specified in the form: W YYYY/MM/DD HH:MM:SS W is the day of the week expressed as a number from zero (Sunday) to six (Saturday). YYYY is the year, including the century. MM is the month expressed as a number from 1 to 12. DD is the day of the month, counting from 1. HH is the hour, from zero to 23. MM is the minute and SS is the second. The time is always in Universal Coordinated Time (UTC), not local time. TThhee _d_y_n_a_m_i_c_-_b_o_o_t_p_-_l_e_a_s_e_-_l_e_n_g_t_h ssttaatteemmeenntt ddyynnaammiicc--bboooottpp--lleeaassee--lleennggtthh _l_e_n_g_t_h;; The _d_y_n_a_m_i_c_-_b_o_o_t_p_-_l_e_a_s_e_-_l_e_n_g_t_h statement is used to set the length of leases dynamically assigned to BOOTP clients. At some sites, it may be possible to assume that a lease is no longer in use if its holder has not used BOOTP or DHCP to get its address within a certain time period. The period is specified in _l_e_n_g_t_h as a num­ ber of seconds. If a client reboots using BOOTP during the timeout period, the lease duration is reset to _l_e_n_g_t_h, so a BOOTP client that boots frequently enough will never lose its lease. Needless to say, this parameter should be adjusted with extreme caution. TThhee _g_e_t_-_l_e_a_s_e_-_h_o_s_t_n_a_m_e_s ssttaatteemmeenntt ggeett--lleeaassee--hhoossttnnaammeess _f_l_a_g;; The _g_e_t_-_l_e_a_s_e_-_h_o_s_t_n_a_m_e_s statement is used to tell dhcpd whether or not to look up the domain name corresponding to the IP address of each address in the lease pool and use that address for the DHCP _h_o_s_t_n_a_m_e option. If _f_l_a_g is true, then this lookup is done for all addresses in the current scope. By default, or if _f_l_a_g is false, no lookups are done. TThhee _u_s_e_-_h_o_s_t_-_d_e_c_l_-_n_a_m_e_s ssttaatteemmeenntt uussee--hhoosstt--ddeeccll--nnaammeess _f_l_a_g;; If the _u_s_e_-_h_o_s_t_-_d_e_c_l_-_n_a_m_e_s parameter is true in a given scope, then for every host declaration within that scope, the name provided for the host declaration will be sup­ plied to the client as its hostname. So, for example, group { use-host-decl-names on; host joe { hardware ethernet 08:00:2b:4c:29:32; fixed-address joe.fugue.com; } } is equivalent to host joe { hardware ethernet 08:00:2b:4c:29:32; fixed-address joe.fugue.com; option host-name "joe"; } An _o_p_t_i_o_n _h_o_s_t_-_n_a_m_e statement within a host declaration will override the use of the name in the host declaration. TThhee _a_u_t_h_o_r_i_t_a_t_i_v_e ssttaatteemmeenntt aauutthhoorriittaattiivvee;; nnoott aauutthhoorriittaattiivvee;; The DHCP server will normally assume that the configura­ tion information about a given network segment is known to be correct and is authoritative. So if a client requests an IP address on a given network segment that the server knows is not valid for that segment, the server will respond with a DHCPNAK message, causing the client to for­ get its IP address and try to get a new one. If a DHCP server is being configured by somebody who is not the network administrator and who therefore does not wish to assert this level of authority, then the statement "not authoritative" should be written in the appropriate scope in the configuration file. Usually, writing nnoott aauutthhoorriittaattiivvee;; at the top level of the file should be sufficient. However, if a DHCP server is to be set up so that it is aware of some networks for which it is authoritative and some networks for which it is not, it may be more appropriate to declare authority on a per-network-segment basis. Note that the most specific scope for which the concept of authority makes any sense is the physical network segment - either a shared-network statement or a subnet statement that is not contained within a shared-network statement. It is not meaningful to specify that the server is author­ itative for some subnets within a shared network, but not authoritative for others, nor is it meaningful to specify that the server is authoritative for some host declara­ tions and not others. TThhee _u_s_e_-_l_e_a_s_e_-_a_d_d_r_-_f_o_r_-_d_e_f_a_u_l_t_-_r_o_u_t_e ssttaatteemmeenntt uussee--lleeaassee--aaddddrr--ffoorr--ddeeffaauulltt--rroouuttee _f_l_a_g;; If the _u_s_e_-_l_e_a_s_e_-_a_d_d_r_-_f_o_r_-_d_e_f_a_u_l_t_-_r_o_u_t_e parameter is true in a given scope, then instead of sending the value speci­ fied in the routers option (or sending no value at all), the IP address of the lease being assigned is sent to the client. This supposedly causes Win95 machines to ARP for all IP addresses, which can be helpful if your router is configured for proxy ARP. If use-lease-addr-for-default-route is enabled and an option routers statement are both in scope, the routers option will be preferred. The rationale for this is that in situations where you want to use this feature, you probably want it enabled for a whole bunch of Windows 95 machines, and you want to override it for a few other machines. Unfortunately, if the opposite happens to be true for you site, you are probably better off not trying to use this flag. TThhee _a_l_w_a_y_s_-_r_e_p_l_y_-_r_f_c_1_0_4_8 ssttaatteemmeenntt aallwwaayyss--rreeppllyy--rrffcc11004488 _f_l_a_g;; Some BOOTP clients expect RFC1048-style responses, but do not follow RFC1048 when sending their requests. You can tell that a client is having this problem if it is not getting the options you have configured for it and if you see in the server log the message "(non-rfc1048)" printed with each BOOTREQUEST that is logged. If you want to send rfc1048 options to such a client, you can set the aallwwaayyss--rreeppllyy--rrffcc11004488 option in that client's host declaration, and the DHCP server will respond with an RFC-1048-style vendor options field. This flag can be set in any scope, and will affect all clients covered by that scope. TThhee _s_e_r_v_e_r_-_i_d_e_n_t_i_f_i_e_r ssttaatteemmeenntt sseerrvveerr--iiddeennttiiffiieerr _h_o_s_t_n_a_m_e;; The server-identifier statement can be used to define the value that is sent in the DHCP Server Identifier option for a given scope. The value specified mmuusstt be an IP address for the DHCP server, and must be reachable by all clients served by a particular scope. The use of the server-identifier statement is not recom­ mended - the only reason to use it is to force a value other than the default value to be sent on occasions where the default value would be incorrect. The default value is the first IP address associated with the physical net­ work interface on which the request arrived. The usual case where the _s_e_r_v_e_r_-_i_d_e_n_t_i_f_i_e_r statement needs to be sent is when a physical interface has more than one IP address, and the one being sent by default isn't appro­ priate for some or all clients served by that interface. Another common case is when an alias is defined for the purpose of having a consistent IP address for the DHCP server, and it is desired that the clients use this IP address when contacting the server. Supplying a value for the dhcp-server-identifier option is equivalent to using the server-identifier statement. RREEFFEERREENNCCEE:: OOPPTTIIOONN SSTTAATTEEMMEENNTTSS DHCP option statements are documented in the ddhhccpp-- ooppttiioonnss((55)) manual page. SSEEEE AALLSSOO dhcpd.conf(5), dhcpd.leases(5), RFC2132, RFC2131. 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.. 1