.\" dhcpd.conf.5 .\" .\" Copyright (c) 1996-1999 Internet Software Consortium. .\" Use is subject to license terms which appear in the file named .\" ISC-LICENSE that should have accompanied this file when you .\" received it. If a file named ISC-LICENSE did not accompany this .\" file, or you are not sure the one you have is correct, you may .\" obtain an applicable copy of the license at: .\" .\" http://www.isc.org/isc-license-1.0.html. .\" .\" This file is part of the ISC DHCP distribution. The documentation .\" associated with this file is listed in the file DOCUMENTATION, .\" included in the top-level directory of this release. .\" .\" Support and other services are available for ISC products - see .\" http://www.isc.org for more information. .TH dhcpd.conf 5 .SH NAME dhcpd.conf - dhcpd configuration file .SH DESCRIPTION The dhcpd.conf file contains configuration information for .IR dhcpd, the Internet Software Consortium DHCP Server. .PP 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-insensitive. Comments may be placed anywhere within the file (except within quotes). Comments begin with the # character and end at the end of the line. .PP The file essentially consists of a list of statements. Statements fall into two broad categories - parameters and declarations. .PP 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 gateway 220.177.244.7). .PP Declarations are used to describe the topology of the network, 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. .PP Declarations about network topology include the \fIshared-network\fR and the \fIsubnet\fR declarations. If clients on a subnet are to be assigned addresses dynamically, a \fIrange\fR declaration must appear within the \fIsubnet\fR declaration. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a \fIhost\fR declaration. If parameters are to be applied to a group of declarations which are not related strictly on a per-subnet basis, the \fIgroup\fR declaration can be used. .PP For every subnet which will be served, and for every subnet to which the dhcp server is connected, there must be one \fIsubnet\fR declaration, which tells dhcpd how to recognize that an address is on that subnet. A \fIsubnet\fR declaration is required for each subnet even if no addresses will be dynamically allocated on that subnet. .PP 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 ethernet until such time as a new physical network can be added. In this case, the \fIsubnet\fR declarations for these two networks must be enclosed in a \fIshared-network\fR declaration. .PP 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 departments on the same subnet. For clients which will be declared explicitly with \fIhost\fR declarations, these declarations can be enclosed in a \fIgroup\fR declaration along with the parameters which are common to that department. For clients whose addresses will be dynamically assigned, class declarations and conditional declarations may be used to group parameter assignments based on information the client sends. .PP When a client is to be booted, its boot parameters are determined by consulting that client's \fIhost\fR declaration (if any), and then consulting the any \fIclass\fR declarations matching the client, followed by the \fIpool\fR, \fIsubnet\fR and \fIshared-network\fR declarations for the IP address assigned to the client. Each of these declarations itself appears within a lexical scope, and all declarations at less specific lexical scopes are also consulted for client option declarations as well. Scopes are never considered twice, and if parameters are declared in more than one scope, the parameter declared in the most specific scope is the one that is used. .PP When dhcpd tries to find a \fIhost\fR declaration for a client, it first looks for a \fIhost\fR declaration which has a \fIfixed-address\fR parameter which matches the subnet or shared network on which the client is booting. If it doesn't find any such entry, it then tries to find an entry which has no \fIfixed-address\fR parameter. .SH EXAMPLES .PP A typical dhcpd.conf file will look something like this: .nf .I global parameters... subnet 204.254.239.0 netmask 255.255.255.224 { \fIsubnet-specific parameters...\fR range 204.254.239.10 204.254.239.30; } subnet 204.254.239.32 netmask 255.255.255.224 { \fIsubnet-specific parameters...\fR range 204.254.239.42 204.254.239.62; } subnet 204.254.239.64 netmask 255.255.255.224 { \fIsubnet-specific parameters...\fR range 204.254.239.74 204.254.239.94; } group { \fIgroup-specific parameters...\fR host zappo.test.isc.org { \fIhost-specific parameters...\fR } host beppo.test.isc.org { \fIhost-specific parameters...\fR } host harpo.test.isc.org { \fIhost-specific parameters...\fR } } .ce 1 Figure 1 .fi .PP 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: .nf option domain-name "isc.org"; option domain-name-servers ns1.isc.org, ns2.isc.org; .ce 1 Figure 2 .fi .PP As you can see in Figure 2, you can specify host addresses in parameters using their domain names rather than their numeric IP addresses. If a given hostname resolves to more than one IP address (for example, if that host has two ethernet interfaces), then where possible, both addresses are supplied to the client. .PP The most obvious reason for having subnet-specific parameters as shown in Figure 1 is that each subnet, of necessity, has its own router. So for the first subnet, for example, there should be something like: .nf option routers 204.254.239.1; .fi .PP 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 legitimate 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. .PP In Figure 1 there is also a \fIgroup\fR statement, which provides 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 supplied to these hosts: .nf option domain-name "test.isc.org"; .fi .PP Also, given the domain they're in, these are probably test machines. If we wanted to test the DHCP leasing mechanism, we might set the lease timeout somewhat shorter than the default: .nf max-lease-time 120; default-lease-time 120; .fi .PP You may have noticed that while some parameters start with the \fIoption\fR keyword, some do not. Parameters starting with the \fIoption\fR 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). .PP In Figure 1, each host had \fIhost-specific parameters\fR. These could include such things as the \fIhostname\fR option, the name of a file to upload (the \fIfilename parameter) and the address of the server from which to upload the file (the \fInext-server\fR 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. .PP Imagine that you have a site with a lot of NCD X-Terminals. 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: .nf 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; } } .fi .SH ADDRESS POOLS .PP The .B pool declaration can be used to specify a pool of addresses that will be treated differently than another pool of addresses, even on the same network segment or subnet. For example, you may want to provide a large set of addresses that can be assigned to DHCP clients that are registered to your DHCP server, while providing a smaller set of addresses, possibly with short lease times, that are available for unknown clients. If you have a firewall, you may be able to arrange for addresses from one pool to be allowed access to the Internet, while addresses in another pool are not, thus encouraging users to register their DHCP clients. To do this, you would set up a pair of pool declarations: .PP .nf subnet 10.0.0.0 netmask 255.255.255.0 { option routers 10.0.0.254; # Unknown clients get this pool. pool { option domain-name-servers bogus.example.com; max-lease-time 300; range 10.0.0.200 10.0.0.253; allow unknown clients; } # Known clients get this pool. pool { option domain-name-servers ns1.example.com, ns2.example.com; max-lease-time 28800; range 10.0.0.5 10.0.0.199; deny unknown clients; } } .fi .PP It is also possible to set up entirely different subnets for known and unknown clients - address pools exist at the level of shared networks, so address ranges within pool declarations can be on different subnets. .PP As you can see in the preceding example, pools can have permit lists that control which clients are allowed access to the pool and which aren't. Each entry in a pool's permit list is introduced with the .I allow or \fIdeny\fR keyword. If a pool has a permit list, then only those clients that match specific entries on the permit list will be elegible to be assigned addresses from the pool. If a pool has a deny list, then only those clients that do not match any entries on the deny list will be elegible. If both permit and deny lists exist for a pool, then only clients that match the permit list and do not match the deny list will be allowed access. .SH ADDRESS ALLOCATION Address allocation is actually only done when a client is in the INIT state and has sent a DHCPDISCOVER message. If the client thinks it has a valid lease and sends a DHCPREQUEST to initiate or renew that lease, the server has only three choices - it can ignore the DHCPREQUEST, send a DHCPNAK to tell the client it should stop using the address, or send a DHCPACK, telling the client to go ahead and use the address for a while. If the server finds the address the client is requesting, and that address is available to the client, the server will send a DHCPACK. If the address is no longer available, or the client isn't permitted to have it, the server will send a DHCPNAK. If the server knows nothing about the, it will remain silent, unless the address is incorrect for the network segment to which the client has been attached and the server is authoritative for that network segment, in which case the server will send a DHCPNAK even though it doesn't know about the address. .PP When the DHCP server allocates a new address for a client (remember, this only happens if the client has sent a DHCPDISCOVER), it first looks to see if the client already has a valid lease on an IP address, or if there is an old IP address the client had before that hasn't yet been reassigned. In that case, the server will take that address and check it to see if the client is still permitted to use it. If the client is no longer permitted to use it, the lease is freed if the server thought it was still in use - the fact that the client has sent a DHCPDISCOVER proves to the server that the client is no longer using the lease. .PP If no existing lease is found, or if the client is forbidden to receive the existing lease, then the server will look in the list of address pools for the network segment to which the client is attached for a lease that is not in use and that the client is permitted to have. It looks through each pool declaration in sequence (all .I range declarations that appear outside of pool declarations are grouped into a single pool with no permit list). If the permit list for the pool allows the client to be allocated an address from that pool, the pool is examined to see if there is an address available. If so, then the client is tentatively assigned that address. Otherwise, the next pool is tested. If no addresses are found that can be assigned to the client, no response is sent to the client. .PP If an address is found that the client is permitted to have, and that has never been assigned to any client before, the address is immediately allocated to the client. If the address is available for allocation but has been previously assigned to a different client, the server will keep looking in hopes of finding an address that has never before been assigned to a client. .SH DHCP FAILOVER This version of the ISC DHCP server supports the DHCP failover protocol as documented in draft-ietf-dhc-failover-07.txt. This is not a final protocol document, and we have not done interoperability testing with other vendors' implementations of this protocol, so you must not assume that this implementation conforms to the standard. If you wish to use the failover protocol, make sure that both failover peers are running the same version of the ISC DHCP server. .PP The failover protocol allows two DHCP servers (and no more than two) to share a common address pool. Each server will have about half of the available IP addresses in the pool at any given time for allocation. If one server fails, the other server will continue to renew leases out of the pool, and will allocate new addresses out of the roughly half of available addresses that it had when communications with the other server were lost. .PP It is possible during a prolonged failure to tell the remaining server that the other server is down, in which case the remaining server will (over time) reclaim all the addresses the other server had available for allocation, and begin to reuse them. This is called putting the server into the PARTNER-DOWN state. .PP When the other server comes back online, it should automatically detect that it has been offline and request a complete update from the server that was running in the PARTNER-DOWN state, and then both servers will resume processing together. .PP It is possible to get into a dangerous situation: if you put one server into the PARTNER-DOWN state, and then *that* server goes down, and the other server comes back up, the other server will not know that the first server was in the PARTNER-DOWN state, and may issue addresses previously issued by the other server to different clients, resulting in IP address conflicts. Before putting a server into PARTNER-DOWN state, therefore, make .I sure that the other server will not restart automatically. .PP The failover protocol defines a primary server role and a secondary server role. There are some differences in how primaries and secondaries act, but most of the differences simply have to do with providing a way for each peer to behave in the opposite way from the other. So one server must be configured as primary, and the other must be configured as secondary, and it doesn't matter too much which one is which. .SH CONFIGURING FAILOVER In order to configure failover, you need to write a peer declaration that configures the failover protocol, and you need to write peer references in each pool declaration for which you want to do failover. You do not have to do failover for all pools on a given network segment. You must not tell one server it's doing failover on a particular address pool and tell the other it is not. You must not have any common address pools on which you are not doing failover. .PP The server currently does very little sanity checking, so if you configure it wrong, it will just fail in odd ways. I would recommend therefore that you either do failover or don't do failover, but don't do any mixed pools. Also, use the same master configuration file for both servers, and have a seperate file that contains the peer declaration and includes the master file. This will help you to avoid configuration mismatches. As our implementation evolves, this will become less of a problem. A basic sample dhcpd.conf file for a primary server might look like this: .PP .nf failover peer "foo" { primary; address anthrax.rc.vix.com; port 519; peer address trantor.rc.vix.com; peer port 520; max-response-delay 60; max-unacked-updates 10; mclt 3600; hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff: 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00; load balance max seconds 3; } include "/etc/dhcpd.master"; .fi .PP The statements in the peer declaration are as follows: .PP .B The .I primary .B and .I secondary .B statements .PP [ \fBprimary\fR | \fBsecondary\fR ] .PP This determines whether the server is primary or secondary, as described earlier under DHCP FAILOVER. .PP .B The .I address .B statement .PP .B address .I address .PP The \fBaddress\fR statement declares the IP address on which the server should listen for connections from its failover peer, and also the value to use for the DHCP Failover Protocol server identifier. Because this value is used as an identifier, it may not be omitted. .PP .B The .I peer address .B statement .PP .B peer address .I address .PP The \fBpeer address\fR statement declares the IP address to which the server should connect to reach its failover peer for failover messages. .PP .B The .I port .B statement .PP .B port .I port-number .PP The \fBport\fR statement declares the TCP port on which the server should listen for connections from its failover peer. This statement may not currently be omitted, because the failover protocol does not yet have a reserved TCP port number. .PP .B The .I peer port .B statement .PP .B peer port .I port-number .PP The \fBpeer port\fR statement declares the TCP port to which the server should connect to reach its failover peer for failover messages. This statement may not be omitted because the failover protocol does not yet have a reserved TCP port number. The port number declared in the \fBpeer port\fR statement may be the same as the port number declared in the \fBport\fR statement. .PP .B The .I max-response-delay .B statement .PP .nf .B max-response-delay .I seconds .fi .PP The \fBmax-response-delay\fR statement tells the DHCP server how many seconds may pass without receiving a message from its failover peer before it assumes that connection has failed. This number should be small enough that a transient network failure that breaks the connection will not result in the servers being out of communication for a long time, but large enough that the server isn't constantly making and breaking connections. This parameter must be specified. .PP .B The .I max-unacked-updates .B statement .PP .B max-unacked-updates .I count .PP The \fBmax-unacked-updates\fR statement tells the DHCP server how many many BINDUPD messages it can send before it receives a BNDACK from the failover peer. We don't have enough operational experience to say what a good value for this is, but 10 seems to work. This parameter must be specified. .PP .B The .I mclt .B statement .PP .B mclt .I seconds .PP The \fBmclt\fR statement defines the Maximum Client Lead Time. It must be specified on the primary, and may not be specified on the secondary. This is the length of time for which a lease may be renewed by either failover peer without contacting the other. The longer you set this, the longer it will take for the running server to recover IP addresses after moving into PARTNER-DOWN state. The shorter you set it, the more load your servers will experience when they are not communicating. A value of something like 3600 is probably reasonable, but again bear in mind that we have no real operational experience with this. .PP .B The .I split .B statement .PP .B split .I index .PP The split statement specifies the split between the primary and secondary for the purposes of load balancing. Whenever a client makes a DHCP request, the DHCP server runs a hash on the client identification. If the hash comes out to less than the split value, the primary answers. If it comes out to equal to or more than the split, the secondary answers. This value should generally be set to 128, and can only be configured on the primary. .PP .B The .I hba .B statement .PP .B hba .I colon-seperated-hex-list .PP The hba statement specifies the split between the primary and secondary as a bitmap rather than a cutoff, which theoretically allows for finer-grained control. In practice, there is probably no need for such fine-grained control, however. An example hba statement: .PP .nf hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff: 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00; .fi .PP .B The .I load balance max seconds .B statement .PP .B load balance max seconds .I seconds .PP This statement allows you to configure a cutoff after which load balancing is disabled. The cutoff is based on the number of seconds since the client sent its first DHCPDISCOVER or DHCPREQUEST message, and only works with clients that correctly implement the \fIsecs\fR field - fortunately most clients do. We recommend setting this to something like 3 or 5. The effect of this is that if one of the failover peers gets into a state where it is responding to failover messages but not responding to some client requests, the other failover peer will take over its client load automatically as the clients retry. .SH CLIENT CLASSING Clients can be seperated into classes, and treated differently depending on what class they are in. This seperation can be done either with a conditional statement, or with a match statement within the class declaration. It is possible to specify a limit on the total number of clients within a particular class or subclass that may hold leases at one time, and it is possible to specify automatic subclassing based on the contents of the client packet. .PP To add clients to classes based on conditional evaluation, you would write an conditional statement to match the clients you wanted in the class, and then put an .B add statement in the conditional's list of statements: .PP .nf if substring (option dhcp-client-identifier, 0, 3) = "RAS" { add "ras-clients"; } .fi .PP A nearly equivalent way to do this is to simply specify the conditional expression as a matching expression in the class statement: .PP .nf class "ras-clients" { match if substring (option dhcp-client-identifier, 0, 3) = "RAS"; } .fi Note that whether you use matching expressions or add statements (or both) to classify clients, you must always write a class declaration for any class that you use. If there will be no match statement and no in-scope statements for a class, the declaration should look like this: .nf class "ras-clients" { } .fi .PP Also, the .B add statement adds the client to the class as the client's scopes are being evaluated - after any address assignment decision has been made. This means that a client that's a member of a class due to an add statement will not be affected by pool permits related to that class - when the pool permit list is computed, the client will not yet be a member of the pool. This is an inconsistency that will probably be addressed in later versions of the DHCP server, but it important to be aware of it at lease for the time being. .SH SUBCLASSES .PP In addition to classes, it is possible to declare subclasses. A subclass is a class with the same name as a regular class, but with a specific submatch expression which is hashed for quick matching. This is essentially a speed hack - the main difference between five classes with match expressions and one class with five subclasses is that it will be quicker to find the subclasses. Subclasses work as follows: .PP .nf class "allocation-class-1" { match pick-first-value (option dhcp-client-identifier, hardware); } class "allocation-class-2" { match pick-first-value (option dhcp-client-identifier, hardware); } subclass "allocation-class-1" 1:8:0:2b:4c:39:ad; subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3; subclass "allocation-class-1" 1:0:0:c4:aa:29:44; subnet 10.0.0.0 netmask 255.255.255.0 { pool { allow members of "allocation-class-1"; range 10.0.0.11 10.0.0.50; } pool { allow members of "allocation-class-2"; range 10.0.0.51 10.0.0.100; } } .fi .PP The data following the class name in the subclass declaration is a constant value to use in matching the match expression for the class. When class matching is done, the server will evaluate the match expression and then look the result up in the hash table. If it finds a match, the client is considered a member of both the class and the subclass. .PP Subclasses can be declared with or without scope. In the above example, the sole purpose of the subclass is to allow some clients access to one address pool, while other clients are given access to the other pool, so these subclasses are declared without scopes. If part of the purpose of the subclass were to define different parameter values for some clients, you might want to declare some subclasses with scopes. .PP In the above example, if you had a single client that needed some configuration parameters, while most didn't, you might write the following subclass declaration for that client: .PP .nf subclass "allocation-class-2" 1:08:00:2b:a1:11:31 { option root-path "samsara:/var/diskless/alphapc"; filename "/tftpboot/netbsd.alphapc-diskless"; } .fi .PP In this example, we've used subclassing as a way to control address allocation on a per-client basis. However, it's also possible to use subclassing in ways that are not specific to clients - for example, to use the value of the vendor-class-identifier option to determine what values to send in the vendor-encapsulated-options option. An example of this is shown under the VENDOR ENCAPSULATED OPTIONS head later on in this document. .SH PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION .PP You may specify a limit to the number of clients in a class that can be assigned leases. The effect of this will be to make it difficult for a new client in a class to get an address. Once a class with such a limit has reached its limit, the only way a new client in that class can get a lease is for an existing client to relinquish its lease, either by letting it expire, or by sending a DHCPRELEASE packet. Classes with lease limits are specified as follows: .PP .nf class "limited-1" { lease limit 4; } .fi .PP This will produce a class in which a maximum of four members may hold a lease at one time. .SH SPAWNING CLASSES .PP It is possible to declare a .I spawning class\fR. A spawning class is a class that automatically produces subclasses based on what the client sends. The reason that spawning classes were created was to make it possible to create lease-limited classes on the fly. The envisioned application is a cable-modem environment where the ISP wishes to provide clients at a particular site with more than one IP address, but does not wish to provide such clients with their own subnet, nor give them an unlimited number of IP addresses from the network segment to which they are connected. .PP Many cable modem head-end systems can be configured to add a Relay Agent Information option to DHCP packets when relaying them to the DHCP server. These systems typically add a circuit ID or remote ID option that uniquely identifies the customer site. To take advantage of this, you can write a class declaration as follows: .PP .nf class "customer" { spawn with option agent.circuit-id; lease limit 4; } .fi .PP Now whenever a request comes in from a customer site, the circuit ID option will be checked against the class's hash table. If a subclass is found that matches the circuit ID, the client will be classified in that subclass and treated accordingly. If no subclass is found matching the circuit ID, a new one will be created and logged in the .B dhcpd.leases file, and the client will be classified in this new class. Once the client has been classified, it will be treated according to the rules of the class, including, in this case, being subject to the per-site limit of four leases. .PP The use of the subclass spawning mechanism is not restricted to relay agent options - this particular example is given only because it is a fairly straightforward one. .SH COMBINING MATCH, MATCH IF AND SPAWN WITH .PP In some cases, it may be useful to use one expression to assign a client to a particular class, and a second expression to put it into a subclass of that class. This can be done by combining the \fBmatch if\fR and \fBspawn with\fR statements, or the \fBmatch if\fR and \fBmatch\fR statements. For example: .PP .nf class "jr-cable-modems" { match if option dhcp-vendor-identifier = "jrcm"; spawn with option agent.circuit-id; lease limit 4; } class "dv-dsl-modems" { match if opton dhcp-vendor-identifier = "dvdsl"; spawn with option agent.circuit-id; lease limit 16; } .fi .PP This allows you to have two classes that both have the same \fBspawn with\fR expression without getting the clients in the two classes confused with each other. .SH DYNAMIC DNS UPDATES .PP The DHCP server has the ability to dynamically update the Domain Name System. Within the configuration files, you can define how you want the Domain Name System to be updated. These updates are RFC 2136 compliant so any DNS server supporting RFC 2136 should be able to accept updates from the DHCP server. .PP The Dynamic DNS update scheme implemented in this version of the ISC DHCP server is an interim implementation, which does not implement any of the standard update methods that have been discussed in the working group, but rather implements some very basic, yet useful, update capabilities. .PP There are three parameters, which may vary according to the scope, that control how DDNS updates will be done. The first two are the .I ddns-domainname and .I ddns-rev-domainname statements. The .I ddns-domainname parameter sets the domain name that will be appended to the client's hostname to form a fully-qualified domain-name (FQDN). For example, if the client's hostname is "hutson" and the .I ddns-domainname is set to "sneedville.edu", then the client's FQDN will be "hutson.sneedville.edu". .PP The .I ddns-rev-domainname parameter sets the domain name that will be appended to the client's reversed IP address to produce a name for use in the client's PTR record. Normally, you would set this to "in-addr.arpa", but this is not required. .PP A third parameter, .I ddns-hostname can be used to specify the hostname that will be used as the client's hostname. If no ddns-hostname is specified in scope, then the server will use a host-name option sent by the client. If the client did not send a host-name option, then if there is a host declaration that applies to the client, the name from that declaration will be used. If none of these applies, the server will not have a hostname for the client, and will not be able to do a DDNS update. .SH HOW DNS UPDATES WORK .PP The client's FQDN, derived as we have described, is used as the name on which an "A" record will be stored. The A record will contain the IP address that the client was assigned in its lease. If there is already an A record with the same name in the DNS server, no update of either the A or PTR records will occur - this prevents a client from claiming that its hostname is the name of some network server. For example, if you have a fileserver called "fs.sneedville.edu", and the client claims its hostname is "fs", no DNS update will be done for that client, and an error message will be logged. .PP If the A record update succeeds, a PTR record update for the assigned IP address will be done, pointing to the A record. This update is unconditional - it will be done even if another PTR record of the same name exists. Since the IP address has been assigned to the DHCP server, this should be safe. .PP Please note that the current implementation assumes clients only have a single network interface. A client with two network interfaces will see unpredictable behaviour. This is considered a bug, and will be fixed in a later release. It may be helpful to enable the .I one-lease-per-client parameter so that roaming clients do not trigger this same behavior. .PP The DHCP protocol normally involves a four-packet exchange - first the client sends a DHCPDISCOVER message, then the server sends a DHCPOFFER, then the client sends a DHCPREQUEST, then the server sends a DHCPACK. In the current version of the server, the server will do a DNS update after it has received the DHCPREQUEST, and before it has sent the DHCPOFFER. It only sends the DNS update if it has not sent one for the client's address before, in order to minimize the impact on the DHCP server. .PP When the client's lease expires, the DHCP server (if it is operating at the time, or when next it operates) will remove the client's A and PTR records from the DNS database. If the client releases its lease by sending a DHCPRELEASE message, the server will likewise remove the A and PTR records. .SH DYNAMIC DNS UPDATE SECURITY .PP When you set your DNS server up to allow updates from the DHCP server, you may be exposing it to unauthorized updates. To avoid this, you should use TSIG signatures - a method of cryptographically signing updates using a shared secret key. As long as you protect the secrecy of this key, your updates should also be secure. Note, however, that the DHCP protocol itself provides no security, and that clients can therefore provide information to the DHCP server which the DHCP server will then use in its updates, with the constraints described previously. .PP The DNS server must be configured to allow updates for any zone that the DHCP server will be updating. For example, let us say that clients in the sneedville.edu domain will be assigned addresses on the 10.10.17.0/24 subnet. In that case, you will need a key declaration for the TSIG key you will be using, and also two zone declarations - one for the zone containing A records that will be updates and one for the zone containing PTR records - for ISC BIND, something like this: .PP .nf key DHCP_UPDATER { algorithm HMAC-MD5.SIG-ALG.REG.INT; secret pRP5FapFoJ95JEL06sv4PQ==; }; zone "example.org" { type master; file "example.org.db"; allow-update { key DHCP_UPDATER; }; }; zone "17.10.10.in-addr.arpa" { type master; file "10.10.17.db"; allow-update { key DHCP_UPDATER; }; }; .fi .PP You will also have to configure your DHCP server to do updates to these zones. To do so, you need to add something like this to your dhcpd.conf file: .PP .nf key DHCP_UPDATER { algorithm HMAC-MD5.SIG-ALG.REG.INT; secret pRP5FapFoJ95JEL06sv4PQ==; }; zone EXAMPLE.ORG. { primary 127.0.0.1; key DHCP_UPDATER; } zone 17.127.10.in-addr.arpa. { primary 127.0.0.1; key DHCP_UPDATER; } .fi .PP You should choose your own secret key, of course. The ISC BIND 8 and 9 distributions come with a program for generating secret keys called dnskeygen. The version that comes with BIND 9 is likely to produce a substantially more random key, so we recommend you use that one even if you are not using BIND 9 as your DNS server. The key above was generated with the command: .nf dnskeygen -H 128 -u -c -n DHCP_UPDATER .fi .PP You may wish to enable logging of DNS transactions on your DNS server. To do so, you might write a logging statement like the following: .PP .nf logging { channel update_debug { file "/var/log/update-debug.log"; severity debug 3; print-category yes; print-severity yes; print-time yes; }; channel security_info { file "/var/log/named-auth.info"; severity info; print-category yes; print-severity yes; print-time yes; }; category update { update_debug; }; category security { security_info; }; }; .fi .PP You must create the /var/log/named-auth.info and /var/log/update-debug.log files before starting the name server. For more information on configuring ISC BIND, consult the documentation that accompanies it. .SH REFERENCE: EVENTS .PP There are three kinds of events that can happen regarding a lease, and it is possible to declare statements that occur when any of these events happen. These events are the commit event, when the server has made a commitment of a certain lease to a client, the release event, when the client has released the server from its commitment, and the expiry event, when the commitment expires. .PP To declare a set of statements to execute when an event happens, you must use the \fBon\fR statement, followed by the name of the event, followed by a series of statements to execute when the event happens, enclosed in braces. Events are used to implement dynamic DNS updates, so you should not define your own event handlers if you are using the built-in dynamic DNS update mechanism. .PP The built-in version of the dynamic DNS update mechanism is in a text string towards the top of server/dhcpd.c. If you want to use events for things other than DNS updates, and you also want DNS updates, you will have to start out by copying this code into your dhcpd.conf file and modifying it. .SH REFERENCE: DECLARATIONS .PP .B The .I shared-network .B statement .PP .nf \fBshared-network\fR \fIname\fR \fB{\fR [ \fIparameters\fR ] [ \fIdeclarations\fR ] \fB}\fR .fi .PP The \fIshared-network\fR statement is used to inform the DHCP server that some IP subnets actually share the same physical network. Any subnets in a shared network should be declared within a \fIshared-network\fR statement. Parameters specified in the \fIshared-network\fR 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. .PP .I Name 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. .PP .B The .I subnet .B statement .PP .nf \fBsubnet\fR \fIsubnet-number\fR \fBnetmask\fR \fInetmask\fR \fB{\fR [ \fIparameters\fR ] [ \fIdeclarations\fR ] \fB}\fR .fi .PP The \fIsubnet\fR 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-specific parameters and to specify what addresses may be dynamically allocated to clients booting on that subnet. Such addresses are specified using the \fIrange\fR declaration. .PP The .I subnet-number should be an IP address or domain name which resolves to the subnet number of the subnet being described. The .I netmask 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. .PP Although a netmask must be given with every subnet declaration, 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. .PP .B The .I range .B statement .PP .nf .B range\fR [ \fBdynamic-bootp\fR ] \fIlow-address\fR [ \fIhigh-address\fR]\fB;\fR .fi .PP For any subnet on which addresses will be assigned dynamically, there must be at least one \fIrange\fR 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 \fIrange\fR statement is declared. The \fIdynamic-bootp\fR 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 single address, \fIhigh-address\fR can be omitted. .PP .B The .I host .B statement .PP .nf \fBhost\fR \fIhostname\fR { [ \fIparameters\fR ] [ \fIdeclarations\fR ] \fB}\fR .fi .PP There must be at least one .B host statement for every BOOTP client that is to be served. .B host statements may also be specified for DHCP clients, although this is not required unless booting is only enabled for known hosts. .PP 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 .I fixed-address parameter, or more than one .B host statement may be specified. .PP If client-specific boot parameters must change based on the network to which the client is attached, then multiple .B host statements should be used. .PP If a client is to be booted using a fixed address if it's possible, but should be allocated a dynamic address otherwise, then a .B host statement must be specified without a .B fixed-address clause. .I hostname should be a name identifying the host. If a \fIhostname\fR option is not specified for the host, \fIhostname\fR is used. .PP \fIHost\fR declarations are matched to actual DHCP or BOOTP clients by matching the \fRdhcp-client-identifier\fR option specified in the \fIhost\fR declaration to the one supplied by the client, or, if the \fIhost\fR declaration or the client does not provide a \fRdhcp-client-identifier\fR option, by matching the \fIhardware\fR parameter in the \fIhost\fR declaration to the network hardware address supplied by the client. BOOTP clients do not normally provide a \fIdhcp-client-identifier\fR, so the hardware address must be used for all clients that may boot using the BOOTP protocol. .PP .B The .I group .B statement .PP .nf \fBgroup\fR { [ \fIparameters\fR ] [ \fIdeclarations\fR ] \fB}\fR .fi .PP 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. .SH REFERENCE: ALLOW AND DENY The .I allow and .I deny statements can be used to control the response of the DHCP server to various sorts of requests. The allow and deny keywords actually have different meanings depending on the context. In a pool context, these keywords can be used to set up access lists for address allocation pools. In other contexts, the keywords simply control general server behaviour with respect to clients based on scope. In a non-pool context, the .I ignore keyword can be used in place of the .I deny keyword to prevent logging of denied requests. .PP .SH ALLOW DENY AND IGNORE IN SCOPE The following usages of allow and deny will work in any scope, although it is not recommended that they be used in pool declarations. .PP .B The .I unknown-clients .B keyword .PP \fBallow unknown-clients;\fR \fBdeny unknown-clients;\fR \fBignore unknown-clients;\fR .PP The \fBunknown-clients\fR flag is used to tell dhcpd whether or not to dynamically assign addresses to unknown clients. Dynamic address assignment to unknown clients is \fBallow\fRed by default. .PP .B The .I bootp .B keyword .PP \fBallow bootp;\fR \fBdeny bootp;\fR \fBignore bootp;\fR .PP The \fBbootp\fR flag is used to tell dhcpd whether or not to respond to bootp queries. Bootp queries are \fBallow\fRed by default. .PP .B The .I booting .B keyword .PP \fBallow booting;\fR \fBdeny booting;\fR \fBignore booting;\fR .PP The \fBbooting\fR 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 \fBallow\fRed, but if it is disabled for a particular client, then that client will not be able to get and address from the DHCP server. .B The .I duplicates .B keyword .PP \fBallow duplicates;\fR \fBdeny duplicates;\fR .PP Host declarations can match client messages based on the DHCP Client Identifer option or based on the client's network hardware type and MAC address. If the MAC address is used, the host declaration will match any client with that MAC address - even clients with different client identifiers. This doesn't normally happen, but is possible when one computer has more than one operating system installed on it - for example, Microsoft Windows and NetBSD or Linux. .PP The \fBduplicates\fR flag tells the DHCP server that if a request is received from a client that matches the MAC address of a host declaration, any other leases matching that MAC address should be discarded by the server, even if the UID is not the same. This is a violation of the DHCP protocol, but can prevent clients whose client identifiers change regularly from holding many leases at the same time. By default, duplicates are \fBallow\fRed. .B The .I declines .B keyword .PP \fBallow declines;\fR \fBdeny declines;\fR \fBignore declines;\fR .PP The DHCPDECLINE message is used by DHCP clients to indicate that the lease the server has offered is not valid. When the server receives a DHCPDECLINE for a particular address, it normally abandons that address, assuming that some unauthorized system is using it. Unfortunately, a malicious or buggy client can, using DHCPDECLINE messages, completely exhaust the DHCP server's allocation pool. The server will reclaim these leases, but while the client is running through the pool, it may cause serious thrashing in the DNS, and it will also cause the DHCP server to forget old DHCP client address allocations. .PP The \fBdeclines\fR flag tells the DHCP server whether or not to honor DHCPDECLINE messages. If it is set to \fBdeny\fR or \fBignore\fR in a particular scope, the DHCP server will not respond to DHCPDECLINE messages. .SH ALLOW AND DENY WITHIN POOL DECLARATIONS .PP The uses of the allow and deny keyword shown in the previous section work pretty much the same way whether the client is sending a DHCPDISCOVER or a DHCPREQUEST message - an address will be allocated to the client (either the old address it's requesting, or a new address) and then that address will be tested to see if it's okay to let the client have it. If the client requested it, and it's not okay, the server will send a DHCPNAK message. Otherwise, the server will simply not respond to the client. If it is okay to give the address to the client, the server will send a DHCPACK message. .PP The primary motivation behind pool declarations is to have address allocation pools whose allocation policies are different. A client may be denied access to one pool, but allowed access to another pool on the same network segment. In order for this to work, access control has to be done during address allocation, not after address allocation is done. .PP When a DHCPREQUEST message is processed, address allocation simply consists of looking up the address the client is requesting and seeing if it's still available for the client. If it is, then the DHCP server checks both the address pool permit lists and the relevant in-scope allow and deny statements to see if it's okay to give the lease to the client. In the case of a DHCPDISCOVER message, the allocation process is done as described previously in the ADDRESS ALLOCATION section. .PP When declaring permit lists for address allocation pools, the following syntaxes are recognized following the allow or deny keyword: .PP \fBknown clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to any client that has a host declaration (i.e., is known). A client is known if it has a host declaration in \fIany\fR scope, not just the current scope. .PP \fBunknown clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to any client that has no host declaration (i.e., is not known). .PP \fBmembers of "\fRclass\fB";\fR .PP If specified, this statement either allows or prevents allocation from this pool to any client that is a member of the named class. .PP \fBdynamic bootp clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to any bootp client. .PP \fBauthenticated clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to any client that has been authenticated using the DHCP authentication protocol. This is not yet supported. .PP \fBunauthenticated clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to any client that has not been authenticated using the DHCP authentication protocol. This is not yet supported. .PP \fBall clients;\fR .PP If specified, this statement either allows or prevents allocation from this pool to all clients. This can be used when you want to write a pool declaration for some reason, but hold it in reserve, or when you want to renumber your network quickly, and thus want the server to force all clients that have been allocated addresses from this pool to obtain new addresses immediately when they next renew. .SH REFERENCE: PARAMETERS .PP .B The .I lease-file-name .B statement .PP .B lease-file-name .I name\fR\fB;\fR .PP .I Name should be the name of the DHCP server's lease file. By default, this is /var/db/dhcpd.leases. This statement \fBmust\fR appear in the outer scope of the configuration file - if it appears in some other scope, it will have no effect. .PP .B The .I pid-file-name .B statement .PP .B pid-file-name .I name\fR\fB;\fR .PP .I Name should be the name of the DHCP server's process ID file. This is the file in which the DHCP server's process ID is stored when the server starts. By default, this is /var/run/dhcpd.pid. Like the lease-file-name statement, this statement must appear in the outer scope of the configuration file. .PP .B The .I default-lease-time .B statement .PP \fBdefault-lease-time\fR \fItime\fR\fB;\fR .PP .I Time 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. .PP .B The .I max-lease-time .B statement .PP \fBmax-lease-time\fR \fItime\fR\fB;\fR .PP .I Time should be the maximum length in seconds that will be assigned to a lease. The only exception to this is that Dynamic BOOTP lease lengths, which are not specified by the client, are not limited by this maximum. .PP .B The .I min-lease-time .B statement .PP \fBmin-lease-time\fR \fItime\fR\fB;\fR .PP .I Time should be the minimum length in seconds that will be assigned to a lease. .PP .B The .I min-secs .B statement .PP \fBmin-secs\fR \fIseconds\fR\fB;\fR .PP .I Seconds should be the minimum number of seconds since a client began trying to acquire a new lease before the DHCP server will respond to its request. The number of seconds is based on what the client reports, and the maximum value that the client can report is 255 seconds. Generally, setting this to one will result in the DHCP server not responding to the client's first request, but always responding to its second request. .PP This can be used to set up a secondary DHCP server which never offers an address to a client until the primary server has been given a chance to do so. If the primary server is down, the client will bind to the secondary server, but otherwise clients should always bind to the primary. Note that this does not, by itself, permit a primary server and a secondary server to share a pool of dynamically-allocatable addresses. .PP .B The .I hardware .B statement .PP \fBhardware\fR \fIhardware-type\fR \fIhardware-address\fR\fB;\fR .PP In order for a BOOTP client to be recognized, its network hardware address must be declared using a \fIhardware\fR clause in the .I host statement. .I hardware-type must be the name of a physical hardware interface type. Currently, only the .B ethernet and .B token-ring types are recognized, although support for a .B fddi hardware type (and others) would also be desirable. The .I hardware-address should be a set of hexadecimal octets (numbers from 0 through ff) seperated by colons. The \fIhardware\fR statement may also be used for DHCP clients. .PP .B The .I filename .B statement .PP \fBfilename\fR \fB"\fR\fIfilename\fR\fB";\fR .PP The \fIfilename\fR statement can be used to specify the name of the initial boot file which is to be loaded by a client. The .I filename should be a filename recognizable to whatever file transfer protocol the client can be expected to use to load the file. .PP .B The .I server-name .B statement .PP \fBserver-name\fR \fB"\fR\fIname\fR\fB";\fR .PP The \fIserver-name\fR statement can be used to inform the client of the name of the server from which it is booting. \fIName\fR should be the name that will be provided to the client. .PP .B The .I next-server .B statement .PP \fBnext-server\fR \fIserver-name\fR\fB;\fR .PP The \fInext-server\fR statement is used to specify the host address of the server from which the initial boot file (specified in the \fIfilename\fR statement) is to be loaded. \fIServer-name\fR should be a numeric IP address or a domain name. If no \fInext-server\fR parameter applies to a given client, the DHCP server's IP address is used. .PP .B The .I fixed-address .B statement .PP \fBfixed-address\fR \fIaddress\fR [\fB,\fR \fIaddress\fR ... ]\fB;\fR .PP The \fIfixed-address\fR statement is used to assign one or more fixed IP addresses to a client. It should only appear in a \fIhost\fR 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 \fIfixed-address\fR statement are on the network on which the client is booting, that client will not match the \fIhost\fR declaration containing that \fIfixed-address\fR statement. Each \fIaddress\fR should be either an IP address or a domain name which resolves to one or more IP addresses. .PP .B The .I dynamic-bootp-lease-cutoff .B statement .PP \fBdynamic-bootp-lease-cutoff\fR \fIdate\fR\fB;\fR .PP The \fIdynamic-bootp-lease-cutoff\fR 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. .PP .I Date should be the date on which all assigned BOOTP leases will end. The date is specified in the form: .PP .ce 1 W YYYY/MM/DD HH:MM:SS .PP 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. .PP .B The .I dynamic-bootp-lease-length .B statement .PP \fBdynamic-bootp-lease-length\fR \fIlength\fR\fB;\fR .PP The \fIdynamic-bootp-lease-length\fR 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 \fIlength\fR as a number of seconds. If a client reboots using BOOTP during the timeout period, the lease duration is reset to \fIlength\fR, so a BOOTP client that boots frequently enough will never lose its lease. Needless to say, this parameter should be adjusted with extreme caution. .PP .B The .I get-lease-hostnames .B statement .PP \fBget-lease-hostnames\fR \fIflag\fR\fB;\fR .PP The \fIget-lease-hostnames\fR 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 \fIhostname\fR option. If \fIflag\fR is true, then this lookup is done for all addresses in the current scope. By default, or if \fIflag\fR is false, no lookups are done. .PP .B The .I use-host-decl-names .B statement .PP \fBuse-host-decl-names\fR \fIflag\fR\fB;\fR .PP If the \fIuse-host-decl-names\fR parameter is true in a given scope, then for every host declaration within that scope, the name provided for the host declaration will be supplied to the client as its hostname. So, for example, .PP .nf 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"; } .fi .PP An \fIoption host-name\fR statement within a host declaration will override the use of the name in the host declaration. .PP .B The .I authoritative .B statement .PP \fBauthoritative;\fR .PP \fBnot authoritative;\fR .PP The DHCP server will normally assume that the configuration information about a given network segment is not known to be correct and is not authoritative. This is so that if a naive user installs a DHCP server not fully understanding how to configure it, it does not send spurious DHCPNAK messages to clients that have obtained addresses from a legitimate DHCP server on the network. .PP Network administrators setting up authoritative DHCP servers for their networks should always write \fBauthoritative;\fR at the top of their configuration file to indicate that the DHCP server \fIshould\fR send DHCPNAK messages to misconfigured clients. If this is not done, clients will be unable to get a correct IP address after changing subnets until their old lease has expired, which could take quite a long time. .PP Usually, writing \fBauthoritative;\fR 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. .PP 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 authoritative 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 declarations and not others. .PP .B The .I always-reply-rfc1048 .B statement .PP \fBalways-reply-rfc1048\fR \fIflag\fR\fB;\fR .PP 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. .PP If you want to send rfc1048 options to such a client, you can set the .B always-reply-rfc1048 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. .PP .B The .I always-broadcast .B statement .PP \fBalways-broadcast\fR \fIflag\fR\fB;\fR .PP The DHCP and BOOTP protocols both require DHCP and BOOTP clients to set the broadcast bit in the flags field of the BOOTP message header. Unfortunately, some DHCP and BOOTP clients do not do this, and therefore may not receive responses from the DHCP server. The DHCP server can be made to always broadcast its responses to clients by setting this flag to 'on' for the relevant scope. To avoid creating excess broadcast traffic on your network, we recommend that you restrict the use of this option to as few clients as possible. For example, the Microsoft DHCP client is known not to have this problem, as are the OpenTransport and ISC DHCP clients. .PP .B The .I one-lease-per-client .B statement .PP \fBone-lease-per-client\fR \fIflag\fR\fB;\fR .PP If this flag is enabled, whenever a client sends a DHCPREQUEST for a particular lease, the server will automatically free any other leases the client holds. This presumes that when the client sends a DHCPREQUEST, it has forgotten any lease not mentioned in the DHCPREQUEST - i.e., the client has only a single network interface .I and it does not remember leases it's holding on networks to which it is not currently attached. Neither of these assumptions are guaranteed or provable, so we urge caution in the use of this statement. .PP .B The .I use-lease-addr-for-default-route .B statement .PP \fBuse-lease-addr-for-default-route\fR \fIflag\fR\fB;\fR .PP If the \fIuse-lease-addr-for-default-route\fR parameter is true in a given scope, then instead of sending the value specified 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. .PP .B The .I server-identifier .B statement .PP \fBserver-identifier \fIhostname\fR\fB;\fR .PP 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 \fBmust\fR be an IP address for the DHCP server, and must be reachable by all clients served by a particular scope. .PP The use of the server-identifier statement is not recommended - 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 network interface on which the request arrived. .PP The usual case where the \fIserver-identifier\fR 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 appropriate 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. .PP Supplying a value for the dhcp-server-identifier option is equivalent to using the server-identifier statement. .PP .B The .I ddns-updates .B statement .PP \fBddns-updates \fIflag\fR\fB;\fR .PP The \fIddns-updates\fR parameter controls whether or not the server will attempt to do a ddns update when a lease is confirmed. Set this to \fIoff\fR if the server should not attempt to do updates within a certain scope. The \fIddns-updates\fR parameter is on by default. .SH SETTING PARAMETER VALUES USING EXPRESSIONS Sometimes it's helpful to be able to set the value of a DHCP server parameter based on some value that the client has sent. To do this, you can use expression evaluation. The .B dhcp-eval(5) manual page describes how to write expressions. To assign the result of an evaluation to an option, define the option as follows: .nf .sp 1 \fImy-parameter \fB= \fIexpression \fB;\fR .fi .PP For example: .nf .sp 1 ddns-hostname = binary-to-ascii (16, 8, "-", substring (hardware, 1, 6)); .fi .SH REFERENCE: OPTION STATEMENTS .PP DHCP option statements are documented in the .B dhcp-options(5) manual page. .SH SEE ALSO dhcpd(8), dhcpd.leases(5), RFC2132, RFC2131. .SH AUTHOR .B dhcpd(8) was written by Ted Lemon under a contract with Vixie Labs. Funding for this project was provided by the Internet Software Consortium. Information about the Internet Software Consortium can be found at .B http://www.isc.org/isc.