dhcpd.conf

NAME

dhcpd.conf - dhcpd configuration file

DESCRIPTION

The dhcpd.conf file contains configuration information for dhcpd, the Internet Systems 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-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. 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 gateway 220.177.244.7). 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. Declarations about network topology include the shared-network and the subnet declarations. If clients on a subnet are to be assigned addresses dynamically, a range declaration must appear within the subnet declaration. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a host declaration. If parameters are to be applied to a group of declarations which are not related strictly on a per-subnet basis, the group declaration can be used. For every subnet which will be served, and for every subnet to which the dhcp server is connected, there must be one subnet declaration, which tells dhcpd how to recognize that an address is on that subnet. A subnet 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 ethernet until such time as a new physical network can be added. In this case, the subnet declarations for these two networks must be enclosed in a shared-network declaration. Note that even when the shared-network declaration is absent, an empty one is created by the server to contain the subnet (and any scoped parameters included in the subnet). For practical purposes, this means that "stateless" DHCP clients, which are not tied to addresses (and therefore subnets) will receive the same configuration as stateful ones. 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 host declarations, these declarations can be enclosed in a group 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. When a client is to be booted, its boot parameters are determined by consulting that client's host declaration (if any), and then consulting any class declarations matching the client, followed by the pool, subnet and shared-network 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. 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. When dhcpd tries to find a host declaration for a client, it first looks for a host declaration which has a fixed-address declaration that lists an IP address that is valid for the subnet or shared network on which the client is booting. If it doesn't find any such entry, it tries to find an entry which has no fixed-address declaration.

EXAMPLES

A typical dhcpd.conf file will look something like this:

global parameters...

subnet 204.254.239.0 netmask 255.255.255.224 {
   subnet-specific parameters...
  range 204.254.239.10 204.254.239.30;
}

subnet 204.254.239.32 netmask 255.255.255.224 {
   subnet-specific parameters...
  range 204.254.239.42 204.254.239.62;
}

subnet 204.254.239.64 netmask 255.255.255.224 {
   subnet-specific parameters...
  range 204.254.239.74 204.254.239.94;
}

group {
   group-specific parameters...
  host zappo.test.isc.org {
     host-specific parameters...
  }
  host beppo.test.isc.org {
     host-specific parameters...
  }
  host harpo.test.isc.org {
     host-specific parameters...
  }
}

 
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, 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. 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:

	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 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. In Figure 1 there is also a group 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:

	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 mechanism, 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 option keyword, some do not. Parameters starting with the option keyword correspond to actual DHCP options, while parameters that do not start with the option keyword either control the behavior 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 host-specific parameters. These could include such things as the hostname option, the name of a file to upload (the filename parameter) and the address of the server from which to upload the file (the next-server 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-Terminals. These terminals come in a variety of models, and you want to specify the boot files for each model. 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; }
}

ADDRESS POOLS

The pool and pool6 declarations 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:

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;
  }
}

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. 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 allow or deny keyword. If a pool has a permit list, then only those clients that match specific entries on the permit list will be eligible 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 eligible. 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. The pool6 declaration is similar to the pool declaration. Currently it is only allowed within a subnet6 declaration, and may not be included directly in a shared network declaration. In addition to the range6 statement it allows the prefix6 statement to be included. You may include range6 statements for both NA and TA and prefix6 statements in a single pool6 statement.

DYNAMIC 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 address, 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. There may be a host declaration matching the client's identification. If that host declaration contains a fixed-address declaration that lists an IP address that is valid for the network segment to which the client is connected, the DHCP server will never do dynamic address allocation. In this case, the client is required to take the address specified in the host declaration. If the client sends a DHCPREQUEST for some other address, the server will respond with a DHCPNAK. 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. 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 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. 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. The DHCP server generates the list of available IP addresses from a hash table. This means that the addresses are not sorted in any particular order, and so it is not possible to predict the order in which the DHCP server will allocate IP addresses. Users of previous versions of the ISC DHCP server may have become accustomed to the DHCP server allocating IP addresses in ascending order, but this is no longer possible, and there is no way to configure this behavior with version 3 of the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION

The DHCP server checks IP addresses to see if they are in use before allocating them to clients. It does this by sending an ICMP Echo request message to the IP address being allocated. If no ICMP Echo reply is received within a second, the address is assumed to be free. This is only done for leases that have been specified in range statements, and only when the lease is thought by the DHCP server to be free - i.e., the DHCP server or its failover peer has not listed the lease as in use. If a response is received to an ICMP Echo request, the DHCP server assumes that there is a configuration error - the IP address is in use by some host on the network that is not a DHCP client. It marks the address as abandoned, and will not assign it to clients. The lease will remain abandoned for a minimum of abandon-lease-time seconds. If a DHCP client tries to get an IP address, but none are available, but there are abandoned IP addresses, then the DHCP server will attempt to reclaim an abandoned IP address. It marks one IP address as free, and then does the same ICMP Echo request check described previously. If there is no answer to the ICMP Echo request, the address is assigned to the client. The DHCP server does not cycle through abandoned IP addresses if the first IP address it tries to reclaim is free. Rather, when the next DHCPDISCOVER comes in from the client, it will attempt a new allocation using the same method described here, and will typically try a new IP address.

DHCP FAILOVER

This version of the ISC DHCP server supports the DHCP failover protocol as documented in draft-ietf-dhc-failover-12.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. 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. 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. You can put the server into the PARTNER-DOWN state either by using the omshell (1) command or by stopping the server, editing the last failover state declaration in the lease file, and restarting the server. If you use this last method, change the "my state" line to:

failover peer name state {
my state partner-down;.
peer state state at date;
}

It is only required to change "my state" as shown above. 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. 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 sure that the other server will not restart automatically. 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.

FAILOVER STARTUP

When a server starts that has not previously communicated with its failover peer, it must establish communications with its failover peer and synchronize with it before it can serve clients. This can happen either because you have just configured your DHCP servers to perform failover for the first time, or because one of your failover servers has failed catastrophically and lost its database. The initial recovery process is designed to ensure that when one failover peer loses its database and then resynchronizes, any leases that the failed server gave out before it failed will be honored. When the failed server starts up, it notices that it has no saved failover state, and attempts to contact its peer. When it has established contact, it asks the peer for a complete copy its peer's lease database. The peer then sends its complete database, and sends a message indicating that it is done. The failed server then waits until MCLT has passed, and once MCLT has passed both servers make the transition back into normal operation. This waiting period ensures that any leases the failed server may have given out while out of contact with its partner will have expired. While the failed server is recovering, its partner remains in the partner-down state, which means that it is serving all clients. The failed server provides no service at all to DHCP clients until it has made the transition into normal operation. In the case where both servers detect that they have never before communicated with their partner, they both come up in this recovery state and follow the procedure we have just described. In this case, no service will be provided to DHCP clients until MCLT has expired.

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. A pool declaration that utilizes failover would look like this:

pool {
	failover peer "foo";
	 pool specific parameters
};

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 separate 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:

failover peer "foo" {
  primary;
  address anthrax.rc.example.com;
  port 519;
  peer address trantor.rc.example.com;
  peer port 520;
  max-response-delay 60;
  max-unacked-updates 10;
  mclt 3600;
  split 128;
  load balance max seconds 3;
}

include "/etc/dhcpd.master";

The statements in the peer declaration are as follows: The primary and secondary statements The address statement The peer address statement The port statement The peer port statement The max-response-delay statement The max-unacked-updates statement The mclt statement The split statement The hba statement The load balance max seconds statement The auto-partner-down statement The Failover pool balance statements.

CLIENT CLASSING

Clients can be separated into classes, and treated differently depending on what class they are in. This separation 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. Classing support for DHCPv6 clients was added in 4.3.0. It follows the same rules as for DHCPv4 except that support for billing classes has not been added yet. To add clients to classes based on conditional evaluation, you can specify a matching expression in the class statement:

class "ras-clients" {
  match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
}

Please note that the values used in match expressions may only come from data or options that are part of the client packet. It is not possible to use values constructed through one or more executable statements. This stems from the fact that client classification occurs before any statements are executed. Attempting to do so will yield indeterminate results. 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:

class "ras-clients" {
}

SUBCLASSES

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:

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;
  }
}

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. 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. 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:

subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
  option root-path "samsara:/var/diskless/alphapc";
  filename "/tftpboot/netbsd.alphapc-diskless";
}

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 in the dhcp-options(5) manual page.

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION

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:

class "limited-1" {
  lease limit 4;
}

This will produce a class in which a maximum of four members may hold a lease at one time.

SPAWNING CLASSES

It is possible to declare a spawning class. 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. 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:

class "customer" {
  spawn with option agent.circuit-id;
  lease limit 4;
}

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 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. 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.

COMBINING MATCH, MATCH IF AND SPAWN WITH

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 match if and spawn with statements, or the match if and match statements. For example:

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 option dhcp-vendor-identifier = "dvdsl";
  spawn with option agent.circuit-id;
  lease limit 16;
}

This allows you to have two classes that both have the same spawn with expression without getting the clients in the two classes confused with each other.

DYNAMIC DNS UPDATES

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. There are two DNS schemes implemented. The interim option is based on draft revisions of the DDNS documents while the standard option is based on the RFCs for DHCP-DNS interaction and DHCIDs. A third option, ad-hoc, was deprecated and has now been removed from the code base. The DHCP server must be configured to use one of the two currently-supported methods, or not to do DNS updates. New installations should use the standard option. Older installations may want to continue using the interim option for backwards compatibility with the DNS database until the database can be updated. This can be done with the ddns-update-style configuration parameter.

THE DNS UPDATE SCHEME

the interim and standard DNS update schemes operate mostly according to work from the IETF. The interim version was based on the drafts in progress at the time while the standard is based on the completed RFCs. The standard RFCs are:

 
RFC 4701 (updated by RF5494)
 
RFC 4702
 
RFC 4703
 

And the corresponding drafts were:

 
draft-ietf-dnsext-dhcid-rr-??.txt
 
draft-ietf-dhc-fqdn-option-??.txt
 
draft-ietf-dhc-ddns-resolution-??.txt
 

The basic framework for the two schemes is similar with the main material difference being that a DHCID RR is used in the standard version while the interim versions uses a TXT RR. The format of the TXT record bears a resemblance to the DHCID RR but it is not equivalent (MD5 vs SHA2, field length differences etc). In these two schemes the DHCP server does not necessarily always update both the A and the PTR records. The FQDN option includes a flag which, when sent by the client, indicates that the client wishes to update its own A record. In that case, the server can be configured either to honor the client´s intentions or ignore them. This is done with the statement allow client-updates; or the statement ignore client-updates;. By default, client updates are allowed. If the server is configured to allow client updates, then if the client sends a fully-qualified domain name in the FQDN option, the server will use that name the client sent in the FQDN option to update the PTR record. For example, let us say that the client is a visitor from the "radish.org" domain, whose hostname is "jschmoe". The server is for the "example.org" domain. The DHCP client indicates in the FQDN option that its FQDN is "jschmoe.radish.org.". It also indicates that it wants to update its own A record. The DHCP server therefore does not attempt to set up an A record for the client, but does set up a PTR record for the IP address that it assigns the client, pointing at jschmoe.radish.org. Once the DHCP client has an IP address, it can update its own A record, assuming that the "radish.org" DNS server will allow it to do so. If the server is configured not to allow client updates, or if the client doesn´t want to do its own update, the server will simply choose a name for the client. By default, the server will choose from the following three values:
1. fqdn option (if present)
2. hostname option (if present)
3. Configured hostname option (if defined). If these defaults for choosing the host name are not appropriate you can write your own statement to set the ddns-hostname variable as you wish. If none of the above are found the server will use the host declaration name (if one) and use-host-decl-names is on. It will use its own domain name for the client. It will then update both the A and PTR record, using the name that it chose for the client. If the client sends a fully-qualified domain name in the fqdn option, the server uses only the leftmost part of the domain name - in the example above, "jschmoe" instead of "jschmoe.radish.org". Further, if the ignore client-updates; directive is used, then the server will in addition send a response in the DHCP packet, using the FQDN Option, that implies to the client that it should perform its own updates if it chooses to do so. With deny client-updates;, a response is sent which indicates the client may not perform updates. Both the standard and interim options also include a method to allow more than one DHCP server to update the DNS database without accidentally deleting A records that shouldn´t be deleted nor failing to add A records that should be added. For the standard option the method works as follows: When the DHCP server issues a client a new lease, it creates a text string that is an SHA hash over the DHCP client´s identification (see RFCs 4701 & 4702 for details). The update attempts to add an A record with the name the server chose and a DHCID record containing the hashed identifier string (hashid). If this update succeeds, the server is done. If the update fails because the A record already exists, then the DHCP server attempts to add the A record with the prerequisite that there must be a DHCID record in the same name as the new A record, and that DHCID record´s contents must be equal to hashid. If this update succeeds, then the client has its A record and PTR record. If it fails, then the name the client has been assigned (or requested) is in use, and can´t be used by the client. At this point the DHCP server gives up trying to do a DNS update for the client until the client chooses a new name. The server also does not update very aggressively. Because each DNS update involves a round trip to the DNS server, there is a cost associated with doing updates even if they do not actually modify the DNS database. So the DHCP server tracks whether or not it has updated the record in the past (this information is stored on the lease) and does not attempt to update records that it thinks it has already updated. This can lead to cases where the DHCP server adds a record, and then the record is deleted through some other mechanism, but the server never again updates the DNS because it thinks the data is already there. In this case the data can be removed from the lease through operator intervention, and once this has been done, the DNS will be updated the next time the client renews. The interim DNS update scheme was written before the RFCs were finalized and does not quite follow them. The RFCs call for a new DHCID RRtype while the interim DNS update scheme uses a TXT record. In addition the ddns-resolution draft called for the DHCP server to put a DHCID RR on the PTR record, but the interim update method does not do this. In the final RFC this requirement was relaxed such that a server may add a DHCID RR to the PTR record.

DDNS IN DUAL STACK ENVIRONMENTS

As described in RFC 4703, section 5.2, in order to perform DDNS in dual stack environments, both IPv4 and IPv6 servers would need to be configured to use the standard update style and participating IPv4 clients MUST convey DUIDs as described in RFC 4361, section 6.1., in their dhcp-client-identifiers. In a nutshell, this mechanism is intended to use globally unique DUIDs to idenfity both IPv4 and IPv6 clients, and where a device has both IPv4 and IPv6 leases it is identified by the same DUID. This allows a dual stack client to use the same FQDN for both mappings, while being protected from updates for other clients by the rules of conflict detection. However, not all IPv4 clients implement this behavior which makes supporting them dual stack environments problematic. In order to address this issue ISC DHCP (as of 4.4.0) supports a new mode of DDNS conflict resolution referred to as Dual Stack Mixed Mode (DSMM). The concept behind DSMM is relatively simple. All dhcp servers of one protocol (IPv4 or v6) use one ddns-update-style (interim or standard) while all servers of the "other" protocol will use the "other" ddns-udpate-style. In this way, all servers of a given protocol are using the same record type (TXT or DHCID) for their DHCID RR entries. This allows conflict detection to be enforced within each protocol without interferring with the other's entries. DSMM modifications now ensure that IPv4 DSMM servers only ever modify A records, their associated PTR records and DHCID records, while DSMM IPv6 severs only modify AAAA records, their associated PTR records, and DHCID records. Note that DSMM is not a perfect solution, it is a compromise that can work well provided all participating DNS updaters play by DSMM rules. As with anything else in life, it only works as well as those who particpate behave. While conflict detection is enabled by default, DSMM is not. To enable DSMM, both update-conflict-detection and ddns-dual-stack-mixed-mode must be true.

PROTECTING DNS ENTRIES FOR STATIC CLIENTS

Built into conflict resolution is the protection of manually made entries for static clients. Per the rules of conflict resolution, a DNS updater may not alter forward DNS entries unless there is a DHCID RR which matches for whom the update is being made. Therefore, any forward DNS entries without a corresponding DHCID RR cannot be altered by such an updater. In some environments, it may be desirable to use only this aspect of conflict resolution and allow DNS updaters to overwrite entries for dynamic clients regardless of what client owns them. In other words, the presence or lack of a DHCID RR is used to determine whether entries may or may not be overwritten. Whether or not the client matches the data value of the DHCID RR is irrelevant. This behavior, off by default, can be configured through the parameter, ddns-guard-id-must-match. As with DSMM, this behavior is can only be enabled if conflict resolution is enabled. This behavior should be considered carefully before electing to use it. There is an additional parameter that can be used with DSMM ddns-other-guard-is-dynamic. When enabled along with DSMM, a server will regard the presence of a DHCID RR of the other style type as indicating that the forward DNS entries for that FQDN should be dynamic and may be overwritten. For example, such a server using interim style could overwrite the DNS entries for an FQDN if there is only a DHDID type DHDID RR for the FQDN. Essentially, if there are dynamic entries for one protocol, that is enough to overcome the static protection of entries for the other protocol. This behavior warrants careful consideration before electing to use it.

DYNAMIC DNS UPDATE SECURITY

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. 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:

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; };
};

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:

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;
}

The primary statement specifies the IP address of the name server whose zone information is to be updated. In addition to the primary statement there are also the primary6 , secondary and secondary6 statements. The primary6 statement specifies an IPv6 address for the name server. The secondaries provide for additional addresses for name servers to be used if the primary does not respond. The number of name servers the DDNS code will attempt to use before giving up is limited and is currently set to three. Note that the zone declarations have to correspond to authority records in your name server - in the above example, there must be an SOA record for "example.org." and for "17.10.10.in-addr.arpa.". For example, if there were a subdomain "foo.example.org" with no separate SOA, you could not write a zone declaration for "foo.example.org." Also keep in mind that zone names in your DHCP configuration should end in a "."; this is the preferred syntax. If you do not end your zone name in a ".", the DHCP server will figure it out. Also note that in the DHCP configuration, zone names are not encapsulated in quotes where there are in the DNS configuration. You should choose your own secret key, of course. The ISC BIND 9 distribution comes with a program for generating secret keys called dnssec-keygen. If you are using BIND 9´s dnssec-keygen, the above key would be created as follows:

	dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

The key name, algorithm, and secret must match that being used by the DNS server. The DHCP server currently supports the following algorithms:

        HMAC-MD5
        HMAC-SHA1
        HMAC-SHA224
        HMAC-SHA256
        HMAC-SHA384
        HMAC-SHA512

You may wish to enable logging of DNS updates on your DNS server. To do so, you might write a logging statement like the following:

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; };
};

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.

REFERENCE: EVENTS

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. To declare a set of statements to execute when an event happens, you must use the on statement, followed by the name of the event, followed by a series of statements to execute when the event happens, enclosed in braces.

REFERENCE: DECLARATIONS

The include statement

  include "filename";

The include statement is used to read in a named file, and process the contents of that file as though it were entered in place of the include statement. The shared-network statement

  shared-network name {
   [  parameters ]
   [  declarations ]
  }

The shared-network 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 shared-network statement. Parameters specified in the shared-network 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. 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. The subnet statement

  subnet subnet-number netmask netmask {
   [  parameters ]
   [  declarations ]
  }

The subnet 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 range declaration. The subnet-number should be an IP address or domain name which resolves to the subnet number of the subnet being described. The 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. 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. The subnet6 statement

  subnet6 subnet6-number {
   [  parameters ]
   [  declarations ]
  }

The subnet6 statement is used to provide dhcpd with enough information to tell whether or not an IPv6 address is on that subnet6. 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. The subnet6-number should be an IPv6 network identifier, specified as ip6-address/bits. The range statement

range [ dynamic-bootp ] low-address [ high-address];

For any subnet on which addresses will be assigned dynamically, there must be at least one range 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 range statement is declared. The dynamic-bootp 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, high-address can be omitted. The range6 statement

range6 low-address high-address;
range6 subnet6-number;
range6 subnet6-number temporary;
range6 address temporary;

For any IPv6 subnet6 on which addresses will be assigned dynamically, there must be at least one range6 statement. The range6 statement can either be the lowest and highest IPv6 addresses in a range6, or use CIDR notation, specified as ip6-address/bits. All IP addresses in the range6 should be in the subnet6 in which the range6 statement is declared. The temporary variant makes the prefix (by default on 64 bits) available for temporary (RFC 4941) addresses. A new address per prefix in the shared network is computed at each request with an IA_TA option. Release and Confirm ignores temporary addresses. Any IPv6 addresses given to hosts with fixed-address6 are excluded from the range6, as are IPv6 addresses on the server itself. The prefix6 statement

prefix6 low-address high-address / bits;

The prefix6 is the range6 equivalent for Prefix Delegation (RFC 3633). Prefixes of bits length are assigned between low-address and high-address. Any IPv6 prefixes given to static entries (hosts) with fixed-prefix6 are excluded from the prefix6. This statement is currently global but it should have a shared-network scope. The host statement

  host hostname {
   [  parameters ]
   [  declarations ]
  }

The host declaration provides a way for the DHCP server to identify a DHCP or BOOTP client. This allows the server to provide configuration information including fixed addresses or, in DHCPv6, fixed prefixes for a specific client. If it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet with fixed v4 addresses, more than one address may be specified in the fixed-address declaration, or more than one host statement may be specified matching the same client. The fixed-address6 declaration is used for v6 addresses. At this time it only works with a single address. For multiple addresses specify multiple host statements. If client-specific boot parameters must change based on the network to which the client is attached, then multiple host declarations should be used. The host declarations will only match a client if one of their fixed-address statements is viable on the subnet (or shared network) where the client is attached. Conversely, for a host declaration to match a client being allocated a dynamic address, it must not have any fixed-address statements. You may therefore need a mixture of host declarations for any given client...some having fixed-address statements, others without. hostname should be a name identifying the host. If a hostname option is not specified for the host, hostname is used. Host declarations are matched to actual DHCP or BOOTP clients by matching the dhcp-client-identifier option specified in the host declaration to the one supplied by the client, or, if the host declaration or the client does not provide a dhcp-client-identifier option, by matching the hardware parameter in the host declaration to the network hardware address supplied by the client. BOOTP clients do not normally provide a dhcp-client-identifier, so the hardware address must be used for all clients that may boot using the BOOTP protocol. DHCPv6 servers can use the host-identifier option parameter in the host declaration, and specify any option with a fixed value to identify hosts. Please be aware that only the dhcp-client-identifier option and the hardware address can be used to match a host declaration, or the host-identifier option parameter for DHCPv6 servers. For example, it is not possible to match a host declaration to a host-name option. This is because the host-name option cannot be guaranteed to be unique for any given client, whereas both the hardware address and dhcp-client-identifier option are at least theoretically guaranteed to be unique to a given client. The group statement

  group {
   [  parameters ]
   [  declarations ]
  }

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.

REFERENCE: ALLOW AND DENY

The allow and 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 behavior with respect to clients based on scope. In a non-pool context, the ignore keyword can be used in place of the deny keyword to prevent logging of denied requests.

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. The unknown-clients keyword
allow unknown-clients;
deny unknown-clients;
ignore unknown-clients; The unknown-clients flag is used to tell dhcpd whether or not to dynamically assign addresses to unknown clients. Dynamic address assignment to unknown clients is allowed by default. An unknown client is simply a client that has no host declaration. The use of this option is now deprecated. If you are trying to restrict access on your network to known clients, you should use deny unknown-clients; inside of your address pool, as described under the heading ALLOW AND DENY WITHIN POOL DECLARATIONS. The bootp keyword
allow bootp;
deny bootp;
ignore bootp; The bootp flag is used to tell dhcpd whether or not to respond to bootp queries. Bootp queries are allowed by default. The booting keyword
allow booting;
deny booting;
ignore booting; The booting 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 allowed, but if it is disabled for a particular client, then that client will not be able to get an address from the DHCP server. The duplicates keyword
allow duplicates;
deny duplicates; Host declarations can match client messages based on the DHCP Client Identifier 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. The duplicates 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 allowed. The declines keyword
allow declines;
deny declines;
ignore declines; 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 eventually reclaim these leases, but not while the client is running through the pool. This may cause serious thrashing in the DNS, and it will also cause the DHCP server to forget old DHCP client address allocations. The declines flag tells the DHCP server whether or not to honor DHCPDECLINE messages. If it is set to deny or ignore in a particular scope, the DHCP server will not respond to DHCPDECLINE messages. The declines flag is only supported by DHCPv4 servers. Given the large IPv6 address space and the internal limits imposed by the server's address generation mechanism we don't think it is necessary for DHCPv6 servers at this time. Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
a) Client renews a specific address:
If a client using a given DUID submits a DHCP REQUEST containing
the last address abandoned by that DUID, the address will be
reassigned to that client.
b) Upon the second restart following an address abandonment. When
an address is abandoned it is both recorded as such in the lease
file and retained as abandoned in server memory until the server
is restarted. Upon restart, the server will process the lease file
and all addresses whose last known state is abandoned will be
retained as such in memory but not rewritten to the lease file.
This means that a subsequent restart of the server will not see the
abandoned addresses in the lease file and therefore have no record
of them as abandoned in memory and as such perceive them as free
for assignment. The total number addresses in a pool, available for a given DUID value, is internally limited by the server's address generation mechanism. If through mistaken configuration, multiple clients are using the same DUID they will competing for the same addresses causing the server to reach this internal limit rather quickly. The internal limit isolates this type of activity such that address range is not exhausted for other DUID values. The appearance of the following error log, can be an indication of this condition:
"Best match for DUID <XX> is an abandoned address, This may be a
result of multiple clients attempting to use this DUID"
where <XX> is an actual DUID value depicted as colon separated
string of bytes in hexadecimal values. The client-updates keyword
allow client-updates;
deny client-updates; The client-updates flag tells the DHCP server whether or not to honor the client's intention to do its own update of its A record. See the documentation under the heading THE DNS UPDATE SCHEME for details. The leasequery keyword
allow leasequery;
deny leasequery; The leasequery flag tells the DHCP server whether or not to answer DHCPLEASEQUERY packets. The answer to a DHCPLEASEQUERY packet includes information about a specific lease, such as when it was issued and when it will expire. By default, the server will not respond to these packets.

ALLOW AND DENY WITHIN POOL DECLARATIONS

The uses of the allow and deny keywords 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. 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. 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. When declaring permit lists for address allocation pools, the following syntaxes are recognized following the allow or deny keywords:
known-clients; 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 any scope, not just the current scope.
unknown-clients; 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).
members of "class"; If specified, this statement either allows or prevents allocation from this pool to any client that is a member of the named class.
dynamic bootp clients; If specified, this statement either allows or prevents allocation from this pool to any bootp client.
authenticated clients; 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.
unauthenticated clients; 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.
all clients; 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.
after time; If specified, this statement either allows or prevents allocation from this pool after a given date. This can be used when you want to move clients from one pool to another. The server adjusts the regular lease time so that the latest expiry time is at the given time+min-lease-time. A short min-lease-time enforces a step change, whereas a longer min-lease-time allows for a gradual change. time is either second since epoch, or a UTC time string e.g. 4 2007/08/24 09:14:32 or a string with time zone offset in seconds e.g. 4 2007/08/24 11:14:32 -7200

REFERENCE: PARAMETERS

The abandon-lease-time statement The adaptive-lease-time-threshold statement The always-broadcast statement The always-reply-rfc1048 statement The authoritative statement The boot-unknown-clients statement The check-secs-byte-order statement

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 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:

 
   my-parameter = expression ;

For example:

 
  ddns-hostname = binary-to-ascii (16, 8, "-",
                                   substring (hardware, 1, 6));

RESERVED LEASES

It's often useful to allocate a single address to a single client, in approximate perpetuity. Host statements with fixed-address clauses exist to a certain extent to serve this purpose, but because host statements are intended to approximate ´static configuration´, they suffer from not being referenced in a littany of other Server Services, such as dynamic DNS, failover, ´on events´ and so forth. If a standard dynamic lease, as from any range statement, is marked ´reserved´, then the server will only allocate this lease to the client it is identified by (be that by client identifier or hardware address). In practice, this means that the lease follows the normal state engine, enters ACTIVE state when the client is bound to it, expires, or is released, and any events or services that would normally be supplied during these events are processed normally, as with any other dynamic lease. The only difference is that failover servers treat reserved leases as special when they enter the FREE or BACKUP states - each server applies the lease into the state it may allocate from - and the leases are not placed on the queue for allocation to other clients. Instead they may only be ´found´ by client identity. The result is that the lease is only offered to the returning client. Care should probably be taken to ensure that the client only has one lease within a given subnet that it is identified by. Leases may be set ´reserved´ either through OMAPI, or through the ´infinite-is-reserved´ configuration option (if this is applicable to your environment and mixture of clients). It should also be noted that leases marked ´reserved´ are effectively treated the same as leases marked ´bootp´.

REFERENCE: OPTION STATEMENTS

DHCP option statements are documented in the dhcp-options(5) manual page.

REFERENCE: EXPRESSIONS

Expressions used in DHCP option statements and elsewhere are documented in the dhcp-eval(5) manual page.

SEE ALSO

dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.

AUTHOR

is maintained by ISC. Information about Internet Systems Consortium can be found at https://www.isc.org.