Defines a global list of IP addresses that also get sent NOTIFY messages whenever a fresh copy of the zone is loaded. This helps to ensure that copies of the zones will quickly converge on ``stealth'' servers. If an also-notify list is given in a zone statement, it will override the options also-notify statement. When a zone notify statement is set to no, the IP addresses in the global also-notify list will not get sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

Forwarding

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external nameservers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

forward

This option is only meaningful if the forwarders list is not empty. A value of first, the default, causes the server to query the forwarders first, and if that doesn't answer the question the server will then look for the answer itself. If only is specified, the server will only query the forwarders.

forwarders

Specifies the IP addresses to be used for forwarding. The default is the empty list (no forwarding).

Forwarding can also be configured on a per-zone basis, allowing for the global forwarding options to be overridden in a variety of ways. You can set particular zones to use different forwarders, or have different forward only/first behavior, or to not forward at all. See the zone statement for more information.

Future versions of BIND 8 will provide a more powerful forwarding system. The syntax described above will continue to be supported.

Name Checking

The server can check domain names based upon their expected client contexts. For example, a domain name used as a hostname can be checked for compliance with the RFCs defining valid hostnames.

Three checking methods are available:

ignore

No checking is done.

warn

Names are checked against their expected client contexts. Invalid names are logged, but processing continues normally.

fail

Names are checked against their expected client contexts. Invalid names are logged, and the offending data is rejected.

The server can check names three areas: master zone files, slave zone files, and in responses to queries the server has initiated. If check-names response fail has been specified, and answering the client's question would require sending an invalid name to the client, the server will send a REFUSED response code to the client.

The defaults are:

    check-names master fail;
    check-names slave warn;
    check-names response ignore;

check-names may also be specified in the zone statement, in which case it overrides the options check-names statement. When used in a zone statement, the area is not specified (because it can be deduced from the zone type).

Access Control

Access to the server can be restricted based on the IP address of the requesting system. See address_match_list for details on how to specify IP address lists.

allow-query

Specifies which hosts are allowed to ask ordinary questions. allow-query may also be specified in the zone statement, in which case it overrides the options allow-query statement. If not specified, the default is to allow queries from all hosts.

allow-transfer

Specifies which hosts are allowed to receive zone transfers from the server. allow-transfer may also be specified in the zone statement, in which case it overrides the options allow-transfer statement. If not specified, the default is to allow transfers from all hosts.

allow-recursion

Specifies which hosts are allowed to make recursive queries through this server. If not specified, the default is to allow recursive queries from all hosts.

blackhole

Specifies a list of addresses that the server will not accept queries from or use to resolve a query. Queries from these addresses will not be responded to.

Interfaces

The interfaces and ports that the server will answer queries from may be specified using the listen-on option. listen-on takes an optional port, and an address_match_list. The server will listen on all interfaces allowed by the address match list. If a port is not specified, port 53 will be used.

Multiple listen-on statements are allowed. For example,

    listen-on { 5.6.7.8; };
    listen-on port 1234 { !1.2.3.4; 1.2/16; };

If no listen-on is specified, the server will listen on port 53 on all interfaces.

Query Address

If the server doesn't know the answer to a question, it will query other nameservers. query-source specifies the address and port used for such queries. If address is * or is omitted, a wildcard IP address (INADDR_ANY) will be used. If port is * or is omitted, a random unprivileged port will be used. The default is

    query-source address * port *;

Note: query-source port applies only to UDP queries, TCP queries always use a random unprivileged port.

Zone Transfers

max-transfer-time-in

Inbound zone transfers (named-xfer processes) running longer than this many minutes will be terminated. The default is 120 minutes (2 hours).

transfer-format

The server supports two zone transfer methods. one-answer uses one DNS message per resource record transferred. many-answers packs as many resource records as possible into a message. many-answers is more efficient, but is only known to be understood by BIND 8.1+ and patched versions of BIND 4.9.5. The default is one-answer. transfer-format may be overridden on a per-server basis by using the server statement.

transfers-in

The maximum number of inbound zone transfers that can be running concurrently. The default value is 10. Increasing transfers-in may speed up the convergence of slave zones, but it also may increase the load on the local system.

transfers-out

This option will be used in the future to limit the number of concurrent outbound zone transfers. It is checked for syntax, but is otherwise ignored.

transfers-per-ns

The maximum number of inbound zone transfers (named-xfer processes) that can be concurrently transferring from a given remote nameserver. The default value is 2. Increasing transfers-per-ns may speed up the convergence of slave zones, but it also may increase the load on the remote nameserver. transfers-per-ns may be overridden on a per-server basis by using the transfers phrase of the server statement.

transfer-source

transfer-source determines which local address will be bound to the TCP connection used to fetch all zones transferred inbound by the server. If not set, it defaults to a system controlled value which will usually be the address of the interface ``closest to'' the remote end. This address must appear in the remote end's allow-transfer option for the zone being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overridden on a per-zone basis by including a transfer-source statement within the zone block in the configuration file.

serial-queries

Slave servers will periodically query master servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the slave server's network bandwidth, but more importantly each query uses a small amount of memory in the slave server while waiting for the master server to respond. The serial-queries option sets the maximum number of concurrent serial-number queries allowed to be outstanding at any given time. The default is four (4). Note: If a server loads a large (tens or hundreds of thousands) number of slave zones, this limit should be raised to the high hundreds or low thousands -- otherwise the slave server may never actually become aware of zone changes in the master servers. Beware, though, that setting this limit arbitrarily high can spend a considerable amount of your slave server's network, CPU, and memory resources. As with all tunable limits, this one should be changed gently and monitored for its effects.

Resource Limits

The server's usage of many system resources can be limited. Some operating systems don't support some of the limits. On such systems, a warning will be issued if the unsupported limit is used. Some operating systems don't support limiting resources, and on these systems a cannot set resource limits on this system message will be logged.

Scaled values are allowed when specifying resource limits. For example, 1G can be used instead of 1073741824 to specify a limit of one gigabyte. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started. See size_spec for more details.

coresize

The maximum size of a core dump. The default is default.

datasize

The maximum amount of data memory the server may use. The default is default.

files

The maximum number of files the server may have open concurrently. The default is unlimited. Note: on some operating systems the server cannot set an unlimited value and cannot determine the maximum number of open files the kernel can support. On such systems, choosing unlimited will cause the server to use the larger of the rlim_max for RLIMIT_NOFILE and the value returned by sysconf(_SC_OPEN_MAX). If the actual kernel limit is larger than this value, use limit files to specify the limit explicitly.

max-ixfr-log-size

Limit the size of the transaction log kept for Incremental Zone Transfer. Default 0 (unlimited).

stacksize

The maximum amount of stack memory the server may use. The default is default.

Periodic Task Intervals

cleaning-interval

The server will remove expired resource records from the cache every cleaning-interval minutes. The default is 60 minutes. If set to 0, no periodic cleaning will occur.

heartbeat-interval

The server will perform zone maintenance tasks for all zones marked dialup yes whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). If set to 0, no zone maintenance for these zones will occur.

interface-interval

The server will scan the network interface list every interface-interval minutes. The default is 60 minutes. If set to 0, interface scanning will only occur when the configuration file is loaded. After the scan, listeners will be started on any new interfaces (provided they are allowed by the listen-on configuration). Listeners on interfaces that have gone away will be cleaned up.

statistics-interval

Nameserver statistics will be logged every statistics-interval minutes. The default is 60. If set to 0, no statistics will be logged.

Topology

All other things being equal, when the server chooses a nameserver to query from a list of nameservers, it prefers the one that is topologically closest to itself. The topology statement takes an address_match_list and interprets it in a special way. Each top-level list element is assigned a distance. Non-negated elements get a distance based on their position in the list, where the closer the match is to the start of the list, the shorter the distance is between it and the server. A negated match will be assigned the maximum distance from the server. If there is no match, the address will get a distance which is further than any non-negated list element, and closer than any negated element. For example,

    topology {
        10/8;
        !1.2.3/24;
        { 1.2/16; 3/8; };
    };

will prefer servers on network 10 the most, followed by hosts on network 1.2.0.0 (netmask 255.255.0.0) and network 3, with the exception of hosts on network 1.2.3 (netmask 255.255.255.0), which is preferred least of all.

The default topology is

    topology { localhost; localnets; };

Resource Record sorting

When returning multiple RRs, the nameserver will normally return them in Round Robin, i.e. after each request, the first RR is put to the end of the list. As the order of RRs is not defined, this should not cause any problems.

The client resolver code should re-arrange the RRs as appropriate, i.e. using any addresses on the local net in preference to other addresses. However, not all resolvers can do this, or are not correctly configured.

When a client is using a local server, the sorting can be performed in the server, based on the client's address. This only requires configuring the nameservers, not all the clients.

The sortlist statement takes an address match list and interprets it even more specially than the topology statement does.

Each top level statement in the sortlist must itself be an explicit address match list with one or two elements. The first element (which may be an IP address, an IP prefix, an ACL name or nested address match list) of each top level list is checked against the source address of the query until a match is found.

Once the source address of the query has been matched, if the top level statement contains only one element, the actual primitive element that matched the source address is used to select the address in the response to move to the beginning of the response. If the statement is a list of two elements, the second element is treated like the address match list in a topology statement. Each top level element is assigned a distance and the address in the response with the minimum distance is moved to the beginning of the response.

In the following example, any queries received from any of the addresses of the host itself will get responses preferring addresses on any of the locally connected networks. Next most preferred are addresses on the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24 network with no preference shown between these two networks. Queries received from a host on the 192.168.1/24 network will prefer other addresses on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries received from a host on the 192.168.4/24 or the 192.168.5/24 network will only prefer other addresses on their directly connected networks.

sortlist {
           { localhost;         // IF   the local host
             { localnets;       // THEN first fit on the
               192.168.1/24;    //      following nets
               { 192,168.2/24; 192.168.3/24; }; }; };
           { 192.168.1/24;      // IF   on class C 192.168.1
             { 192.168.1/24;    // THEN use .1, or .2 or .3
               { 192.168.2/24; 192.168.3/24; }; }; };
           { 192.168.2/24;      // IF   on class C 192.168.2
             { 192.168.2/24;    // THEN use .2, or .1 or .3
               { 192.168.1/24; 192.168.3/24; }; }; };
           { 192.168.3/24;      // IF   on class C 192.168.3
             { 192.168.3/24;    // THEN use .3, or .1 or .2
               { 192.168.1/24; 192.168.2/24; }; }; };
           { { 192.168.4/24; 192.168.5/24; }; // if .4 or .5, prefer that net
           };
};

sortlist {
            { localhost; localnets; };
            { localnets; };
};

  [ class class_name ][ type type_name ][ name "FQDN" ] order ordering

    rrset-order {
	class IN type A name "rc.vix.com" order random;
        order cyclic;
    };

    rrset-order { class ANY type ANY name "*" order cyclic ; };