BIND 8 is much more configurable than previous release of BIND. There are entirely new areas of configuration, such as access control lists and categorized logging. Many options that previously applied to all zones can now be used selectively. These features, plus a consideration of fu- ture configuration needs led to the creation of a new configuration file format.
General Syntax
A BIND 8 configuration consists of two general features, statements and comments. All statements end with a semicolon. Many statements can con- tain substatements, which are each also terminated with a semicolon.
The following statements are supported:
logging specifies what the server logs, and where the log messages are sent
options controls global server configuration options and sets defaults for oth- er statements
zone defines a zone
acl defines a named IP address matching list, for access control and other uses
key specifies key information for use in authentication and authorization
trusted-keys defines DNSSEC keys that are preconfigured into the server and implic- itly trusted
server sets certain configuration options for individual remote servers
controls declares control channels to be used by the ndc utility
include includes another file
The logging and options statements may only occur once per configuration, while the rest may appear numerous times. Further detail on each state- ment is provided in individual sections below.
Comments may appear anywhere that whitespace may appear in a BIND config- uration file. To appeal to programmers of all kinds, they can be written in C, C++, or shell/perl constructs.
C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.
C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:
/* This is the start of a comment. This is still part of the comment. /* This is an incorrect attempt at nesting a comment. */ This is no longer in any comment. */
C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:
// This is the start of a comment. The next line // is a new comment, even though it is logically // part of the previous comment.
Shell-style (or perl-style, if you prefer) comments start with the char- acter # (hash or pound or number or octothorpe or whatever) and continue to the end of the physical line, like C++ comments. For example:
# This is the start of a comment. The next line # is a new comment, even though it is logically # part of the previous comment.
WARNING: you cannot use the ; (semicolon) character to start a comment such as you would in a zone file. The semicolon indicates the end of a configuration statement, so whatever follows it will be interpreted as the start of the next statement.
Converting from BIND 4.9.x
BIND 4.9.x configuration files can be converted to the new format by us- ing src/bin/named/named-bootconf, a shell script that is part of the BIND 8.2.x source kit.
Described below are elements used throughout the BIND configuration file documentation. Elements which are only associated with one statement are described only in the section describing that statement.acl_name The name of an address_match_list as defined by the acl statement.
address_match_list A list of one or more ip_addr, ip_prefix, key_id, or acl_name elements, as described in the ADDRESS MATCH LISTS section.
dotted-decimal One or more integers valued 0 through 255 separated only by dots (``.''), such as 123, 45.67 or 89.123.45.67.
domain_name A quoted string which will be used as a DNS name, for example "my.test.domain".
path_name A quoted string which will be used as a pathname, such as "zones/master/my.test.domain".
ip_addr An IP address in with exactly four elements in dotted-decimal notation.
ip_port An IP port number. number is limited to 0 through 65535, with values below 1024 typically restricted to root-owned processes. In some cases an asterisk (``*'') character can be used as a placeholder to select a random high-numbered port.
ip_prefix An IP network specified in dotted-decimal form, followed by ``/'' and then the number of bits in the netmask. E.g. 127/8 is the network 127.0.0.0 with netmask 255.0.0.0. 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240.
key_name A string representing the name of a shared key, to be used for transac- tion security.
number A non-negative integer with an entire range limited by the range of a C language signed integer (2,147,483,647 on a machine with 32 bit inte- gers). Its acceptable value might further be limited by the context in which it is used.
size_spec A number, the word unlimited, or the word default.
The maximum value of size_spec is that of unsigned long integers on the machine. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started.
A number can optionally be followed by a scaling factor: K or k for kilobytes, M or m for megabytes, and G or g for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively.
Integer storage overflow is currently silently ignored during conver- sion of scaled values, resulting in values less than intended, possibly even negative. Using unlimited is the best way to safely set a really large number.
yes_or_no Either yes or no. The words true and false are also accepted, as are the numbers 1 and 0.
Syntaxaddress_match_list = 1*address_match_element
address_match_element = [ "!" ] ( address_match_list / ip_address / ip_prefix / acl_name / "key " key_id ) ";"
Definition and Usage
Address match lists are primarily used to determine access control for various server operations. They are also used to define priorities for querying other nameservers and to set the addresses on which named will listen for queries. The elements which constitute an address match list can be any of the following:
+o an ip-address (in dotted-decimal notation,
+o an ip-prefix (in the '/'-notation),
+o A key_id, as defined by the key statement,
+o the name of an address match list previously defined with the acl statement, or
+o another address_match_list.
Elements can be negated with a leading exclamation mark (``!''), and the match list names any, none, localhost and localnets are predefined. More information on those names can be found in the description of the acl statement.
The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate ac- cess without regard to a host or network address. Nonetheless, the term ``address match list'' is still used throughout the documentation.
When a given IP address or prefix is compared to an address match list, the list is traversed in order until an element matches. The interpreta- tion of a match depends on whether the list is being used for access con- trol, defining listen-on ports, or as a topology, and whether the element was negated.
When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match at all in the list, access is denied. The clauses allow-query, allow-transfer, allow- update, allow-recursion, and blackhole all use address match lists like this. Similarly, the listen-on option will cause the server to not ac- cept queries on any of the machine's addresses which do not match the list.
When used with the topology option, a non-negated match returns a dis- tance based on its position on the list (the closer the match is to the start of the list, the shorter the distance is between it and the serv- er). 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.
Because of the first-match aspect of the algorithm, an element that de- fines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; !1.2.3.13 the 1.2.3.13 element is completely useless, because the algorithm will match any lookup for 1.2.3.13 to the 1.2.3/24 element. Using !1.2.3.13; 1.2.3/24 fixes that problem by having 1.2.3.13 blocked by the negation but all other 1.2.3.* hosts fall through.
Syntaxlogging { [ channel channel_name { ( file path_name [ versions ( number | unlimited ) ] [ size size_spec ] | syslog ( kern | user | mail | daemon | auth | syslog | lpr | news | uucp | cron | authpriv | ftp | local0 | local1 | local2 | local3 | local4 | local5 | local6 | local7 ) | null );
[ severity ( critical | error | warning | notice | info | debug [ level ] | dynamic ); ] [ print-category yes_or_no; ] [ print-severity yes_or_no; ] [ print-time yes_or_no; ] }; ] [ category category_name { channel_name; [ channel_name; ... ] }; ] ... };
Definition and Usage
The logging statement configures a wide variety of logging options for the nameserver. Its channel phrase associates output methods, format op- tions and severity levels with a name that can then be used with the category phrase to select how various classes of messages are logged.
Only one logging statement is used to define as many channels and cate- gories as are wanted. If there are multiple logging statements in a con- figuration, the first defined determines the logging, and warnings are issued for the others. If there is no logging statement, the logging configuration will be:
logging { category default { default_syslog; default_debug; }; category panic { default_syslog; default_stderr; }; category packet { default_debug; }; category eventlib { default_debug; }; };
The logging configuration is established as soon as the logging statement is parsed. If you want to redirect messages about processing of the en- tire configuration file, the logging statement must appear first. Even if you do not redirect configuration file parsing messages, we recommend always putting the logging statement first so that this rule need not be consciously recalled if you ever do need want the parser's messages relo- cated.
The channel phrase
All log output goes to one or more ``channels''; you can make as many of them as you want.
Every channel definition must include a clause that says whether messages selected for the channel go to a file, to a particular syslog facility, or are discarded. It can optionally also limit the message severity lev- el that will be accepted by the channel (default is info), and whether to include a time stamp generated by named, the category name, or severity level. The default is not to include any of those three.
The word null as the destination option for the channel will cause all messages sent to it to be discarded; other options for the channel are meaningless.
The file clause can include limitations both on how large the file is al- lowed to become, and how many versions of the file will be saved each time the file is opened.
The size option for files is simply a hard ceiling on log growth. If the file ever exceeds the size, then named will just not write anything more to it until the file is reopened; exceeding the size does not automati- cally trigger a reopen. The default behavior is to not limit the size of the file.
If you use the version logfile option, then named will retain that many backup versions of the file by renaming them when opening. For example, if you choose to keep 3 old versions of the file lamers.log then just be- fore it is opened lamers.log.1 is renamed to lames.log.2, lamers.log.0 is renamed to lamers.log.1, and lamers.log is renamed to lamers.log.0. No rolled versions are kept by default; any existing log file is simply ap- pended. The unlimited keyword is synonymous with 99 in current BIND re- leases. Example usage of size and versions options:
channel an_example_level { file "lamers.log" versions 3 size 20m; print-time yes; print-category yes; };
The argument for the syslog clause is a syslog facility as described in the syslog(SLIB) manual page. How syslogd will handle messages sent to this facility is described in the syslog.conf(ADM) manual page. If you have a system which uses a very old version of syslog that only uses two argu- ments to the openlog() function, then this clause is silently ignored.
The severity clause works like syslog's ``priorities'', except that they can also be used if you are writing straight to a file rather than using syslog. Messages which are not at least of the severity level given will not be selected for the channel; messages of higher severity levels will be accepted.
If you are using syslog, then the syslog.conf priorities will also deter- mine what eventually passes through. For example, defining a channel fa- cility and severity as daemon and debug but only logging daemon.warning via syslog.conf will cause messages of severity info and notice to be dropped. If the situation were reversed, with named writing messages of only warning or higher, then syslogd would print all messages it received from the channel.
The server can supply extensive debugging information when it is in de- bugging mode. If the server's global debug level is greater than zero, then debugging mode will be active. The global debug level is set either by starting the named server with the -d flag followed by a positive in- teger, or by sending the running server the SIGUSR1 signal (for example, by using ndc trace). The global debug level can be set to zero, and de- bugging mode turned off, by sending the server the SIGUSR2 signal (as with ndc notrace). All debugging messages in the server have a debug lev- el, and higher debug levels give more more detailed output. Channels that specify a specific debug severity, e.g.
channel specific_debug_level { file "foo"; severity debug 3; };
will get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with dynamic severity use the server's global level to determine what messages to print.
If print-time has been turned on, then the date and time will be logged. print-time may be specified for a syslog channel, but is usually point- less since syslog also prints the date and time. If print-category is requested, then the category of the message will be logged as well. Fi- nally, if print-severity is on, then the severity level of the message will be logged. The print- options may be used in any combination, and will always be printed in the following order: time, category, severity. Here is an example where all three print- options are on:
28-Apr-1997 15:05:32.863 default: notice: Ready to answer queries.
There are four predefined channels that are used for named 's default logging as follows. How they are used used is described in the next sec- tion, The category phrase.
channel default_syslog { syslog daemon; # send to syslog's daemon facility severity info; # only send priority info and higher };
channel default_debug { file "named.run"; # write to named.run in the working directory # Note: stderr is used instead of "named.run" # if the server is started with the -f option. severity dynamic; # log at the server's current debug level };
channel default_stderr { # writes to stderr file "<stderr>"; # this is illustrative only; there's currently # no way of specifying an internal file # descriptor in the configuration language. severity info; # only send priority info and higher };
channel null { null; # toss anything sent to this channel };
Once a channel is defined, it cannot be redefined. Thus you cannot alter the built-in channels directly, but you can modify the default logging by pointing categories at channels you have defined.
The category phrase
There are many categories, so you can send the logs you want to see wher- ever you want, without seeing logs you don't want. If you don't specify a list of channels for a category, then log messages in that category will be sent to the default category instead. If you don't specify a de- fault category, the following ``default default'' is used:
category default { default_syslog; default_debug; };
As an example, let's say you want to log security events to a file, but you also want keep the default logging behavior. You'd specify the fol- lowing:
channel my_security_channel { file "my_security_file"; severity info; }; category security { my_security_channel; default_syslog; default_debug; };
To discard all messages in a category, specify the null channel:
category lame-servers { null; }; category cname { null; };
The following categories are available:
default The catch-all. Many things still aren't classified into categories, and they all end up here. Also, if you don't specify any channels for a category, the default category is used instead. If you do not define the default category, the following definition is used: category default { default_syslog; default_debug; };
config High-level configuration file processing.
parser Low-level configuration file processing.
queries A short log message is generated for every query the server receives.
lame-servers Messages like ``Lame server on ...''
statistics Statistics.
panic If the server has to shut itself down due to an internal problem, it will log the problem in this category as well as in the problem's na- tive category. If you do not define the panic category, the following definition is used: category panic { default_syslog; default_stderr; };
update Dynamic updates.
ncache Negative caching.
xfer-in Zone transfers the server is receiving.
xfer-out Zone transfers the server is sending.
db All database operations.
eventlib Debugging info from the event system. Only one channel may be speci- fied for this category, and it must be a file channel. If you do not define the eventlib category, the following definition is used: category eventlib { default_debug; };
packet Dumps of packets received and sent. Only one channel may be specified for this category, and it must be a file channel. If you do not define the packet category, the following definition is used: category packet { default_debug; };
notify The NOTIFY protocol.
cname Messages like ``... points to a CNAME''.
security Approved/unapproved requests.
os Operating system problems.
insist Internal consistency check failures.
maintenance Periodic maintenance events.
load Zone loading messages.
response-checks Messages arising from response checking, such as ``Malformed response ...'', ``wrong ans. name ...'', ``unrelated additional info ...'', ``invalid RR type ...'', and ``bad referral ...''.
Syntaxoptions { [ version version_string; ] [ directory path_name; ] [ named-xfer path_name; ] [ dump-file path_name; ] [ memstatistics-file path_name; ] [ pid-file path_name; ] [ statistics-file path_name; ] [ auth-nxdomain yes_or_no; ] [ deallocate-on-exit yes_or_no; ] [ dialup yes_or_no; ] [ fake-iquery yes_or_no; ] [ fetch-glue yes_or_no; ] [ has-old-clients yes_or_no; ] [ host-statistics yes_or_no; ] [ host-statistics-max number; ] [ multiple-cnames yes_or_no; ] [ notify yes_or_no; ] [ recursion yes_or_no; ] [ rfc2308-type1 yes_or_no; ] [ use-id-pool yes_or_no; ] [ treat-cr-as-space yes_or_no; ] [ also-notify yes_or_no; ] [ forward ( only | first ); ] [ forwarders { [ in_addr ; [ in_addr ; ... ] ] }; ] [ check-names ( master | slave | response ) ( warn | fail | ignore); ] [ allow-query { address_match_list }; ] [ allow-recursion { address_match_list }; ] [ allow-transfer { address_match_list }; ] [ blackhole { address_match_list }; ] [ listen-on [ port ip_port ] { address_match_list }; ] [ query-source [ address ( ip_addr | * ) ] [ port ( ip_port | * ) ] ; ] [ lame-ttl number; ] [ max-transfer-time-in number; ] [ max-ncache-ttl number; ] [ min-roots number; ] [ serial-queries number; ] [ transfer-format ( one-answer | many-answers ); ] [ transfers-in number; ] [ transfers-out number; ] [ transfers-per-ns number; ] [ transfer-source ip_addr; ] [ maintain-ixfr-base yes_or_no; ] [ max-ixfr-log-size number; ] [ coresize size_spec ; ] [ datasize size_spec ; ] [ files size_spec ; ] [ stacksize size_spec ; ] [ cleaning-interval number; ] [ heartbeat-interval number; ] [ interface-interval number; ] [ statistics-interval number; ] [ topology { address_match_list }; ] [ sortlist { address_match_list|fR }; ] [ rrset-order { order_spec ; [ order_spec ; ... [ [ }; };
Definition and Usage
The options statement sets up global options to be used by BIND. This statement may appear at only once in a configuration file; if more than one occurrence is found, the first occurrence determines the actual op- tions used, and a warning will be generated. If there is no options statement, an options block with each option set to its default will be used.
Pathnames
version The version the server should report via the ndc command or via a query of name version.bind in class chaos. The default is the real version number of ths server, but some server operators prefer the string ( surely you must be joking ).
directory The working directory of the server. Any non-absolute pathnames in the configuration file will be taken as relative to this directory. The default location for most server output files (e.g. named.run) is this directory. If a directory is not specified, the working directory de- faults to ., the directory from which the server was started. The di- rectory specified should be an absolute path.
named-xfer The pathname to the named-xfer program that the server uses for inbound zone transfers. If not specified, the default is system dependent (e.g. /usr/sbin/named-xfer ).
dump-file The pathname of the file the server dumps the database to when it re- ceives SIGINT signal (as sent by ndc dumpdb ). If not specified, the default is named_dump.db.
memstatistics-file The pathname of the file the server writes memory usage statistics to on exit, if deallocate-on-exit is yes. If not specified, the default is named.memstats.
pid-file The pathname of the file the server writes its process ID in. If not specified, the default is operating system dependent, but is usually /var/run/named.pid or /etc/named.pid. The pid-file is used by programs like ndc that want to send signals to the running nameserver.
statistics-file The pathname of the file the server appends statistics to when it re- ceives SIGILL signal (from ndc stats). If not specified, the default is named.stats.
Boolean Options
auth-nxdomain If yes, then the AA bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is yes. Do not turn off auth-nxdomain unless you are sure you know what you are doing, as some older software won't like it.
deallocate-on-exit If yes, then when the server exits it will painstakingly deallocate ev- ery object it allocated, and then write a memory usage report to the memstatistics-file. The default is no, because it is faster to let the operating system clean up. deallocate-on-exit is handy for detecting memory leaks.
dialup If yes, then the server treats all zones as if they are doing zone transfers across a dial on demand dialup link, which can be brought up by traffic originating from this server. This has different effects according to zone type and concentrates the zone maintenance so that it all happens in a short interval, once every heartbeat-interval and hopefully during the one call. It also suppresses some of the normal zone maintenance traffic. The default is no. The dialup option may al- so be specified in the zone statement, in which case it overrides the options dialup statement.
If the zone is a master then the server will send out NOTIFY request to all the slaves. This will trigger the zone up to date checking in the slave (providing it supports NOTIFY) allowing the slave to verify the zone while the call us up.
If the zone is a slave or stub then the server will suppress the zone regular zone up to date queries and only perform the when the heartbeat-interval expires.
fake-iquery If yes, the server will simulate the obsolete DNS query type IQUERY. The default is no.
fetch-glue If yes (the default), the server will fetch ``glue'' resource records it doesn't have when constructing the additional data section of a re- sponse. fetch-glue no can be used in conjunction with recursion no to prevent the server's cache from growing or becoming corrupted (at the cost of requiring more work from the client).
has-old-clients Setting the option to yes, is equivalent to setting the following three options: auth-nxdomain yes ;, maintain-ixfr-base yes ;, and rfc2308-type1 no; has-old-clients with auth-nxdomain, maintain-ixfr- base, and rfc2308-type1 is order dependant.
host-statistics If yes, then statistics are kept for every host that the the nameserver interacts with. The default is no. Note: turning on host-statistics can consume huge amounts of memory.
maintain-ixfr-base If yes, an IXFR database file is kept for all dynamically updated zones. This enables the server to answer IXFR queries which can speed up zone transfers enormously. The default is no.
multiple-cnames If yes, then multiple CNAME resource records will be allowed for a do- main name. The default is no. Allowing multiple CNAME records is against standards and is not recommended. Multiple CNAME support is available because previous versions of BIND allowed multiple CNAME records, and these records have been used for load balancing by a num- ber of sites.
notify If yes (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes. The use of NOTIFY speeds conver- gence between the master and its slaves. Slave servers that receive a NOTIFY message and understand it will contact the master server for the zone and see if they need to do a zone transfer, and if they do, they will initiate it immediately. The notify option may also be specified in the zone statement, in which case it overrides the options notify statement.
recursion If yes, and a DNS query requests recursion, then the server will at- tempt to do all the work required to answer the query. If recursion is not on, the server will return a referral to the client if it doesn't know the answer. The default is yes. See also fetch-glue above.
rfc2308-type1 If yes, the server will send NS records along with the SOA record for negative answers. You need to set this to no if you have an old BIND server using you as a forwarder that does not understand negative an- swers which contain both SOA and NS records or you have an old version of sendmail. The correct fix is to upgrade the broken server or send- mail. The default is no.
use-id-pool If yes, the server will keep track of its own outstanding query ID's to avoid duplication and increase randomness. This will result in 128KB more memory being consumed by the server. The default is no.
treat-cr-as-space If yes, the server will treat CR characters the same way it treats a space or tab. This may be necessary when loading zone files on a UNIX system that were generated on an NT or DOS machine. The default is no.
Also-Notify
also-notify 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 notifi- cation 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 ac- cess to the Internet, but wish to look up exterior names anyway. For- warding occurs only on those queries for which the server is not authori- tative 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 section 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 con- texts. 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 ques- tion 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 state- ment, 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 re- questing system or via shared secret keys. See ADDRESS MATCH LISTS for details on how to specify access criteria.
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 de- fault is to allow queries from all hosts.
allow-recursion Specifies which hosts are allowed to ask recursive questions. If not specified, the default is to allow recursive 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.
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 inter- faces 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; };
will enable the nameserver on port 53 for the IP address 5.6.7.8, and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4.
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 oth- er 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 unpriv- ileged port will be used. The default is query-source address * port *;
Note: query-source currently applies only to UDP queries; TCP queries always use a wildcard IP address and 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 con- currently. 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 concur- rent outbound zone transfers. It is checked for syntax, but is oth- erwise 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-serv- er 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 re- mote end. This address must appear in the remote end's allow- transfer option for the zones being transferred, if one is specified. This statement sets the transfer-source for all zones, but can be overriden on a per-zone basis by including a transfer-source statement within the zone block in the configuration file.
Resource Limits
The server's usage of many system resources can be limited. Some oper- ating systems don't support some of the limits. On such systems, a warning will be issued if the unsupported limit is used. Some operat- ing systems don't support limiting resources, and on these systems a set resource limits on this system message will be logged.
Scaled values are allowed when specifying resource limits. For exam- ple, 1G can be used instead of 1073741824 to specify a limit of one gi- gabyte. unlimited requests unlimited use, or the maximum available amount. default uses the limit that was in force when the server was started. See the definition of size_spec in the DOCUMENTATION DEFINITIONS section for more details.
coresize The maximum size of a core dump. The default value is default.
datasize The maximum amount of data memory the server may use. The default value is default.
files The maximum number of files the server may have open concurrently. The default value is unlimited. Note that 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 sys- tems, choosing unlimited will cause the server to use the larger of the rlim_max from getrlimit(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 The max-ixfr-log-size will be used in a future release of the server to limit the size of the transaction log kept for Incremental Zone Transfer.
stacksize The maximum amount of stack memory the server may use. The default value 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 min- utes. 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. Af- ter the scan, listeners will be started on any new interfaces (pro- vided 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 min- utes. 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 topologi- cally 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 net- work 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 config- ured.
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 configur- ing the nameservers, not all the clients.
The sortlist statement takes an address match list and interprets it even more specially than the statement does.
Each top level statement in the sortlist must itself be an explicit ad- dress 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 re- sponse 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 address- es 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 net- works. 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 }; };
The following example will give reasonable behaviour for the local host and hosts on directly connected networks. It is similar to the behavior of the address sort in BIND 4.9.x. Responses sent to queries from the local host will favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network will prefer addresses on that same network. Responses to other queries will not be sorted.
sortlist { { localhost; localnets; }; { localnets; }; };
RRset Ordering
When multiple records are returned in an answer it may be useful to configure the order the records are placed into the response. For exam- ple the records for a zone might be configured to always be returned in the order they are defined in the zone file. Or perhaps a random shuf- fle of the records as they are returned is wanted. The rrset-order statement permits configuration of the ordering made of the records in a multiple record response. The default, if no ordering is defined, is a cyclic ordering (round robin).
An order_spec is defined as follows:
[ class class_name ][ type type_name ][ name "FQDN" ] order ordering
If no class is specified, the default is ANY. If no Ictype is speci- fied, the default is ANY. If no name is specified, the default is "*".
The legal values for ordering are:
fixed Records are returned in the order they are defined in the zone file. random Records are returned in some random order. cyclic Records are returned in a round-robin order.
For example:
rrset-order { class IN type A name "rc.vix.com" order random; order cyclic; };
will cause any responses for type A records in class IN that have "rc.vix.com" as a suffix, to always be returned in random order. All other records are returned in cyclic order.
If multiple rrset-order statements appear, they are not combined--the last one applies.
If no rrset-order statement is specified, a default one of:
rrset-order { class ANY type ANY name "*" order cyclic ; };
is used.
Tuning
lame-ttl Sets the number of seconds to cache a lame server indication. 0 dis- ables caching. Default is 600 (10 minutes). Maximum value is 1800 (30 minutes)
max-ncache-ttl To reduce network traffic and increase performance the server store negative answers. max-ncache-ttl is used to set a maximum retention time for these answers in the server is seconds. The default max- ncache-ttl is 10800 seconds (3 hours). max-ncache-ttl cannot exceed the maximum retention time for ordinary (positive) answers (7 days) and will be silently truncated to 7 days if set to a value which is greater that 7 days.
min-roots The minimum number of root servers that is required for a request for the root servers to be accepted. Default is 2.
Syntaxzone domain_name [ ( in | hs | hesiod | chaos ) ] { type master; file path_name; [ check-names ( warn | fail | ignore ); ] [ allow-update { address_match_list }; ] [ allow-query { address_match_list }; ] [ allow-transfer { address_match_list }; ] [ dialup yes_or_no; ] [ notify yes_or_no; ] [ also-notify { ip_addr; [ ip_addr; ... ] }; [ pubkey number number number string; ] };
zone domain_name [ ( in | hs | hesiod | chaos ) ] { type ( slave | stub ); [ file path_name; ] masters [ port ip_port ] { ip_addr; [ ip_addr; ... ] }; [ check-names ( warn | fail | ignore ); ] [ allow-update { address_match_list }; ] [ allow-query { address_match_list }; ] [ allow-transfer { address_match_list }; ] [ transfer-source ip_addr; ] [ max-transfer-time-in number; ] [ notify yes_or_no; ] [ also-notify { ip_addr; [ ip_addr; ... ] }; [ pubkey number number number string; ] };
zone domain_name [ ( in | hs | hesiod | chaos ) ] { type forward; [ forward ( only | first ); ] [ forwarders { [ ip_addr ; [ ip_addr ; ... ] ] }; ] [ check-names ( warn | fail | ignore ); ] };
zone "." [ ( in | hs | hesiod | chaos ) ] { type hint; file path_name; [ check-names ( warn | fail | ignore ); ] };
Definition and Usage
The zone statement is used to define how information about particular DNS zones is managed by the server. There are five different zone types.
master The server has a master copy of the data for the zone and will be able to provide authoritative answers for it.
slave A slave zone is a replica of a master zone. The masters list specifies one or more IP addresses that the slave contacts to update its copy of the zone. If a port is specified then checks to see if the zone is current and zone transfers will be done to the port given. If file is specified, then the replica will be written to the named file. Use of the file clause is highly recommended, since it often speeds server startup and eliminates a needless waste of bandwidth.
stub A stub zone is like a slave zone, except that it replicates only the NS records of a master zone instead of the entire zone.
forward A forward zone is used to direct all queries in it to other servers, as described in THE OPTIONS STATEMENT section. The specification of op- tions in such a zone will override any global options declared in the options statement.
If either no forwarders clause is present in the zone or an empty list for forwarders is given, then no forwarding will be done for the zone, cancelling the effects of any forwarders in the options statement. Thus if you want to use this type of zone to change only the behavior of the global forward option, and not the servers used, then you also need to respecify the global forwarders.
hint The initial set of root nameservers is specified using a hint zone. When the server starts up, it uses the root hints to find a root name- server and get the most recent list of root nameservers.
Note: previous releases of BIND used the term primary for a master zone, secondary for a slave zone, and cache for a hint zone.
Classes
The zone's name may optionally be followed by a class. If a class is not specified, class in (for "internet"), is assumed. This is correct for the vast majority of cases.
The hesiod class is for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on. More information can be found at ftp://athena-dist.mit.edu/pub/ATHENA/usenix/athena_changes.PS. The key- word hs is a synonym for hesiod.
Another MIT development was CHAOSnet, a LAN protocol created in the mid-1970s. It is still sometimes seen on LISP stations and other hard- ware in the AI community, and zone data for it can be specified with the chaos class.
Options
check-names See the subsection on Name Checking in THE OPTIONS STATEMENT.
allow-query See the description of allow-query in the Access Control subsection of THE OPTIONS STATEMENT.
allow-update Specifies which hosts are allowed to submit Dynamic DNS updates to the server. The default is to deny updates from all hosts.
allow-transfer See the description of allow-transfer in the Access Control subsection of THE OPTIONS STATEMENT.
transfer-source transfer-source determines which local address will be bound to the TCP connection used to fetch this zone. If not set, it defaults to a sys- tem 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 this zone if one is specified.
max-transfer-time-in See the description of max-transfer-time-in in the Zone Transfers sub- section of THE OPTIONS STATEMENT.
dialup See the description of dialup in the Boolean Options subsection of THE OPTIONS STATEMENT.
notify See the description of notify in the Boolean Options subsection of the THE OPTIONS STATEMENT.
also-notify also-notify is only meaningful if notify is active for this zone. The set of machines that will receive a DNS NOTIFY message for this zone is made up of all the listed nameservers for the zone (other than the pri- mary master) plus any IP addresses specified with also-notify. also- notify is not meaningful for stub zones. The default is the empty list.
forward forward is only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first would allow a normal lookup to be tried.
forwarders The forwarders option in a zone is used to override the list of global forwarders. If it is not specified in a zone of type forward, no for- warding is done for the zone; the global options are not used.
pubkey The DNSSEC flags, protocol, and algorithm are specified, as well as a base-64 encoded string representing the key.
Syntaxacl name { address_match_list };
Definition and Usage
The acl statement creates a named address match list. It gets its name from a primary use of address match lists: Access Control Lists (ACLs).
Note that an address match list's name must be defined with acl before it can be used elsewhere; no forward references are allowed.
The following ACLs are built-in:
any Allows all hosts.
none
Denies all hosts.
localhost Allows the IP addresses of all interfaces on the system.
localnets Allows any host on a network for which the system has an interface.
Syntaxkey key_id { algorithm algorithm_id; secret secret_string; };
Definition and Usage
The key statement defines a key ID which can be used in a server state- ment to associate a method of authentication with a particular name serv- er that is more rigorous than simple IP address matching. A key ID must be created with the key statement before it can be used in a server defi- nition or an address match list.
The algorithm_id is a string that specifies a security/authentication al- gorithm. secret_string is the secret to be used by the algorithm, and is treated as a base-64 encoded string. It should go without saying, but probably can't, that if you have secret_string 's in your named.conf, then it should not be readable by anyone but the superuser.
Syntaxtrusted-keys { [ domain_name flags protocol algorithm key; ] };
Definition and Usage
The trusted-keys statement is for use with DNSSEC-style security, origi- nally specified in RFC 2065. DNSSEC is meant to provide three distinct services: key distribution, data origin authentication, and transaction and request authentication. A complete description of DNSSEC and its use is beyond the scope of this document, and readers interested in more in- formation should start with RFC 2065 and then continue with the Internet Drafts available at http://www.ietf.org/ids.by.wg/dnssec.html.
Each trusted key is associated with a domain name. Its attributes are the non-negative integral flags, protocol, and algorithm, as well as a base-64 encoded string representing the key.
Any number of trusted keys can be specified.
Syntaxserver ip_addr { [ bogus yes_or_no; ] [ transfers number; ] [ transfer-format ( one-answer | many-answers ); ] [ keys { key_id [ key_id ... ] }; ] };
Definition and Usage
The server statement defines the characteristics to be associated with a remote name server.
If you discover that a server is giving out bad data, marking it as bogus will prevent further queries to it. The default value of bogus is no.
If the server supports IXFR you can tell named to attempt to perform a IXFR style zone transfer by specifing support-ixfr yes. The default value of support-ixfr is no.
The server supports two zone transfer methods. The first, 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. You can specify which method to use for a server with the transfer-format option. If transfer-format is not specified, the transfer-format specified by the options statement will be used.
The transfers will be used in a future release of the server to limit the number of concurrent in-bound zone transfers from the specified server. It is checked for syntax but is otherwise ignored.
The keys clause is used to identify a key_id defined by the key state- ment, to be used for transaction security when talking to the remote server. The key statememnt must come before the server statement that references it.
The keys statement is intended for future use by the server. It is checked for syntax but is otherwise ignored.
Syntaxcontrols { [ inet ip_addr port ip_port allow { address_match_list; }; ] [ unix path_name perm number owner number group number; ] };
Definition and Usage
The controls statement declares control channels to be used by system ad- ministrators to affect the operation of the local name server. These control channels are used by the ndc utility to send commands to and re- trieve non-DNS results from a name server.
A unix control channel is a FIFO in the file system, and access to it is controlled by normal file system permissions. It is created by named with the specified file mode bits (see chmod(C)), user and group owner. Note that, unlike chmod, the mode bits specified for perm will normally have a leading 0 so the number is interpreted as octal. Also note that the user and group ownership specified as owner and group must be given as numbers, not names. It is recommended that the permissions be re- stricted to administrative personnel only, or else any user on the system might be able to manage the local name server.
An inet control channel is a TCP/IP socket accessible to the Internet, created at the specified ip_port on the specified ip_addr. Modern telnet clients are capable of speaking directly to these sockets, and the con- trol protocol is ARPAnet-style text. It is recommended that 127.0.0.1 be the only ip_addr used, and this only if you trust all non-privileged users on the local host to manage your name server.
Syntaxinclude path_name;
Definition and Usage
The include statement inserts the specified file at the point that the include statement is encountered. It cannot be used within another statement, though, so a line such as acl internal_hosts { include internal_hosts.acl; }; is not allowed.
Use include to break the configuration up into easily-managed chunks. For example:
include "/etc/security/keys.bind"; include "/etc/acls.bind";
could be used at the top of a BIND configuration file in order to include any ACL or key information.
Be careful not to type ``#include'', like you would in a C program, be- cause ``#'' is used to start a comment.
The simplest configuration file that is still realistically useful is one which simply defines a hint zone that has a full path to the root servers file.zone "." in { type hint; file "/var/named/root.cache"; };
Here's a more typical real-world example.
/* * A simple BIND 8 configuration */
logging { category lame-servers { null; }; category cname { null; }; };
options { directory "/var/named"; };
controls { inet * port 52 allow { any; }; // a bad idea unix "/var/run/ndc" perm 0600 owner 0 group 0; // the default };
zone "isc.org" in { type master; file "master/isc.org"; };
zone "vix.com" in { type slave; file "slave/vix.com"; masters { 10.0.0.53; }; };
zone "0.0.127.in-addr.arpa" in { type master; file "master/127.0.0"; };
zone "." in { type hint; file "root.cache"; };
/etc/named.conf
The BIND 8 named configuration file.
named(ADMN), ndc(ADMN)