The Internet Domain Name System (DNS) consists of the syntax to specify the names of entities in the Internet in a hierarchical manner, the rules used for delegating authority over names, and the system implementation that actually maps names to Internet addresses. DNS data is maintained in a group of distributed hierarchical databases. The Berkeley Internet Name Domain (BIND) implements an Internet nameserver for a number of operating systems. This document provides basic information about the installation and care of the Internet Software Consortium (ISC) BIND version 9 software package for system administrators. In this document, Section 1 introduces the basic DNS and BIND concepts. Section 2 describes resource requirements for running BIND in various environments. Information in Section 3 is task-oriented in its presentation and is organized functionally, to aid in the process of installing the BIND 9 software. The task-oriented section is followed by Section 4, which contains more advanced concepts that the system administrator may need for implementing certain options. Section 5 describes the BIND 9 lightweight resolver. The contents of Section 6 are organized as in a reference manual to aid in the ongoing maintenance of the software. Section 7 addresses security considerations, and Section 8 contains troubleshooting help. The main body of the document is followed by several Appendices which contain useful reference information, such as a Bibliography and historic information related to BIND and the Domain Name System. In this document, we use the following general typographic conventions: To describe: We use the style: a pathname, filename, URL, hostname, mailing list name, or new term or concept Italic literal user input Fixed Width Bold variable user input Fixed Width Italic program output Fixed Width Bold The following conventions are used in descriptions of the BIND configuration file: To describe: We use the style: keywords Sans Serif Bold variables Sans Serif Italic "meta-syntactic" information (within brackets when optional) Fixed Width Italic Command line input Fixed Width Bold Program output Fixed Width Optional input Text is enclosed in square brackets The purpose of this document is to explain the installation and basic upkeep of the BIND software package, and we begin by reviewing the fundamentals of the domain naming system as they relate to BIND. BIND consists of a nameserver (or "daemon") called named and a resolver library. The BIND server runs in the background, servicing queries on a well known network port. The standard port for the User Datagram Protocol (UDP) and Transmission Control Protocol (TCP), usually port 53, is specified in /etc/services. The resolver is a set of routines residing in a system library that provides the interface that programs can use to access the domain name services. A nameserver (NS) is a program that stores information about named resources and responds to queries from programs called resolvers which act as client processes. The basic function of an NS is to provide information about network objects by answering queries. With the nameserver, the network can be broken into a hierarchy of domains. The name space is organized as a tree according to organizational or administrative boundaries. Each node of the tree, called a domain, is given a label. The name of the domain is the concatenation of all the labels of the domains from the root to the current domain. This is represented in written form as a string of labels listed from right to left and separated by dots. A label need only be unique within its domain. The whole name space is partitioned into areas called zones, each starting at a domain and extending down to the leaf domains or to domains where other zones start. Zones usually represent administrative boundaries. For example, a domain name for a host at the company Example, Inc. would be: ourhost.example.com where com is the top level domain to which ourhost.example.com belongs, example is a subdomain of com, and ourhost is the name of the host. The specifications for the domain nameserver are defined in the RFC 1034, RFC 1035 and RFC 974. These documents can be found in /usr/src/etc/named/doc in 4.4BSD or are available via File Transfer Protocol (FTP) from ftp://www.isi.edu/in-notes/ or via the Web at http://www.ietf.org/rfc/. (See Appendix C for complete information on finding and retrieving RFCs.) It is also recommended that you read the related man pages: named and resolver. As we stated previously, a zone is a point of delegation in the DNS tree. A zone consists of those contiguous parts of the domain tree for which a domain server has complete information and over which it has authority. It contains all domain names from a certain point downward in the domain tree except those which are delegated to other zones. A delegation point has one or more NS records in the parent zone, which should be matched by equivalent NS records at the root of the delegated zone. To properly operate a nameserver, it is important to understand the difference between a zone and a domain. For instance, consider the example.com domain which includes names such as host.aaa.example.com and host.bbb.example.com even though the example.com zone includes only delegations for the aaa.example.com and bbb.example.com zones. A zone can map exactly to a single domain, but could also include only part of a domain, the rest of which could be delegated to other nameservers. Every name in the DNS tree is a domain, even if it is terminal, that is, has no subdomains. Every subdomain is a domain and every domain except the root is also a subdomain. The terminology is not intuitive and we suggest that you read RFCs 1033, 1034 and 1035 to gain a complete understanding of this difficult and subtle topic. Though BIND is a Domain Nameserver, it deals primarily in terms of zones. The master and slave declarations in the named.conf file specify zones, not domains. When you ask some other site if it is willing to be a slave server for your domain, you are actually asking for slave service for some collection of zones. Each zone will have one primary master (also called primary) server which loads the zone contents from some local file edited by humans or perhaps generated mechanically from some other local file which is edited by humans. There there will be some number of slave (also called secondary) servers, which load the zone contents using the DNS protocol (that is, the secondary servers will contact the primary and fetch the zone data using TCP). This set of servers — the primary and all of its secondaries — should be listed in the NS records in the parent zone and will constitute a delegation. This set of servers must also be listed in the zone file itself, usually under the @ name which indicates the top level or root of the current zone. You can list servers in the zone's top-level @ NS records that are not in the parent's NS delegation, but you cannot list servers in the parent's delegation that are not present in the zone's @. Any servers listed in the NS records must be configured as authoritative for the zone. A server is authoritative for a zone when it has been configured to answer questions for that zone with authority, which it does by setting the "authoritative answer" (AA) bit in reply packets. A server may be authoritative for more than one zone. The authoritative data for a zone is composed of all of the Resource Records (RRs) — the data associated with names in a tree-structured name space — attached to all of the nodes from the top node of the zone down to leaf nodes or nodes above cuts around the bottom edge of the zone. Adding a zone as a type master or type slave will tell the server to answer questions for the zone authoritatively. If the server is able to load the zone into memory without any errors it will set the AA bit when it replies to queries for the zone. See RFCs 1034 and 1035 for more information about the AA bit. A DNS server can be master for some zones and slave for others or can be only a master, or only a slave, or can serve no zones and just answer queries via its cache. Master servers are often also called primaries and slave servers are often also called secondaries. Both master/primary and slave/secondary servers are authoritative for a zone. All servers keep data in their cache until the data expires, based on a Time To Live (TTL) field which is maintained for all resource records. The primary master server is the ultimate source of information about a domain. The primary master is an authoritative server configured to be the source of zone transfer for one or more secondary servers. The primary master server obtains data for the zone from a file on disk. A slave server, also called a secondary server, is an authoritative server that uses zone transfers from the primary master server to retrieve the zone data. Optionally, the slave server obtains zone data from a cache on disk. Slave servers provide necessary redundancy. All secondary/slave servers are named in the NS RRs for the zone. Some servers are caching only servers. This means that the server caches the information that it receives and uses it until the data expires. A caching only server is a server that is not authoritative for any zone. This server services queries and asks other servers, who have the authority, for the information it needs. Instead of interacting with the nameservers for the root and other domains, a forwarding server always forwards queries it cannot satisfy from its authoritative data or cache to a fixed list of other servers. The forwarded queries are also known as recursive queries, the same type as a client would send to a server. There may be one or more servers forwarded to, and they are queried in turn until the list is exhausted or an answer is found. A forwarding server is typically used when you do not wish all the servers at a given site to interact with the rest of the Internet servers. A typical scenario would involve a number of internal DNS servers and an Internet firewall. Servers unable to pass packets through the firewall would forward to the server that can do it, and that server would query the Internet DNS servers on the internal server's behalf. An added benefit of using the forwarding feature is that the central machine develops a much more complete cache of information that all the workstations can take advantage of. There is no prohibition against declaring a server to be a forwarder even though it has master and/or slave zones as well; the effect will still be that anything in the local server's cache or zones will be answered, and anything else will be forwarded using the forwarders list. A stealth server is a server that answers authoritatively for a zone, but is not listed in that zone's NS records. Stealth servers can be used as a way to centralize distribution of a zone, without having to edit the zone on a remote nameserver. Where the master file for a zone resides on a stealth server in this way, it is often referred to as a "hidden primary" configuration. Stealth servers can also be a way to keep a local copy of a zone for rapid access to the zone's records, even if all "official" nameservers for the zone are inaccessible. DNS hardware requirements have traditionally been quite modest. For many installations, servers that have been pensioned off from active duty have performed admirably as DNS servers. The DNSSEC and IPv6 features of BIND 9 may prove to be quite CPU intensive however, so organizations that make heavy use of these features may wish to consider larger systems for these applications. BIND 9 is now fully multithreaded, allowing full utilization of multiprocessor systems for installations that need it. CPU requirements for BIND 9 range from i486-class machines for serving of static zones without caching, to enterprise-class machines if you intend to process many dynamic updates and DNSSEC signed zones, serving many thousands of queries per second. The memory of the server has to be large enough to fit the cache and zones loaded off disk. Future releases of BIND 9 will provide methods to limit the amount of memory used by the cache, at the expense of reducing cache hit rates and causing more DNS traffic. It is still good practice to have enough memory to load all zone and cache data into memory — unfortunately, the best way to determine this for a given installation is to watch the nameserver in operation. After a few weeks the server process should reach a relatively stable size where entries are expiring from the cache as fast as they are being inserted. Ideally, the resource limits should be set higher than this stable size. For nameserver intensive environments, there are two alternative configurations that may be used. The first is where clients and any second-level internal nameservers query a main nameserver, which has enough memory to build a large cache. This approach minimizes the bandwidth used by external name lookups. The second alternative is to set up second-level internal nameservers to make queries independently. In this configuration, none of the individual machines needs to have as much memory or CPU power as in the first alternative, but this has the disadvantage of making many more external queries, as none of the nameservers share their cached data. ISC BIND 9 compiles and runs on the following operating systems: IBM AIX 4.3 Compaq Digital/Tru64 UNIX 4.0D HP HP-UX 11 IRIX64 6.5 Red Hat Linux 6.0, 6.1 Sun Solaris 2.6, 7, 8 (beta) FreeBSD 3.4-STABLE NetBSD-current with "unproven" pthreads In this section we provide some suggested configurations along with guidelines for their use. We also address the topic of reasonable option setting. The following sample configuration is appropriate for a caching-only name server for use by clients internal to a corporation. All queries from outside clients are refused. // Two corporate subnets we wish to allow queries from. acl "corpnets" { 192.168.4.0/24; 192.168.7.0/24; }; options { directory "/etc/namedb"; // Working directory pid-file "named.pid"; // Put pid file in working dir allow-query { "corpnets"; }; }; // Root server hints zone "." { type hint; file "root.hint"; }; // Provide a reverse mapping for the loopback address 127.0.0.1 zone "0.0.127.in-addr.arpa" { type master; file "localhost.rev"; notify no; }; This sample configuration is for an authoritative-only server that is the master server for "example.com" and a slave for the subdomain "eng.example.com". options { directory "/etc/namedb"; // Working directory pid-file "named.pid"; // Put pid file in working dir allow-query { any; }; // This is the default recursion no; // Do not provide recursive service }; // Root server hints zone "." { type hint; file "root.hint"; }; // Provide a reverse mapping for the loopback address 127.0.0.1 zone "0.0.127.in-addr.arpa" { type master; file "localhost.rev"; notify no; }; // We are the master server for example.com zone "example.com" { type master; file "example.com.db"; // IP addresses of slave servers allowed to transfer example.com allow-transfer { 192.168.4.14; 192.168.5.53; }; }; // We are a slave server for eng.example.com zone "eng.example.com" { type slave; file "eng.example.com.bk"; // IP address of eng.example.com master server masters { 192.168.4.12; }; }; Primitive load balancing can be achieved in DNS using multiple A records for one name. For example, if you have three WWW servers with network addresses of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the following means that clients will connect to each machine one third of the time: Name TTL CLASS TYPE Resource Record (RR) Data www 600 IN A 10.0.0.1 600 IN A 10.0.0.2 600 IN A 10.0.0.3 When a resolver queries for these records, BIND will rotate them and respond to the query with the records in a different order. In the example above, clients will randomly receive records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients will use the first record returned and discard the rest. For more detail on ordering responses, check the rrset-order substatement in the options statement, see . This substatement is not supported in BIND 9, and only the ordering scheme described above is available. DNS Notify is a mechanism that allows master nameservers to notify their slave servers of changes to a zone's data. In response to a NOTIFY from a master server, the slave will check to see that its version of the zone is the current version and, if not, initiate a transfer. DNS Notify is fully documented in RFC 1996. See also the description of the zone option also-notify, see . For more information about notify, see . There are several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the nameserver daemon. We describe several in this section dig The domain information groper (dig) is a command line tool that can be used to gather information from the Domain Name System servers. Dig has two modes: simple interactive mode for a single query, and batch mode which executes a query for each in a list of several query lines. All query options are accessible from the command line. dig @server domain query-type query-class +query-option -dig-option %comment The usual simple use of dig will take the form dig @server domain query-type query-class For more information and a list of available commands and options, see the dig man page. host The host utility provides a simple DNS lookup using a command-line interface for looking up Internet hostnames. By default, the utility converts between host names and Internet addresses, but its functionality can be extended with the use of options. host -aCdlrTwv -c class -N ndots -t type -W timeout -R retries hostname server For more information and a list of available commands and options, see the host man page. nslookup nslookup is a program used to query Internet domain nameservers. nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query nameservers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain. nslookup -option host-to-find - server Interactive mode is entered when no arguments are given (the default nameserver will be used) or when the first argument is a hyphen (`-') and the second argument is the host name or Internet address of a nameserver. Non-interactive mode is used when the name or Internet address of the host to be looked up is given as the first argument. The optional second argument specifies the host name or address of a nameserver. Due to its arcane user interface and frequently inconsistent behavior, we do not recommend the use of nslookup. Use dig instead. named-checkconf Checks the syntax of named.conf. named-checkconf filename named-checkzone Performs syntax and consistency checks on a individual zone. named-checkzone -dq -c class zone filename Administrative tools play an integral part in the management of a server. rndc The remote name daemon control (rndc) program allows the system administrator to control the operation of a nameserver. If you run rndc without any options it will display a usage message as follows: rndc -c config -s server -p port -y key command command command is one of the following: reload Reload configuration file and zones. reload zone class view Reload the given zone. refresh zone class view Schedule zone maintenance for the given zone. stats Write server statistics to the statistics file. querylog Toggle query logging. dumpdb Dump the current contents of the cache (or caches if there are multiple views) into the file named by the dump-file option (by default, named_dump.db). stop Stop the server, making sure any recent changes made through dynamic update or IXFR are first saved to the master files of the updated zones. halt Stop the server immediately. Recent changes made through dynamic update or IXFR are not saved to the master files, but will be rolled forward from the journal files when the server is restarted. In BIND 9.1, rndc does not yet support all the commands of the BIND 8 ndc utility. Additonal commands will be added in future releases. A configuration file is required, since all communication with the server is authenticated with digital signatures that rely on a shared secret, and there is no way to provide that secret other than with a configuration file. The default location for the rndc configuration file is /etc/rndc.conf, but an alternate location can be specified with the -c option. The format of the configuration file is similar to that of named.conf, but limited to only three statements, the options, key and server statements. These statements are what associate the secret keys to the servers with which they are meant to be shared. The order of statements is not significant. The options statement has two clauses: default-server and default-key. default-server takes a host name or address argument and represents the server that will be contacted if no -s option is provided on the command line. default-key takes the name of key as its argument, as defined by a key statement. In the future a default-port clause will be added to specify the port to which rndc should connect. The key statement names a key with its string argument. The string is required by the server to be a valid domain name, though it need not actually be hierarchical; thus, a string like "rndc_key" is a valid name. The key statement has two clauses: algorithm and secret. While the configuration parser will accept any string as the argument to algorithm, currently only the string "hmac-md5" has any meaning. The secret is a base-64 encoded string, typically generated with either dnssec-keygen or mmencode. The server statement uses the key clause to associate a key-defined key with a server. The argument to the server statement is a host name or address (addresses must be double quoted). The argument to the key clause is the name of the key as defined by the key statement. A port clause will be added to a future release to specify the port to which rndc should connect on the given server. A sample minimal configuration file is as follows: key rndc_key { algorithm "hmac-md5"; secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K"; }; options { default-server localhost; default-key rndc_key; }; This file, if installed as /etc/rndc.conf, would allow the command: $ rndc reload to connect to 127.0.0.1 port 953 and cause the nameserver to reload, if a nameserver on the local machine were running with following controls statements: controls { inet 127.0.0.1 allow { localhost; } keys { rndc_key; }; }; and it had an identical key statement for rndc_key. Certain UNIX signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the kill command. SIGHUP Causes the server to read named.conf and reload the database. SIGTERM Causes the server to clean up and exit. SIGINT Causes the server to clean up and exit. Dynamic update is the term used for the ability under certain specified conditions to add, modify or delete records or RRsets in the master zone files. Dynamic update is fully described in RFC 2136. Dynamic update is enabled on a zone-by-zone basis, by including an allow-update or update-policy clause in the zone statement. Updating of secure zones (zones using DNSSEC) is modelled after the simple-secure-update proposal, a work in progress in the DNS Extensions working group of the IETF. (See http://www.ietf.org/html.charters/dnsext-charter.html for information about the DNS Extensions working group.) SIG and NXT records affected by updates are automatically regenerated by the server using an online zone key. Update authorization is based on transaction signatures and an explicit server policy. The zone files of dynamic zones must not be edited by hand. The zone file on disk at any given time may not contain the latest changes performed by dynamic update. The zone file is written to disk only periodically, and changes that have occurred since the zone file was last written to disk are stored only in the zone's journal (.jnl) file. BIND 9 currently does not update the zone file when it exits as BIND 8 does, so editing the zone file manually is unsafe even when the server has been shut down. The incremental zone transfer (IXFR) protocol is a way for slave servers to transfer only changed data, instead of having to transfer the entire zone. The IXFR protocol is documented in RFC 1995. See When acting as a master, BIND 9 supports IXFR for those zones where the necessary change history information is available. These include master zones maintained by dynamic update and slave zones whose data was obtained by IXFR, but not manually maintained master zones nor slave zones obtained by performing a full zone transfer (AXFR). When acting as a slave, BIND 9 will attempt to use IXFR unless it is explicitly disabled. For more information about disabling IXFR, see the description of the request-ixfr clause of the server statement. Setting up different views, or visibility, of DNS space to internal and external resolvers is usually referred to as a Split DNS setup. There are several reasons an organization would want to set up its DNS this way. One common reason for setting up a DNS system this way is to hide "internal" DNS information from "external" clients on the Internet. There is some debate as to whether or not this is actually useful. Internal DNS information leaks out in many ways (via email headers, for example) and most savvy "attackers" can find the information they need using other means. Another common reason for setting up a Split DNS system is to allow internal networks that are behind filters or in RFC 1918 space (reserved IP space, as documented in RFC 1918) to resolve DNS on the Internet. Split DNS can also be used to allow mail from outside back in to the internal network. Here is an example of a split DNS setup: Let's say a company named Example, Inc. (example.com) has several corporate sites that have an internal network with reserved Internet Protocol (IP) space and an external demilitarized zone (DMZ), or "outside" section of a network, that is available to the public. Example, Inc. wants its internal clients to be able to resolve external hostnames and to exchange mail with people on the outside. The company also wants its internal resolvers to have access to certain internal-only zones that are not available at all outside of the internal network. In order to accomplish this, the company will set up two sets of nameservers. One set will be on the inside network (in the reserved IP space) and the other set will be on bastion hosts, which are "proxy" hosts that can talk to both sides of its network, in the DMZ. The internal servers will be configured to forward all queries, except queries for site1.internal, site2.internal, site1.example.com, and site2.example.com, to the servers in the DMZ. These internal servers will have complete sets of information for site1.example.com, site2.example.com, site1.internal, and site2.internal. To protect the site1.internal and site2.internal domains, the internal nameservers must be configured to disallow all queries to these domains from any external hosts, including the bastion hosts. The external servers, which are on the bastion hosts, will be configured to serve the "public" version of the site1 and site2.example.com zones. This could include things such as the host records for public servers (www.example.com and ftp.example.com), and mail exchange (MX) records (a.mx.example.com and b.mx.example.com). In addition, the public site1 and site2.example.com zones should have special MX records that contain wildcard (`*') records pointing to the bastion hosts. This is needed because external mail servers do not have any other way of looking up how to deliver mail to those internal hosts. With the wildcard records, the mail will be delivered to the bastion host, which can then forward it on to internal hosts. Here's an example of a wildcard MX record: * IN MX 10 external1.example.com. Now that they accept mail on behalf of anything in the internal network, the bastion hosts will need to know how to deliver mail to internal hosts. In order for this to work properly, the resolvers on the bastion hosts will need to be configured to point to the internal nameservers for DNS resolution. Queries for internal hostnames will be answered by the internal servers, and queries for external hostnames will be forwarded back out to the DNS servers on the bastion hosts. In order for all this to work properly, internal clients will need to be configured to query only the internal nameservers for DNS queries. This could also be enforced via selective filtering on the network. If everything has been set properly, Example, Inc.'s internal clients will now be able to: Look up any hostnames in the site1 and site2.example.com zones. Look up any hostnames in the site1.internal and site2.internal domains. Look up any hostnames on the Internet. Exchange mail with internal AND external people. Hosts on the Internet will be able to: Look up any hostnames in the site1 and site2.example.com zones. Exchange mail with anyone in the site1 and site2.example.com zones. Here is an example configuration for the setup we just described above. Note that this is only configuration information; for information on how to configure your zone files, see Internal DNS server config: acl internals { 172.16.72.0/24; 192.168.1.0/24; }; acl externals { bastion-ips-go-here; }; options { ... ... forward only; forwarders { // forward to external servers bastion-ips-go-here; }; allow-transfer { none; }; // sample allow-transfer (no one) allow-query { internals; externals; }; // restrict query access allow-recursion { internals; }; // restrict recursion ... ... }; zone "site1.example.com" { // sample slave zone type master; file "m/site1.example.com"; forwarders { }; // do normal iterative // resolution (do not forward) allow-query { internals; externals; }; allow-transfer { internals; }; }; zone "site2.example.com" { type slave; file "s/site2.example.com"; masters { 172.16.72.3; }; forwarders { }; allow-query { internals; externals; }; allow-transfer { internals; }; }; zone "site1.internal" { type master; file "m/site1.internal"; forwarders { }; allow-query { internals; }; allow-transfer { internals; } }; zone "site2.internal" { type slave; file "s/site2.internal"; masters { 172.16.72.3; }; forwarders { }; allow-query { internals }; allow-transfer { internals; } }; External (bastion host) DNS server config: acl internals { 172.16.72.0/24; 192.168.1.0/24; }; acl externals { bastion-ips-go-here; }; options { ... ... allow-transfer { none; }; // sample allow-transfer (no one) allow-query { internals; externals; }; // restrict query access allow-recursion { internals; externals; }; // restrict recursion ... ... }; zone "site1.example.com" { // sample slave zone type master; file "m/site1.foo.com"; allow-query { any; }; allow-transfer { internals; externals; }; }; zone "site2.example.com" { type slave; file "s/site2.foo.com"; masters { another_bastion_host_maybe; }; allow-query { any; }; allow-transfer { internals; externals; } }; In the resolv.conf (or equivalent) on the bastion host(s): search ... nameserver 172.16.72.2 nameserver 172.16.72.3 nameserver 172.16.72.4 This is a short guide to setting up Transaction SIGnatures (TSIG) based transaction security in BIND. It describes changes to the configuration file as well as what changes are required for different features, including the process of creating transaction keys and using transaction signatures with BIND. BIND primarily supports TSIG for server to server communication. This includes zone transfer, notify, and recursive query messages. Resolvers based on newer versions of BIND 8 have limited support for TSIG. TSIG might be most useful for dynamic update. A primary server for a dynamic zone should use access control to control updates, but IP-based access control is insufficient. Key-based access control is far superior, see . The nsupdate program supports TSIG via the -k and -y command line options. A shared secret is generated to be shared between host1 and host2. An arbitrary key name is chosen: "host1-host2.". The key name must be the same on both hosts. The following command will generate a 128 bit (16 byte) HMAC-MD5 key as described above. Longer keys are better, but shorter keys are easier to read. Note that the maximum key length is 512 bits; keys longer than that will be digested with MD5 to produce a 128 bit key. dnssec-keygen -a hmac-md5 -b 128 -n HOST host1-host2. The key is in the file Khost1-host2.+157+00000.private. Nothing directly uses this file, but the base-64 encoded string following "Key:" can be extracted from the file and used as a shared secret: Key: La/E5CjG9O+os1jq0a2jdA== The string "La/E5CjG9O+os1jq0a2jdA==" can be used as the shared secret. The shared secret is simply a random sequence of bits, encoded in base-64. Most ASCII strings are valid base-64 strings (assuming the length is a multiple of 4 and only valid characters are used), so the shared secret can be manually generated. Also, a known string can be run through mmencode or a similar program to generate base-64 encoded data. This is beyond the scope of DNS. A secure transport mechanism should be used. This could be secure FTP, ssh, telephone, etc. Imagine host1 and host 2 are both servers. The following is added to each server's named.conf file: key host1-host2. { algorithm hmac-md5; secret "La/E5CjG9O+os1jq0a2jdA=="; }; The algorithm, hmac-md5, is the only one supported by BIND. The secret is the one generated above. Since this is a secret, it is recommended that either named.conf be non-world readable, or the key directive be added to a non-world readable file that is included by named.conf. At this point, the key is recognized. This means that if the server receives a message signed by this key, it can verify the signature. If the signature succeeds, the response is signed by the same key. Since keys are shared between two hosts only, the server must be told when keys are to be used. The following is added to the named.conf file for host1, if the IP address of host2 is 10.1.2.3: server 10.1.2.3 { keys { host1-host2. ;}; }; Multiple keys may be present, but only the first is used. This directive does not contain any secrets, so it may be in a world-readable file. If host1 sends a message that is a request to that address, the message will be signed with the specified key. host1 will expect any responses to signed messages to be signed with the same key. A similar statement must be present in host2's configuration file (with host1's address) for host2 to sign request messages to host1. BIND allows IP addresses and ranges to be specified in ACL definitions and allow-{ query | transfer | update } directives. This has been extended to allow TSIG keys also. The above key would be denoted key host1-host2. An example of an allow-update directive would be: allow-update { key host1-host2. ;}; This allows dynamic updates to succeed only if the request was signed by a key named "host1-host2.". You may want to read about the more powerful update-policy statement in . The processing of TSIG signed messages can result in several errors. If a signed message is sent to a non-TSIG aware server, a FORMERR will be returned, since the server will not understand the record. This is a result of misconfiguration, since the server must be explicitly configured to send a TSIG signed message to a specific server. If a TSIG aware server receives a message signed by an unknown key, the response will be unsigned with the TSIG extended error code set to BADKEY. If a TSIG aware server receives a message with a signature that does not validate, the response will be unsigned with the TSIG extended error code set to BADSIG. If a TSIG aware server receives a message with a time outside of the allowed range, the response will be signed with the TSIG extended error code set to BADTIME, and the time values will be adjusted so that the response can be successfully verified. In any of these cases, the message's rcode is set to NOTAUTH. TKEY is a mechanism for automatically generating a shared secret between two hosts. There are several "modes" of TKEY that specify how the key is generated or assigned. BIND implements only one of these modes, the Diffie-Hellman key exchange. Both hosts are required to have a Diffie-Hellman KEY record (although this record is not required to be present in a zone). The TKEY process must use signed messages, signed either by TSIG or SIG(0). The result of TKEY is a shared secret that can be used to sign messages with TSIG. TKEY can also be used to delete shared secrets that it had previously generated. The TKEY process is initiated by a client or server by sending a signed TKEY query (including any appropriate KEYs) to a TKEY-aware server. The server response, if it indicates success, will contain a TKEY record and any appropriate keys. After this exchange, both participants have enough information to determine the shared secret; the exact process depends on the TKEY mode. When using the Diffie-Hellman TKEY mode, Diffie-Hellman keys are exchanged, and the shared secret is derived by both participants. BIND 9 partially supports DNSSEC SIG(0) transaction signatures as specified in RFC 2535. SIG(0) uses public/private keys to authenticate messages. Access control is performed in the same manner as TSIG keys; privileges can be granted or denied based on the key name. When a SIG(0) signed message is received, it will only be verified if the key is known and trusted by the server; the server will not attempt to locate and/or validate the key. BIND 9 does not ship with any tools that generate SIG(0) signed messages. Cryptographic authentication of DNS information is possible through the DNS Security (DNSSEC) extensions, defined in RFC 2535. This section describes the creation and use of DNSSEC signed zones. In order to set up a DNSSEC secure zone, there are a series of steps which must be followed. BIND 9 ships with several tools that are used in this process, which are explained in more detail below. In all cases, the "-h" option prints a full list of parameters. Note that the DNSSEC tools require the keyset and signedkey files to be in the working directory, and that the tools shipped with BIND 9.0.x are not fully compatible with the current ones. There must also be communication with the administrators of the parent and/or child zone to transmit keys and signatures. A zone's security status must be indicated by the parent zone for a DNSSEC capable resolver to trust its data. For other servers to trust data in this zone, they must either be statically configured with this zone's zone key or the zone key of another zone above this one in the DNS tree. The dnssec-keygen program is used to generate keys. A secure zone must contain one or more zone keys. The zone keys will sign all other records in the zone, as well as the zone keys of any secure delegated zones. Zone keys must have the same name as the zone, a name type of ZONE, and must be usable for authentication. It is recommended that zone keys be mandatory to implement a cryptographic algorithm; currently the only key mandatory to implement an algorithm is DSA. The following command will generate a 768 bit DSA key for the child.example zone: dnssec-keygen -a DSA -b 768 -n ZONE child.example. Two output files will be produced: Kchild.example.+003+12345.key and Kchild.example.+003+12345.private (where 12345 is an example of a key tag). The key file names contain the key name (child.example.), algorithm (3 is DSA, 1 is RSA, etc.), and the key tag (12345 in this case). The private key (in the .private file) is used to generate signatures, and the public key (in the .key file) is used for signature verification. To generate another key with the same properties (but with a different key tag), repeat the above command. The public keys should be inserted into the zone file with $INCLUDE statements, including the .key files. The dnssec-makekeyset program is used to create a key set from one or more keys. Once the zone keys have been generated, a key set must be built for transmission to the administrator of the parent zone, so that the parent zone can sign the keys with its own zone key and correctly indicate the security status of this zone. When building a key set, the list of keys to be included and the TTL of the set must be specified, and the desired signature validity period of the parent's signature may also be specified. The list of keys to be inserted into the key set may also included non-zone keys present at the top of the zone. dnssec-makekeyset may also be used at other names in the zone. The following command generates a key set containing the above key and another key similarly generated, with a TTL of 3600 and a signature validity period of 10 days starting from now. dnssec-makekeyset -t 3600 -e +864000 Kchild.example.+003+12345 Kchild.example.+003+23456 One output file is produced: keyset-child.example.. This file should be transmitted to the parent to be signed. It includes the keys, as well as signatures over the key set generated by the zone keys themselves, which are used to prove ownership of the private keys and encode the desired validity period. The dnssec-signkey program is used to sign one child's keyset. If the child.example zone has any delegations which are secure, for example, grand.child.example, the child.example administrator should receive keyset files for each secure subzone. These keys must be signed by this zone's zone keys. The following command signs the child's key set with the zone keys: dnssec-signkey keyset-grand.child.example. Kchild.example.+003+12345 Kchild.example.+003+23456 One output file is produced: signedkey-grand.child.example.. This file should be both transmitted back to the child and retained. It includes all keys (the child's keys) from the keyset file and signatures generated by this zone's zone keys. The dnssec-signzone program is used to sign a zone. Any signedkey files corresponding to secure subzones should be present, as well as a signedkey file for this zone generated by the parent (if there is one). The zone signer will generate NXT and SIG records for the zone, as well as incorporate the zone key signature from the parent and indicate the security status at all delegation points. The following command signs the zone, assuming it is in a file called zone.child.example. By default, all zone keys which have an available private key are used to generate signatures. dnssec-signzone -o child.example zone.child.example One output file is produced: zone.child.example.signed. This file should be referenced by named.conf as the input file for the zone. Unlike in BIND 8, data is not verified on load in BIND 9, so zone keys for authoritative zones do not need to be specified in the configuration file. The public key for any security root must be present in the configuration file's trusted-keys statement, as described later in this document. BIND 9 fully supports all currently defined forms of IPv6 name to address and address to name lookups. It will also use IPv6 addresses to make queries when running on an IPv6 capable system. For forward lookups, BIND 9 supports both A6 and AAAA records. The use of AAAA records is deprecated, but it is still useful for hosts to have both AAAA and A6 records to maintain backward compatibility with installations where AAAA records are still used. In fact, the stub resolvers currently shipped with most operating system support only AAAA lookups, because following A6 chains is much harder than doing A or AAAA lookups. For IPv6 reverse lookups, BIND 9 supports the new "bitstring" format used in the ip6.arpa domain, as well as the older, deprecated "nibble" format used in the ip6.int domain. BIND 9 includes a new lightweight resolver library and resolver daemon which new applications may choose to use to avoid the complexities of A6 chain following and bitstring labels, see . The AAAA record is a parallel to the IPv4 A record. It specifies the entire address in a single record. For example, $ORIGIN example.com. host 3600 IN AAAA 3ffe:8050:201:1860:42::1 While their use is deprecated, they are useful to support older IPv6 applications. They should not be added where they are not absolutely necessary. The A6 record is more flexible than the AAAA record, and is therefore more complicated. The A6 record can be used to form a chain of A6 records, each specifying part of the IPv6 address. It can also be used to specify the entire record as well. For example, this record supplies the same data as the AAAA record in the previous example: $ORIGIN example.com. host 3600 IN A6 0 3ffe:8050:201:1860:42::1 A6 records are designed to allow network renumbering. This works when an A6 record only specifies the part of the address space the domain owner controls. For example, a host may be at a company named "company." It has two ISPs which provide IPv6 address space for it. These two ISPs fully specify the IPv6 prefix they supply. In the company's address space: $ORIGIN example.com. host 3600 IN A6 64 0:0:0:0:42::1 company.example1.net. host 3600 IN A6 64 0:0:0:0:42::1 company.example2.net. ISP1 will use: $ORIGIN example1.net. company 3600 IN A6 0 3ffe:8050:201:1860:: ISP2 will use: $ORIGIN example2.net. company 3600 IN A6 0 1234:5678:90ab:fffa:: When host.example.com is looked up, the resolver (in the resolver daemon or caching name server) will find two partial A6 records, and will use the additional name to find the remainder of the data. When an A6 record specifies the address of a name server, it should use the full address rather than specifying a partial address. For example: $ORIGIN example.com. @ 14400 IN NS ns0 14400 IN NS ns1 ns0 14400 IN A6 0 3ffe:8050:201:1860:42::1 ns1 14400 IN A 192.168.42.1 It is recommended that IPv4-in-IPv6 mapped addresses not be used. If a host has an IPv4 address, use an A record, not an A6, with ::ffff:192.168.42.1 as the address. While the use of nibble format to look up names is deprecated, it is supported for backwards compatiblity with existing IPv6 applications. When looking up an address in nibble format, the address components are simply reversed, just as in IPv4, and ip6.int. is appended to the resulting name. For example, the following would provide reverse name lookup for a host with address 3ffe:8050:201:1860:42::1. $ORIGIN 0.6.8.1.1.0.2.0.0.5.0.8.e.f.f.3.ip6.int. 1.0.0.0.0.0.0.0.0.0.0.0.2.4.0.0 14400 IN PTR host.example.com. Bitstring labels can start and end on any bit boundary, rather than on a multiple of 4 bits as in the nibble format. They also use ip6.arpa rather than ip6.int. To replicate the previous example using bitstrings: $ORIGIN \[x3ffe805002011860/64].ip6.arpa. \[x0042000000000001/64] 14400 IN PTR host.example.com. In IPV6, the same host may have many addresses from many network providers. Since the trailing portion of the address usually remains constant, DNAME can help reduce the number of zone files used for reverse mapping that need to be maintained. For example, consider a host which has two providers (example.net and example2.net) and therefore two IPv6 addresses. Since the host chooses its own 64 bit host address portion, the provider address is the only part that changes: $ORIGIN example.com. host A6 64 ::1234:5678:1212:5675 cust1.example.net. A6 64 ::1234:5678:1212:5675 subnet5.example2.net. $ORIGIN example.net. cust1 A6 48 0:0:0:dddd:: ipv6net.example.net. ipv6net A6 0 aa:bb:cccc:: $ORIGIN example2.net. subnet5 A6 48 0:0:0:1:: ipv6net2.example2.net. ipv6net2 A6 0 6666:5555:4:: This sets up forward lookups. To handle the reverse lookups, the provider example.net would have: $ORIGIN \[x00aa00bbcccc/48].ip6.arpa. \[xdddd/16] DNAME ipv6-rev.example.com. and example2.net would have: $ORIGIN \[x666655550004/48].ip6.arpa. \[x0001/16] DNAME ipv6-rev.example.com. example.com needs only one zone file to handle both of these reverse mappings: $ORIGIN ipv6-rev.example.com. \[x1234567812125675/64] PTR host.example.com. Traditionally applications have been linked with a stub resolver library that sends recursive DNS queries to a local caching name server. IPv6 introduces new complexity into the resolution process, such as following A6 chains and DNAME records, and simultaneous lookup of IPv4 and IPv6 addresses. These are hard or impossible to implement in a traditional stub resolver. Instead, BIND 9 provides resolution services to local clients using a combination of a lightweight resolver library and a resolver daemon process running on the local host. These communicate using a simple UDP-based protocol, the "lightweight resolver protocol" that is distinct from and simpler than the full DNS protocol. To use the lightweight resolver interface, the system must run the resolver daemon lwresd. By default, applications using the lightweight resolver library will make UDP requests to the IPv4 loopback address (127.0.0.1) on port 921. The address can be overriden by lwserver lines in /etc/resolv.conf. The daemon will try to find the answer to the questions "what are the addresses for host foo.example.com?" and "what are the names for IPv4 address 10.1.2.3?" The daemon currently only looks in the DNS, but in the future it may use other sources such as /etc/hosts, NIS, etc. The lwresd daemon is essentially a caching-only name server that answers requests using the lightweight resolver protocol rather than the DNS protocol. Because it needs to run on each host, it is designed to require no or minimal configuration. Unless configured otherwise, it uses the name servers listed on nameserver lines in /etc/resolv.conf as forwarders, but is also capable of doing the resolution autonomously if none are specified. The lwresd daemon may also be configured with a named.conf style configuration file, in /etc/lwresd.conf by default. A name server may also be configured to act as a lightweight resolver daemon using the lwres statement in named.conf. BIND 9 configuration is broadly similar to BIND 8.x; however, there are a few new areas of configuration, such as views. BIND 8.x configuration files should work with few alterations in BIND 9, although more complex configurations should be reviewed to check if they can be more efficiently implemented using the new features found in BIND 9. BIND 4 configuration files can be converted to the new format using the shell script contrib/named-bootconf/named-bootconf.sh. Following is a list of elements used throughout the BIND configuration file documentation: 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, see . domain_name A quoted string which will be used as a DNS name, for example "my.test.domain". dotted_decimal One or more integers valued 0 through 255 separated only by dots (`.'), such as 123, 45.67 or 89.123.45.67. ip4_addr An IPv4 address with exactly four elements in dotted_decimal notation. ip6_addr An IPv6 address, such as fe80::200:f8ff:fe01:9742. ip_addr An ip4_addr or ip6_addr. 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 as an ip_addr, followed by a slash (`/') and then the number of bits in the netmask. Trailing zeros in a ip_addr may omitted. For example, 127/8 is the network 127.0.0.0 with netmask 255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240. key_id A domain_name representing the name of a shared key, to be used for transaction security. key_list A list of one or more key_ids, separated by semicolons and ending with a semicolon. 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 integers). Its acceptable value might further be limited by the context in which it is used. path_name A quoted string which will be used as a pathname, such as zones/master/my.test.domain. 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. An unlimited size_spec requests unlimited use, or the maximum available amount. A default size_spec 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 conversion 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. dialup_option One of yes, no, notify, notify-passive, refresh or passive. When used in a zone, notify-passive, refresh, and passive are restricted to slave and stub zones. address_match_list = address_match_list_element ; address_match_list_element; ... address_match_list_element = ! (ip_address /length | key key_id | acl_name | { address_match_list } ) 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: an IP address (IPv4 or IPv6) an IP prefix (in the `/'-notation) a key ID, as defined by the key statement the name of an address match list previously defined with the acl statement a nested address match list enclosed in braces 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 access 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 interpretation of a match depends on whether the list is being used for access control, 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, access is denied. The clauses allow-notify, allow-query, allow-transfer, allow-update and blackhole all use address match lists this. Similarly, the listen-on option will cause the server to not accept queries on any of the machine's addresses which do not match the list. When used with the topology clause, a non-negated match returns a distance 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 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. Because of the first-match aspect of the algorithm, an element that defines 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. The BIND 9 comment syntax allows for comments to appear anywhere that white space may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in C, C++, or shell/perl constructs. /* This is a BIND comment as in C */ // This is a BIND comment as in C++ # This is a BIND comment as in common UNIX shells and perl Comments may appear anywhere that whitespace may appear in a BIND configuration file. 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 character # (number sign) and continue to the end of the physical line, as in 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. A BIND 9 configuration consists of statements and comments. Statements end with a semicolon. Statements and comments are the only elements that can appear without enclosing braces. Many statements contain a block of substatements, which are also terminated with a semicolon. The following statements are supported: acl defines a named IP address matching list, for access control and other uses. controls declares control channels to be used by the rndc utility. include includes a file. key specifies key information for use in authentication and authorization using TSIG. logging specifies what the server logs, and where the log messages are sent. options controls global server configuration options and sets defaults for other statements. server sets certain configuration options on a per-server basis. trusted-keys defines trusted DNSSEC keys. view defines a view. zone defines a zone. The logging and options statements may only occur once per configuration. acl acl-name { address_match_list }; The acl statement assigns a symbolic name to an 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 Matches all hosts. none Matches no hosts. localhost Matches the IP addresses of all interfaces on the system. localnets Matches any host on a network for which the system has an interface. controls { inet ( ip_addr | * ) port ip_port allow address_match_list keys key_list ; inet ...; }; The controls statement declares control channels to be used by system administrators to affect the operation of the local nameserver. These control channels are used by the rndc utility to send commands to and retrieve non-DNS results from a nameserver. An inet control channel is a TCP/IP socket accessible to the Internet, created at the specified ip_port on the specified ip_addr. If no port is specified, port 953 is used by default. "*" cannot be used for ip_port. The ability to issue commands over the control channel is restricted by the allow and keys clauses. Connections to the control channel are permitted based on the address permissions in address_match_list. key_id members of the address_match_list are ignored, and instead are interpreted independently based the key_list. Each key_id in the key_list is allowed to be used to authenticate commands and responses given over the control channel by digitally signing each message between the server and a command client (See in ). All commands to the control channel must be signed by one of its specified keys to be honored. The UNIX control channel type of BIND 8 is not supported in BIND 9.0.0, and is not expected to be added in future releases. If it is present in the controls statement from a BIND 8 configuration file, a non-fatal warning will be logged. include filename; The include statement inserts the specified file at the point that the include statement is encountered. The include statement facilitates the administration of configuration files by permitting the reading or writing of some things but not others. For example, the statement could include private keys that are readable only by a nameserver. key key_id { algorithm string; secret string; }; The key statement defines a shared secret key for use with TSIG, see . The key_id, also known as the key name, is a domain name uniquely identifying the key. It can be used in a "server" statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret. The algorithm_id is a string that specifies a security/authentication algorithm. The only algorithm currently supported with TSIG authentication is hmac-md5. The secret_string is the secret to be used by the algorithm, and is treated as a base-64 encoded string. logging { [ channel channel_name { ( file path name [ versions ( number | unlimited ) ] [ size size spec ] | syslog syslog_facility | stderr | 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 ; ... ] }; ] ... }; The logging statement configures a wide variety of logging options for the nameserver. Its channel phrase associates output methods, format options 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 categories as are wanted. If there is no logging statement, the logging configuration will be: logging { category "default" { "default_syslog"; "default_debug"; }; }; In BIND 9, the logging configuration is only established when the entire configuration file has been parsed. In BIND 8, it was established as soon as the logging statement was parsed. When the server is starting up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the "-g" option was specified. 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 destination clause that says whether messages selected for the channel go to a file, to a particular syslog facility, to the standard error stream, or are discarded. It can optionally also limit the message severity level that will be accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name and/or severity level (the default is not to include any). The null destination clause causes all messages sent to the channel to be discarded; in that case, other options for the channel are meaningless. The file destination clause directs the channel to a disk file. It can include limitations both on how large the file is allowed 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 not write anything more to it until the file is reopened; exceeding the size does not automatically trigger a reopen. The default behavior is not to limit the size of the file. If you use the version log file 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 before it is opened lamers.log.1 is renamed to lamers.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 appended. The unlimited keyword is synonymous with 99 in current BIND releases. Example usage of the size and versions options: channel "an_example_channel" { file "example.log" versions 3 size 20m; print-time yes; print-category yes; }; The syslog destination clause directs the channel to the system log. Its argument is a syslog facility as described in the syslog man page. How syslog will handle messages sent to this facility is described in the syslog.conf man page. If you have a system which uses a very old version of syslog that only uses two arguments 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 determine what eventually passes through. For example, defining a channel facility 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 stderr destination clause directs the channel to the server's standard error stream. This is intended for use when the server is running as a foreground process, for example when debugging a configuration. The server can supply extensive debugging information when it is in debugging 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 integer, or by running rndc trace. the latter method is not yet implemented The global debug level can be set to zero, and debugging mode turned off, by running ndc notrace. All debugging messages in the server have a debug level, and higher debug levels give more detailed output. Channels that specify a specific debug severity, for example: 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 pointless since syslog also prints the date and time. If print-category is requested, then the category of the message will be logged as well. Finally, 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-Feb-2000 15:05:32.863 general: notice: running There are four predefined channels that are used for named's default logging as follows. How they are used is described in . channel "default_syslog" { syslog daemon; // end 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 stderr; severity info; // only send priority info // and higher }; channel "null" { null; // toss anything sent to // this channel }; The default_debug channel normally writes to a file named.run in the server's working directory. For security reasons, when the "-u" command line option is used, the named.run file is created only after named has changed to the new UID, and any debug output generated while named is starting up and still running as root is discarded. If you need to capture this output, you must run the server with the "-g" option and redirect standard error to a file. 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. There are many categories, so you can send the logs you want to see wherever 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 default 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 following: 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 "xfer-out" { "null"; }; category "notify" { "null"; }; Following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future BIND releases. default The default category defines the logging options for those categories where no specific configuration has been defined. general The catch-all. Many things still aren't classified into categories, and they all end up here. database Messages relating to the databases used internally by the name server to store zone and cache data. security Approval and denial of requests. config Configuration file parsing and processing. resolver DNS resolution, such as the recursive lookups performed on behalf of clients by a caching name server. xfer-in Zone transfers the server is receiving. xfer-out Zone transfers the server is sending. notify The NOTIFY protocol. client Processing of client requests. network Network operations. update Dynamic updates. queries Queries. This is the grammar of the lwres statement in the named.conf file: lwres { listen-on { ip_addr port ip_port ; ip_addr port ip_port ; ... }; view view_name; search { domain_name ; ip_addr ; ... }; ndots number; }; The lwres statement configures the name server to also act as a lightweight resolver server, see . There may be be multiple lwres statements configuring lightweight resolver servers with different properties. The listen-on statement specifies a list of addresses (and ports) that this instance of a lightweight resolver daemon should accept requests on. If this statement is omitted, requests will be accepted on 127.0.0.1, port 53. The view statement binds this instance of a lightweight resolver daemon to a view in the DNS namespace, so that the response will be constructed in the same manner as a normal DNS query matching this view. If this statement is omitted, the default view is used, and if there is no default view, an error is triggered. The search statement is equivalent to the search statement in /etc/resolv.conf. It provides a list of domains which are appended to relative names in queries. The ndots statement is equivalent to the ndots statement in /etc/resolv.conf. It indicates the minimum number of dots in a relative domain name that should result in an exact match lookup before search path elements are appended. This is the grammar of the options statement in the named.conf file: options { version version_string; directory path_name; named-xfer path_name; tkey-domain domainname; tkey-dhkey key_name key_tag; dump-file path_name; memstatistics-file path_name; pid-file path_name; statistics-file path_name; zone-statistics yes_or_no; auth-nxdomain yes_or_no; deallocate-on-exit yes_or_no; dialup dialup_option; fake-iquery yes_or_no; fetch-glue yes_or_no; has-old-clients yes_or_no; host-statistics yes_or_no; multiple-cnames yes_or_no; notify yes_or_no | explicit; recursion yes_or_no; rfc2308-type1 yes_or_no; use-id-pool yes_or_no; maintain-ixfr-base yes_or_no; forward ( only | first ); forwarders { in_addr ; in_addr ; ... }; check-names ( master | slave | response )( warn | fail | ignore ); allow-notify { address_match_list }; allow-query { address_match_list }; allow-transfer { address_match_list }; allow-recursion { address_match_list }; blackhole { address_match_list }; listen-on port ip_port { address_match_list }; listen-on-v6 port ip_port { address_match_list }; query-source address ( ip_addr | * ) port ( ip_port | * ) ; max-transfer-time-in number; max-transfer-time-out number; max-transfer-idle-in number; max-transfer-idle-out number; tcp-clients number; recursive-clients number; serial-queries number; transfer-format ( one-answer | many-answers ); transfers-in number; transfers-out number; transfers-per-ns number; transfer-source (ip4_addr | *) port ip_port ; transfer-source-v6 (ip6_addr | *) port ip_port ; notify-source (ip4_addr | *) port ip_port ; notify-source-v6 (ip6_addr | *) port ip_port ; also-notify { ip_addr port ip_port ; ip_addr port ip_port ; ... }; 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 }; rrset-order { order_spec ; order_spec ; ... }; lame-ttl number; max-ncache-ttl number; max-cache-ttl number; sig-validity-interval number ; min-roots number; use-ixfr yes_or_no ; treat-cr-as-space yes_or_no ; min-refresh-time number ; max-refresh-time number ; min-retry-time number ; max-retry-time number ; port ip_port; additional-from-auth yes_or_no ; additional-from-cache yes_or_no ; }; The options statement sets up global options to be used by BIND. This statement may appear only once in a configuration file. If more than one occurrence is found, the first occurrence determines the actual options 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. version The version the server should report via a query of name version.bind in class chaos. The default is the real version number of this server. 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 defaults to `.', the directory from which the server was started. The directory specified should be an absolute path. named-xfer This option is obsolete. It was used in BIND 8 to specify the pathname to the named-xfer program. In BIND 9, no separate named-xfer program is needed; its functionality is built into the name server. tkey-domain The domain appended to the names of all shared keys generated wi