Chapter 7. Domain Name Server (207)

This topic has a weight of 8 points.

Basic DNS server configuration (207.1)

Resources and further reading: Albitz01, DNShowto, BINDfaq.

LPIC 2 objective 207.1 (3 points)

Candidates should be able to configure BIND to function as a caching-only DNS server. This objective includes the ability to convert older BIND configuration files to newer format, managing a running server and configuring logging.

Key Knowledge Areas:

  • BIND 9.x configuration files, terms and utilities.

  • Defining the location of the BIND zone files in BIND configuration files.

  • Reloading modified configuration and zone file.

  • Awareness of dnsmasq, djbdns and PowerDNS as alternate name servers.

The following is a partial list of used files, terms, and utilities:

  • /etc/named.conf

  • /var/named/*

  • /usr/sbin/rndc

  • kill

  • dig

  • host

Name-server components in BIND

Name servers like BIND (Berkeley Internet Name Domain system) are part of a worldwide DNS system that resolves machine names to IP addresses.

In the early days of the Internet, host name to IP address mappings were maintained by the Network Information Center (NIC) in a single file called HOSTS.TXT. This file was then distributed by FTP to other Internet connected hosts.

Due to the vast amount of hosts being connected to the Internet over time, another name resolving system was developed known as Domain Names. This system incorporated design goals like distributed updates and local caching, to increase resolving performance. Because of these features, every nameserver needs to be specifically configured for it's purpose. The following terms are apparent when configuring nameserver software like BIND:

Zones are the equivalent of domains. Zone configuration files consist of hostnames and IP address information. Nameserver software responds to requests on port 53, and translates DNS (host- or domain-)names to IP addresses. It can also translate IP addresses into DNS names, this is called a 'reverse DNS lookup' (rDNS). In order for rDNS to work, a so called pointer DNS record (PTR record) has to exist for the host being queried.

We distinguish authoritive nameservers, recursive nameservers and so called resolvers. The authoritive nameserver for a zone is the nameserver which administrates the zone configuration. It is therefore sometimes also referred to as the zone master. A recursive nameserver is a nameserver that resolves zones for which it is not authoritive for at other nameservers. The resolver is the part of the nameserver and DNS client software which performs the actual queries. In general, these are libraries as part of DNS software.

Table Major BIND components lists the most relevant parts of BIND software on a system. Note that directories may vary across distributions.

Table 7.1. Major BIND components

ComponentDescription
Program /usr/sbin/namedthe real name server
Program /usr/sbin/rndcname daemon control program
File named.confBIND configuration file
Program /etc/init.d/binddistribution specific startfile
Directory /var/namedworking directory for named


Resolving is controlled by the file nsswitch.conf which is discussed in Chapter 10, Network Client Management (210) .

BIND components will be discussed below.

The named.conf file

The file named.conf is the main configuration file of BIND. It is the first configuration file read by named, the DNS name daemon.

Note

BIND 8 configuration files should work with BIND 9, although some modifications might be necessary. Some options do not work with BIND 9. BIND 9 offers new areas of configuration, please consult the BIND documentation and manpages when upgradading from BIND 8 to BIND 9.

Note

BIND 4 configuration files can be converted to BIND 9 format using a script called named-bootconf.sh

Location of named.conf

According to LPI the location of the file named.conf is in the /etc directory. However, the location may vary across distributions. For example in the Debian Linux distribution named.conf is located in the /etc/bind directory.

A caching-only name server

A caching-only name server resolves names, which are also stored in a cache, so that they can be accessed faster when the nameserver is asked to resolve these names again. But this is what every name server does. The difference is that this is the only task a caching-only name server performs. It does not serve out zones, except for a few internal ones.

This is an example of a caching-only named.conf file. The version below is taken from the Debian bind package (some comments removed).

options {
        directory "/var/named";

        // query-source address * port 53;

        // forwarders {
        //      0.0.0.0;
        // };
};

// reduce log verbosity on issues outside our control
logging {
        category lame-servers { null; };
        category cname { null; };
};

// prime the server with knowledge of the root servers
zone "." {
        type hint;
        file "/etc/bind/db.root";
};

// be authoritative for the localhost forward and reverse zones, and for
// broadcast zones as per RFC 1912

zone "localhost" {
        type master;
        file "/etc/bind/db.local";
};

zone "127.in-addr.arpa" {
        type master;
        file "/etc/bind/db.127";
};

zone "0.in-addr.arpa" {
        type master;
        file "/etc/bind/db.0";
};

zone "255.in-addr.arpa" {
        type master;
        file "/etc/bind/db.255";
};

// add entries for other zones below here

The Debian bind package that contains this file, will provide a fully functional caching-only name server. BIND packages of other manufacturers will provide the same functionality.

Syntax

The named.conf file contains statements that start with a keyword plus an opening curly brace { and end with a closing curly brace }. A statement may contain other statements. The forwarders statement is an example of this. A statement may also contain IP addresses or the file word followed by a filename. These simple statements must be terminated by a semi-colon (;).

All kinds of comments are allowed, e.g., // and # as end of line comments. See the named.conf(5) manual page for details.

Note

The ; is NOT valid as a comment sign in named.conf. However, it is a comment sign in BIND zone files, like the file /etc/bind/db.local from the named.conf example above. An example BIND zone file can be found in the section called “The db.local file”

The options statement

Of the many possible entries (see named.conf(5)) inside an options statement, only directory, forwarders, forward, version and dialup will be discussed below.

Note

There can be only one options statement in a named.conf file.

Notable keywords in named.conf

directory

Specifies the working directory for the name daemon. A common value is /var/named. Also, zone files without a directory part are looked up in this directory.

Recent distributions separate the configuration directory from the working directory. In a recent Debian Linux distribution, for example, the working directory is specified as /var/cache/bind, but all the configuration files can be found in /etc/bind. All zone files can also be found in the latter directory and must be specified with their directory part, as can be seen in the named.conf example above.

forwarders

The forwarders statement contains one or more IP addresses of name servers to query. How these IP addresses are used is specified by the forward statement described below.

The default is no forwarders. Resolving is done through the worldwide (or company local) DNS system.

Usually the specified name servers are the same the Service Provider uses.

forward

The forward works only when forwarders are specified.

Two values can be specified: forward first; (default) and forward only;. With forward first, the query is sent first to the specified name-server IP addresses and if this fails it should perform lookups elsewhere. With forward only, queries are limited only to the specified name-server IP addresses.

An example with both forwarders and forward:

    options {
        // ...

        forwarders {
            123.12.134.2;
            123.12.134.3;
        }

        forward only;

        // ...
    }

In this example bind is told to query only the name servers 123.12.134.2 and 123.12.134.3.

version

It is possible to query the version from a running name server:


$ dig @ns12.zoneedit.com version.bind chaos txt

; <<>> DiG 9.8.3-P1 <<>> @ns12.zoneedit.com version.bind chaos txt
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 59790
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;version.bind.			CH	TXT

;; ANSWER SECTION:
VERSION.BIND.		0	CH	TXT	"8.4.X"

;; Query time: 169 msec
;; SERVER: 209.62.64.46#53(209.62.64.46)
;; WHEN: Tue Jun 25 11:38:48 2013
;; MSG SIZE  rcvd: 60

Note that the BIND version is shown in the output. Because some BIND versions have known exploits, the BIND version is sometimes kept hidden. The version specification:

    version "not revealed";

or

    version none;

inside the options statement leads to not revealed responses on version queries.

dialup

When a name server sits behind a firewall, that connects to the outside world through a dialup connection, some maintenance normally done by name servers might be unwanted. Examples of unwanted actions are: sending heartbeat packets, zone transfers with a nameserver on the other side of the firewall.

The following example, also inside the options part, stops external zone maintenance:

    heartbeat-interval 0;
    dialup yes; // NOTE: This actually *stops* dialups!

Many more options can be placed inside the options block. Refer to the manual pages for details.

Depending on the distribution used, a seperate bind.conf.options file might be used which holds all the options for the BIND configuration. The main configuration file named.conf has to include this separate file though, which can be accomplished by adding the following line to named.conf:

include "/etc/bind/named.conf.options";

Other separate configuration files like named.conf.log or named.conf.default-zones may be nested this way as well.

The logging statement

The BIND (version 8 and 9) logging system is too elaborate to discuss in detail here. An important difference between the two has to do with parsing the log configuration. BIND 8 used to parse the logging statement and start the logging configuration right away. BIND 9 only establishes the logging configuration after the entire configuration file has been parsed. While starting up, the server sends all logging messages regarding syntax errors in the configuration file to the default channels. These errors may be redirected to standard error output if the -g option has been given during startup.

The distinction between categories and channels is an important part of logging.

A channel is an output specification. The null channel, for example, dismisses any output sent to the channel.

A category is a type of data. The category security is one of many categories. To log messages of type (category) security, for example, to the default_syslog channel, use the following:

    logging {
        category security { default_syslog; };
        // ...
    };

To turn off logging for certain types of data, send it to the null channel, as is done in the example named.conf shown earlier:

    logging {
            category lame-servers { null; };
            category cname { null; };
    };

This means that messages of types lame-servers and cname are being discarded.

There are reasonable defaults for logging. This means that a named.conf without logging statement is possible.

Note

A maximum of one logging statement is allowed in a named.conf file.

Predefined zone statements

A zone defined in named.conf can be referred to using the @ symbol inside the corresponding zone file. The @ is called the current origin. For example,

    zone "127.in-addr.arpa" {
            type master;
            file "/etc/bind/db.127";
    };

will result in a current origin of 127.in-addr.arpa that is available as @ in file /etc/bind/db.127.

Details about zone files, as well as how to create your own zone files and statements will be covered in the section called “Create And Maintain DNS Zones (207.2)”.

The named name server daemon

The named name server daemon is the program that communicates with other name servers to resolve names. It accepts queries, looks in its cache and queries other name servers if it does not yet have the answer. Once it finds an answer in its own cache or database or receives an answer from another nameserver, it sends the answer back to the name server that sent the query in the first place.

Table Table 7.2, “ Controlling named lists ways to control the named name server.


The rndc program

The rndc (Remote Name Daemon Control) program can be used to control the named name server daemon, locally as well as remotely. It requires a /etc/rndc.key file which contains a key.

key "rndc-key" {
	algorithm hmac-md5;
	secret "tyZqsLtPHCNna5SFBLT0Eg==";
};

options {
	default-key "rndc-key";
	default-server 127.0.0.1;
	default-port 953;
};
    

The name server configuration file /etc/named.conf needs to have the same key added as well to allow a host with this key to control the name server. The relevant part of that file is shown below.

key "rndc-key" {
 	algorithm hmac-md5;
 	secret "tyZqsLtPHCNna5SFBLT0Eg==";
};
 
controls {
	inet 127.0.0.1 port 953
		allow { 127.0.0.1; } keys { "rndc-key"; };
};

The secret itself will never be transmitted over the network. Both ends calculate a hash using the algorithm declared after the algorithm keyword and compare hashes.

Depending on the distribution used, the configuration files might also be stored in /etc/bind/*. The rndc.key file should be owned by root:bind and have a mode of 640. The main bind configuration file, named.conf should have a line that includes the keyfile. Running the program rndc-confgen will create a key in case none is available on the system.

A command to the name server can be given as a parameter to rndc, e.g.: rndc reload. This will request the name server to reload its configuration and zone files. All commands specified in this way are understood by the name daemon. The help command presents a list of commands understood by the name server.

While not discussed here rndc may be used to manage several name servers remotely. Consult the man pages and the "BIND 9 Administrator Reference Manual" for more information on rndc and BIND.

Sending signals to named

It is possible to send signals to the named process to control its behaviour. A full list of signals can be found in the named manpage. One example is the SIGHUP signal, that causes named to reload named.conf and the database files.

Signals are sent to named with the kill command, e.g.,

    kill -HUP 217

This sends a SIGHUP signal to a named process with process id 217, which triggers a reload.

Controlling named with a start/stop script

Most distributions will come with a start/stop script that allows you to start, stop or control named manually, e.g., /etc/init.d/bind in Debian or /etc/init.d/named in RedHat.

Note

Red Hat (based) systems have the service command which can be used instead. service uses the same set of parameters, so you might, for example, say:

# service named reload

Table Table 7.3, “/etc/init.d/bind parameters” lists parameters which a current version of /etc/init.d/bind accepts.

Table 7.3. /etc/init.d/bind parameters

ParameterDescription
startstarts named
stopstops named
restartstops and restarts named
reloadreloads configuration
force-reloadsame as restart


dnsmasq

dnsmasq is both a lightweight DNS forwarder and DHCP server. dnsmasq supports static and dynamic DHCP leases and supports BOOTP/TFTP/PXE network boot protocols.

$ apt-cache search dnsmasq
dnsmasq - Small caching DNS proxy and DHCP/TFTP server
dnsmasq-base - Small caching DNS proxy and DHCP/TFTP server
dnsmasq-utils - Utilities for manipulating DHCP leases

djbdns

djbdns - Daniel J. Bernstein DNS - was build due to frustrations with repeated BIND security holes. Besides holding a DNS cache, DNS server and DNS client djbdns also includes several DNS debugging tools. The source code was released into the public domain in 2007. There have been several forks, one of which is dbndns, the fork of the Debian Project.

PowerDNS

PowerDNS is a Dutch supplier of DNS software and services. The PowerDNS software is open source (GPL), and comes packaged with many distributions as pdns, powerdns-server or pdns-server. The system allows multiple backends to allow access to DNS configuration data, including a simple backend that accepts BIND style files.

$ apt-cache search pdns
pdns-backend-geo - geo backend for PowerDNS
pdns-backend-ldap - LDAP backend for PowerDNS
pdns-backend-lua - lua backend for PowerDNS
pdns-backend-mysql - generic MySQL backend for PowerDNS
pdns-backend-pgsql - generic PostgreSQL backend for PowerDNS
pdns-backend-pipe - pipe/coprocess backend for PowerDNS
pdns-backend-sqlite - sqlite backend for PowerDNS
pdns-backend-sqlite3 - sqlite backend for PowerDNS
pdns-server - extremely powerful and versatile nameserver
pdns-server-dbg - debugging symbols for PowerDNS
pdns-recursor - PowerDNS recursor
pdns-recursor-dbg - debugging symbols for PowerDNS recursor
pdnsd - Proxy DNS Server

The dig and host utilities

The Internet Systems Consortium (ICS) has deprecated nslookup in favor of host and dig. However, nslookup is still widely used due to longevity of older Unix releases. It remains part of most Linux distributions too.

Both dig and host commands can be used to query nameservers, it's a matter of preference which one to use for which occasion. dig has far more options and provides a more elaborate output by default. The help for both commands should give some insights in the differences:

$ host
Usage: host [-aCdlriTwv] [-c class] [-N ndots] [-t type] [-W time]
            [-R number] [-m flag] hostname [server]
       -a is equivalent to -v -t ANY
       -c specifies query class for non-IN data
       -C compares SOA records on authoritative nameservers
       -d is equivalent to -v
       -l lists all hosts in a domain, using AXFR
       -i IP6.INT reverse lookups
       -N changes the number of dots allowed before root lookup is done
       -r disables recursive processing
       -R specifies number of retries for UDP packets
       -s a SERVFAIL response should stop query
       -t specifies the query type
       -T enables TCP/IP mode
       -v enables verbose output
       -w specifies to wait forever for a reply
       -W specifies how long to wait for a reply
       -4 use IPv4 query transport only
       -6 use IPv6 query transport only
       -m set memory debugging flag (trace|record|usage)
$ dig -h
Usage:  dig [@global-server] [domain] [q-type] [q-class] {q-opt}
            {global-d-opt} host [@local-server] {local-d-opt}
            [ host [@local-server] {local-d-opt} [...]]
Where:  domain	  is in the Domain Name System
        q-class  is one of (in,hs,ch,...) [default: in]
        q-type   is one of (a,any,mx,ns,soa,hinfo,axfr,txt,...) [default:a]
                 (Use ixfr=version for type ixfr)
        q-opt    is one of:
                 -x dot-notation     (shortcut for reverse lookups)
                 -i                  (use IP6.INT for IPv6 reverse lookups)
                 -f filename         (batch mode)
                 -b address[#port]   (bind to source address/port)
                 -p port             (specify port number)
                 -q name             (specify query name)
                 -t type             (specify query type)
                 -c class            (specify query class)
                 -k keyfile          (specify tsig key file)
                 -y [hmac:]name:key  (specify named base64 tsig key)
                 -4                  (use IPv4 query transport only)
                 -6                  (use IPv6 query transport only)
                 -m                  (enable memory usage debugging)
        d-opt    is of the form +keyword[=value], where keyword is:
                 +[no]vc             (TCP mode)
                 +[no]tcp            (TCP mode, alternate syntax)
                 +time=###           (Set query timeout) [5]
                 +tries=###          (Set number of UDP attempts) [3]
                 +retry=###          (Set number of UDP retries) [2]
                 +domain=###         (Set default domainname)
                 +bufsize=###        (Set EDNS0 Max UDP packet size)
                 +ndots=###          (Set NDOTS value)
                 +edns=###           (Set EDNS version)
                 +[no]search         (Set whether to use searchlist)
                 +[no]showsearch     (Search with intermediate results)
                 +[no]defname        (Ditto)
                 +[no]recurse        (Recursive mode)
                 +[no]ignore         (Don't revert to TCP for TC responses.)
                 +[no]fail           (Don't try next server on SERVFAIL)
                 +[no]besteffort     (Try to parse even illegal messages)
                 +[no]aaonly         (Set AA flag in query (+[no]aaflag))
                 +[no]adflag         (Set AD flag in query)
                 +[no]cdflag         (Set CD flag in query)
                 +[no]cl             (Control display of class in records)
                 +[no]cmd            (Control display of command line)
                 +[no]comments       (Control display of comment lines)
                 +[no]question       (Control display of question)
                 +[no]answer         (Control display of answer)
                 +[no]authority      (Control display of authority)
                 +[no]additional     (Control display of additional)
                 +[no]stats          (Control display of statistics)
                 +[no]short          (Disable everything except short
                                      form of answer)
                 +[no]ttlid          (Control display of ttls in records)
                 +[no]all            (Set or clear all display flags)
                 +[no]qr             (Print question before sending)
                 +[no]nssearch       (Search all authoritative nameservers)
                 +[no]identify       (ID responders in short answers)
                 +[no]trace          (Trace delegation down from root)
                 +[no]dnssec         (Request DNSSEC records)
                 +[no]nsid           (Request Name Server ID)
                 +[no]sigchase       (Chase DNSSEC signatures)
                 +trusted-key=####   (Trusted Key when chasing DNSSEC sigs)
                 +[no]topdown        (Do DNSSEC validation top down mode)
                 +[no]multiline      (Print records in an expanded format)
                 +[no]onesoa         (AXFR prints only one soa record)
        global d-opts and servers (before host name) affect all queries.
        local d-opts and servers (after host name) affect only that lookup.
        -h                           (print help and exit)
        -v                           (print version and exit)

As demonstrated, the dig command provides the broader range of options. Without options though, the provided information is quite similar:

$ host zonetransfer.me
zonetransfer.me has address 217.147.180.162
zonetransfer.me mail is handled by 20 ASPMX2.GOOGLEMAIL.COM.
zonetransfer.me mail is handled by 20 ASPMX3.GOOGLEMAIL.COM.
zonetransfer.me mail is handled by 20 ASPMX4.GOOGLEMAIL.COM.
zonetransfer.me mail is handled by 20 ASPMX5.GOOGLEMAIL.COM.
zonetransfer.me mail is handled by 0 ASPMX.L.GOOGLE.COM.
zonetransfer.me mail is handled by 10 ALT1.ASPMX.L.GOOGLE.COM.
zonetransfer.me mail is handled by 10 ALT2.ASPMX.L.GOOGLE.COM.
$ dig zonetransfer.me

; <<>> DiG 9.8.4-rpz2+rl005.12-P1 <<>> zonetransfer.me
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31395
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 1

;; QUESTION SECTION:
;zonetransfer.me.		IN	A

;; ANSWER SECTION:
zonetransfer.me.	7193	IN	A	217.147.180.162

;; AUTHORITY SECTION:
zonetransfer.me.	7193	IN	NS	ns12.zoneedit.com.
zonetransfer.me.	7193	IN	NS	ns16.zoneedit.com.

;; ADDITIONAL SECTION:
ns12.zoneedit.com.	1077	IN	A	209.62.64.46

;; Query time: 6 msec
;; SERVER: 213.154.248.156#53(213.154.248.156)
;; WHEN: Thu Jun 27 07:30:36 2013
;; MSG SIZE  rcvd: 115
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