14.6. DNS — Domain Name System

DNS (domain name system) is needed to resolve the domain and host names into IP addresses. In this way, the IP address is assigned to the host name earth, for example. Before setting up your own name server, read the general information about DNS in Section 14.1.3. “Domain Name System”. The configuration examples below are only valid for BIND 9, which is the new default DNS server with SUSE LINUX.

14.6.1. Starting the Name Server BIND

On a SUSE LINUX system, the name server BIND (short for Berkeley Internet Name Domain) comes preconfigured so it can be started right after installation without any problem. If you already have a functioning Internet connection and have entered as the name server address for localhost in /etc/resolv.conf, you normally already have a working name resolution without needing to know the DNS of the provider. BIND carries out the name resolution via the root name server, a notably slower process. Normally, the DNS of the provider should be entered with its IP address in the configuration file /etc/named.conf under forwarders to ensure effective and secure name resolution. If this works so far, the name server runs as a pure caching-only name server. Only when you configure its own zones will it become a proper DNS. A simple example of this is included in the documentation as /usr/share/doc/packages/bind/sample-config.

However, do not set up any official domains until assigned one by the responsible institution. Even if you have your own domain and it is managed by the provider, you are better off not to use it, as BIND would otherwise not forward any more requests for this domain. The provider's web server, for example, would not be accessible for this domain.

To start the name server, enter the command rcnamed start as root. If “done” appears to the right in green, named, as the name server process is called, has been started successfully. Test the name server immediately on the local system with the host or dig programs, which should return localhost as the default server with the address If this is not the case, /etc/resolv.conf probably contains an incorrect name server entry or the file does not exist at all. For the first test, enter host, which should always work. If you get an error message, use rcnamed status to see whether the server is actually running. If the name server does not start or behaves in an unexpected way, you can usually find the cause in the log file /var/log/messages.

To use the name server of the provider or one already running on your network as the forwarder, enter the corresponding IP address or addresses in the options section under forwarders. The addresses included in File 14.10. “Forwarding Options in named.conf” are just examples. Change these entries according to your own setup.

Example 14.10. Forwarding Options in named.conf

options { 
        directory "/var/lib/named";
        forwarders {;; };
        listen-on {;; };
        allow-query { 127/8; 192.168.0/24; };
        notify no;

The options entry is followed by entries for the zone, for localhost, 0.0.127.in-addr.arpa, and the type hint entry under “.”, which should always be present. The corresponding files do not need to be modified and should work as is. Also make sure that each entry is closed with a “;” and that the curly braces are in the correct places. After changing the configuration file /etc/named.conf or the zone files, tell BIND to reread them with the command rcnamed reload. You can achieve the same by stopping and restarting the name server with the command rcnamed restart. The server can also be stopped at any time by entering the command rcnamed stop.

14.6.2. The Configuration File /etc/named.conf

All the settings for the BIND name server itself are stored in the file /etc/named.conf. However, the zone data for the domains to handle, consisting of the host names, IP addresses, and so on, are stored in separate files in the /var/lib/named directory. The details of this are described further below.

The /etc/named.conf is roughly divided into two areas. One is the options section for general settings and the other consists of zone entries for the individual domains. A logging section as well as acl (access control list) entries are optional. Comment lines begin with a “#” sign or “//”. A minimalistic /etc/named.conf looks like File 14.11. “A Basic /etc/named.conf”.

Example 14.11. A Basic /etc/named.conf

options { 
        directory "/var/lib/named"; 
        forwarders {; };
        notify no;

zone "localhost" in {
       type master;
       file "localhost.zone";

zone "0.0.127.in-addr.arpa" in {
        type master;
        file "127.0.0.zone";

zone "." in {
        type hint;
        file "root.hint";

14.6.3. Important Configuration Options

directory "/var/lib/named";

specifies the directory where BIND can find the files containing the zone data.


specifies the name servers (mostly of the provider) to which DNS requests should forwarded if they cannot be resolved directly.

forward first;

causes DNS requests to be forwarded before an attempt is made to resolve them via the root name servers. Instead of forward first, forward only can be written to have all requests forwarded and none sent to the root name servers. This makes sense for firewall configurations.

listen-on port 53;;;

tells BIND to which network interface and port to listen. The port 53 specification can be left out, as 53 is the default port. If this entry is completely omitted, BIND accepts requests on all interfaces.

listen-on-v6 port 53 {any; };

tells BIND on which port it should listen for IPv6 client requests. The only alternative to any is none. As far as IPv6 is concerned, the server only accepts a wild card address.

query-source address * port 53;

This entry is necessary if a firewall is blocking outgoing DNS requests. This tells BIND to post requests externally from port 53 and not from any of the high ports above 1024.

query-source-v6 address * port 53;

tells BIND which port to use for IPv6 queries.

allow-query; 192.168.1/24;;

defines the networks from which clients can post DNS requests. The /24 at the end is an abbreviated expression for the netmask, in this case,

allow-transfer ! *;;

controls which hosts can request zone transfers. In the example, such requests are completely denied with ! *. Without this entry, zone transfers can be requested from anywhere without restrictions.

statistics-interval 0;

In the absence of this entry, BIND generates several lines of statistical information per hour in /var/log/messages. Specify 0 to completely suppress such statistics or specify an interval in minutes.

cleaning-interval 720;

This option defines at which time intervals BIND clears its cache. This triggers an entry in /var/log/messages each time it occurs. The time specification is in minutes. The default is 60 minutes.

interface-interval 0;

BIND regularly searches the network interfaces for new or nonexisting interfaces. If this value is set to 0, this is not done and BIND only listens at the interfaces detected at start-up. Otherwise, the interval can be defined in minutes. The default is 60 minutes.

notify no;

no prevents other name servers from being informed when changes are made to the zone data or when the name server is restarted.

14.6.4. The Configuration Section Logging

What, how, and where logging takes place can be extensively configured in BIND. Normally, the default settings should be sufficient. File 14.12. “Entry to Disable Logging” represents the simplest form of such an entry and completely suppresses any logging.

Example 14.12. Entry to Disable Logging

logging {
        category default { null; };

14.6.5. Zone Entry Structure

Example 14.13. Zone Entry for my-domain.de

zone "my-domain.de" in {
      type master;
      file "my-domain.zone";
      notify no;

After zone, the name of the domain to administer is specified, my-domain.de, followed by in and a block of relevant options enclosed in curly braces, as shown in File 14.13. “Zone Entry for my-domain.de”. To define a slave zone, the type is simply switched to slave and a name server is specified that administers this zone as master (which, in turn, may be a slave of another master), as shown in File 14.14. “Zone Entry for other-domain.de”.

Example 14.14. Zone Entry for other-domain.de

zone "other-domain.de" in {
      type slave;
      file "slave/other-domain.zone";
      masters {; }; 

The zone options:

type master;

By specifying master, tell BIND that the zone is handled by the local name server. This assumes that a zone file has been created in the correct format.

type slave;

This zone is transferred from another name server. Must be used together with masters.

type hint;

The zone . of the hint type is used to set the root name servers. This zone definition can be left as is.

file my-domain.zone or file “slave/other-domain.zone”;

This entry specifies the file where zone data for the domain is located. This file is not required for a slave, as this data is fetched from another name server. To differentiate master and slave files, the directory slave is specified for the slave files.

masters {; };

This entry is only needed for slave zones. It specifies from which name server the zone file should be transferred.

allow-update {! *; };

This option controls external write access, which would allow clients to make a DNS entry — something not normally desirable for security reasons. Without this entry, zone updates are not allowed at all. The above entry achieves the same because ! * effectively bans any such activity.

14.6.6. Structure of Zone Files

Two types of zone files are needed. One serves to assign IP addresses to host names and the other does the reverse — supplies a host name for an IP address.

The . has an important meaning in the zone files. If host names are given without ending with a ., the zone is appended. Thus, complete host names specified with a complete domain must end with a . so the domain is not added to it again. A missing period or one in the wrong place is probably the most frequent cause of name server configuration errors.

The first case to consider is the zone file world.zone, responsible for the domain world.cosmos, shown in File 14.15. “File /var/lib/named/world.zone”.

Example 14.15. File /var/lib/named/world.zone

world.cosmos. IN SOA      gateway  root.world.cosmos. (
            2003072441  ; serial
            1D          ; refresh
            2H          ; retry
            1W          ; expiry
            2D )        ; minimum

            IN NS       gateway
            IN MX       10 sun

gateway     IN A
            IN A
sun         IN A
moon        IN A
earth       IN A
mars        IN A
www         IN CNAME    moon
Line 1:

$TTL defines the default time to live that should apply to all the entries in this file. In this example, entries are valid for a period of two days (2 D).

Line 2:

This is where the SOA control record begins:

  • The name of the domain to administer is world.cosmos in the first position. This ends with a ., because otherwise the zone would be appended a second time. Alternatively, a @ can be entered here, in which case the zone would be extracted from the corresponding entry in /etc/named.conf.

  • After IN SOA is the name of the name server in charge as master for this zone. The name is expanded from gateway to gateway.world.cosmos, because it does not end with a ..

  • An e-mail address of the person in charge of this name server will follow. Because the @ sign already has a special meaning, . is entered here instead, so for root@world.cosmos the entry must read root.world.cosmos.. Again the . sign must be included at the end to prevent the zone from being added.

  • The ( is used to include all lines up to ) into the SOA record.

Line 3:

The serial number is an arbitrary number that is increased each time this file is changed. It is needed to inform the secondary name servers (slave servers) of changes. For this, a ten-digit number of the date and run number, written as YYYYMMDDNN, has become the customary format.

Line 4:

The refresh rate specifies the time interval at which the secondary name servers verify the zone serial number. In this case, one day.

Line 5:

The retry rate specifies the time interval at which a secondary name server, in case of error, attempts to contact the primary server again. Here, two hours.

Line 6:

The expiration time specifies the time frame after which a secondary name server discards the cached data if it has not regained contact to the primary server. Here, it is a week.

Line 7:

The last entry in the SOA record specifies the negative caching TTL — the time for which results of unresolved DNS queries from other servers may be cached.

Line 9:

The IN NS specifies the name server responsible for this domain. gateway is extended to gateway.world.cosmos because it does not end with a .. There can be several lines like this — one for the primary and one for each secondary name server. If notify is not set to no in /etc/named.conf, all the name servers listed here will be informed of the changes made to the zone data.

Line 10:

The MX record specifies the mail server that accepts, processes, and forwards e-mails for the domain world.cosmos. In this example, this is the host sun.world.cosmos. The number in front of the host name is the preference value. If there are multiple MX entries, the mail server with the smallest value is taken first and, if mail delivery to this server fails, an attempt will be made with the next higher value.

Lines 12–17:

These are the actual address records where one or more IP addresses are assigned to host names. The names are listed here without a . because they do not include their domain, so world.cosmos is added to all of them. Two IP addresses are assigned to the host gateway, because it has two network cards. Wherever the host address is a traditional one (IPv4), the record is marked with an A. If the address is an IPv6 address, the entry is marked with A6. (The previous token for IPv6 addresses was AAAA, which is now obsolete.)

Line 18:

The alias www can be used to address mond (CNAME = canonical name).

The pseudodomain in-addr.arpa is used for the reverse lookup of IP addresses into host names. It is appended to the network part of the address in reverse notation. So 192.168.1 is resolved into 1.168.192.in-addr.arpa. See File 14.16. “Reverse Lookup”.

Example 14.16. Reverse Lookup

1.168.192.in-addr.arpa. IN SOA gateway.world.cosmos. root.world.cosmos. (
                        2003072441      ; serial
                        1D              ; refresh
                        2H              ; retry
                        1W              ; expiry
                        2D )            ; minimum

                        IN NS           gateway.world.cosmos.

1                       IN PTR          gateway.world.cosmos.
2                       IN PTR          earth.world.cosmos.
3                       IN PTR          mars.world.cosmos.
Line 1:

$TTL defines the standard TTL that applies to all entries here.

Line 2:

The configuration file is supposed to activate reverse lookup for the network Given that the zone is called 1.168.192.in-addr.arpa, we would not want to add this to the host names. Therefore, all host names are entered in their complete form — with their domain and with a . at the end. The remaining entries correspond to those described for the previous world.cosmos example.

Lines 3–7:

See the previous example for world.cosmos.

Line 9:

Again this line specifies the name server responsible for this zone. This time, however, the name is entered in its complete form with the domain and a . at the end.

Lines 11–13:

These are the pointer records hinting at the IP addresses on the respective hosts. Only the last part of the IP address is entered at the beginning of the line, without the . at the end. Appending the zone to this (without the .in-addr.arpa) results in the complete IP address in reverse order.

Normally, zone transfers between different versions of BIND should be possible without any problem.

14.6.7. Secure Transactions

Secure transactions can be carried out with the help of transaction signatures (TSIGs) based on shared secret keys (also called TSIG keys). This section describes how to generate and use such keys.

Secure transactions are needed for the communication between different servers and for the dynamic update of zone data. Making the access control dependent on keys is much more secure than merely relying on IP addresses.

A TSIG key can be generated with the following command (for details, see man dnssec-keygen):

dnssec-keygen -a hmac-md5 -b 128 -n HOST host1-host2

This creates two files with names similar to these:

Khost1-host2.+157+34265.private Khost1-host2.+157+34265.key

The key itself (a string like ejIkuCyyGJwwuN3xAteKgg==) is found in both files. To use it for transactions, the second file (Khost1-host2.+157+34265.key) must be transferred to the remote host, preferably in a secure way (using scp, for instance). On the remote server, the key must be included in the file /etc/named.conf to enable a secure communication between host1 and host2:

key host1-host2. {
 algorithm hmac-md5;
 secret ";ejIkuCyyGJwwuN3xAteKgg==;

Make sure the permissions of /etc/named.conf are properly restricted. The default for this file is 0640, with the owner being root and the group named. As an alternative, move the keys to an extra file with specially limited permissions, which is then included from /etc/named.conf.

To enable the server host1 to use the key for host2 (which has the address in our example), the server's /etc/named.conf must include the following rule:

server {
  keys { host1-host2. ;};

Analogous entries must be included in the configuration files of host2.

In addition to any ACLs that are defined for IP addresses and address ranges, add TSIG keys for these to enable transaction security. The corresponding entry could look like this:

allow-update { key host1-host2. ;};

This topic is discussed in more detail in the BIND Administrator Reference Manual under update-policy.

14.6.8. Dynamic Update of Zone Data

The term dynamic update refers to operations by which entries in the zone files of a master server are added, changed, or deleted. This mechanism is described in RFC 2136. Dynamic update is configured individually for each zone entry by adding an optional allow-update or update-policy rule. Zones to update dynamically should not be edited by hand.

Transmit the entries to update to the server with the command nsupdate. For the exact syntax of this command, check the manual page for nsupdate(man 8 nsupdate). For security reasons, any such update should be performed using TSIG keys, as described in Section 14.6.7. “Secure Transactions”.

14.6.9. DNSSEC

DNSSEC, or DNS security, is described in RFC 2535. The tools available for DNSSEC are discussed in the BIND Manual.

A zone considered secure must have one or several zone keys associated with it. These are generated with dnssec-keygen, just like the host keys. Currently the DSA encryption algorithm is used to generate these keys. The public keys generated should be included in the corresponding zone file with an $INCLUDE rule.

All keys generated are packaged into one set, using the command dnssec-makekeyset, which must then be transferred to the parent zone in a secure manner. On the parent, the set is signed with dnssec-signkey. The files generated by this command are then used to sign the zones with dnssec-signzone, which in turn generates the files to include for each zone in /etc/named.conf.

14.6.10. For More Information

For additional information, refer to the BIND Administrator Reference Manual, which is installed under /usr/share/doc/packages/bind/. Consider additionally consulting the RFCs referenced by the manual and the manual pages included with BIND.