31 The Domain Name System #

To install a DNS server, start YaST and select Software › Software Management. Choose View › Patterns and select DHCP and DNS Server. Confirm the installation of the dependent packages to finish the installation process.

Alternatively use the following command on the command line:

> sudo zypper in -t pattern dhcp_dns_server

Use the YaST DNS module to configure a DNS server for the local network. When starting the module for the first time, a wizard starts, prompting you to make a few decisions concerning administration of the server. Completing this initial setup produces a basic server configuration. Use the expert mode to deal with more advanced configuration tasks, such as setting up ACLs, logging, TSIG keys, and other options.

31.3.2 Expert Configuration #

  

After starting the module, YaST opens a window displaying several configuration options. Completing it results in a DNS server configuration with the basic functions in place:

31.3.2.1 Start-Up #

  

Under Start-Up, define whether the DNS server should be started when the booting the system or manually. To start the DNS server immediately, click Start DNS Server Now. To stop the DNS server, click Stop DNS Server Now. To save the current settings, select Save Settings and Reload DNS Server Now. You can open the DNS port in the firewall with Open Port in Firewall and modify the firewall settings with Firewall Details.

By selecting LDAP Support Active, the zone files are managed by an LDAP database. Any changes to zone data written to the LDAP database are picked up by the DNS server when it is restarted or prompted to reload its configuration.

31.3.2.2 Forwarders #

  

If your local DNS server cannot answer a request, it tries to forward the request to a Forwarder, if configured so. This forwarder may be added manually to the Forwarder List. If the forwarder is not static like in dial-up connections, netconfig handles the configuration. For more information about netconfig, see man 8 netconfig.

31.3.2.3 Basic Options #

  

In this section, set basic server options. From the Option menu, select the desired item then specify the value in the corresponding text box. Include the new entry by selecting Add.

31.3.2.4 Logging #

  

To set what the DNS server should log and how, select Logging. Under Log Type, specify where the DNS server should write the log data. Use the system-wide log by selecting System Log or specify a different file by selecting File. In the latter case, additionally specify a name, the maximum file size in megabytes and the number of log file versions to store.

Further options are available under Additional Logging. Enabling Log All DNS Queries causes every query to be logged, in which case the log file could grow extremely large. For this reason, it is not a good idea to enable this option for other than debugging purposes. To log the data traffic during zone updates between DHCP and DNS server, enable Log Zone Updates. To log the data traffic during a zone transfer from master to slave, enable Log Zone Transfer. See Figure 31.4, “DNS Server: Logging”.

Figure 31.4: DNS Server: Logging #

  

31.3.2.5 ACLs #

  

Use this dialog to define ACLs (access control lists) to enforce access restrictions. After providing a distinct name under Name, specify an IP address (with or without netmask) under Value in the following fashion:

{ 192.168.1/24; }

The syntax of the configuration file requires that the address ends with a semicolon and is put into curly braces.

31.3.2.6 TSIG Keys #

  

The main purpose of TSIGs (transaction signatures) is to secure communications between DHCP and DNS servers. They are described in Section 31.8, “Secure Transactions”.

To generate a TSIG key, enter a distinctive name in the field labeled Key ID and specify the file where the key should be stored (Filename). Confirm your choices with Generate.

To use a previously created key, leave the Key ID field blank and select the file where it is stored under Filename. After that, confirm with Add.

31.3.2.7 DNS Zones (Adding a Slave Zone) #

  

To add a slave zone, select DNS Zones, choose the zone type Slave, write the name of the new zone, and click Add.

In the Zone Editor sub-dialog under Master DNS Server IP, specify the master from which the slave should pull its data. To limit access to the server, select one of the ACLs from the list.

31.3.2.8 DNS Zones (Adding a Master Zone) #

  

To add a master zone, select DNS Zones, choose the zone type Master, write the name of the new zone, and click Add. When adding a master zone, a reverse zone is also needed. For example, when adding the zone example.com that points to hosts in a subnet 192.168.1.0/24, you should also add a reverse zone for the IP-address range covered. By definition, this should be named 1.168.192.in-addr.arpa.

31.3.2.9 DNS Zones (Editing a Master Zone) #

  

To edit a master zone, select DNS Zones, select the master zone from the table, and click Edit. The dialog consists of several pages: Basics (the one opened first), NS Records, MX Records, SOA, and Records.

The basic dialog, shown in Figure 31.5, “DNS Server: Zone Editor (Basics)”, lets you define settings for dynamic DNS and access options for zone transfers to clients and slave name servers. To permit the dynamic updating of zones, select Allow Dynamic Updates and the corresponding TSIG key. The key must have been defined before the update action starts. To enable zone transfers, select the corresponding ACLs. ACLs must have been defined already.

In the Basics dialog, select whether to enable zone transfers. Use the listed ACLs to define who can download zones.

Figure 31.5: DNS Server: Zone Editor (Basics) #

  

Zone Editor (NS Records)

The NS Records dialog allows you to define alternative name servers for the zones specified. Make sure that your own name server is included in the list. To add a record, enter its name under Name Server to Add then confirm with Add. See Figure 31.6, “DNS Server: Zone Editor (NS Records)”.

Figure 31.6: DNS Server: Zone Editor (NS Records) #

  

Zone Editor (MX Records)

To add a mail server for the current zone to the existing list, enter the corresponding address and priority value. After doing so, confirm by selecting Add. See Figure 31.7, “DNS Server: Zone Editor (MX Records)”.

Figure 31.7: DNS Server: Zone Editor (MX Records) #

  

Zone Editor (SOA)

This page allows you to create SOA (start of authority) records. For an explanation of the individual options, refer to Example 31.6, “The /var/lib/named/example.com.zone File”. Changing SOA records is not supported for dynamic zones managed via LDAP.

Figure 31.8: DNS Server: Zone Editor (SOA) #

  

Zone Editor (Records)

This dialog manages name resolution. In Record Key, enter the host name then select its type. The A type represents the main entry. The value for this should be an IP address (IPv4). Use AAAA for IPv6 addresses. CNAME is an alias. Use the types NS and MX for detailed or partial records that expand on the information provided in the NS Records and MX Records tabs. These three types resolve to an existing A record. PTR is for reverse zones. It is the opposite of an A record, for example:

hostname.example.com. IN A 192.168.0.1
1.0.168.192.in-addr.arpa IN PTR hostname.example.com.

31.3.2.9.1 Adding Reverse Zones #

  

To add a reverse zone, follow this procedure:

1. Start YaST › DNS Server › DNS Zones.

2. If you have not added a master forward zone, add it and Edit it.

3. In the Records tab, fill the corresponding Record Key and Value, then add the record with Add and confirm with OK. If YaST complains about a non-existing record for a name server, add it in the NS Records tab.

   

   Figure 31.9: Adding a Record for a Master Zone #

     

4. Back in the DNS Zones window, add a reverse master zone.

   

   Figure 31.10: Adding a Reverse Zone #

     

5. Edit the reverse zone, and in the Records tab, you can see the PTR: Reverse translation record type. Add the corresponding Record Key and Value, then click Add and confirm with OK.

   

   Figure 31.11: Adding a Reverse Record #

     

   Add a name server record if needed.

Tip: Editing the Reverse Zone

After adding a forward zone, go back to the main menu and select the reverse zone for editing. There in the tab Basics activate the check box Automatically Generate Records From and select your forward zone. That way, all changes to the forward zone are automatically updated in the reverse zone.

On a SUSE® Linux Enterprise Server system, the name server BIND (Berkeley Internet Name Domain) comes preconfigured, so it can be started right after installation without any problems. Normally, if you already have an Internet connection and entered 127.0.0.1 as the name server address for localhost in /var/run/netconfig/resolv.conf, you already have a working name resolution without needing to know the DNS of the provider. BIND carries out 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 it becomes a proper DNS. Find a simple example documented in /usr/share/doc/packages/bind/config.

Tip: Automatic Adaptation of the Name Server Information

Depending on the type of Internet connection or the network connection, the name server information can automatically be adapted to the current conditions. To do this, set the NETCONFIG_DNS_POLICY variable in the /etc/sysconfig/network/config file to auto.

However, do not set up an official domain until one is assigned to you by the responsible institution. Even if you have your own domain and it is managed by the provider, you are better off not using it, because BIND would otherwise not forward requests for this domain. The Web server at the provider, for example, would not be accessible for this domain.

To start the name server, enter the command systemctl start named as root. Check with systemctl status named whether 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 127.0.0.1. If this is not the case, /var/run/netconfig/resolv.conf probably contains an incorrect name server entry or the file does not exist. For the first test, enter host 127.0.0.1, which should always work. If you get an error message, use systemctl status named to see whether the server is actually running. If the name server does not start or behaves unexpectedly, check the output of journalctl -e.

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 Example 31.1, “Forwarding Options in named.conf” are examples only. Adjust these entries to your own setup.

options {
        directory "/var/lib/named";
        forwarders { 10.11.12.13; 10.11.12.14; };
        listen-on { 127.0.0.1; 192.168.1.116; };
        allow-query { 127/8; 192.168/16 };
        notify no;
};

The options entry is followed by entries for the zone, localhost, and 0.0.127.in-addr.arpa. The type hint entry under “.” should always be present. The corresponding files do not need to be modified and should work as they are. 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 systemctl reload named. Achieve the same by stopping and restarting the name server with systemctl restart named. Stop the server at any time by entering systemctl stop named.

All the settings for the BIND name server itself are stored in the /etc/named.conf file. 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 later.

/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 and acl (access control list) entries are optional. Comment lines begin with a # sign or //. A minimal /etc/named.conf is shown in Example 31.2, “A Basic /etc/named.conf”.

options {
        directory "/var/lib/named";
        forwarders { 10.0.0.1; };
        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";
};

logging {
        category default { null; };
};

zone "example.com" in {
      type master;
      file "example.com.zone";
      notify no;
};

zone "example.net" in {
      type slave;
      file "slave/example.net.zone";
      masters { 10.0.0.1; }; 
};

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

Tip: Using the Dot (Period, Fullstop) in Zone Files

The "." has an important meaning in the zone files. If host names are given without a final dot (.), the zone is appended. Complete host names specified with a full domain name must end with a dot (.) to avoid having the domain added to it again. A missing or wrongly placed "." is probably the most frequent cause of name server configuration errors.

The first case to consider is the zone file example.com.zone, responsible for the domain example.com, shown in Example 31.6, “The /var/lib/named/example.com.zone File”.

$TTL 2D 1
example.com. IN SOA      dns  root.example.com. ( 2
             2003072441  ; serial 3
             1D          ; refresh 4
             2H          ; retry 5
             1W          ; expiry 6
             2D )        ; minimum 7

             IN NS       dns 8
             IN MX       10 mail dns 9
gate         IN A        192.168.5.1 10
             IN A        10.0.0.1
dns          IN A        192.168.1.116
mail         IN A        192.168.3.108
jupiter      IN A        192.168.2.100
venus        IN A        192.168.2.101
saturn       IN A        192.168.2.102
mercury      IN A        192.168.2.103
ntp          IN CNAME    dns 11
dns6         IN A6  0    2002:c0a8:174::

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).
2	This is where the SOA (start of authority) control record begins: The name of the domain to administer is example.com in the first position. This ends with ".", because otherwise the zone would be appended a second time. Alternatively, @ 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 dns to dns.example.com, because it does not end with a ".". An e-mail address of the person in charge of this name server follows. Because the @ sign already has a special meaning, "." is entered here instead. For root@example.com the entry must read root.example.com.. The "." must be included at the end to prevent the zone from being added. The ( includes all lines up to ) into the SOA record.
3	The serial number is a 10 digit number. It must be changed each time this file is changed. It is needed to inform the secondary name servers (slave servers) of changes. For this, a 10 digit number of the date and run number, written as YYYYMMDDNN, has become the customary format (YYYY = year, MM = month and DD = day. NN is a sequence number in case you update it more than once on the given day).
4	The refresh rate specifies the time interval at which the secondary name servers verify the zone serial number. In this case, one day.
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.
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, a week.
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.
8	The IN NS specifies the name server responsible for this domain. dns is extended to dns.example.com 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 are informed of the changes made to the zone data.
9	The MX record specifies the mail server that accepts, processes, and forwards e-mails for the domain example.com. In this example, this is the host mail.example.com. 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. If mail delivery to this server fails, the entry with the next-smallest value is used.
10	This and the following lines 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 example.com is added to all of them. Two IP addresses are assigned to the host gate, as it has two network cards. Wherever the host address is a traditional one (IPv4), the record is marked with A. If the address is an IPv6 address, the entry is marked with AAAA. Note: IPv6 Syntax The IPv6 record has a slightly different syntax than IPv4. Because of the fragmentation possibility, it is necessary to provide information about missed bits before the address. To fill up the IPv6 address with the needed number of “0”, add two colons at the correct place in the address. pluto AAAA 2345:00C1:CA11::1234:5678:9ABC:DEF0 pluto AAAA 2345:00D2:DA11::1234:5678:9ABC:DEF0
11	The alias ntp can be used to address dns (CNAME means canonical name).

The pseudo domain 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 is resolved into 168.192.in-addr.arpa. See Example 31.7, “Reverse Lookup”.

$TTL 2D 1
168.192.in-addr.arpa.   IN SOA dns.example.com. root.example.com. ( 2
                        2003072441      ; serial
                        1D              ; refresh
                        2H              ; retry
                        1W              ; expiry
                        2D )            ; minimum

                        IN NS           dns.example.com. 3

1.5                     IN PTR          gate.example.com. 4
100.3                   IN PTR          www.example.com.
253.2                   IN PTR          cups.example.com.

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

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 31.8, “Secure Transactions”.

Secure transactions can be made with 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 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.

Generate a TSIG key with the following command (for details, see man tsig-keygen):

> sudo tsig-keygen -a hmac-md5 host1-host2 > host1-host2.key

This creates a file with the name host1-host2.key with contents that may look as follows:

key "host1-host2" {                       |
    algorithm hmac-md5;
    secret "oHpBLgtcZso6wxnRTWdJMA==";
};

The file must be transferred to the remote host, preferably in a secure way (using scp, for example). To enable a secure communication between host1 and host2, the key must be included in the /etc/named.conf file on both the local and the remote server.

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

Warning: File Permissions of /etc/named.conf

Make sure that 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 include an external file, use:

include  "filename"

Replace filename with an absolute path to your file with keys.

To enable the server host1 to use the key for host2 (which has the address 10.1.2.3 in this example), the server's /etc/named.conf must include the following rule:

server 10.1.2.3 {
  keys { host1-host2. ;};
};

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

Add TSIG keys for any ACLs (access control lists, not to be confused with file system ACLs) that are defined for IP addresses and address ranges 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.

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, as are the host keys. The DSA encryption algorithm is currently used to generate these keys. The public keys generated should be included in the corresponding zone file with an $INCLUDE rule.

With the command dnssec-signzone, you can create sets of generated keys (keyset- files), transfer them to the parent zone in a secure manner, and sign them. This generates the files to include for each zone in /etc/named.conf.

For more information, see the BIND Administrator Reference Manual from the bind-doc package, which is installed under /usr/share/doc/packages/bind/arm. Consider additionally consulting the RFCs referenced by the manual and the manual pages included with BIND. /usr/share/doc/packages/bind/README.SUSE contains up-to-date information about BIND in SUSE Linux Enterprise Server.