Configuring the Name Service

About DNS and BIND

The Domain Name System (DNS) is a network-based service that maps (resolves) domain names to IP addresses. For a small, isolated network, you could use entries in the /etc/hosts file to provide the mapping, but most networks that are connected to the Internet use DNS.

DNS is a hierarchical and distributed database, where each level of the hierarchy is delimited by a period (.). Consider the following fully qualified domain name (FQDN):

wiki.us.mydom.com.

The root domain, represented by the final period in the FQDN, is usually omitted, except in DNS configuration files:

wiki.us.mydom.com

In this example, the top-level domain is com, mydom is a subdomain of com, us is a subdomain of mydom, and wiki is the host name. Each of these domains are grouped into zones for administrative purposes. A DNS server, or name server, stores the information that is needed to resolve the component domains inside a zone. In addition, a zone's DNS server stores pointers to the DNS servers that are responsible for resolving each subdomain.

If a client outside the us.mydom.com domain requests that its local name server resolve a FQDN such as wiki.us.mydom.com into an IP address for which the name server is not authoritative, the name server queries a root name server for the address of a name server that is authoritative for the com domain. Querying this name server returns the IP address of a name server for mydom.com. In turn, querying this name server returns the IP address of the name server for us.oracle.com, and querying this final name server returns the IP address for the FQDN. This process is known as a recursive query, where the local name server handles each referral from an external name server to another name server on behalf of the resolver.

Iterative queries rely on the resolver being able to handle the referral from each external name server to trace the name server that is authoritative for the FQDN. Most resolvers use recursive queries and so cannot use name servers that support only iterative queries. Fortunately, most

Oracle Linux provides the Berkeley Internet Name Domain (BIND) implementation of DNS. The bind package includes the DNS server daemon (named), tools for working with DNS such as rndc, and a number of configuration files, including the following:

/etc/named.conf: Contains settings for named and lists the location and characteristics of the zone files for your domain. Zone files are usually stored in /var/named.
/etc/named.rfc1912.zones: Contains several zone sections for resolving local loopback names and addresses.
/var/named/named.ca: Contains a list of the root authoritative DNS servers.

About Types of Name Servers

You can configure several types of name server using BIND, including:

Master name server: Authoritative for one or more domains, a primary (master) name server maintains its zone data in several database files, and can transfer this information periodically to any backup (slave) name servers that are also configured in the zone. An organization might maintain two primary name servers for a zone: one primary server outside the firewall to provide restricted information about the zone for publicly accessible hosts and services, and a hidden or stealth primary server inside the firewall that holds details of internal hosts and services.
Slave name server: Acting as a backup to a primary name server, a backup name server maintains a copy of the zone data, which it periodically refreshes from the primary server's copy.
Stub name server: A primary name server for a zone might also be configured as a stub name server that maintains information about the primary and backup name servers of child zones.
Caching-only name server: Performs queries on behalf of a client and stores the responses in a cache after returning the results to the client. It is not authoritative for any domains and the information that it records is limited to the results of queries that it has cached.
Forwarding name server: Forwards all queries to another name server and caches the results, which reduces local processing, external access, and network traffic.

In practice, a name server can be a combination of several of these types in complex configurations.

About DNS Configuration Files

Domains are grouped into zones and zones are configured through the use of zone files. Zone files store information about domains in the DNS database. Each zone file contains directives and resource records. Optional directives apply settings to a zone or instruct a name server to perform certain tasks. Resource records specify zone parameters and define information about the systems (hosts) in a zone.

For examples of BIND configuration files, see /usr/share/doc/bind-version/sample/ .

/etc/named.conf

The main configuration file for named is /etc/named.conf, which contains settings for named and the top-level definitions for zones, for example:

include "/etc/rndc.key";

controls {
    inet 127.0.0.1 allow { localhost; } keys { "rndc-key"; }
};

zone "us.mydom.com" {
    type master;
    file "master-data";
    allow-update { key "rndc-key"; };
    notify yes;
};

zone "mydom.com" IN {
    type slave;
    file "sec/slave-data";
    allow-update { key "rndc-key"; };
    masters {10.1.32.1;};
};

zone "2.168.192.in-addr.arpa" IN {
    type master;
    file "reverse-192.168.2";
    allow-update { key “rndc-key”; };
    notify yes;
};

The include statement allows external files to be referenced so that potentially sensitive data such as key hashes can be placed in a separate file with restricted permissions.

The controls statement defines access information and the security requirements that are necessary to use the rndc command with the named server:

inet: Specifies which hosts can run rndc to control named. In this example, rndc must be run on the local host (127.0.0.1).
keys: Specifies the names of the keys that can be used. The example specifies using the key named rndc-key, which is defined in /etc/rndc.key. Keys authenticate various actions by named and are the primary method of controlling remote access and administration.

The zone statements define the role of the server in different zones.

The following zone options are used:

type

Specifies that this system is the primary name server for the zone us.mydom.com and a backup server for mydom.com. 2.168.192.in-addr.arpa is a reverse zone for resolving IP addresses to host names. See About Resource Records for Reverse-name Resolution.

file

Specifies the path to the zone file relative to /var/named. The zone file for us.mydom.com is stored in /var/named/master-data and the transferred zone data for mydom.com is cached in /var/named/sec/slave-data.

allow-update

Specifies that a shared key must exist on both the primary and backup name servers for a zone transfer to take place from the primary server to the backup. The following is an example record for a key in /etc/rndc.key:

key "rndc-key" {
    algorithm hmac-md5;
    secret "XQX8NmM41+RfbbSdcqOejg==";
};

You can use the rndc-confgen -a command to generate a key file.

notify

Specifies whether to notify the backup name servers when the zone information is updated.

masters

Specifies the primary name server for a backup name server.

The next example is taken from the default /etc/named.conf file that is installed with the bind package, and which configures a caching-only name server.

options {
    listen-on port 53 { 127.0.0.1; };
    listen-on-v6 port 53 { ::1; };
    directory       "/var/named";
    dump-file       "/var/named/data/cache_dump.db";
    statistics-file "/var/named/data/named_stats.txt";
    memstatistics-file "/var/named/data/named_mem_stats.txt";
    allow-query { localnets; };
    recursion yes;

    dnssec-enable yes;
    dnssec-validation yes;
    dnssec-lookaside auto;

    /* Path to ISC DLV key */
    bindkeys-file "/etc/named.iscdlv.key";

    managed-keys-directory "/var/named/dynamic";
};

logging {
    channel default_debug {
        file "data/named.run";
        severity dynamic;
    };
};

zone "." IN {
    type hint;
    file "named.ca";
};

include "/etc/named.rfc1912.zones";
include "/etc/named.root.key";

The options statement defines global server configuration options and sets defaults for other statements.

listen-on: The port on which named listens for queries.
directory: Specifies the default directory for zone files if a relative pathname is specified.
dump-file: Specifies where named dumps its cache if it crashes.
statistics-file: Specifies the output file for the rndc stats command.
memstatistics-file: Specifies the output file for named memory-usage statistics.
allow-query: Specifies which IP addresses may query the server. localnets specifies all locally attached networks.
recursion: Specifies whether the name server performs recursive queries.
dnssec-enable: Specifies whether to use secure DNS (DNSSEC).
dnssec-validation: Whether the name server should validate replies from DNSSEC-enabled zones.
dnssec-lookaside: Whether to enable DNSSEC Lookaside Validation (DLV) using the key in /etc/named.iscdlv.key defined by bindkeys-file.

The logging section enables logging of messages to /var/named/data/named.run. The severity parameter controls the logging level, and the dynamic value means that this level can be controlled by using the rndc trace command.

The zone section specifies the initial set of root servers using a hint zone. This zone specifies that named should consult /var/named/named.ca for the IP addresses of authoritative servers for the root domain (.).

For more information, see the named.conf(5) manual page and the BIND documentation in /usr/share/doc/bind-version/arm.

About Resource Records in Zone Files

A resource record in a zone file contains the following fields, some of which are optional depending on the record type:

Name

Domain name or IP address.

TTL (time to live)

The maximum time that a name server caches a record before it checks whether a newer one is available.

Class

Always IN for Internet.

Type

Type of record, for example:

A (address): IPv4 address corresponding to a host.
AAAA (address): IPv6 address corresponding to a host.
CNAME (canonical name): Alias name corresponding to a host name.
MX (mail exchange): Destination for email addressed to the domain.
NS (name server): Fully qualified domain name of an authoritative name server for a domain.
PTR (pointer): Host name corresponding to an IP address for address to name lookups (reverse-name resolution).
SOA (start of authority): Authoritative information about a zone, such as the primary name server, the email address of the domain's administrator, and the domain's serial number. All records following a SOA record relate to the zone that it defines up to the next SOA record.

Data

The information that the record stores, such as an IP address in an A record, or a host name in a CNAME or PTR record.

The following example shows the contents of a typical zone file such as /var/named/master-data:

$TTL 86400        ; 1 day
@ IN SOA dns.us.mydom.com. root.us.mydom.com. (
            57 ; serial
            28800 ; refresh (8 hours)
            7200 ; retry (2 hours)
            2419200 ; expire (4 weeks)
            86400 ; minimum (1 day)
            )
              IN  NS      dns.us.mydom.com.

dns           IN  A       192.168.2.1
us.mydom.com  IN  A       192.168.2.1
svr01         IN  A       192.168.2.2
www           IN  CNAME   svr01
host01        IN  A       192.168.2.101
host02        IN  A       192.168.2.102
host03        IN  A       192.168.2.103
...

A comment on a line is preceded by a semicolon (;).

The $TTL directive defines the default time-to-live value for all resource records in the zone. Each resource record can define its own time-to-live value, which overrides the global setting.

The SOA record is mandatory and included the following information:

us.mydom.com: The name of the domain.
dns.us.mydom.com.: The fully qualified domain name of the name server, including a trailing period (.) for the root domain.
root.us.mydom.com.: The email address of the domain administrator.
serial: A counter that, if incremented, tells named to reload the zone file.
refresh: The time after which a primary name server notifies backup name servers that they should refresh their database.
retry: If a refresh fails, the time that a backup name server should wait before attempting another refresh.
expire: The maximum elapsed time that a backup name server has to complete a refresh before its zone records are no longer considered authoritative and it will stop answering queries.
minimum: The minimum time for which other servers should cache information obtained from this zone.

An NS record declares an authoritative name server for the domain.

Each A record specifies the IP address that corresponds to a host name in the domain.

The CNAME record creates the alias www for svr01.

For more information, see the BIND documentation in /usr/share/doc/bind-version/arm.

About Resource Records for Reverse-name Resolution

Forward resolution returns an IP address for a specified domain name. Reverse-name resolution returns a domain name for a specified IP address. DNS implements reverse-name resolution by using the special in-addr.arpa and ip6.arpa domains for IPv4 and IPv6.

The characteristics for a zone's in-addr.arpa or ip6.arpa domains are usually defined in /etc/named.conf, for example:

zone "2.168.192.in-addr.arpa" IN {
    type master;
    file "reverse-192.168.2";
    allow-update { key “rndc-key”; };
    notify yes;
};

The zone's name consists of in-addr.arpa preceded by the network portion of the IP address for the domain with its dotted quads written in reverse order.

If your network does not have a prefix length that is a multiple of 8, see RFC 2317 for the format that you should use instead.

The PTR records in in-addr.arpa or ip6.arpa domains define host names that correspond to the host portion of the IP address. The following example is taken from the /var/named/reverse-192.168.2 zone file:

$TTL 86400        ;
@ IN SOA dns.us.mydom.com. root.us.mydom.com. (
            57 ;
            28800 ;
            7200 ;
            2419200 ;
            86400 ;
            )
              IN  NS      dns.us.mydom.com.

1             IN  PTR     dns.us.mydom.com.
1             IN  PTR     us.mydom.com.
2             IN  PTR     svr01.us.mydom.com.
101           IN  PTR     host01.us.mydom.com.
102           IN  PTR     host02.us.mydom.com.
103           IN  PTR     host03.us.mydom.com.
...

For more information, see the BIND documentation in /usr/share/doc/bind-version/arm.

Configuring a Name Server

By default, the BIND installation allows you to configure a caching-only name server using the configuration settings that are provided in /etc/named.conf and files that it includes. This procedure assumes that you will either use the default settings or configure new named configuration and zone files.

To configure a name server:

Install the bind package:
```
sudo yum install bind
```
If NetworkManager is enabled on the system, edit the /etc/sysconfig/network-scripts/ifcfg-interface file, and add the following entry:
```
DNS1=127.0.0.1
```
This line causes NetworkManager to add the following entry to /etc/resolv.conf when the network service starts:
```
nameserver 127.0.0.1
```
This entry points the resolver at the local name server.

If you have disabled NetworkManager, edit /etc/resolv.conf to include the nameserver 127.0.0.1 entry.
If required, modify the named configuration and zone files.

Configure the system firewall to allow incoming TCP connections to port 53 and incoming UDP datagrams on port 53:

sudo firewall-cmd --zone=zone --add-port=53/tcp --add-port=53/udp
sudo firewall-cmd --permanent --zone=zone --add-port=53/tcp --add-port=53/udp

Restart the network service, restart the named service, and configure named to start following system reboots:
```
sudo systemctl restart network
sudo systemctl start named
sudo systemctl enable named
```

Administering the Name Service

The rndc command allows you to administer the named service, either locally or from a remote machine (if permitted in the controls section of the /etc/named.conf file). To prevent unauthorized access to the service, rndc must be configured to listen on the selected port (by default, port 953), and both named and rndc must have access to the same key. To generate a suitable key, use the rndc-confgen command:

# rndc-confgen -a
wrote key file "/etc/rndc.key"

To ensure that only root can read the file:

# chmod o-rwx /etc/rndc.key

To check the status of the named service:

# rndc status
number of zones: 3
debug level: 0
xfers running: 0
xfers deferred: 0
soa queries in progress: 0
query logging is OFF
recursive clients: 0/1000
tcp clients: 0/100
server is up and running

If you modify the named configuration file or zone files, rndc reload instructs named to reload the files:

# rndc reload
server reload successful

For more information, see the named(8), rndc(8) and rndc-confgen(8) manual pages.

Performing DNS Lookups

The host utility is recommended for performing DNS lookups. Without any arguments, host displays a summary of its command-line arguments and options. For example, look up the IP address for host01:

host host01

Perform a reverse lookup for the domain name that corresponds to an IP address:

host 192.168.2.101

Query DNS for the IP address that corresponds to a domain:

host dns.us.mydoc.com

Use the -v and -t options to display verbose information about records of a certain type:

host -v -t MX  www.mydom.com

Trying "www.mydom.com"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49643
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;www.mydom.com.			IN	MX

;; ANSWER SECTION:
www.mydom.com.		135	IN	CNAME	www.mydom.com.acme.net.
www.mydom.com.acme.net. 1240 IN	CNAME	d4077.c.miscacme.net.

;; AUTHORITY SECTION:
c.miscacme.net.	2000	IN	SOA	m0e.miscacme.net. hostmaster.misc.com. ...

Received 163 bytes from 10.0.0.1#53 in 40 ms

The -a option (equivalent to -v -t ANY) displays all available records for a zone:

host -a www.us.mydom.com

Trying "www.us.mydom.com"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40030
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.us.mydom.com.			IN	ANY

;; ANSWER SECTION:
www.us.mydom.com.		263	IN	CNAME	www.us.mydom.acme.net.

Received 72 bytes from 10.0.0.1#53 in 32 ms

For more information, see the host(1) manual page.