Data File Resource Record Format

All the data files used by the DNS daemon in.named are written in standard resource record format. Each DNS data file must contain certain resource records. This section describes the DNS data files and the resource records each file should contain.

Standard Resource Record Format

In the standard resource record format, each line of a data file is called a resource record (RR), which contains the following fields separated by white space:

namettlclassrecord-typerecord-specific-data

The order of the fields is always the same; however, the first two are optional (as indicated by the brackets), and the contents of the last vary according to the record-type field.

The name Field

The first field is the name of the domain that applies to the record. If this field is left blank in a given RR, it defaults to the name of the previous RR.

A domain name in a zone file can be either a fully qualified name, terminated with a dot, or a relative name, in which case the current domain is appended to it.

The ttl Field

The second field is an optional time-to-live field. This specifies how long (in seconds) this data will be cached in the database before it is disregarded and new information is requested from a server. By leaving this field blank, the ttl defaults to the minimum time specified in the Start-Of-Authority (SOA) resource record.

If the ttl value is set too low, the server will incur a lot of repeat requests for data refreshment; if, on the other hand, the ttl value is set too high, changes in the information will not be timely distributed.

Most ttl values should be initially set to between a day (86400) and a week (604800). Then, depending on the frequency of actual change of the information, you can change the appropriate ttl values to reflect that frequency. Also, if you have some ttl values that have very high numbers because you know they relate to data that rarely changes. When you know that the data is now about to change, reset the ttl to a low value (3600 to 86400) until the change takes place. Then change it back to the original high value.

All RR's with the same name, class, and type should have the same ttl value.

The class Field

The third field is the record class. Only one class is currently in use: IN for the TCP/IP protocol family.

The record-type Field

The fourth field states the resource record type. There are many types of RR's; the most commonly used types are discussed in Resource Record Types.

The record-specific-data Field

The contents of the record-specific-data field depend on the type of the particular resource record.

Although case is preserved in names and data fields when loaded into the name server, all comparisons and lookups in the name server database are case insensitive. However, this situation might change in the future; thus, you should be consistent in your use of lower and uppercase.

Special Resource Record Characters

The following characters have special meanings:

Most resource records have the current origin appended to names if they are not terminated by a dot (.) This is useful for appending the current domain name to the data, such as machine names, but might cause problems when you do not want this to happen. You should use a fully qualified name ending in a period if the name is not in the domain for which you are creating the data file.

Control Entries

The only lines that do not conform to the standard RR format in a data file are control-entry lines. There are two kinds of control entries: $INCLUDE() and $ORIGIN().

$INCLUDE

An include line begins with $INCLUDE in column 1, and is followed by a file name (known as the $INCLUDE file). This feature is particularly useful for separating different types of data into multiple files as in this example:

$INCLUDE /etc/named/data/mailboxes

The line is interpreted as a request to load the /etc/named/data/mailboxes file at that point. The $INCLUDE command does not cause data to be loaded into a different zone or tree. The command allows for data for a given zone to be organized in separate files. For example, mailbox data might be kept separately from host data using this mechanism.

Use of $INCLUDE statements and files is optional. You can use as many as you wish, or none at all.

$ORIGIN()

The $ORIGIN command is a way of changing the origin in a data file. The line starts in column 1, and is followed by a domain name. It resets the current origin for relative domain names (for example, not fully qualified names) to the stated name. This is useful for putting more than one domain in a data file.

You cannot use $ORIGIN() for putting more than one zone in a data file.

Use of $ORIGIN commands in a data file is optional. If there is no $ORIGIN() statement the default origin for DNS data files is the domain named in the second field of the master or slave line of the named.conf file.

Resource Record Types

The most commonly used types of resource records are listed in Table 5–7. They are usually entered in the order shown in Table 5–7, but that is not a requirement.

Start-of-Authority record (SOA)

Example 5–19 shows the syntax of a start-of-authority (SOA) resource record.

Example 5–19 SOA Record Format

name class SOA origin person-in-charge ( serial number
	 refresh
 retry
 expire
 ttl)		 

The SOA record designates the start of a zone. The zone ends at the next SOA record. The SOA record fields are described below.

name

This field indicates the name of the zone. Note that the zone name must end with a trailing dot. For example: doc.com. is correct, while doc.com is wrong.

class

This field is the address class. For example, IN for Internet (the most commonly used class).

SOA

This field is the type of this resource record.

origin

This field is the name of the host where this data file resides. Note that this host name must end in a trailing dot. For example, dnsmaster.doc.com. is correct, but dnsmaster.doc.com is wrong.

person-in-charge

This field is the email address of the person responsible for the name server. For example, kjd.nismaster.doc.com. Again, this name must end with a trailing dot.

serial

This field is the version number of this data file. You must increment this number whenever you make a change to the data: slave servers use the serial field to detect whether the data file has been changed since the last time they copied the file from the master server.

refresh

This field indicates how often, in seconds, a slave name server should check with the master name server to see if an update is needed. For example, 7200 indicates a period of two hours.

retry

This field indicates how long, in seconds, a slave server is to retry after a failure to check for a refresh.

expire

This field is the upper limit, in seconds, that a slave name server is to use the data before it expires for lack of getting a refresh.

ttl

This field is the default number of seconds to be used for the time-to-live field on resource records that do not have a ttl specified elsewhere.

There should only be one SOA record per zone. Example 5–20 is a sample SOA resource record.

Example 5–20 Sample SOA Resource Record

;name class 		SOA 	origin				 person-in-charge
doc.com. IN		SOA	dnsmaster.doc.com. root.nismaster.doc.com. (
							101			;Serial
							7200		;Refresh
							3600		;Retry
							432000		;Expire	
							86400)		;Minimum			 )

Name Server (NS)

Example 5–21 shows the syntax of a name-server (NS) resource record:

Example 5–21 NS Record Format

domainname [optional TTL] class NS name-server-name

The name-server record lists by name a server responsible for a given domain. The name field lists the domain that is serviced by the listed name server. If no name field is listed, then it defaults to the last name listed. One NS record should exist for each master and slave server for the domain. Example 5–22 is a sample NS resource record.

Example 5–22 Sample NS Resource Record

;domainname    [TTL] 	 class	NS	nameserver	
doc.com        90000     IN	NS 	sirius.doc.com.

Address (A)

Example 5–23 shows the syntax of an address (A) resource record:

Example 5–23 Address Record Format

	
machinename	[optional TTL] class A	address

The A record lists the address for a given machine. The name field is the host name, and the address is the IP address. One A record should exist for each address of the machine (in other words, routers, or gateways require at least two entries, a separate entry including the IP address assigned to each network interface).

Example 5–24 Sample Address Record

;machinename	[TTL]	class	A	address
sirius		IN		A	123.45.6.1

Host Information (HINFO)

Example 5–25 shows the syntax of a host-information (HINFO) resource record:

Example 5–25 HINFO Record Format

[optional name] [optional TTL] 	class	HINFO hardware	OS

The HINFO contains host-specific data. It lists the hardware and operating environment that are running at the listed host. If you want to include a space in the machine name or in the entry in the hardware field, you must surround the entry with quotes. The name field specifies the name of the host. If no name is specified, it defaults to the last in.named host. One HINFO record should exist for each host. Example 5–26 is a sample HINFO resource record.

Example 5–26 Sample HINFO Resource Record

;[name]   [TTL] class HINFO   hardware    OS
                IN    HINFO   Sparc-10    UNIX

Because the HINFO field provides information about the machines on your network, many sites consider it a security risk and no longer use it.

Well-Known Services (WKS)

Example 5–27 shows the syntax of a well-known services (WKS) resource record:

Example 5–27 WKS Record Format

[Optional name] [TTL] class WKS address protocol-list-of-services

The WKS record describes the well-known services supported by a particular protocol at a specified address. The list of services and port numbers come from the list of services specified in the services database. Only one WKS record should exist per protocol per address. Example 5–28 is an example of a WKS resource record.

Example 5–28 Sample WKS Resource Record

;[name]	[TTL]	class	WKS	address 	 protocol-list-of-services
altair		IN	WKS	123.45.6.1	 TCP ( smtp discard rpc
 sftp uucp-path systat daytime
 netstat qotd nntp doc.com )

The WKS record is optional. For security reasons, most sites no longer provide this information.

Canonical Name (CNAME)

Example 5–29 shows the syntax of a canonical-name (CNAME) resource record.

Example 5–29 CNAME Record Format

 nickname [optional TTL] class CNAME	canonical-name

The CNAME specifies a nickname or alias for a canonical name. A nickname should be unique. All other resource records should be associated with the canonical name and not with the nickname. Do not create a nickname and then use it in other resource records. Nicknames are particularly useful during a transition period, when a machine's name has changed but you want to permit people using the old name to reach the machine. Nicknames can also be used to identify machines that serve some specific purpose such as a mail server. Example 5–30 is a sample CNAME resource record.

Example 5–30 Sample CNAME Resource Record

;nickname      [TTL]	class	CNAME	canonical-name
mailhost		IN	CNAME	antares.doc.com

Pointer Record (PTR)

Example 5–31 shows the syntax for a PTR resource record.

Example 5–31 PTR Record Format

 special-name	[optional TTL]  class	 PTR-real-name

A pointer record allows special names to point to some other location in the domain. In the example, PTRs are used mainly in the in-addr.arpa. records for the translation of an address (the special name) to a real name. When translating an address, if the domain is fully qualified only the machine identification number need be specified. PTR names should be unique to the zone. The PTR records Example 5–32 sets up reverse pointers for the special in-addr.arpa domain.

Example 5–32 Sample PTR Resource Record

;special name   [TTL]	class	PTR-real-name
1			IN	PTR sirius.doc.com.

Mail Exchanger (MX)

Example 5–33 shows the syntax for a mail-exchanger (MX) resource record.

Example 5–33 MX Record Format

name [optional TTL] class	MX preference-value mailer-exchanger

The MX resource records are used to specify a machine that knows how to deliver mail to a domain or specific machines in a domain. There might be more than one MX resource record for a given name. In Example 5–34, Seismo.CSS.GOV. (note the fully qualified domain name) is a mail gateway that knows how to deliver mail to Munnari.OZ.AU. Other machines on the network cannot deliver mail directly to Munnari. Seismo and Munnari might have a private connection or use a different transport medium. The preference-value field indicates the order a mailer should follow when there is more than one way to deliver mail to a single machine. The value 0 (zero) indicates the highest preference. If there is more than one MX resource record for the same name, records might or might not have the same preference value.

You can use names with the wildcard asterisk (*) for mail routing with MX records. There are likely to be servers on the network that state that any mail to a domain is to be routed through a relay. In Example 5–34, all mail to hosts in domain foo.com is routed through RELAY.CS.NET. You do this by creating a wildcard resource record, which states that the mail exchanger for *.foo.com is RELAY.CS.NET. The asterisk will match any host or subdomain of foo.com, but it will not match foo.com itself.

Example 5–34 Sample MX Resource Record

;name	 	[TTL]	class		MX	 preference mailer-exchanger
Munnari.OZ.AU.		IN		MX	 0	Seismo.CSS.GOV.
foo.com.		IN		MX	 10	RELAY.CS.NET.
*.foo.com.		IN		MX	 20	RELAY.CS.NET.