Standard Resource Record Format for TCP/IP

Purpose

Defines the format of lines in the named data files.

Description

Records in the named files are called resource records. Files using the standard resource record format are:

• DOMAIN data file
• DOMAIN reverse data file
• DOMAIN cache file
• DOMAIN local file

Resource records in the named files have the following general format:

{Name} {TTL} AddressClass RecordType RecordSpecificData

Although case distinctions are kept when loading databases, all queries to the name server database are case insensitive.

The following characters have special meanings:

There are two special types of lines that are not data lines. Instead they specify special processing. These lines are the $INCLUDE and $ORIGIN lines.

$INCLUDE /usr/named/data/mailbox

Resource Record Types

Following is a list of the resource record types used in the named data files:

• Start of authority record
• Name server record
• Address record
• Host information record
• Well-known services record
• Canonical name record
• IN-ADDR.ARPA record
• Domain-name pointer record
• Gateway PTR record
• Mailbox record
• Mail rename name record
• Mailbox information record
• Mail group member record
• Mail exchanger record

Start of Authority Record

The start of authority (SOA) record indicates the start of a zone of authority. There should be only one start of authority record per zone, indicated by a value of SOA in the RecordType field. However, the SOA record for the zone should be in each named.data and named.rev file on each name server in the zone. Its structure corresponds to the following format:

{Name}{TTL} AddressClass RecordType Origin PersonInCharge @ IN SOA merl.century.com jane.merl.century.com (1.1 ;Serial 3600 ;Refresh 600 ;Retry 3600000 ;Expire 86400) ;Minimum

Fields

Name Server Record

The name server record specifies the name server responsible for a given domain. There should be one name server record for each primary server for the domain, indicated by a value of NS in the RecordType field. The name server record can be in the named.data file, the named.rev file, the named.ca file, and the named.local file. Its structure corresponds to the following format:

{Name} {TTL} AddressClass RecordType NameServerName

              IN         NS       arthur.century.com

Fields

Address Record

The address record specifies the address for the host and is indicated by a value of A in the RecordType field. Address records can be entries in the named.ca, named.data, and named.rev files. Its structure corresponds to the following format:

{Name} {TTL} AddressClass RecordType Address

arthur             IN           A      132.10.8.1
                   IN           A       10.0.4.1 

Fields

aus.century.com.      IN      NS   fran.aus.century.com.
fran.aus.century.com. IN      A    192.9.201.14

Host Information Record

The host information (HINFO) record lists host specific information, and is indicated by HINFO in the RecordType field. This lists the hardware and operating system that are running at the specified host. Note that the hardware and operating system information is separated by a single space. There must be one host information record for each host. The HINFO record is a valid entry in the named.data and the named.rev files. Its structure corresponds to the following format:

{Name} {TTL} AddressClass RecordType Hardware OS

Fields

Well-Known Services Record

The well-known services (WKS) record lists the well-known services supported by a particular protocol at a specified address. This record is indicated by WKS in the RecordType field. Although TCP/IP provides the record for backward compatibility, it is now obsolete.

The services and port numbers come from the list of services in the /etc/services file. There should be only one WKS record per protocol per address. The WKS record is a valid entry in the named.data file. Its structure corresponds to the following format:

{Name}{TTL} AddressClass RecordType Address Protocol ListOfServices

              IN      WKS      125.10.0.4   UDP    (who route timed domain)
              IN      WKS      125.10.0.4   TCP    (echo  telnet ftp netstat finger) 

Fields

Canonical Name Record

The canonical name record specifies an alias for a canonical name (CNAME), and is indicated by CNAME in the RecordType field. The CNAME record is the only Resource record that can use the alias of a canonical name. All other resource records must use the full canonical (or domain) name. The CNAME record is a valid entry in the named.data file. For each CNAME record, there must be a corresponding address (A) record. Its structure corresponds to the following format:

{Aliases} {TTL} AddressClass RecordType CanonicalName

knight                 IN             CNAME               lancelot
john                   IN             CNAME               lancelot

Fields

IN-ADDR.ARPA Record

The structure of names in the domain system is set up in a hierarchical fashion. The address of a name can be found by tracing down the domain structure, contacting a server for each label in the name. Because the structure is based on names, there is no easy way to translate a host address back into its host name.

In order to allow simple reverse translation, the IN-ADDR.ARPA domain was created. This domain uses host addresses as part of a name that points to the data for that host. The IN-ADDR.ARPA domain provides an index to the resource records of each host based on its address. There are subdomains within the IN-ADDR.ARPA domain for each network, based on network number. Also, to maintain consistency and natural groupings, the 4 octets of a host number are reversed. The IN-ADDR.ARPA domain is defined by the IN-ADDR.ARPA record in the named.boot files and the DOMAIN hosts data file.

For example, the ARPANET is net 10, which means that there is a domain called 10.in-addr.arpa. Within this domain, there is a PTR resource record at 51.0.0.10.IN-ADDR, which points to the resource records for the host sri-nic.arpa (whose address is 10.0.0.51). Since the NIC is also on the MILNET (net 26, address 26.0.0.73), there is also a PTR resource record at 73.0.0.26.in-addr.arpa that points to the same resource records for SRI-NIC.ARPA. The format of these special pointers is defined in the following section on PTR resource records, along with the examples for the NIC.

Domain-Name Pointer Record

The PTR record is a valid entry in the named.rev file. Its structure corresponds to the following format:

{Aliases} {TTL} AddressClass RecordType RealName

  7.0              IN           PTR        arthur.century.com.

Fields

Gateway PTR Record

The IN-ADDR domain is also used to locate gateways on a particular network. Gateways have the same kind of PTR resource records as hosts, but they also have other PTR records used to locate them by network number alone. These records have 1, 2, or 3 octets as part of the name, depending on whether they are class A, B, or C networks, respectively.

gw.csl.sri.com.       IN     A    10.2.0.2
                      IN     A     128.18.1.1
                      IN     A     192.12.33.2

In addition, this gateway has one of the following pairs of number-to-name translation pointers and gateway location pointers in each of the three different zones (one for each network). In each example, the number-to-name pointer is listed first, followed by the gateway location pointer.

Class A

2.0.2.10.in-addr.arpa.     IN     PTR    gw.csl.sri.com.
10.in-addr.arpa.           IN     PTR    gw.csl.sri.com.

Class B

1.1.18.128.in-addr.arpa.   IN     PTR    gw.csl.sri.com.
18.128.in-addr.arpa.       IN     PTR    gw.csl.sri.com.

Class C

2.33.12.192.in-addr.arpa.  IN     PTR    gw.csl.sri.com.
33.12.192.in-addr.arpa.    IN     PTR    gw.csl.sri.com.

For example, a user named elizabeth used the following resource record to have her mail delivered to host venus.abc.aus.century.com:

elizabeth                 IN    MB     venus.abc.aus.century.com. 

Mailbox Record

The mailbox (MB) record defines the machine where a user wants to receive mail, and is indicated by MB in the RecordType field. The MB record is a valid entry in the named.data file. Its structure corresponds to the following format:

{Aliases} {TTL} AddressClass RecordType Machine

 jane            IN             MB          merlin.century.com

Fields

Mail Rename Name Record

The mail rename (MR) name record allows a user to receive mail addressed to a list of aliases. This record is indicated by MR in the RecordType field. The MR record is a valid entry in the named.data file. Its structure corresponds to the following format:

{Aliases} {TTL} AddressClass RecordType CorrespondingMB

 merlin               IN            MR                   jane

Fields

Mailbox Information Record

The mailbox information (MINFO) record creates a mail group for a mailing list, and is indicated by MINFO in the RecordType field. This record usually has a corresponding mail group record, but may also be used with a mailbox record. The MINFO record is a valid entry in the named.data file. Its structure corresponds to the following format:

{Name} {TTL} AddressClass RecordType Requests Maintainer

postmaster          IN         MINFO      post-request  greg.century.com

Fields

Mail Group Member Record

The mail group member (MG) record lists the members of a mail group. This record is indicated by MG in the RecordType field. The MG record is a valid entry in the named.data file. Its structure corresponds to the following format:

{MailGroupName} {TTL} AddressClass RecordType MemberName

  dept               IN         MG            Tom

Fields

Mail Exchanger Record

The mail exchanger (MX) records identify machines (gateways) that know how to deliver mail to a machine that is not directly connected to the network. This record is indicated by MX in the RecordType field. Wildcard names containing an * (asterisk) can be used for mail routing with MX records. There may be servers on the network that state that any mail to a domain is to be routed through a relay. The MX record is a valid entry in the named.data file. Its structure corresponds to the following format:

{Name} {TTL} AddressClass RecordType PrefValue MailExchanger

Ann.bus.com       IN            MX         0      Hamlet.Century.Com
*.dev.bus.com     IN            MX         0      Lear.Century.Com

Fields

Examples

The following is an example of a mailing list:

dept         IN       MINFO     dept-request jane.merlin.century.com
             IN       MG        greg.arthur.century.com
             IN       MG        tom.lancelot.century.com
             IN       MG        gary.guinevere.century.com
             IN       MG        kent.gawain.century.com