PTR resource record

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PTR (short for 'Pointer') resource records assign one or more host names to a given IP address in the Domain Name System . In a sense, they represent the counterpart to the classic assignment of one or more IP addresses to a given host name via A or AAAA resource record . Domain owners cannot use PTR records independently via a DNS service (such as BIND ) Instead, changes must be requested from the respective provider .

PTR resource records are a central element of the reverse DNS . They are commonly used in the following cases:

  • in the in-addr.arpa zone (for the reverse lookup of IPv4 addresses),
  • in the zone ip6.arpa (for the reverse lookup of IPv6 addresses) and
  • in other zones for hostnames to which a CNAME resource record from one of the aforementioned zones points.
  • in connection with DNS-Based Service Discovery for the listing of existing resources in the network

construction

IP address
IP address in reverse order of the components plus corresponding zone (see above); other hostnames are also possible through delegation.
TTL
Time To Live: Indicates how long (in seconds) this resource record is valid in a cache (optional; if not specified, a previously explicitly defined default value - via a $ TTL instruction at the beginning of the zone file - is assumed).
IN
Class: Internet (optional; alternatives are the classes HS for HESIOD and CH for CHAOS, two experimental network projects that have been obsolete for a long time, IN is assumed if not specified).
PTR
Type
Surname
the name assigned to the IP address

Example (IPv4) 

1.0.0.10.in-addr.arpa. 3600 IN  PTR  example.com.

In this example, a client asking a DNS server for the IP address 10.0.0.1 would reply example.com. receive.

Example (IPv6) 

1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. 3600 IN PTR test-ipv6.example.com.

In this example, a client asking for the IPv6 address 2001: db8 :: 1 would receive test-ipv6.example.com as a response .

Individual evidence

  1. ↑ RFC 3596 DNS Extensions to Support IP , Version 6, IP6.ARPA Domain
  2. ↑ RFC 2317 Classless IN-ADDR.ARPA delegation , Section 5.2 Alternative naming conventions .
  3. ↑ RFC 6763 DNS-Based Service 4.1 Structured Service Instance Names .
  4. ↑ RFC 2317 Classless IN-ADDR.ARPA delegation