What are the PTR record and reverse DNS?

Short answer

A PTR (Pointer) record maps an IP address to a name; since it is the reverse of a forward lookup, it is referred to as reverse DNS (RFC 1035). For IPv4, the record name is in a reversed address format such as 10.113.0.203.in-addr.arpa; for IPv6, it is kept under ip6.arpa, and this reverse zone is usually located at the service provider that owns the address block. The trust checks of e-mail servers and the readability of logs rely on PTR. In TR7 GTM, PTR is a record type in the Record Type list and its value is entered as a full name ending with a dot. In LB and GTM DNS health checks, when Reverse DNS (PTR) is turned on, the query is made from the address to the name.

Where It Is Used in TR7

Where	Field / label	What for
GTM → GTM Services → service name → Records → Add	Record Type = PTR; Record cell (Must end with dot(.))	Address → name mapping
Load Balancer → Profiles → Health Checks (DNS)	Reverse DNS (PTR) (off by default)	Checking the DNS server with a reverse query; while it is on, the DNS record type field is hidden
GTM → Health Checks (DNS)	Reverse DNS (PTR)	The same reverse query check
Load Balancer → Profiles → Health Checks (DNS)	DNS Record Type = PTR	Querying a record of the PTR type by name

Forward and Reverse Lookup

	Forward lookup	Reverse lookup
Question	At which address is www.example.com?	Which name does 203.0.113.10 belong to?
Record type	A / AAAA	PTR
Zone	The domain's zone (example.com)	The reverse zone of the address block (113.0.203.in-addr.arpa)
Who manages the zone	The domain owner	Usually the owner of the address block (service provider)

Common Misconceptions

• "The PTR record is created automatically when an A record is added." Forward and reverse records are kept in separate zones; PTR is defined separately.
• "Writing the PTR record into the domain's zone is enough." Reverse queries go to the reverse zone of the address block; in most cases this zone is managed by the service provider, and the provider is asked to write the PTR record there.
• "A health check with Reverse DNS (PTR) on queries an A record." While it is on, the query is made from the address to the name and the DNS Record Type selection is not used.

How it is done in the documentation

• Records · The Record Type Specific Fields
• Health Checks · Reverse DNS (PTR)
• GTM Health Checks · Reverse DNS (PTR)

Related questions

• How do I health check DNS servers?
• What is the difference between an A record and an AAAA record?
• What is a DNS zone; how does it relate to a domain and a DNS record?