A reverse DNS lookup takes an IP address and returns the domain name associated with that IP. A forward DNS lookup does just the opposite.
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A reverse DNS lookup is a DNS query for the domain name associated with a given IP address. This accomplishes the opposite of the more commonly used forward DNS lookup, in which the DNS system is queried to return an IP address.
Standards from the Internet Engineering Task Force (IETF) suggest that every domain should be capable of reverse DNS lookup, but as reverse lookups are not critical to the normal function of the Internet, they are not a hard requirement. As such, reverse DNS lookups are not universally adopted.
Reverse DNS lookups query DNS servers for a PTR (pointer) record; if the server does not have a PTR record, it cannot resolve a reverse lookup. PTR records store IP addresses with their segments reversed, and they append ".in-addr.arpa" to that. For example if a domain has an IP address of 192.0.2.1, the PTR record will store the domain's information under 22.214.171.124.in-addr.arpa.
In IPv6, the latest version of the Internet Protocol, PTR records are stored within the ".ip6.arpa" domain instead of ".in-addr.arpa."
Reverse lookups are commonly used by email servers. Email servers check and see if an email message came from a valid server before bringing it onto their network. Many email servers will reject messages from any server that does not support reverse lookups or from a server that is highly unlikely to be legitimate. Spammers often use IP addresses from hijacked machines, which means there will be no PTR record. Or, they may use dynamically assigned IP addresses that lead to server domains with highly generic names.
Logging software also employs reverse lookups in order to provide users with human-readable domains in their log data, as opposed to a bunch of numeric IP addresses.
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