Reverse DNS records translate IP addresses into domain names, unlike forward DNS records that translate domain names into IP addresses. They are used for troubleshooting, system monitoring, and anti-spam purposes. PTR records are handled by ISPs, while domain owners are responsible for A records. Matching reverse and forward DNS records can indicate a legitimate domain.
A reverse Domain Name System (DNS) record is a feature of DNS technology that translates an Internet Protocol (IP) address into a domain name. This is in contrast to the more widely used forward DNS record, which translates a human-readable domain into a series of numbers understood by software on the web. Separate records are kept for both types of lookups and have different uses. Reverse DNS records can be used for network troubleshooting or anti-spam purposes, but are usually not associated with web browsing.
DNS associates human-readable domain names with the IP addresses used by web browsers and other software. Millions of computers around the world store and exchange DNS records, creating the Internet equivalent of a global telephone directory. In most cases, the system is used to translate a web address or domain name, such as example.com, into an IP address such as 192.0.32.10. This action is also known as a forward DNS lookup. A reverse DNS record is used to do the opposite: translate a known IP address into a domain name.
DNS uses separate records for DNS forward lookups and the less common reverse lookups; example.com may point to 192.0.32.10 in a forward lookup, but 192.0.32.10 will not necessarily point to example.com in a reverse lookup. In technical jargon, a reverse DNS record is known as a “Pointer Record” (PTR), while the forward record is known as an “address” (A) record. The former is often handled by an Internet Service Provider (ISP) or web hosting company, while a domain owner is responsible for the latter. A PTR may also contain entries for either IPv4 addresses, as used in the previous examples, or addresses from more recent IPv6.
A reverse DNS record is most commonly used for troubleshooting, system monitoring, and anti-spam. Some domains may omit this type of record because it has no impact on web browsing and many other types of internet traffic. Vintage network tools like traceroute and ping use reverse DNS records, as do many business management tools and network backups. Many anti-spam tools look at a domain’s PTR when evaluating incoming mail and can block messages that appear to come from domains with a mismatch or those commonly associated with low-quality ISPs. Conversely, a reverse DNS record that matches its forward DNS cousin can be a strong indicator of a legitimate domain, and many email tools are more likely to accept messages from domains with such a match.
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