What is PTR and how does it work?
Chris Buxton
cbuxton at menandmice.com
Wed Jul 11 15:27:00 UTC 2001
At 4:07 AM +0000 7/11/01, Cejet50 wrote:
>What is PTR and how does it work?
A PTR record is a mapping of IP address to name. It normally works like this:
If the machine at 192.168.0.1 has the name host1.example.com, then
the PTR record should look like this:
1.0.168.192.in-addr.arpa. PTR host1.example.com.
Some notes:
- Notice that the octets of the IP address are in reverse order.
- Normally, this record is likely to be found in a zone named
"0.168.192.in-addr.arpa.", so the name of the record could be
abbreviated as "1" (with no trailing dot).
- In the case of a classless subnet delegation, the name of the zone
is generally changed to something arbitrary.
- In the case of a machine having multiple names, only one should
appear in a PTR record - the IP address should have exactly one PTR
record. Anything else will, at best, simply not work as intended.
PTR records are used for a variety of purposes. The most important
use, unfortunately, is as a security measure: Some servers won't talk
to machines that don't have proper PTR records. This is a widespread
(but not universal) practice, especially for mail servers. The idea
is that many spammers don't have control of their PTR records, but
they try to forge their hostnames; a PTR record check may discover
this, allowing the destination mail server to reject the connection.
____________________________________________________________________
Chris Buxton <cbuxton at menandmice.com>
Men & Mice <http://www.menandmice.com/> offers:
- DNS training, including Active Directory
- QuickDNS, a DNS management system (now supporting Solaris)
- DNS Expert, a DNS analysis and troubleshooting utility
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