The Purposes of Reverse DNS (PTR Records)

Often times the question comes up as to what purpose reverse DNS really does for us. To be honestly, I haven't found anything that ptr records MUST be set up for. However, here are some uses that you may encounter them being used for:

• Network Troubleshooting using tools such as traceroute, ping, or the "Received" header field in SMTP emails. Many web sites use tools such as these to help track users as a reverse DNS name will often mean a lot more to a webmaster than an IP will when trying to figure out if someone is the same user, so ISP's having rDNS set up when they hand out IPs is useful.
• Along the same lines of network troubleshooting is System logs or Monitoring tools. These often receive the IP address and will try to do a reverse lookup to put in the logs. Again quite useful for system admins or network admins as an IP might not mean much to them. Email anti-spam techniques often use domain names for the rDNS record to see if these domains have rDNS records of something that doesn't match the domain of the sender, or if the record looks like it's coming from a dynamic IP address. According to spamhaus, a global email blacklist database, these are things that could raise the spam score of an email.
• Microsoft DNS servers by default add reverse dns lookup. This is a very useful tool for internal debugging as said above, however when it boils down to setting up this record for your webhosting domains, if you aren't planning on sending email from that IP it's probably not worth your effort to add the records.

Another common issue is that many webhosts do not support ptr record setups. If however you have a dedicated IP then your webhost may be able to have the ptr record added. There is no restriction on the number of ptr records for a domain.

If you wish to look up a rdns (pointer) record for an IP address you can use the nslookup command and use the following commands:

nslookup
set type=ptr
[enter ip address and hit enter]