What is Reverse DNS?

Posted by / Sep 23, 2014 / Categories: DNS

Reverse DNS

Reverse DNS is IP address to domain name mapping – the opposite of forward (normal) DNS which maps domain names to IP addresses.

Reverse DNS is separate from forward DNS.
Forward DNS for “abc.com” pointing to IP address “1.2.3.4”, does not necessarily mean that reverse DNS for IP “1.2.3.4” also points to “abc.com”.
This comes from two separate sets of data.

A special PTR-record type is used to store reverse DNS entries. The name of the PTR-record is the IP address with the segments reversed + “.in-addr.arpa”.
For example the reverse DNS entry for IP 1.2.3.4 would be stored as a PTR-record for “4.3.2.1.in-addr.arpa”.

Reverse DNS is also different from forward DNS in who points the zone (domain name) to your DNS server.
With forward DNS, you point the zone to your DNS server by registering that domain name with a registrar.
With reverse DNS, Bhost must point (or “sub-delegate”) the zone (“….in-addr.arpa”) to your DNS server.
Without this sub-delegation from Bhost your reverse zone will not work.

Reverse DNS is mostly used by humans for such things as tracking where a web-site visitor came from, or where an e-mail message originated etc.
It is typically not as critical in as forward DNS – visitors will still reach your web-site just fine without any reverse DNS for your web-server IP or the visitor’s IP.

Why do I need it?

Reverse DNS is really important for one particular application.
Many e-mail servers on the Internet are configured to reject incoming e-mails from any IP address which does not have reverse DNS.
So if you run your own e-mail server, reverse DNS must exist for the IP address that outgoing e-mail is sent from.
It does not matter what the reverse DNS record for your IP address points to as long as it is there. If you host multiple domains on one e-mail server, just setup reverse DNS to point to whichever domain name you consider primary.
(e-mail servers checking for reverse DNS do recognise that it is normal to host many domains on a single IP address and it would be impossible to list all those domains in reverse DNS for the IP).

How do I set it up?

You must already have set up your forward DNS records to enable this feature.

1) Log in to your Bhost server control panel and select the VPS you want to manage.

2) Click on the Network tab

3) Click [Edit] for the IP address you want to add the reverse DNS to.

4) In the popup add the domain you want to allocate to this IP address. This popup will complain if your forward DNS records are not correctly set up.

How to check reverse DNS?

This Reverse Lookup tool will do a reverse IP lookup. If you type in an IP address, it will attempt to locate a dns PTR record for that IP address. You can then click on the results to find out more about that IP Address. Please note that in general, Bhost must setup and maintain these Reverse DNS records (i.e. PTR records) for you.

About the author
Dave is BHost's Docker guru. If you have any questions about BHost Apps or Docker services on BHost then get in touch - support@BHost.net