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Knowledge base Reverse DNS

Reverse DNS is the system by which you can give a name to each of your IP addresses. Normally DNS gives an IP address to a name, but reverse DNS is the other way around. Sometimes it is necessary to have a name associated with an IP address, and then have that IP address correctly associated with a name. This is because some systems and tools on the internet will check the address (e.g. some ftp servers and some email servers). You should only set reverse DNS to domain names you control.

The way this works is that the IP address is turned in to a domain, e.g. 217.169.0.1 would be turned in to 1.0.169.217.in-addr.arpa. Any machine wanting to know the name for 217.169.0.1 would look up the PTR DNS record on 1.0.169.217.in-addr.arpa.

We are delegated the address blocks we control by RIPE, e.g. we manage 0.169.217.in-addr.arpa. (and many others). If you had, for example, a block of addresses from 217.169.0.0 to 217.169.0.3 then you might want to give names to those addresses.

This may sound rather complicated, but we have several ways to manage your reverse DNS as follows.

Reverse DNS from forward DNS

If you have a domain with us, you can create entries in the domain tagged as A+reverse rather than just A. This tells our DNS server to automatically fill in a PTR record the other way around. This is the simplest way to set specific names for each IP on your network. Even if your domain is not with us, contact support and we can add it just for reverse entries.

Auto-reverse name

By default we create a reverse entry and a matching forward DNS entry so that your IP has a name associated with it. This is set up in our control pages as auto reverse DNS.

Reverse DNS by NS records

We can delegate the reverse DNS for each of your addresses. This means you would have a zone file for each IP, e.g. 0.0.169.217.in-addr.arpa., 1.0.169.217.in-addr.arpa., 2.0.169.217.in-addr.arpa., 3.0.169.217.in-addr.arpa. each of which would contain one PTR record identifying the name of the machine. Remember that you also have a WAN address which may be completely different from your other addresses, and the reverse DNS is also delegated to your name servers for this too.

To do this, simply put one or two name servers in the settings under your login on the Control Pages. There are options for up to 6 name servers.

Reverse DNS by CNAME

We can also delegate the reverse DNS using CNAME records. This allows you to manage your DNS using only one zone file instead of one for each IP address. In the above example this would mean we have a CNAME record pointing 0.0.169.217.in-addr.arpa. to 0.0-3.0.169.217.in-addr.arpa., and 1.0.169.217.in-addr.arpa. to 1.0-3.0.169.217.in-addr.arpa. and so on. We then delegate 0-3.0.169.217.in-addr.arpa. as a zone to you and you can create PTR records for 0, 1, 2 and 3 within that zone.

Your WAN address and any other single addresses are still individually delegated as their own zone as above.

To do this, simply put up to six name servers in the settings on the login page in the Control Pages and and select the CNAME option.

Reverse for /24 or larger

If you have a block of 256 addresses or more, then reverse DNS delegation is done at the /24 level and setting CNAME or NS has the same effect. This means you are responsible for an entire reverse zone without the need for special CNAME entries, etc. Most people do not have such a large block of addresses.

Web based edit of reverse settings

Using the CNAME method above we can create reverse DNS zone on our primary DNS server. We can set this up with a password in the same way as any other domain, allowing you to edit the zone using the web based control pages at http://aa.net.uk/login.html . This way you don't need to run your own DNS servers.

To do this, ask technical support to set up the domain for you and provide a password.

Reverse DNS for IP6 addresses

If you have an IP6 assignment from us then you can set up reverse DNS in the same way. You can create forward entries that are AAAA+reverse which will automatically create reverse DNS entries in the same way as A+reverse as shown above. We can also delegate reverse DNS to you.