Reverse DNS: Difference between revisions

Normal (forward) DNS is a system that allows you to look up information about a domain/host name. For example, you might want to look up the IP address for the name <code>my.firebrick.co.uk</code>. To do this, a normal DNS lookup is done for an '''A''' record called <code>my.firebrick.co.uk</code> and you get the answer <code>217.169.0.1</code>.

Reverse DNS is about finding a name for an IP address. The system is quite simple, the IP address is converted to a name, andbut athe lookup is done in the usual way. The record type for a reverse DNS lookup is a '''PTR''' record not an A record.

Because of the way DNS works, control is delegated at each level, so <code>my.firebrick.co.uk</code> is delegated so that the name servers for <code>co.uk</code> tell the name servers for <code>firebrick.co.uk</code> and so on. This is normally only at a couple of levels but it could be that at each level control of the domains below that level (i.e. with anything added to the start of the domain) areis delegated to a new name server.

With IP addresses the control is delegated the other way, e.g. <code>217.''x.x.x''</code> is delegated to RIPE, and <code>217.169.0.''x''</code> is delegated to AAISP. To allow DNS to be used to turn IP addresses in tointo names, the reverse DNS name for an IP address is backwards. For example, <code>217.169.0.1</code> is <code>1.0.169.217.in-addr.arpa </code>. This means that <code>217.in-addr.arpa</code> is delegated to RIPE and <code>0.169.217.in-addr.arpa</code> is delegated to AAISP.

Auto reverse is the default option that works by AAISP filling in a PTR record for each IP4IPv4 address and a corresponding forward A record as well so that all of your IP addresses will have a valid reverse entry automatically. This is mainly for customers who are not interested in setting up any reverse entries but need something in place to avoid problems with some servers. If you also use A+reverse records in a domain then your IPs will have two PTR records, both valid.

if you have the IPs <code>192.0.2.1</code> and <code>2001:DB8::1</code> reverses will be created of:

<code>1.2.0.192.in-addr.arpa</code>

<code>1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa</code>

==2. A+Reverse or AAAA+Reverse DNS records on a Domain We Host ==

=== Reverse lookup ...if your domain is hosted elsewhere ===

If you administer a domain that is not hosted with us, then '''we''' can add an entry into our control system specifying your choice of domain name (optionally including subdomains) specifically to then allow you to set up entries on our systems that will define a number of individual reverse lookup mappings from your AAISP addresses to individual names below your ‘base’ domain name. There is no charge for this., An'''however exampleit ismust givenbe inrequested'''; thethis nextcan section.be Onconfusing our systemsas you will definenot afind numberthe ofrelevant records, one (or more) per address of yours, and we append your chosen domain nameconfiguration to eachwhich tothis formsection arefers reversewill lookup (PTR) record that we define and publish for you. You can ask for several of your domain names tonot be setvisible upuntil on our systemsrequested. Such a domain name can include your choice of subdomain in your request.

For example, suppose you have an IP address from us such as <code>198.51.100.10</code> and you wish to define a PTR mapping (reverse lookup) link to the name <code>fred-workstation.your-domain.example.com</code>.Important: If this domain is not hosted by us, you must first have asked staff to set up <code>your-domain.example.com</code> in our control system, but you only need to do so once. Then you would go into our control panel system, go to ''"domainsDomains"'' and select the entry <code>your-domain.example.com</code> which we will have added for you following your request. Now for each individual reverse lookup entry you wish to define, enter for example the name <code>"fred-workstation"</code> into the first edit box for the hostname. The value is not restricted to be just one single dnsDNS label; it could contain dots for subdomains. Then enter the ipIP address in the edit box below, in this case <code>198.51.100.10</code>. You must remember to then click '''OK''' or '''Apply''' to create the record. You will see that a new record is now listed below.

Notice there is an option to select "ipv4 reverse" or "ipv6 reverse" lookup - ipv4IPv4 is correct in this example case. The procedure for ipv6IPv6 address reverse lookups is the same, just click the pop-out menu toand select "ipv6 reverse".

WeIf you have a /24 or larger block (few customers will!) we can create a suitable <code>in-addr.arpa</code> zone on your account, and from within that you can add PTR records. We can do a similar thing for [[IPv6]] too where we'll set up <code>''x.x.x.x.''0.b.8.0.1.0.0.2.ip6.arpa</code> zone for you. Contact support staff for this.

==4. Generic Per -IP Block==

Support staff can add a name on a per -IP block basis and we'll automatically create reverse records, e.g., if your block is <code>81.187.81.0/29</code> and we add the name <code>hosts.testing.me.uk</code>, then we'll create reverse records such as:

<code>0.hosts.testing.me.uk</code>

<code>1.hosts.testing.me.uk</code>

<code>2.hosts.testing.me.uk</code>

etc...…

Contact Supportsupport Staffstaff for this.

===1. Delegation by NS===

Delegation by NS works by putting your name server in our DNS for each of your addresses. e.g. if you had <code>217.169.0.0-3</code> then we would put your name servers for each entry <code>0.0.169.217.in-addr.arpa</code>, <code>1.0.169.217.in-addr.arpa</code>, <code>2.0.169.217.in-addr.arpa</code>, <code>3.0.169.217.in-addr.arpa </code>. This would mean you can create 4four separate zone files each of which has to normal SOA records, etc., and a single PTR record with the name for that IP address. This is logically the correct way of doing it as the reverse DNS zone is delegated at each level of control right down to the IP address level. It is rather tedious to set up lots of zone files though, especially if you have, say, 128 addresses.

===2. Delegation by CNAME ===

Delegation by CNAME is a way to delegate a block of addresses to you so that you only have one zone file to worry about. The way this works is that we put a CNAME record for each address indicating that the answer is found under a different name. We then delegate that different name to your name servers. There are several ways to do this, but we use the system of <code>''first''-''last''.''restofzone''.in-addr.arpa</code>. e.g. if you had <code>217.169.0.0</code>-<code>3</code> we would delegate a zone <code>0-3.0.169.217.in-addr.arpa</code> to your name server(s) and add CNAME entries for each IP, e.g. <code>1.0.169.217.in-addr.arpa</code> with CNAME to <code>1.0-3.0.169.217.in-addr.arpa </code>.

===IPv6 Reverse Delegation===

===Reverse Delegation for /24 or larger===