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Reverse DNS: Difference between revisions
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→About Reverse DNS: clean up, typos fixed: For example → For example, (2)
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==About Reverse DNS==
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 my.firebrick.co.uk. To do this a normal DNS lookup is done for an A record called my.firebrick.co.uk and you get the answer 217.169.0.1.
Reverse DNS is about finding a name for an IP address. The system is quite simple, the IP address is converted to a name and a lookup 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 my.firebrick.co.uk is delegated so that the name servers for co.uk tell the name servers for firebrick.co.uk 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) are delegated to a new name server.
With IP addresses the control is delegated the other way, e.g. 217.x.x.x is delegated to RIPE, and 217.169.0.x is delegated to AAISP. To allow DNS to be used to turn IP addresses in to names, the reverse DNS name for an IP address is backwards. For example, 217.169.0.1 is 1.0.169.217.in-addr.arpa . This means that 217.in-addr.arpa is delegated to RIPE and 0.169.217.in-addr.arpa is delegated to AAISP.
To delegate your IP addresses to you we have to find a way to delegate within the block of 256 addresses we have received from RIPE. Few customers have a complete block of 256 addresses. Those that do can simply be set up so that their own name servers are used in the delegation from RIPE. For anyone with less than 256 addresses we have to find a way to give you some of the addresses within a block - which DNS does not allow.
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