Thursday, 30 July 2009

Classless IN-ADDR.ARPA delegation

In some cases you may be allocated a block of IP addresses smaller than a /24. This is referred to as a Classless Delegation and is explained in technical detail in RFC 2317.

For example, if your ISP allocated the block 192.168.5.224/27 (the 32 IP addresses from 192.168.5.224 to 192.168.5.255), the reverse DNS for this zone would be served out of a zone named

    224-27.5.168.192.in-addr.arpa

Once the zone is created, you'll be able to create PTR entries within this zone for each individual IP address. As an example, to map the IP address 192.168.5.226 to mail.example.com, you would enter:

Host: 226
TTL: 86400
Type: PTR
Data: mail.example.com

Once you have setup the Custom DNS zone for this block, you need to have the block delegated to us. Contact your ISP and have them create NS records for the zone:

   224-27.5.168.192.in-addr.arpa.   86400  IN      NS      ns1.mydyndns.org.
224-27.5.168.192.in-addr.arpa. 86400 IN NS ns2.mydyndns.org.
224-27.5.168.192.in-addr.arpa. 86400 IN NS ns3.mydyndns.org.
224-27.5.168.192.in-addr.arpa. 86400 IN NS ns4.mydyndns.org.
224-27.5.168.192.in-addr.arpa. 86400 IN NS ns5.mydyndns.org.

Your ISP will ALSO need to create individual CNAME records for each IP address they have delegated to you. These CNAME records map the IP addresses into the new name space that has been defined via the 224-27.5.168.192.in-addr.arpa zone.

   224.5.168.192.in-addr.arpa.   86400  IN     CNAME      224.224-27.5.168.192.in-addr.arpa.
225.5.168.192.in-addr.arpa. 86400 IN CNAME 225.224-27.5.168.192.in-addr.arpa.
226.5.168.192.in-addr.arpa. 86400 IN CNAME 226.224-27.5.168.192.in-addr.arpa.
etc...

Until those changes are made, DNS queries for those IP addresses won't come to us, so it is not enough to simply create the zone. You MUST get it delegated to us and have the CNAME entries created in order for the reverse DNS to function.

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