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1.8 Registering a Reverse-Mapping Domain

1.8.1 Problem

You want to register the reverse-mapping domain that corresponds to your network.

1.8.2 Solution

Start by determining whether your reverse-mapping domain is already registered and, if not, which of its parent domains is registered. If your network is, say, 192.168.0/24, try looking up an SOA record for 0.168.192.in-addr.arpa. If you find an SOA record, then your network's reverse-mapping domain has been registered. If your network is part of a larger block of networks that your ISP owns, you may find that your ISP has registered it. Contact your ISP and ask them to change the delegation for that domain to your name servers. If you're not sure which email address to use for your ISP, the SOA record will show you the email address (in the second RDATA field) of a technical contact. You can also use the whois service offered by one of the regional address registries to look up contact information for your network, including phone numbers; see this recipe's "Discussion" for details.

If you don't find an SOA record, peel off the domain name's leading label and a dot and try looking up an SOA record for the result; in this example, you'd look up an SOA record for 168.192.in-addr.arpa. If that turns up an SOA record, use that record's email address or the associated whois information (again, see the "Discussion") to find out whom to contact to have your domain delegated. If there's no SOA record, keep peeling off those labels until you find one. If you get all the way to in-addr.arpa, you may need to contact your regional address registry to register your network and the corresponding reverse-mapping zone with them.

1.8.3 Discussion

The three regional address registries are APNIC, which serves Asia and the Pacific, ARIN, which handles the Americas, sub-Saharan Africa and the Caribbean, and RIPE, which deals with Europe and Saharan Africa. Each registry runs its own whois service, which contains information about all of that registry's registered networks. Here's a list of the registries' whois web pages and the domain names of their whois servers:


The web page is at http://www.apnic.net/apnic-bin/whois2.pl; the whois server is at whois.apnic.net


The web page at http://www.arin.net/whois/index.html; the whois server is at whois.arin.net


The web page is at http://www.ripe.net/ripencc/pub-services/db/whois/whois.html; the whois server is at whois.ripe.net

Unfortunately, life is a little more complicated for those of us with networks that have network masks whose bit-lengths aren't integer multiples of eight. If you have a network that's smaller than a /24, you'll have to contact your ISP and ask them to follow the instructions in RFC 2317 (described in Section 6.4) to delegate control of the reverse-mapping information for your network to you. If you have a network larger than a /24, you'll end up with more than one reverse-mapping domain for your network. For example, if you run 10.0.0/22, you'll need to have all four of the following domains delegated to you:

  • 0.0.10.in-addr.arpa

  • 1.0.10.in-addr.arpa

  • 2.0.10.in-addr.arpa

  • 3.0.10.in-addr.arpa

Woe unto the poor hostmaster who must set up reverse-mapping for a network like 10.0.0/17!

If the length of your network mask isn't evenly divisible by eight and you're trying to determine which of your domain's parents are registered, start by rounding your network's netmask down to the nearest even multiple of eight and looking up an SOA record for the corresponding network. For example, for 10.0.0/22, round down to 10.0/16 and look up an SOA record for 0.10.in-addr.arpa.

1.8.4 See Also

Recipes Section 6.3 and Section 6.4, for delegating subdomains of reverse-mapping domains, and Section 6.4, for handling networks smaller than a /24.

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