Wrapping up DNS

A DNS record type is the pointer, or PTR record. This record type reverse maps addresses to names, so for an example network, the PTR records would be:

2.120.65.206.in-addr.arpa IN PTR alice.redqueen.com

3.120.65.206.in-addr.arpa IN PTR kitten.redqueen.com

4.120.65.206.in-addr.arpa IN PTR hatter.redqueen.com

5.120.65.206.in-addr.arpa IN PTR dormouse.redqueen.com

6.120.65.206.in-addr.arpa IN PTR dormouse.redqueen.com

The addresses on the left side are reversed and the in-addr.arpa domain is added. This data goes in the database that resolves reverse lookups - 206.65.120.db (www.nwfusion. com, DocFinder: 3736).

Now we have the forward lookup database for our domain - redqueen.db, and our reverse lookup database - 206.65.120.db.

The final thing we need to do is set up a loopback network database. The loopback address - 127.0.0.1 - is used when a machine wants to send and receive packets to and from itself. In our example, this data is stored in the file 127.0.0.db and looks like this:

0.0.127.in-addr.arpa. IN SOA alice. redqueen.com. admin.redqueen.com. (

200103051053 ; Serial

86400 ; 1 day refresh

3600 ; 1 hour retry

604800 ; 1 week expiration

86400 ) ; 1 day TTL

0.0.127.in-addr.arpa. IN NS alice. redqueen.com.

1.0.0.127.in-addr. IN PTR localhost.

That final line is how the name server knows that "localhost" is mapped to 127.0.0.1 (the reverse of the record "localhost.redqueen.com IN A 127.0. 0.1" in the database redqueen.db).

One thing we haven't discussed is how DNS works with e-mail. There's a special record type - the Mail Exchanger, or MX record - for specifying mail servers for a domain.

An MX record looks like this:

redqueen.com. IN MX 1 borogrove. com.

This means borogrove.com will accept or relay mail for the domain redqueen.com. The value "1" is used when there are multiple mail servers for a given name to determine the order servers should be used in. If we had several servers:

redqueen.com. IN MX 1 borogrove1. blackqueen.com.

redqueen.com. IN MX 2 borogrove2. blackqueen.com.

redqueen.com. IN MX 5 hatter.blackqueen.com.

The lowest-numbered server would be tried first and, failing that, the next-highest-numbered server, and so on. Again, there's a lot more behind the mechanisms of MX records and how mail servers interact with them that we will forgo due to space constraints.

Anyway, that should get you started with a DNS server.