We know what MX records are, we know what SOA records are, so now we want to learn what other records are available. Every time you query DNS for something, you’re asking for a record and according to Wikipedia there are 39 record types.  Not all of these are commonly used, so let’s talk about the common ones.

When you ask a name server for an IP address for a domain, you’re asking for an A record.  The way to make this query is:

Alpha:~ computerlamp$ host -t A www.google.com
www.google.com has address 209.85.144.105
www.google.com has address 209.85.144.103
www.google.com has address 209.85.144.147
www.google.com has address 209.85.144.106
www.google.com has address 209.85.144.99
www.google.com has address 209.85.144.104

See that -t flag?  That tells the command host to look for the record type A.

We’ve discussed name servers before too.  How do I find out the name servers for google.com?  The DNS record to query for a name server is the NS record.  That means the query looks like:

Alpha:~ computerlamp$ host -t NS www.google.com
google.com name server ns1.google.com.
google.com name server ns2.google.com.
google.com name server ns3.google.com.
google.com name server ns4.google.com.

If you can ask for an IP address for a domain, can you ask for a domain if you have an IP address?  Well, yes, you can.  That’s a PTR record.  That’s pronounced pointer record, by the way.  It’s the reverse of an A record and is known as a reverse lookup.

And once again, you can look it up with the host command:

Alpha:~ computerlamp$ host -t PTR 209.85.144.105
105.144.85.209.in-addr.arpa domain name pointer qv-in-f105.1e100.net.

There’s a couple of weird things about that response.  First of all, I used an IP address from the responses for www.google.com, but I got a different domain back.  Which is actually just fine.  One IP address can have a lot of domains with A records for it, but only one reverse record.

Second, it reversed my domain!   You see that

105.144.85.209.in-addr.arpa

response?  That looks weird, doesn’t it?  Well, it’s one of the underpinnings of DNS that makes all things work nicely.  All PTR records are in the domain in-addr.arpa.  Remember, it’s the Domain Name System, so there has to be a domain. We take that IP address, reverse it so that instead of 209.85.144.105 we get 105.144.85.209 and stick it in front of that domain.  That’s what we’re actually querying for when we make a reverse lookup.