Reverse DNS Records

[FOBSIDE]

I have a mail server, which is colocated. The reverse DNS is not set up for this IP address, but I can request to have it set up. The problem is that I don't know how it should be formatted. There is an "A Record" and a "PTR Record" as far as I can tell, but I am not understanding how these entries should be formatted. Does anyone know this stuff?

Thanks in advance.

 

[cleverhandle]

That depends on the name server and how it's configured. On BIND9, for the zone "168.192.in-addr.arpa", I have entries like...

1.1 IN PTR gateway.domain.org.
1.0 IN PTR desktop.domain.org.

Basically, you reverse the IP address (the desktop above has IP 192.168.0.1) and give a PTR to the complete hostname of the machine, including a trailing dot. Again, some of the details depend on the configuration, but that may give you something to go on. The colo service should be able to help you with details if you give them a call.

That being said, a lot of times ISP's will handle this with a CNAME or some other kind of workaround for manageability purposes (ISP's don't like delegating reverse zones for small netblocks). So the "official" style above may not actually apply to you. Again, give the service a call.

 

[FOBSIDE]

Let me see if I got this right. You have a router/firewall of some sort that takes a public IP on the WAN side. On the LAN side, you have addresses specified in the 192.168.0.x range. I don't get why you have 1.1 and 1.0 for the entries. Is that right?

 

[cleverhandle]

Originally posted by: FOBSIDE
Let me see if I got this right. You have a router/firewall of some sort that takes a public IP on the WAN side. On the LAN side, you have addresses specified in the 192.168.0.x range.

Yes, this particular example is from an internal zone. Regular public IP's follow the same format though.

I don't get why you have 1.1 and 1.0 for the entries. Is that right?

Yes, it's right. When a machine does a reverse lookup on the IP 192.168.0.1, it looks for a record for "1.0.168.192.in-addr.arpa". In my BIND configuration, the "168.192.in-addr.arpa" is specified in the server's config file, so the zone file itself doesn't need to include that part. Thus, the zone file only needs the first part - "1.0".

Again, a lot of this is implementation-specific. Are you sure your provider is really expecting you to deal with this level of detail?

 

[bgroff]

If your server is colocated, you shouldn't have to worry about what the DNS entries look like. Just tell the provider what the host name should be for the IP address. I.E. the IP address 128.210.10.11 should be expert.cc.purdue.edu. You can test it like this:

C:\>nslookup -query=ptr 128.210.10.11
Server: ns2.auburnes.net
Address: 208.10.128.150

Non-authoritative answer:
11.10.210.128.in-addr.arpa name = expert.ics.purdue.edu