http://ralph.glblnet.com/ is both a master and a slave
http://ralph.glblnet.com/ is both a master and a slave nameserver, but for different domains. Forward and Reverse DNS You may have heard of or otherwise encountered the concepts of forward and reverse DNS. Forward DNS is what you do when you have a hostname and you look up an IP address. You saw examples of forward DNS in the A records in our dig examples: ………………………………………………………………………………………. nostarch.com. 2h13m2s IN A 66.80.60.21 ………………………………………………………………………………………. The A means that this is an address record, or forward DNS. This is known as an A record or an address record. Reverse DNS is what you do when you have an IP address and want a hostname. For example, suppose your system logs show that someone keeps trying to connect to your SSH server from the IP address 66.80.60.21, and you want to know the name of that host. You can look up IP addresses using dig’s -x option. Much of the output will look the same as a forward lookup, but the answer is considerably different: ………………………………………………………………………………………. # dig x 66.80.60.21 … ;; ANSWER SECTION: 21.60.80.66.in-addr.arpa. 2h24m IN PTR www.megapathdsl.net. # ………………………………………………………………………………………. Examining this output we see that, for historical reasons we won’t delve into, IP addresses are displayed in reverse order and as part of the domain in-addr.arpa when you’re doing a reverse lookup. Next we have the usual time-to-live data and the IN for Internet data. The interesting part is the PTR or pointer record, which tells us that an IP address “points to” a name. Basically, this is the canonical, most correct hostname for an IP address. This is much like a phone system; again, while many people can share a phone number, it’s only registered to one person. Forward and reverse DNS are generally expected to match, but since many hosts can share one IP address, an A record does not necessarily need a matching PTR record. For example, we saw earlier that http://nostarch.com/ has an IP of 66.80.60.21, but the hostname associated with that IP address is http://www.megapathdsl.net/. The part that must match is the A record for http://www.megapathdsl.net/. If the hostname given by a reverse lookup does not have a matching forward record, DNS is not correctly configured, and the tools that rely upon DNS checking, such as certain configurations of TCP wrappers, will reject connections from this system. Fortunately, automated tools exist to check forward and reverse DNS matches. 277
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