Domain Name Service (DNS for short) is that magical protocol that allows your computer to turn meaningless domain names like www.slackware.com into meaningful IP address like 64.57.102.34. Computers can't route packets to www.slackware.com, but they can route packets to that domain name's IP address. This gives us a convenient way to remember machines. Without DNS we'd have to keep a mental database of just what IP address belongs to what computer, and that's assuming the IP address doesn't change. Clearly using names for computers is better, but how do we map names to IP addresses?
host(1) can do this for us. host is used to map names to IP addresses. It is a very quick and simple utility without a lot of functions.
% host www.slackware.com www.slackware.com is an alias for slackware.com. slackware.com has address 64.57.102.34
But let's say for some reason we want to map an IP address to a domain name; what then?
nslookup is a tried and true program that has weathered the ages. nslookup has been deprecated and may be removed from future releases. There is not even a man page for this program.
% nslookup 64.57.102.34 Note: nslookup is deprecated and may be removed from future releases. Consider using the `dig' or `host' programs instead. Run nslookup with the `-sil[ent]' option to prevent this message from appearing. Server: 192.168.1.254 Address: 192.168.1.254#53 Non-authoritative answer: www.slackware.com canonical name = slackware.com. Name: slackware.com Address: 64.57.102.34
The meanest dog in the pound, the domain information groper, dig(1) for short, is the go-to program for finding DNS information. dig can grab just about anything from a DNS server including reverse lookups, A, CNAME, MX, SP, and TXT records. dig has many command line options and if you're not familiar with it you should read through it's extensive man page.
% dig @192.168.1.254 www.slackware.com mx ; <<>> DiG 9.2.2 <<>> @192.168.1.254 www.slackware.com mx ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26362 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 2 ;; QUESTION SECTION: ;www.slackware.com. IN MX ;; ANSWER SECTION: www.slackware.com. 76634 IN CNAME slackware.com. slackware.com. 86400 IN MX 1 mail.slackware.com. ;; AUTHORITY SECTION: slackware.com. 86400 IN NS ns1.cwo.com. slackware.com. 86400 IN NS ns2.cwo.com. ;; ADDITIONAL SECTION: ns1.cwo.com. 163033 IN A 64.57.100.2 ns2.cwo.com. 163033 IN A 64.57.100.3 ;; Query time: 149 msec ;; SERVER: 192.168.1.254#53(192.168.1.254) ;; WHEN: Sat Nov 6 16:59:31 2004 ;; MSG SIZE rcvd: 159
This should give you an idea how dig works. “@192.168.1.254” specifies the dns server to use. “www.slackware.com” is the domain name I am performing a lookup on, and “mx” is the type of lookup I am performing. The above query tells me that e-mail to www.slackware.com will instead be sent to mail.slackware.com for delivery.