09.13
Using Dig Instead of NSLookup
If you’re an old-timer you probably find yourself automatically typing in nslookup when you go to troubleshoot DNS issues. It’s a familiar interface and you don’t have time to learn a bunch of new command line switches that do the exact same thing. Besides, nslookup has been threatening depracation for a long time now, yet it is still in every new version of Linux that comes out. Luckily, dig is not to tough to learn. You should resolve yourself today to learn it’s syntax because it really can save you some time.
Here is the very basic usage syntax:
> dig [@server-name] [domain] [q-type] host
See there, that is totally painless. Let’s learn by example. Here is how you would look up all the mail exchanger(mx) records for gmail.com:
$user@localhost:>> dig gmail.com mx
That would produce these results:
; <<>> DiG 9.3.1 <<>> gmail.com mx ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15796 ;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 4, ADDITIONAL: 4 ;; QUESTION SECTION: ;gmail.com. IN MX ;; ANSWER SECTION: gmail.com. 2638 IN MX 5 gmail-smtp-in.l.google.com. gmail.com. 2638 IN MX 10 alt1.gmail-smtp-in.l.google.com. gmail.com. 2638 IN MX 10 alt2.gmail-smtp-in.l.google.com. gmail.com. 2638 IN MX 50 gsmtp163.google.com. gmail.com. 2638 IN MX 50 gsmtp183.google.com. ;; AUTHORITY SECTION: gmail.com. 45177 IN NS ns3.google.com. gmail.com. 45177 IN NS ns4.google.com. gmail.com. 45177 IN NS ns1.google.com. gmail.com. 45177 IN NS ns2.google.com. ;; ADDITIONAL SECTION: ns1.google.com. 174016 IN A 216.239.32.10 ns2.google.com. 174016 IN A 216.239.34.10 ns3.google.com. 174016 IN A 216.239.36.10 ns4.google.com. 174016 IN A 216.239.38.10 ;; Query time: 330 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Tue Sep 12 15:55:28 2006 ;; MSG SIZE rcvd: 294
If you wanted to look up the name servers(ns) for gmail.com, but use a specific google DNS server(@servername) to do it, you could do this:
$user@localhost:>> dig @ns2.google.com gmail.com ns ; <<>> DiG 9.3.1 <<>> @ns2.google.com gmail.com ns ; (1 server found) ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17405 ;; flags: qr aa rd; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 4 ;; QUESTION SECTION: ;gmail.com. IN NS ;; ANSWER SECTION: gmail.com. 345600 IN NS ns2.google.com. gmail.com. 345600 IN NS ns3.google.com. gmail.com. 345600 IN NS ns4.google.com. gmail.com. 345600 IN NS ns1.google.com. ;; ADDITIONAL SECTION: ns2.google.com. 345600 IN A 216.239.34.10 ns3.google.com. 345600 IN A 216.239.36.10 ns4.google.com. 345600 IN A 216.239.38.10 ns1.google.com. 345600 IN A 216.239.32.10 ;; Query time: 105 msec ;; SERVER: 216.239.34.10#53(216.239.34.10) ;; WHEN: Tue Sep 12 16:39:20 2006 ;; MSG SIZE rcvd: 170
If you don’t specify a server with the @servername switch, then it defaults to whatever server is listed in resolv.conf. The real power of dig though is in it’s global options. For instance, the +short option makes dig output only the results instead of printing in a report type format. It’s quite useful when combined with bash scripting. For example, if you wanted to store all of gmail’s name servers in an array and then ping them in turn, you could do something like this:
#!/bin/sh
gmailers=(`dig +short gmail.com ns`)
count=1
max=${#gmailers[*]}
while [ $count -lt $max ]
do
ping -c1 ${gmailers[$count]}
count=$((count+1))
done
Learning dig only takes a few minutes but it can save you tons of time down the road. Do yourself a favor the next time you need to troubleshoot an e-mail problem and you reach for nslookup. Type man dig instead and make yourself learn it.
