DNS troubleshooting and reconnaissance

The main tool in Linux to troubleshoot DNS services is dig, which comes pre-installed in almost all Linux distributions. If you don't have dig in your distribution, you can install it with apt-get install dnsutils. The use of this tool is pretty simple, as can be seen here:

Dig <request value you are making> <the request type you are making>  +<additional request types>

So, to find name server records for a company (we'll check sans.org), we'll make an ns query against sans.org, as follows:

$ dig sans.org ns
; <<>> DiG 9.16.1-Ubuntu <<>> sans.org ns
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27639
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;sans.org.         ...