Using Dig
Sometime back, I stumbled across this command that helps you lookup your public IP address from the terminal:
dig +short myip.opendns.com @resolver1.opendns.com
and it sent me down a fun rabbit hole of the dig command. Here are some things I found:
What's dig?
dig - short for Domain Information Groper - provides a nice CLI to find DNS/DNS-related information about a domain name, a server or just an address. It comes as part of the dnsutils package
Setting up
Chances of dig being installed before-hand on your system are pretty slim, though you could check with:
which dig
If it's not present, it can easily be installed using your distro's package manager viz:
Debian/Ubuntu: sudo apt install dnsutils
ArchLinux/Arch-based distros: sudo pacman -S bind-tools
CentOS/Fedora: sudo yum install bind-utils
Dig-ging in
Using dig is as simple as running:
dig mchl.xyz
The above command results in the following:
; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> mchl.xyz
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50289
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;mchl.xyz. IN A
;; ANSWER SECTION:
mchl.xyz. 20 IN A 134.209.226.211
;; Query time: 1226 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Dec 30 12:17:19 WAT 2019
;; MSG SIZE rcvd: 53
That's quite verbose, let's tone it down with:
dig mchl.xyz +short
Now, we get:
134.209.226.211
which is really the IP address of the domain mchl.xyz.
Getting Specific Record Type
You can specify the record type you want to dig such as the NS (Name Server), A (IP address), MX (Mail Exchange) records and you'd get only the matching result. For instance, the following gives us the NS records for mchl.xyz
dig mchl.xyz NS
; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> mchl.xyz ns
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49605
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;mchl.xyz. IN NS
;; ANSWER SECTION:
mchl.xyz. 2827 IN NS dns1.p07.nsone.net.
mchl.xyz. 2827 IN NS dns4.p07.nsone.net.
mchl.xyz. 2827 IN NS dns2.p07.nsone.net.
mchl.xyz. 2827 IN NS dns3.p07.nsone.net.
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Dec 30 12:30:11 WAT 2019
;; MSG SIZE rcvd: 126
while this gives us the MX servers for github.com
dig github.com MX
; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> GITHUB.COM MX
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60722
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;GITHUB.COM. IN MX
;; ANSWER SECTION:
GITHUB.COM. 3600 IN MX 10 ALT3.ASPMX.L.GOOGLE.COM.
GITHUB.COM. 3600 IN MX 5 ALT1.ASPMX.L.GOOGLE.COM.
GITHUB.COM. 3600 IN MX 5 ALT2.ASPMX.L.GOOGLE.COM.
GITHUB.COM. 3600 IN MX 10 ALT4.ASPMX.L.GOOGLE.COM.
GITHUB.COM. 3600 IN MX 1 ASPMX.L.GOOGLE.COM.
;; Query time: 1175 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Dec 30 12:31:00 WAT 2019
;; MSG SIZE rcvd: 154
Reverse DNS (rDNS) Lookup
Given an IP address, dig can find the associated domain name when used with the -x flag (indicating a reverse lookup) Here is an example:
dig -x 146.112.62.105 +short
and the result is:
www.opendns.com.
opendns.com.
NB: DNS isn't always two ways i.e, that a domain points to an IP address doesn't always mean that IP address will resolve to the same domain name, or even resolve to any domain name at all unless it is explicitly set up to do that via a corresponding PTR record.
Finding your public IP address
Using dig combined with a pre-configured server, you can find your public IP address over DNS, as opposed to the HTTP mechanism employed by “IP-finder” webistes. Remember how our first example gave us the IP address of the domain, some servers are instead configured to respond with the originating IP address - which is your network public IP address. For example, the command below queries the OpenDNS “myip” server:
dig +short myip.opendns.com @resolver1.opendns.com
and gives the result below on my local network:
41.203.73.121
We can achieve same thing using Google's DNS server like so:
dig TXT o-o.myaddr.l.google.com @ns1.google.com +short
Conclusion
The dig command is a really handy tool if you're looking to troubleshoot or simply dive into DNS stuff, In case you want to go deeper, here are some resources: