Get AS(Autonomous System) Number By domain name

If you followed my last post HERE you might be wondering “That’s great and all, but how do I find the AS number in the first place?”.

If you are running Linux it’s easy:

First install dig and whois:

If you are using Centos/Fedora:


yum install bind-utils whois -y

 
In case you are using Debian/Ubuntu:


apt install dnsutils whois -y

 
After that to get the IP number associated with the domain.


dig +short facebook.com | head -1

 
The output should be an IP address in example:


31.13.91.36

 
With the IP address in hand to get the ASN you just need to run:


whois -h whois.radb.net '31.13.91.36' | grep -i origin | tr -s " " | cut -d " " -f2

 
Voila the output should be the AS number(s)


AS32934

 
And with most things on the internet these lists are not static so you should build a script to update them from time to time.

Sources:

https://stackoverflow.com/questions/11164672/list-of-ip-space-used-by-facebook
https://gist.github.com/normoes/829d65866c8bf6d32b13f020479b172b
https://developers.facebook.com/docs/sharing/webmasters/crawler