While sending out our notifications of Legacy Resolver Shutdowns following DNS Flag Day, we have had a lot of questions from non-managed customers on how to find their DNS Server settings on their servers.
Below is a step-by-step guide on how to locate your DNS Server settings on a Linux server that is NOT Ubuntu 18.04. We're not including Ubuntu 18.04 here, because, well, you won't be pointing at the wrong DNS Resolvers if you have Ubuntu 18.04.
1) You will want to SSH into your server, and become root.
2) Edit the /etc/resolv.conf file. You can use whatever editor you like; vim, nano, heck - you can use emacs if you're that way inclined!. In the case of the below, it is pointing at Indigo Legacy Resolvers, and needs to be changed.
3) Update it to point to the correct servers, and then save the file.
4) If this IS NOT a CentOS box, you're done. If this is CentOS, you should have a quick look in your /etc/sysconfig/network-scripts/ifcfg-* files, and see if they have DNS1= and DNS2= lines. If they do, you will need to also update those lines to the new Resolvers.
5) You're done!
5.1) If you are using Ubuntu 18.04, then you want to check /etc/netplan/50-cloud-init-yaml, in the 'nameservers:' section