Issue: Upstream DNS provider fails to resolve some domain names. Troubleshooting confirms the issue is with upstream DNS partners, and not your server.
Solution: Edit the resolv.conf file to use more reliable DNS servers.
In example below Level 3 nameservers will be used. And the file may be edited to look like the following example:
search your-webserver-domain.com nameserver 188.8.131.52 nameserver 184.108.40.206 nameserver 220.127.116.11 nameserver 18.104.22.168
Save the file. To test, ping one of the domain names that were previously not reachable (from your server):
ping -c 4 domainname.com