Today, a customer approached us after a Host Server Down complaining that, although the server is up again their website and application were down & not working. Even though the server was online and functioning correctly.
The customer discovered that the source of the issue was that there /etc/resolv.conf is blank, this means that they will not be able to resolve DNS A/PTR/CNAME record hostnames into a resolved IP. This is called hostname to IP resolution. Its means that if /etc/resolv.conf is blank and the customer uses hostnames in their calls, such a failure will break the connectivity due to failure to resolve to IP to communicate on the TCP stack.
There is actually a very simple way to prevent the /etc/resolv.conf file from being changed. But first, it’s important to understand why /etc/resolv.conf is being reset.
On All Rackspace cloud-servers there is a process called nova-agent, and when the server starts up, the /etc/resolv.conf file will be reset along with the networking configuration. This happens each time your server is restarted and is used to set new networking details, specifically if you take an image and build server on a new ip address or if your server is live-migrated to a new host, it makes sure on the next reboot it comes up with correct networking detail transparently. However this can cause some issues, such as in this case with the /etc/resolv.conf file. Fortunately there are some novel ways of preventing your /etc/resolv.conf being modified after you have added the correct nameservers you desire to it.
You can use the chattr immutable file setting to stop processes from modifying it after you have made the changes to your resolv.conf that are desired;
Set Immutable File
chattr +i /etc/resolv.conf
Un-set Immutable File
chattr -i /etc/resolv.conf
This /etc/resolv.conf issue is a common problem, however using immutable file flag and chattr should prevent it from being changed ever again.