Centos – How to persist resolv.conf options such as rotate, timeout in CentOS

centosdomain-name-systemresolv.conf

CentOS will wipe out any manual changes made to /etc/resolv.conf periodically. The defaults in Linux are poor in terms of failing over in a reasonable time (query name servers in same order every time, 5 second timeout, 2 retries).

Hence, the first DNS in your resolv.conf is essentially critical path. If it fails you can be looking at 10 seconds before you fail over.

These defaults are tweakable (see resolv.conf man page), but how can any changes be made permanent in CentOS and persist through reboots etc.?

Best Answer

The answer can be found in the /sbin/dhclient-script:

if [ -n "${RES_OPTIONS}" ]; then
    echo "options ${RES_OPTIONS}" >> ${rscf}
fi

But, it's not terribly obvious where you can set RES_OPTIONS to make the script pick it up - some things like the search domain can be set in the ifcfg-ethX file, but resolver options are set elsewhere. The file you want is in fact /etc/sysconfig/network. To set the relevant options, add something like this line to that file:

RES_OPTIONS="rotate timeout:1 retries:1"

That will set the timeout to 1 second, use a single retry and tell the client to rotate its resolvers per request instead of sequentially going through the list each time.

If you would like to have the changes take effect immediately, then issue a service network restart command and check out your new /etc/resolv.conf in all its glory. Here's what mine looked like when testing this out:

# cat /etc/resolv.conf 
; generated by /sbin/dhclient-script
search example.com
options rotate timeout:1 retries:1
nameserver 10.1.1.2
nameserver 10.1.1.1
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