Resolv.conf

The resolver can load up to 3 name servers. If your server needs to find the mx record of gmail.com, it will first need to resolve gmail.com locally.

• using a resolver from resolv.conf
• dig @8.8.8.8 gmail.com -t mx +short

  result: alt1.gmail-smtp-in.l.google.com

• connect to alt1.gmail-smtp-in.l.google.com
• dig @8.8.4.4 alt1.gmail-smtp-in.l.google.com +short

  result: 209.85.227.27

• connect to 209.85.227.27:25

Optimized resolv.conf

nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 4.2.2.2
option rotate 
option timeout:1

option rotate - load balance the queries across the 3 listed servers
option timeout:1 - sets the lookup timeout to 1 sec (default 5s)

Reference : http://www.edwin.io/optimized-resolv-conf

http://linux.die.net/man/5/resolv.conf