Changing the DNS server on a RT-AC66U / RT-N66U

dnsmasq.conf

# Add these, though they didn't seem to take effect for me.
server=8.8.8.8
server=8.8.4.4

resolv.conf

# nameserver 75.75.75.75
# nameserver 75.75.76.76
nameserver 8.8.8.8
nameserver 8.8.4.4

router-dnsmasq-guide.md

Router DNS settings

It appears that /etc/ and /tmp/etc/ are the same.

# Connect to the router.
telnet 192.168.1.1
# Edit as needed.
vi /tmp/etc/dnsmasq.conf
vi /tmp/etc/resolv.conf
# Restart dnsmasq.
killall dnsmasq && dnsmasq --log-async --no-poll
# Verify that the reported "Server" is the expected DNS server.
nslookup www.google.com

These links were helpful:

• http://www.snbforums.com/threads/adding-unblock-us-dns-using-dnsmasq.8266/#post-49996
• https://wiki.archlinux.org/index.php/Dnsmasq

you should use ASUSWRT-Merlin firmware..

You can also use https://github.com/jacklul/asuswrt-scripts with the stock firmware. I'm using the scripts in that repo to replicate all the functionality needed from ASUSWRT-Merlin including dnsmasq manipulation and allowing config to survive reboots. I needed to do this because there is no ASUSWRT-Merlin version for the RT-AX89X afaik.