DNS Settings on Linux

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Manual DNS

There may be times you need to change your server's DNS settings, either because you find they're misconfigured or because you want to use your own. Fortunately there isn't a lot of work involved in changing the DNS servers, just a quick edit in the right place.

resolv.conf

On Linux the DNS servers the system uses for name resolution are defined in the file:

/etc/resolv.conf

It's spelled just like that, with no "e" at the end of "resolv".
In that file we'll want to have at least one "nameserver" line (two is better, so we have a fallback). Each line defines a DNS server.
The name servers will be prioritized in the order the system finds them in the file. Use the IP addresses of the name servers when entering them, since the system won't know what to do with domain names until after it knows how to get to the DNS servers.
Open resolv.conf with an editor like nano to make the necessary changes (if it doesn't exist already this will create the file for us):

sudo nano /etc/resolv.conf

Rackspace Cloud Hong Kong

If your Cloud Server is in the Hong Kong (HKG) datacenter you should use:

nameserver 120.136.32.63
nameserver 120.136.32.62

Rackspace Cloud UK

If you're on Rackspace Cloud UK you can use our closest DNS servers by making the contents of the /etc/resolv.conf file read:

nameserver 83.138.151.80
nameserver 83.138.151.81

Rackspace Cloud USA - ORD

If your Cloud Server is in the US Chicago (ORD) datacenter you should use:

nameserver 173.203.4.8
nameserver 173.203.4.9

Rackspace Cloud USA - DFW

If your Cloud Server is in the US Dallas/Fort Worth (DFW) datacenter you should use:

nameserver 72.3.128.240
nameserver 72.3.128.241

Rackspace Cloud AUS - SYD

If your Cloud Server is in the Sydney, Australia datacenter you should use:

nameserver 119.9.60.63
nameserver 119.9.60.62

A quick test

Once you have your DNS servers set, save the file. And you're done. No reboot required.
The easiest way to make sure your new settings are good ones is to try to ping a domain name:

ping -c 3 rackspace.com

You should see a result like:

PING rackspace.com (173.203.44.122) 56(84) bytes of data.
64 bytes from 173.203.44.122: icmp_req=1 ttl=249 time=25.3 ms
64 bytes from 173.203.44.122: icmp_req=2 ttl=249 time=25.2 ms
64 bytes from 173.203.44.122: icmp_req=3 ttl=249 time=25.2 ms
 
--- rackspace.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 25.236/25.292/25.392/0.147 ms

If you get an "unknown host" message back you should double-check the IP addresses you set as your DNS servers.

IPv6

If you're using IPv6 on your server you may need to add the IPv6 addresses of your name servers to resolv.conf.  You can see if a DNS server has an IPv6 address with two steps.
First, use "host" to get the name of the server:

$ host 72.3.128.240
240.128.3.72.in-addr.arpa domain name pointer cachens1.dfw1.rackspace.com.

Then use the domain name you got back in another "host" lookup:

$ host cachens1.dfw1.rackspace.com
cachens1.dfw1.rackspace.com has address 72.3.128.240
cachens1.dfw1.rackspace.com has IPv6 address 2001:4800:d::1

If an IPv6 address is returned you can add that as another "nameserver" line in resolv.conf, as in:

nameserver 2001:4800:d::1

Then test as above, using the "ping6" command instead of the regular "ping" command to force the system to use IPv6.

Summary

It's that simple - change or add those name servers and once the settings are saved the system will use them right away. Make sure the changes take with a quick ping to test it and you'll be done.