Configure Caching DNS server on UBUNTU


By Sam Davis on Sep 10, 2007 filed under Guides, Ubuntu
From: www.zaphu.com

Step 1: Install BIND DNS server on Ubuntu

There are two ways to install BIND on Ubuntu. If you are performing a fresh installation of Ubuntu Server Edition (version 7.04 at time of writing) as per this post, at some point the install shell will ask if you wish to install a DNS and/or LAMP server. Select DNS (and LAMP if you so desire using the arrow keys and spacebar) and continue (using tab and enter). On the other hand, if you have already completed the installation of your LAMP server then use Ubuntu’s built in package management program aptitude to install BIND. Open a terminal and type

sudo aptitude update
sudo aptitude install bind9

You may need to insert the Ubuntu install CD to perform this installation.


Step 2: Configure BIND Caching DNS server

By default, BIND installs on Ubuntu configured to act as a caching DNS server. However, you need to edit the configuration options file /etc/bind/named.conf.options to specify a public DNS server operating on the wide area network (WAN) to which un-cached domain names should be forwarded. Open this file with the text editor of your choice (I use vi here).

sudo vi /etc/bind/named.conf.options

Uncomment and edit the forwarders section of this file to point to your internet service provider’s DNS server. You may enter multiple DNS server addresses (separated by semicolons) if you desire. When finished, the forwarders section should look like the following with the xxx.xxx.xxx.xxx replaced with the appropriate IP address(es).

forwarders {
xxx.xxx.xxx.xxx;
xxx.xxx.xxx.xxx;
};

You must also edit the /etc/resolv.conf configuration file of all machines on your LAN (including the DNS server itself) to point to your new DNS server. Open this file

vi /etc/resolv.conf

and add

nameserver xxx.xxx.xxx.xxx

to the top of the file where xxx.xxx.xxx.xxx is the IP address of your new DNS server. When configuring the DNS server itself, change the nameserver address to 127.0.0.1, which points to localhost. You may delete any additional nameserver lines appearing in the resolv.conf file although it may be prudent to leave lines in place that point to your ISP’s DNS server so that client machines continue to function in the event of your server going offline (just make sure your DNS server is listed first). To implement the changes to your DNS server, restart BIND.

sudo /etc/init.d/bind9 restart

Finally, test your server by typing the following command in a terminal on any machine on your LAN configured to use your new DNS server.

Near the end of the output of this command there should be a line that reads Query time: 24 ms (of course the actual time may be different). Execute the dig www.zaphu.com command again and you should notice that the query time significantly decreased indicating that your DNS server is caching DNS information for www.zaphu.com. Note that BIND caches DNS information to RAM and not disk. In most cases this will not be a problem since most machines have plenty of memory and old records are purged from memory after a period of time. However, if you expect your server to get a lot of traffic you may want to periodically flush the cache using

sudo rndc -s localhost flush

or set the maximum amount of memory to use (in essence forcing overflow data to be deleted before it expires) by setting the max-cache-size option in the configuration file.

Congratulations! you are finished setting up your Ubuntu caching name server. See my next post where I discuss configuring a master DNS server to serve hostnames to machines on your LAN.