This guide is based on various community forum posts. Special thanks to Jonathan Wagener for the Ubuntu specific instructions

This guide is intended as a relatively easy step by step guide to:

• Install and configure memcached and memcache PHP extension on Ubuntu 11.04 and higher for use with Drupal and the Drupal Memcache module.
• This is a very basic single memcached server setup but a good starting point for getting memcached working. 

Requirements:

• Ubuntu 11.04 or later installed on your machine. 
• PHP 5.2.0 or higher installed

1. Install memcached on your server.

• Open the Terminal Window and enter :

sudo apt-get install memcached libmemcached-tools

2. Install memcache PHP extension using PECL.

• PECL is great for installing PHP extensions.

sudo apt-get install php5-dev php-pear make

• After you have installed PECL on your system, open the Terminal Window and enter :

sudo pecl install memcache

3. Add memcache.so to php.ini

• We must instruct PHP to load the extension.
• You can do this by adding a file named memcache.ini to the configuration directory /etc/php5/conf.d
• Open the Terminal Window and enter :

sudo nano /etc/php5/conf.d/memcache.ini

• Add the following line to the file and save :

extension=memcache.so

• If you intend to use memcached with Drupal also add the following line to your php.ini or memcache.ini file and save :

memcache.hash_strategy="consistent"

4. Open firewall port 11211.

• The default port for the memcached server is TCP port 11211.
• Configure your firewall to open port 11211 for TCP traffic.

5. Configure the memcached allowed memory.

• All memcached configuration settings can be found in /etc/memcached.conf
• The default memory setting for memcached is 64 MB. 
• Depending on the amount of RAM available on the server allocate a block of memory to memcached.
• Open the Terminal Window and enter :

sudo nano /etc/memcached.conf

• Change the following line FROM-

# Start with a cap of 64 megs of memory. It's reasonable, and the daemon default
# Note that the daemon will grow to this size, but does not start out holding this much
# memory
-m 64

• TO the following by changing the -m 64 to -m 4096 to allow memcached 4 GB of RAM. Adjust the size in MB according to the memory that you have available. Save the file when done.

# Start with a cap of 64 megs of memory. It's reasonable, and the daemon default
# Note that the daemon will grow to this size, but does not start out holding this much
# memory
-m 4096

6. Start the memcached service.

• Open the Terminal Window and enter :

sudo service memcached start

• OR on older systems :

sudo /etc/init.d/memcached start

7. Restart Apache.

• Open the Terminal Window and enter :

sudo service apache2 restart

• OR on older systems :

sudo /etc/init.d/apache2 restart

8. Check to see if memcached server is active and listening on port 11211.

• Open the Terminal Window and enter :

netstat -tap | grep memcached

9. Check the status and stats with memstat tool

• Part of the memcached package is a handy tool called memstat.
• You need to specify the host IP and port. In this case the host IP is 127.0.0.1 and the port 1211.
• Open the Terminal Window and enter :

memstat 127.0.0.1:11211

10. Activate the Drupal memcached module.

• Install the Drupal Memcache module and activate. For more complete instructions visit the Drupal Memcache Documentation
• Edit settings.php in your Drupal installation to include memcache.inc
• For Drupal 6, edit the settings.php file and add the following :

$conf['cache_inc'] ='sites/all/modules/memcache/memcache.inc';

• For Drupal 7, edit the settings.php file and add the following :

$conf['cache_backends'][] = 'sites/all/modules/memcache/memcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['memcache_key_prefix'] = 'something_unique';

* note : Replace the "something_unique" in the last line with your own unique memcache key prefix. The memcache_key_prefix is also needed for both Drupal 6 & 7 in a multi-site environment if you would like to use memcached for more than one Drupal installation on the same server.