Install Varnish in Drupal 7

Learn Installation of Varnish Cache Server for Drupal 7 in few Steps. 

1.Update Apt-get 

sudo apt-get update

2.Install Varnish 

sudo apt-get install varnish

3.Edit your Varnish Page

sudo vi /etc/default/varnish

Add this to the top of the file to start varnishd at boot:

START=yes

Then look for these settings and edit to match:

DAEMON_OPTS="-a :80 \
-T localhost:6082 \
-f /etc/varnish/default.vcl \
-S /etc/varnish/secret \
-s file,/var/lib/varnish/$INSTANCE/varnish_storage.bin,128M"

4. Now Edit ports.conf file 

sudo vi /etc/apache2/ports.conf

Update settings to match:

NameVirtualHost *:8080
Listen 8080

4. Also Edit default file

sudo vi /etc/apache2/sites-availbable/default

Update settings to match:

<VirtualHost *:8080>

5. Restart Apache2 and Varnish Servers

sudo service apache2 restart 

sudo service varnish restart 

6. Update default.vcl file 

 sudo vi  /etc/varnish/default.vcl 

backend default {
    .host = "127.0.0.1";
    .port = "8080";
    .max_connections = 250;
    .connect_timeout = 300s;
    .first_byte_timeout = 300s;
    .between_bytes_timeout = 300s;
}

5. Restart Apache2 and Varnish Servers

sudo service apache2 restart

sudo service varnish restart

6. Download Drupal Varnish Module from the below URI
https://drupal.org/project/varnish 
Add the followint to sites/default/settings.php 

$conf['cache_backends'][] = 'sites/all/modules/varnish/varnish.cache.inc';
$conf['cache_class_cache_page'] = 'VarnishCache';
$conf['reverse_proxy'] = TRUE;
$conf['page_cache_invoke_hooks'] = FALSE;
$conf['cache'] = 1;
$conf['cache_lifetime'] = 0;
$conf['page_cache_maximum_age'] = 21600;
$conf['reverse_proxy_header'] = 'HTTP_X_FORWARDED_FOR';
$conf['reverse_proxy_addresses'] = array('127.0.0.1');
$conf['omit_vary_cookie'] = TRUE;