In this tutorial we will learn about,how to install and configure Varnish Cache server on CentOS/RHEL 6.x. We will install varnish 3.0 version in CentOS 6.x . I am writing this basic tutorial to setup your first varnish in server.Here we will use rpm packages for installation.The rpm packages are available as per your operating system architecture.
Here I am using noarch for example.

Varnish is a famous web accelerator which helps to speed up your server by web caching technique.

I am setting the varnish in same server where web server is running

Download Link: http://repo.varnish-cache.org/redhat/
Note: Always select latest available stable package

My Operating System details:

Operating System:CentOS release 6.4 (Final)
arch : x86_64
IP Address: 192.168.1.22
Web Server: Apache version 2.2.15-29

To install and setup the varnish follow tthe given steps

Step 1: Download the rpm package with wget into your system (Login with root in your system)

wget http://repo.varnish-cache.org/redhat/varnish-3.0/el6/noarch/varnish-release-3.0-1.el6.noarch.rpm

Step 2: Install the varnish by using yum command

yum install varnish

Step 3: Take the backup of original default.vcl file

 cp -p /etc/varnish/default.vcl /etc/varnish/default.vcl.orig

Step 4: Bydefault varnish runs in port no. 6021
To check in which port no. it is running use netstat command. Before this start the varnish

/etc/init.d/varnish start

[root@varnish ~]# netstat -tanp|grep varnish
tcp        0      0 0.0.0.0:6081                0.0.0.0:*                   LISTEN      1640/varnishd       
tcp        0      0 127.0.0.1:6082              0.0.0.0:*                   LISTEN      1639/varnishd       
tcp        0      0 :::6081                     :::*                        LISTEN      1640/varnishd       
[root@varnish ~]#

Step 5: Because bydefault webserver runs in port no. 80 we will change this port no. so that varnish can cache the web application.And we will run the Varnish in port no. 80.
Hence there is customization of port in both Varnish and Apache

Change the port no. of Varnish to port 80

Edit the file /etc/sysconfig/varnish and search for keyword VARNISH_LISTEN_PORT=6021. Here we will change the port no. 6021 to 80.Now the value of VARNISH_LISTEN_PORT must be 80.See given below

vi /etc/sysconfig/varnish

VARNISH_LISTEN_PORT=80

Now check the default.vcl file and given below lines must be uncommented.Because we are doing practical in same server where apache is running hence we are not changing the IP address as 127.0.0.1.
Here in default.vcl file change the port no. 6021 to 8880

FAQ: Why port no. 8880 ?
Answer: Port no. 8880 is not specific one but in next step I am going to change the bydefault port no. of apache web server to 8880 hence I am using it. You can give other port no. but must be same port no. in apache config file also plus it must be above port no. 1024

vi /etc/varnish/default.vcl

backend default {
  .host = "127.0.0.1";
  .port = "8880";
}

Step 6: Change the port no. of apache to port 8880.

Note: Virtual hosting in apache is of two types Name based and IP based.You have to configure the web server as per your requirement. For eg. purpose I will use IP based virtual host and write few lines in VirtualHost tag.
If you are using Name based Virtual host then in httpd.conf file ,you have to change port no. in line having keyword VirtualNameHost

Take the backup of original httpd.conf file

cp -p /etc/httpd/conf/httpd.conf /etc/httpd/conf/httpd.conf.orig

Now change the Listening port no. 80 of apache to 8880. Find the line having keyword Listen

vi /etc/httpd/conf/httpd.conf

Listen *:8880

Now configuring our first VirtualHost
In last line add the following given lines in /etc/httpd/conf/httpd.conf file

vi /etc/httpd/conf/httpd.conf


    ServerAdmin sharad@example.com
    DocumentRoot /var/www/html/
    ErrorLog logs/sample-error.log
    CustomLog logs/sampleaccess_log common

For testing purpose I am creating a index.html file If you already have index.html file DO NOT FOLLOW this step.

echo "This is a test file for varnish" >> /var/www/html/index.html

Step 7: Now restart the Varnish and apache

/etc/init.d/varnish restart ;/etc/init.d/httpd restart;/etc/init.d/varnish restart

Now check the webpage by typing IP Address of your server in URL of web browser

http://192.168.1.22 (Replace the IP address with your web server IP Address)

From remote system run the below given command

curl -I http://ip-address-of-your-webserver

Below given reference is from my server: Hit the command 2-3 times. The Age value should not be constantly ZERO,it must be changeing always. If it is always showing zero, it means Varnish is not caching.(then again check the all configuration)
In below reference you can also see webpage is delivered by Varnish
Via: 1.1 varnish

linux@tuxworld:/tmp$ curl -I http://192.168.1.22
HTTP/1.1 200 OK
Server: Apache/2.2.15 (CentOS)
Last-Modified: Wed, 25 Sep 2013 15:59:46 GMT
ETag: "24d18-20-4e73757a7d276"
Content-Type: text/html; charset=UTF-8
Content-Length: 32
Accept-Ranges: bytes
Date: Wed, 25 Sep 2013 16:05:24 GMT
X-Varnish: 1980745874 1980745871
Age: 4
Via: 1.1 varnish
Connection: keep-alive

linux@tuxworld:/tmp$ 

Enable logging of Varnish

After doing all above configuration you may notice Varnish is not doing logging.

[root@varnish log]# ls -l /var/log/varnish/
total 0
[root@varnish log]#

To enable varnish logging start the service varnishlog and varnishncsa

starting varnishlog service

[root@varnish log]# /etc/init.d/varnishlog start
Starting varnish logging daemon:                           [  OK  ]
[root@varnish log]# ls -l /var/log/varnish/
total 4
-rw-r--r-- 1 root root 168 Sep 25 21:40 varnish.log
[root@varnish log]#

starting varnishncsa service

[root@varnish log]# /etc/init.d/varnishncsa start
Starting varnish ncsa logging daemon:                      [  OK  ]
[root@varnish log]# ls -l /var/log/varnish/
total 4
-rw-r--r-- 1 root root 1008 Sep 25 21:41 varnish.log
-rw-r--r-- 1 root root    0 Sep 25 21:41 varnishncsa.log
[root@varnish log]#