Friday, 24 February 2012

Understanding Virtual Host Concept in Tomcat

Hi in this post we will see how to setup virtual host in Apache Tomcat server. Virtual Host is in-built feature that allows to deploy multiple website(domains) in single instance of tomcat server. The main benefit in this way is its cost effective.


I am going to deploy 3 website with following domain names in single tomcat

The following diagram is my outline.
Outline structure of Virtual Host Concept in Tomcat
Here my tomcat IP address or any IP address allocated my ISP. but it should be public IP address.

How all domain names are pointing to my Tomcat?
                  When we purchase the domain name we need to update the our tomcat IP address to it. like

or we can simulate same DNS Setup through hosts file in both Linux and Windows. In Linux tha file is located at /etc/hosts

Now How Setup Virtual Host Concept?

Before going to setup the virtual host. first take look at the server.xml file in conf folder in tomcat directory.





here <Engine> tag specified which engine is responsible for executing servlet. Here Catalina is the Engine.
<Host> tag  specify the domain name and web apps base location. here default domain name is localhost and web apps base location is webapps folder in tomcat directory. here name attribute to specify the domain name and appbase attribute to specify the location of domain specific web apps folder path.

 Now we need to add more <Host> tags to represent to our domains

<Host name="" appbase="ramki_webapps" />
<Host name="" appbase="krishnan_webapps" />
<Host name="" appbase="blog_webapps" />

Then we need to copy the webapps folder in tomcat and paste it anywhere and rename it to ramki_webapps, krishnan_webapps, blog_webapps and update the path in <Host> tag

Modifies server.xml file


Simulate the DNS
Open the /etc/hosts file through root privilege and add following entry

deploy the websites to respective web apps folder and start the tomcat.

now open the browser and type then its shows the ramk website content. Other two sites and works respective webapps.

In above diagram represent when we access the tomcat server consult with server.xml file and serves the files from ramki_webapps directory.

How is Virtual Host Works
       Here big question all websites are pointed to same tomcat. How tomcat can distinguished the request. (i.e) how tomcat knows browser requested or

The answer is based Host header field in HTTP request.
when we accssed then browser make HTTP request. and the request look like this

GET / HTTP/1.1 
Proxy-Connection: keep-alive 
User-Agent: Mozilla/5.0 (Windows NT 6.2) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 
Accept-Encoding: gzip,deflate,sdch 
Accept-Language: en-US,en;q=0.8

here Host Field contain domain name

when tomcat receive the request from any browser, it read the Host field and understand which domain we requested, then consult the server.xml file and delegate to appropriate Host process thread

check my screen cast for setup

Comments are welcomed


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