Multi-Homing

Multi-Homing is a method for distributing the load of your web-site across multiple servers using a technique called a DNS Round Robin.

• Cost Effective Load Distribution
  For high traffic Web sites, a DNS Round Robin cycles through a list of IP addresses associated with the domain name of your web site. Thus, each DNS lookup on your domain name sends the request to a different IP in your DNS zone file. This distributes the requests for your Web site to the different servers hosting your site, decreasing the number of requests that come to a single Virtual Private Server.

• Redundant Backup and Access
  By mirroring your Web site on two different machines, it decreases the already small chance that your site content will be lost if one of your servers experiences a catastrophic failure. In addition, if one of your Virtual Private Servers does go down, the mirrored server will decrease the effect of downtime significantly.

Configuration

To configure your web-site for Multi-Homing, follow these steps.

1. Prepare Additional Servers
   If necessary, order a new server to serve as the second (or third, etc.) host for the site. You will want to set the server hostname to be different from the primary host. You might consider using a scheme such as host1, host2, etc., or you can give each host server a distinctly different name. It is very important that you are able to distinguish between the separate hosts in the case of a problem.

   On the second host, configure the apache server to handle the domain name of the web-site. In addition to the primary domain name, you should configure a specific cname to access this specific server. For example, you may want to configure the server to use the domain host2.YOUR-DOMAIN.NAME. (If you are configuring the site as a Virtual Subhost you will need to add the cname domain to the opening VirtualHost tag. Otherwise, you only need to create the DNS record for the cname.)

   You will only want the primary server to handle email for the domain. In order to make sure mail is handled properly, make sure that the domain is not listed in the ~/etc/sendmail.cf or ~/etc/local-host-names files.

2. Create a CNAME for the secondary server
   In order to access your primary and secondary servers specifically, you should create a DNS record pointing specifically at each server. To do this, create an A record for each separate host similar to the following in the zone file for the site's domain name.

   host1   IN  A  1.1.1.1
   host2   IN  A  2.2.2.2
   

3. Copy Content to the New Server
   Once the secondary server is properly configured, SSH or Telnet to the primary server, and cd to the DocumentRoot of the web site. In the following example, we will assume this is ~/usr/local/etc/httpd/htdocs/. You will need to adapt the commands to your specific configuration.

   % cd ~/usr/local/etc/httpd/htdocs

   The easiest way to transfer the files is to create a single, compressed, tar file which you can copy over in one move. To do this, use the following commands (remember to adapt the commands to your specific configuration).

   % cd ..
   % tar -cvf mysite.tar htdocs
   

   This will create a file named mysite.tar in your ~/usr/local/etc/httpd/ directory. You now need to copy the content to the second server. In this example, username is the Virtual Private Server user account the second server. (this command should all go on one line when you type it in).

   % scp mysite.tar username@host2.YOUR-DOMAIN.NAME:/usr/local/etc/httpd/mysite.tar

   You will be prompted to enter the password for username, and then the file will be transfered to the new server.

   Finally, connect via SSH or Telnet to the secondary server, and cd to the ~/usr/local/etc/httpd directory. If you type ls, you should find the mysite.tar file. You will then need to extract the web site files from the tarfile. You can use the following command to do this.

   tar -xvf mysite.tar

   The wget program is also an excellent tool for mirroring your site.

4. Test the New Site
   Go to the new site in your favorite web browser using the specific CNAME you configured earlier and check to see that everything loads properly.

5. Go Live
   Once you are certain that the secondary server is running properly, you can set up the DNS to do round robbining. To do this, create an additional A record in your DNS Zone file for the main domain name, and point it at your secondary server. You should now have two A records for your domain, something like this:

   @  IN  A  1.1.1.1
   @  IN  A  2.2.2.2
   

Please note: the information on this page applies to ITS web hosting plans. It may or may not apply to other environments. If you are looking for a feature described here, or better support from your hosting provider, please consider hosting your site with ITS!

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