Knowledgebase: Migrations
Using Host File For Migration Testing
Posted by Lee Blakely on 09 December 2006 08:34 PM
A common problem is how to test a website that has been installed on a new server when the domain name for the site is still pointing to the old server. An easy solution to this problem is to use a "hosts" file to force your web browser to go to the new server instead of the old server.

What is the "hosts" file?

The short answer is that the Hosts file is like an address book. When you type an address like www.yahoo.com into your browser, the Hosts file is consulted to see if you have the IP address, or "telephone number," for that site. If you do, then your computer will "call it" and the site will open. If not, your computer will ask your ISP's (internet service provider) computer for the phone number before it can "call" that site. Most of the time, you do not have addresses in your "address book," because you have not put any there. Therefore, most of the time your computer asks for the IP address from your ISP to find sites.

Here's a longer and more technical description:

The "Hosts" file in Windows and other operating systems is used to associate host names with IP addresses. Host names are the www.yahoo.com addresses that you see every day. IP addresses are numbers that mean the same thing as the www words - the computers use the numbers to actually find the sites, but we have words like www.yahoo.com so humans do not need to remember the long strings of numbers when they want to visit a site.

For instance, the host name for Yahoo! is www.yahoo.com, while its IP address is 204.71.200.67 Either address will take you to Yahoo!'s site, but the www address will first have to be translated into the IP address. If you type in the IP address directly, your computer will not have to look it up.

A series of steps are used when searching for IP addresses that go with these host names. The first step, and the one that concerns us here, is the hosts file on your local computer. The Hosts file tells your computer what the name is in numbers so the computer can go find it. If the IP address is found in your Hosts file, the computer will stop looking and go to that site, but if it is not it will ask a DNS computer (domain name server) for the information.

Where is the "hosts" file located?

On Linux/Unix/Mac:

/etc/hosts

On Windows:

Windows 95/98/Me c:\windows\hosts

Windows NT/2000/XP Pro c:\winnt\system32\drivers\etc\hosts

Windows XP Home c:\windows\system32\drivers\etc\hosts

NOTE: Hosts is the name of the hosts file and not another directory name. It does not have an extension (extensions are the .exe, .txt, .doc, etc. endings to filenames).

NOTE: In Windows machines you may not already have a hosts file. If this is the case there will most likely be a sample hosts file called hosts.sam that you can rename to hosts and use as you wish.

How can I change my "hosts" file?

Open the file with a text editor like Notepad.

The HOSTS file is a text file that contains IP addresses separated by at least once space and then a domain name, with each entry on its own line. For example, imagine that we wanted to make it so that if you typed in www.google.com, instead of going to Google we would go to www.yahoo.com. In order to do this you would need to find out one of the IP addresses of Yahoo and map www.google.com to that IP address.

One of the IP addresses for Yahoo is 216.109.118.69. If we wanted to map Google to that IP address we would add an entry into our HOSTS file as follows:

216.109.118.69 www.google.com

NOTE: When inputting entries in the hosts file there must be at least one space between the IP address and the domain name. You should not use any web notations such as \, /, or http://. You can disable a specific entry by putting a # sign in front of it.

You may be wondering why this would work as we said previously that when you need to resolve a domain name to an IP address the device will use its configured DNS servers. Normally this is true, but on most operating system the default configuration is that any mappings contained in the Hosts file overrides any information that would be retrieved from a DNS server. In fact, if there is a mapping for a domain name in a hosts file, then your computer will not even bother querying the DNS servers that are authoritative for that domain, but instead read the IP address directly from the HOSTS file. It is also important to note that when you add entries to your HOSTS file they automatically start working. There is no need to reboot or enter another command to start using the entries in the HOSTS file.

How do I use the "hosts" file for testing?

Since we can use the hosts file to redirect a domain to a different IP address, it's a perfect tool for testing website migrations.

Let's assume that your domain is "acme.com" and you are moving to a new server where the IP address will be 192.168.10.115.

1. Open your "hosts" file in Notepad or other text editor.

2. Add the following lines to the file:

192.168.10.115 www.acme.com
192.168.10.115 acme.com

3. Save the changes.

4. Open your web browser and enter http://www.acme.com.

You'll get the site on the new server instead of the old server.

NOTE: Don forget to remove the hosts file entries once you're finished testing.
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