Before we get to the specifics of what's happening, it's useful to take a step back and understand the mechanics of DNS and how it relates to your blog. Let's assume for a moment your blog is published to http://www.yourdomain.com/. There are several parts to that URL that are important for this discussion:
- yourdomain.com: The domain you registered, from your registrar.
- www: the CNAME (often referred to as the subdomain), commonly configured by default by your registrar, to point to a nameserver.
- Nameserver: the server that stores the right IP address to direct users to for the CNAME
- IP Address: the numerical address (something like 188.8.131.52) that identifies the server which hosts the content
When someone types "www.yourdomain.com" into their browser, the CNAME is passed to the nameserver, which translates the CNAME into an IP address, which then receives the request (for the homepage, for instance) and sends it back.
Important note: For this illustration, we're using the example of someone who's moving from www.yourdomain.com (hosted by someone else) to www.yourdomain.com (hosted by Blogger). If you are considering setting your blog up on a subdomain (i.e., blog.yourdomain.com), your setup will be slightly different. The purpose of this post is to explain Blogger's Missing Files Host; later posts (and the migration tool) will provide more guidance about addressing specific situations.When you move a CNAME (in this example, let's assume that you were hosting www.yourdomain.com with "Joe's webhost", and have opted to move www.yourdomain.com to Blogger), you're simply instructing the Internet to direct requests for "www" to Blogger's IP address, not Joe's Webhost.
The important thing to recognize at this point is that Joe's Webhost still has your old content, but there is no URL to request the old content. Blogger's Missing Files Host is designed to address this, by watching for requests to us that 404 (meaning we can't find them) and rewriting them to look for them in another location.
Using the above example, let's spell this out:
- Old setup: www.yourdomain.com --> 184.108.40.206 (maintained by Joe's Webhost)
- New setup: CNAME "www" to point to Blogger, which results in requests for www.yourdomain.com --> Blogger's IP address
- Create a CNAME (we'll use "files") to point to Joe's Webhost (Joe's Webhost can help you with this, our help file discussing CNAMEs is here)
- In Blogger's Custom Domain options, enable the Missing Files Host and input "http://files.yourdomain.com"
Result? You have a PDF stored on your domain today (managed by Joe) at http://www.yourdomain.com/uploads/resume.pdf. Once you point "www" to Blogger, requests for that URL will fail - we don't have that content on our servers. By enabling the Missing Files Host, requests will first go to us, and when we can't find the content in question, we'll automatically redirect the request to the backup URL, in this case: http://files.yourdomain.com/uploads/resume.pdf. This will be invisible to the end user, and will ensure that all of your old content will surface as intended.