Wednesday, February 3, 2010

Missing Files Host: what it does, why it helps

For people contemplating a switch to custom domains, one good question that's shown up repeatedly in the comments concerns files other than blog posts. These could be images, PDFs, Word docs, etc. - and if you're moving to a custom domain, you will want to think about what happens to those files.

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 There are several parts to that URL that are important for this discussion:

  • 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 that identifies the server which hosts the content

When someone types "" 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 (hosted by someone else) to (hosted by Blogger). If you are considering setting your blog up on a subdomain (i.e.,, 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 with "Joe's webhost", and have opted to move 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: --> (maintained by Joe's Webhost)
  • New setup: CNAME "www" to point to Blogger, which results in requests for --> 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 ""
Result? You have a PDF stored on your domain today (managed by Joe) at 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: This will be invisible to the end user, and will ensure that all of your old content will surface as intended.


  1. Thanks very much for this, Rick.

    One question: In my current setup, at, I have all my images at I have all of my MP3s in directories below There are other things in other subdirectories.

    The question is, if I move over to a blogger custom domain, will I be forced to pick one single location for all this content on my own hosting provider? That would be a whole lot of work, and would break a lot of existing links to my content from other sites around the web.

  2. Related to this, I can't get the CNAME to work. This is specifically for moving my blog over to a custom domain. Not under the '' but '' subdomain.

    I create the CNAME for '' pointing to and then add the four A records for the 'naked site', ''. Then, nothing. In the Blogger settings panel, Blogger tells me that my DNS setup is not correct.

    At a loss.

    It might be 0.5% of all Blogger users who use FTP, all 8 blogs I run through Blogger do.

  3. Using a subdomain is what I'm most interested in for some blogs.

  4. @Brad
    Your setup sounds like a perfect fit for the missing files host feature. You can leave all of your files where they are and add a CNAME (aka subdomain) pointed at your FTP host. For example, you could pick "". You would then enter "" as the missing files host.

    When Blogger servers see a request for the original url, say, they will send a redirect to the browser causing it to load This should be seamless and work with any file type and directory structure you have.

    Hope this helps,

  5. Will this work for new files uploaded after migration? Or only for files that were there before the move?

    ie. is this a permanent, general mapping or a set of specific mapped filenames?

    Could this solution be used for .css, .js, favicons etc?

  6. I've got quite the little quandry. I'm currently using FTP, and my URL is set like this:

    It's already like that. My question is...can I run the migration tool first, and then change the CNAME in my DNS server to point to you, and just allow for normal propagation? I BELEIVE this is the only way I can allow for no downtime on this.

  7. Same question as eDiets, as well as a couple more:

    I have a fairly photo-intensive blog, photos in every post, uploaded to a directory within my subdomain. Is this missing files host solution going to cause my pages to download more slowly?

    To be honest, I was planning to move to WordPress sometime this year because the non-blog part of my site is being converted to WP. If I go the Custom Domain/MFH route now, won't it be tougher for me down the road to move to WP? Because my files will be scattered with only your MFH to unite them? (I know this is not your problem, but this is the dilemma Blogger has put me in with only 2 months notice).

  8. I'm a little confused about how this will effect different sections of my website.

    My website is located at All of this is hosted on my own servers. As you can see, the blog aspect of the website is the website's "main" content when you arrive.

    However, I also have different sections of my website that are located in the top menu bar of the website ("Images" "Discography" "Videos" "Movies" "Tours" "Timeline" "Lyrics" etc...)

    Is there some way that I can somehow use Blogger's "custom domains" to host my blog and still have these sections of the rest of my website somewhere, all under the same domain name as it currently is?

    I'm a little confused about how that will work and how to set it up. Thanks!!

  9. This still doesn't address non-text content on blogs for backing up a google-hosted blog. How can this be done outside of wget?

  10. This is a pretty clever solution, and probably what I would use if I chose to stay with blogger. There are various other reasons I've decided not to do that, but once again, I'm impressed with the effort and thought the Blogger team has put into this migration.

  11. What about all my 'other' content? How will this help? If I remap my domain name to point to BlogSpot/Custom Domain and the user attempts to access:, how will your missing file catcher work? Will it point the user to:
    so they can run the hosted Application?

  12. @Rich Gautier - yes they will be redirected. My blog is already using the missing files host. So you can see a demo of the same by going to and you can see the redirection happening.. :)[Watch the url in the address bar]

  13. @Aneesh (and the Blogger team) - This is good news. I still have a problem in that my main blog uses .ASP files and I will have to convert to the basic blog style, but at least I'll be able to get redirection and use my main domain name for my website.

    My remaining problem is getting re-indexed at the new location by Google.......

  14. @RickKlau I went through the steps to switch from FTP to Custom Domain. I went from FTPing to to using the subdomain I did nothing with the 5 years of images that have been uploaded to my domain, but all the links are broken - none of the images are showing up now. I thought the links would not break??

  15. Does this mean that everyone who has subscribed to the RSS feed has to use a new address? - Sam Smith, Progressive Review

  16. So as to make this all clearer, can you point to some examples of sites that are using subdomains, so that the blog is inside their site: To be clearer--point me to a, which has non-blog pages on it, in which is in use for the blog portion of the site.

  17. Hi,
    I have just moved to as mentioned, using 'A' domain registries. Everything works just fine except for one thing - our flash header.

    The Flash header exists on, and the blogger template fetches it from that url. However, no "internal" (i.e. links work, only external ones.
    The first thing to do was to swap the relative urls to absolute, but that didn't help. What I found was that all external links seem to work. Either it's that, or it's the fact that those have a target=_blank
    Maybe Flash doesn't allow for calling a flash from one CNAME and use it on another, within the same domain? Or is it the fact that the sub domains exist on different ip-addresses (www:dreamhost, blog:google)?

    I'd be very grateful if anyone here could come with hints/tricks on how to solve this.

  18. Like other people my blog is a part of my site: I have lots of media and other files in the same main directory as my blog. There's no way I'm redirecting my DNS, etc to Blogger as my entire site will go offline (except for the blog of course).

    IF I decide to go with a subdomain for the blog, e.g. and move the blog there I'm assuming my main site which is hosted on my servers will not be changed at all.

    With my blog though will Google/Blogger transfer Page Rank from the old blog post URLs to the new sub domain URLs and will they forward people going to the old locations to the new post locations on the sub domain?

  19. Hi, is there a SEO penalty for this?

  20. Hi all - just noting that the FAQ is updated, addressing a number of recent questions raised on this and other posts.

  21. What happens when something 404s on the hosted blog, then 404s again on the missing files blog... does the user get the missing file domain's 404 message, or some generic Blogger 404 message? If it's a Blogger page can we customise it?

  22. @200ok: You'll get a 404 message from your remote host, which is customizable.

  23. @Rick Still haven't found an answer to my question as to why all my photo links broke. I didn't move any of my photos from the location that blogger FTPd them to. I thought they were all supposed to show up unless you moved them to use Missing File Host. Is the only fix for me to use Missing File Host?

  24. @Rick I meant to add that my site is www dot lifeonthewater dot net.

  25. @Kristen: It looks like you previously used www lifeonthewater net as your FTP blog URL, meaning that you are using the same URL now as you were before. That means that image URLs are now being sent to Blogger's hosting servers which it doesn't know what to do with ( is one example) - this is why you need to use a missing files host so that when those requests result in a 404 not found error, we can fall back to a backup address.

    If you set up something like as a secondary domain (following the instructions on this post, and/or in the help doc), then you can define that as your missing files host and then when we get requests for files like we can rewrite them to and those will go to your prior webhost.

    Hope that helps.

  26. HI, I'm gonna try this once more too. I never received an answer as to why my links in my flash header doesn't work anymore. Only the external links work, but links "internally", i.e. from (i.e. CNAME'd blogspot-site) to doesn't work at all. I've asked around on Flash-forums and they doesn't seem to know why either.
    Maybe you guys do?

  27. Note: Comments are closed on this post. Please direct general questions to the FAQ page, and specific problems with the migration tool to the issue tracker. More details about support for the FTP migration are here.