Links: 2-23-2007

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Links: 2-5-2007

The Referer header, intranets and privacy

I’ve discussed meaningful URL’s a number of times on this site: one of the biggest benefits of a good blog URL is that you can infer who posted the article, when it was posted and what the blog post is about. For the most part this is all ‘a good thing’. But when you’re blogging on an intranet and you create a blog post that results in a URL like this:

and then in the blog post you put a couple links to your competition and embed a picture of their latest product, you’re potentially letting secrets through the firewall without evening knowing it. See, HTTP has this really nice mechanism for specifying both a) what page an image is loading in and b) what page the user was on when they clicked on a link to visit the next page. It’s called the HTTP referer and it’s commonly used for good: web statistics packages (like Google Analytics or AWStats) use the referer header to show you click paths through your site and to show you what other websites are linking to you. A typical request in an Apache HTTPD log file might look something like this: - - [06/Feb/2007:01:54:32 -0500] "GET /blogs/aaron/ HTTP/1.1" 200 34659 "" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:; .NET CLR 2.0.50727) Gecko/20061204 Firefox/"

but back to the point at hand: if you’re using blogs or wikis or anything that might produce a clean, understandable, meaningful URL and you or your company are serious about security, you’ll want to make sure that HTTP Referers are blocked because you really don’t want the president of your company breathing down your neck on a Monday morning because your competition just called… and they know. Here’s how:

  • Force anyone / everyone reading your internal site to use a Firefox plugin called RefControl, which allows you to control what gets sent in the referer field per website. Unless you’re the IT guy and you can force people to use this plugin, it’s doubtful this would work.
  • Force all of your outgoing links through what’s called a dereferer. Again, this is unwieldy, can probably be subverted and may not work for images. (you can do the same thing by modifying your Firefox config, but the plugin is easier)
  • Use HTTPS for all the pages on your intranet because RFC 2616 states that:

    Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.

    which means that even if someone does create a link to your competition’s website on the intranet, your competition won’t find out.

On a semi-related note, here are a couple things I learned from reading this article by Eric Lawrence (creator of the fine HTTP Fiddler Tool for Windows):

  • Fiddler has a really cool diff feature where you can select two sessions, right click and select WinDiff from the menu
  • somehow he’s got Firefox hooked up to Fiddler… I gotta learn how.
  • is reserved by RFC2606 specifically for the purpose of blog posts like this. Try the link. Who knew?