Four Tips for Setting up HTTP File Downloads

calendarMarch 24, 2010 in Firefox , HTTP , HTTPS , HttpWatch , Internet Explorer , Javascript , Optimization

Web sites don’t just contain pages; sometimes you need to provide files that users can download. Putting a file on your web server and linking to it from an HTML page is just the first step. You also need to be aware of the HTTP response headers that affect file downloads.

These four tips cover some of the issues you may run into:

Tip #1: Forcing a Download and Controlling the File Name

Providing a download link in the HTML is easy:

<a href="">Download</a>

It works well for binary files like setup programs and ZIP archives that the browser doesn’t know how to display. A dialog is displayed allowing the user to save the file locally:

IE File Save Dialog

The trouble is that the browser behaves differently if the file is something that it can display itself. For example, if you link to a plain text file the browser just opens it and doesn’t prompt to save the download:

Plain Text in IE

You can force the use of the file download dialog by adding the following response header:

Content-Disposition: attachment; filename=<file name.ext>

The header also allows you to control the default file name. This can be handy if you’re generating the content in something like getfile.aspx but you want to supply a more meaningful file name to the user.

For static content you can manually configure the additional header in your web server. For example, here’s the setting in IIS:


For dynamically generated content you would need to add this header in the page’s server side code.

After adding the header, the browser will always prompt the user to download the file:


Tip #2: Use Effective HTTP Caching

Like any other content, it’s worth setting up HTTP caching to maximize the speed of download and minimize your bandwidth costs. Usually content needs to expire immediately or be cached forever.

Our example download of the HTTP spec (RFC2616) could be cached forever because it is not expected to change. You can see here in HttpWatch we have set up a far futures Expires value and set Cache-Control to public :


This allows future downloads of the file to be delivered from the local browser cache or an intermediate proxy. If the file is subject to frequent changes, you may want to expire it immediately so that a fresh copy is always downloaded. You can do this by setting Expires to -1 or any date in the past.

Tip #3: Don’t break HTTPS downloads in IE

It’s tempting to use the no-store and no-cache directives with the Cache-Control response header to prevent any caching of a file that is often updated:

Cache-Control: no-store, no-cache

This works in Firefox, but watch out for Internet Explorer. It interprets these flags as meaning that the content should never be saved to the disk when HTTPS is being used and causes the file download dialog to hang at 0% for several minutes:


It eventually displays an error message:


There’s more information about this problem and other possible causes in a post on Eric Lawrence’s IEInternals blog.

Tip #4: Don’t Forget to Setup Analytics

You’ll probably want to track file downloads along with other metrics from your web site. Javascript based solutions such as Google Analytics are very popular, but will not show file downloads by default. This is because downloading a file does not cause any Javascript to be executed.

With Google Analytics you need to add an onlick handler to enable download tracking:

<a onclick="pageTracker._trackPageview('/httpwatch.exe');" href="...">Download</a>

You can see the Google Analytics call being made just before the file download starts:


Using Firefox, HttpWatch and Internet Explorer on Vista 64-bit

calendarMay 9, 2009 in Automation , C# , Firefox , HttpWatch , Internet Explorer , Javascript

The 64-bit version of Windows Vista is becoming increasing popular due to its ability to access more than 4 GB of physical memory. To an ordinary Windows user, there are few noticeable differences from the 32-bit version because it can run both 32-bit and 64-bit programs.

In this post we wanted to answer some of the questions you may have about Windows Vista 64-bit and the effect it has on the use of Firefox, HttpWatch and Internet Explorer (IE).

How can I tell if a program is running in 32-bit or 64-bit mode?

The Task Manager on Vista 64-bit shows 32-bit tasks with a ‘*32′ after the process name. If there’s no ‘*32′ after the name then it is running in 64-bit mode. This screen shot shows a 32-bit process called page_check running on Vista 64-bit:

32-bit Program Running on Vista 64-bit

Why does IE run in 32-bit mode by default on Vista 64-bit?

Clicking on the launch bar IE icon in Vista 64-bit starts the 32-bit version of IE:

32-bit version of IE on Vista 64-bit

A 64-bit version of IE is available but you have to explicitly select it from the Start menu:

64-bit version of IE

Microsoft has done this because the 64-bit version of IE can only load 64-bit browser extensions and most  extensions, such as the Adobe Flash player, are only available in 32-bit.

Here’s what Adobe says about 64-bit support:

Adobe Flash Player is not supported for playback in a 64-bit browser. However, you can run Flash Player in a 32-bit browser running on a 64-bit operating system.

Adobe is working on Flash Player support for 64-bit platforms as part of our ongoing commitment to the cross-platform compatibility of Flash Player. We expect to provide native support for 64-bit platforms in an upcoming release of Flash Player following Flash Player 10.

Without the Flash player, you can’t use sites like YouTube or BBC iPlayer. That’s a non-starter for many web users and the reason why 32-bit IE is the default on Vista 64-bit.

Microsoft is also putting off 64-bit support in Silverlight until other 64-bit extensions are available:

Right now our plan is to run SL in 32-bit mode (and not have a 64-bit native version). This is mostly because other browser plug-ins (and most browsers) don’t support 64-bit yet. We are looking at adding native 64-bit support in the future though.

Can I run Firefox in 64-bit mode on Vista 64?

There’s currently no official build of Firefox for 64-bit Windows as stated in a Mozilla support forum:

There are no releases or nightly builds of 64 bit Firefox for Windows from Mozilla as the earliest release to likely have 64-bit versions is Firefox 4.0. The only option you have is either roll your own or use a third-party build.

There’s little motivation for the browser vendors to go 64-bit because unlike other applications, such as memory hungry video editors or database servers, web browsers will not see significant performance benefits in 64-bit mode and there is also the problem with the lack of 64-bit extensions.

Can I use HttpWatch on Vista 64-bit?

Yes, you can use HttpWatch on Vista 64-bit but it is currently only available in 32-bit. This means that the HttpWatch plug-in can only be used in the 32-bit edition of IE. We will provide 64-bit plug-ins when Adobe, Microsoft and Mozilla provide significant 64-bit support.

The automation interface can be used from 64-bit programs without making any changes so as long as you have HttpWatch version 6.1.32 or later.

In some cases, such as accessing a .hwl file opened with the OpenLog method, you may see reduced performance in a 64-bit program. This is because the 64-bit API support in HttpWatch uses a separate WIN32 process rather than directly loading the API. To avoid this issue make sure that your client program runs 32-bit (see below).

Can I use the HttpWatch version 5.x automation on Vista 64-bit?

This will work, but you will need to ensure that your program runs in 32-bit mode. Otherwise, you will see this error when you attempt to create an instance of the HttpWatch controller object:

Unhandled Exception: System.Runtime.InteropServices.COMException (0x80040154): Retrieving
the COM class factory for component with CLSID {C4CEDB78-2B64-4703-99BE-A037A849D703}
failed due to the following error: 80040154.

If you’re seeing this error with HttpWatch version 6 make sure that you’ve installed version 6.1.32 or later.

How can I get my HttpWatch automation program to run 32-bit?

As mentioned above, you will need to ensure that a program using the HttpWatch automation API runs in 32-bit mode if:

  • You are using HttpWatch version 5.x
  • You want to improve data access performance on log files that are opened using the OpenLog method.

.Net projects (e.g. C# or VB.Net) have a default configuration setting of ‘Any CPU’. This causes them to automatically run as a  64-bit process on Vista 64-bit. To force them to always run 32-bit you’ll need to change this setting to ‘x86′ in Visual Studio:


Script based programs, such as Javascript or VBScript, that use the Windows Scripting Host will normally run in 64-bit on Vista 64-bit because cscript.exe and wscript.exe are  64-bit programs. You can force the use of 32-bit by using the script programs that are installed in the Windows\SysWow64 directory:


The sample automation code that is installed with HttpWatch version 6.1.32 or later, uses the ‘x86’ setting  in C# projects and includes instructions on how to run the Javascript samples in 32-bit mode.

Google Adds HTTPS Support to the Ajax Libraries API

calendarNovember 27, 2008 in HTTP , HTTPS , HttpWatch , Javascript , Optimization

Our blog post ‘Test Drive of the Google Hosted Ajax Libraries‘ looked at using Google’s CDN (Content Delivery Network) to serve up commonly used libraries such as jQuery and Prototype.

One significant advantage, that we found, was that downloading a library from a different hostname (i.e. helps to avoid the HTTP connection limiting behavior that causes blocking in browsers:

There are other significant advantages to using Google hosted libraries:

  • The libraries are hosted on Google’s high speed global network providing fast access from most locations world wide
  • HTTP compression minimizes the size of the download
  • Minimized versions of the each library are available to further reduce download size
  • The library that your site uses may already be in the user’s browser cache if the user has visited another site that uses the Google hosted libraries
  • You can specify which version of a library should be used with a hard coded URL or allow for automatic version upgrades using the google.load() function
  • Google picks up the bandwidth bill for the hosted Javascript libraries

However, there was one issue that stopped us using the Google CDN. Its lack of HTTPS support caused this warning to be generated on secure pages:

The good news is that Google has added SSL support and we were able to change the script tag on our Ajax Demo page to:

<script type="text/javascript"

So, you can now have all the performance benefits of the Google CDN without the userability issues on secure pages.

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