Using a VPN Clobbers IE 8 Performance

calendarDecember 7, 2009 in Firefox , HttpWatch , Internet Explorer

A significant change in IE 8 was the increase in the number of active connections per hostname from two to six. This allowed pages with many embedded resources (e.g. images, CSS or JavaScript) to be loaded much more quickly because more requests could be executed in parallel; reducing queuing and the amount of Blocked time seen in HttpWatch:

Six connections per host in IE 8

Recently, we noticed that IE 8 sometimes reverts back to the earlier limit of two connections per hostname. The connectivity enhancements in IE 8 are documented as requiring broadband connections:


But we were seeing the number of active connections being reduced over a fast broadband connection.

Eventually, we found that this occurred whenever a VPN connection was active. Suprisingly, the VPN connection was not actually being used by IE 8 because it did not have the default gateway flag set:


Unchecking this option ensures that only traffic for the target network goes over the VPN connection. Other traffic, such as access to public internet web sites, will go through your normal network connection even when the VPN is connected.

It appears that the dialup/broadband detection in IE 8 is too simplistic. If you are using any non-network card based connection, including VPNs or connections through high speed 3G modems, then IE 8 assumes you are using a slow dialup connection. This applies even if no network traffic from IE 8 goes through the connection.

The effect on performance can be significant. For an unoptimized site that requires lots of round-trips, page load time may be increased by 50% or more. Here’s a screen shot from HttpWatch of this blog being loaded in IE 8:


With a Windows VPN client connected the load time nearly doubles due to the change in connection limiting:

IE 8 Page Load with VPN

This is definitely something to watch out for if you are working remotely using a VPN connection into your office. Fortunately, there are some simple workarounds:

  • Use Firefox as it is not affected by connection speed. It always uses up to six connections per hostname.
  • Use a third party VPN client (e.g. OpenVPN) instead of the standard Windows VPN client. IE 8 shouldn’t detect this as a dialup connection.

Even more problems with the IE 8 mixed content warning

calendarSeptember 17, 2009 in HTTPS , HttpWatch , Internet Explorer

We have previously written about the pointless and confusing ‘Do you want to view only the webpage content that was delivered securely‘ message in IE 8. It is displayed by default when a secure web page attempts to use non-secure content such as images, javascript or CSS. That post has been so popular that it attracts 40% of the traffic to this blog.

The IE 8 mixed content dialog is pointless because 99.9% of web users just want it to go away and let them get on with what they were doing. For the 0.1% of web surfers who do care, it is confusing because of the way it is worded:

IE 8 Security Warning

The blog post described how you can disable this warning and from the comments it looks like many users are now doing this.

Even if you do this, IE still silently performs the check and hides the re-assuring padlock icon that you normally see on HTTPS pages:

No Padlock icon when mixed content present

This could be disturbing for anyone on a checkout page who is about to enter their credit card details. So if you’re web site developer you really need to avoid using mixed content – even for users who have disabled this warning. Firefox has the mixed content warning turned off by default. Let’s hope Microsoft do the same turn in the next version of IE.

You can normally fix the mixed content warning by ensuring that all the content on a secure page is served up with HTTPS. In HttpWatch you can quickly check a page by using a forced refresh to look for URLs starting with ‘http;’ :

Mixed Content in HttpWatch

However, a customer contacted us recently because they were still getting the mixed content warning even though they had no HTTP URLs on their secure page. After some investigation it was found that this commonly used javascript technique was causing the problem:

// Causes mixed content message in IE on a secure page
document.write("<script id="__ie_onload" src="javascript:void(0)"></script>");
document.getElementById("__ie_onload").onreadystatechange = function()
     if (this.readyState == "complete") domReady();

It’s a trick used to emulate a DOMContentLoaded event in IE.  A security warning occurs because of the use the “javascript:” protocol even though no download occurs.

The fix is to use //: in the src attribute in the same way as popular javascript libraries such as jQuery and prototype. This does cause a harmless ERROR_INVALID_URL entry in HttpWatch, but it avoids the mixed content message:

// Does not cause a mixed content message in IE on a secure page
document.write("<script id="__ie_onload" src="//:"></script>");
document.getElementById("__ie_onload").onreadystatechange = function()
     if (this.readyState == "complete") domReady();

Fixing the IE 8 warning – ‘Do you want to view only the webpage content that was delivered securely?’

calendarApril 23, 2009 in HTTPS , Internet Explorer

In a previous blog post, we talked about the problem of using HTTP based resources, such as images, on a secure HTTPS page. Internet Explorer interrupts the download and displays a confirmation dialog whenever it detects the use of mixed content on a secure page.

In IE 7 and ealier, this dialog would cause annoyance to users but generally didn’t cause any other significant problems. This was because it was worded in such a way that most users would click on the Yes button and allow non-secure content to be downloaded.

However, the wording in the IE 8 version of this dialog has changed:

IE8 Security Warning

To download the content a user would now have to click on the No button. As we know, most people using the web only scan text and avoid reading it if at all possible! They will usually go for the Yes button if there is not an OK button.

Some sites are going to find that their secure pages in IE 8 have the following problems:

  • Any non-secure HTTP image beacons used for analytics data gathering will often be ignored
  • The page may not display or even work correctly if it relies on non-secure images, CSS or Javascript

Therefore, avoiding mixed content on HTTPS pages is even more important now that IE 8 has been released. It often becomes an issue when using third party services such as analytics or Content Delivery Networks (CDN). For example, we avoided the use of Google hosted Ajax libraries on our site until Google added HTTPS support.

As mention in the previous blog post, an IE user you can disable this warning by:

  1. Going  to Tools->Internet Options->Security
  2. Select the Security tab
  3. Click on the Internet zone icon at the top of the tab page
  4. Click the Custom Level button
  5. In the Miscellaneous section change Display mixed content to Enable
  6. Repeat steps 1 – 5 for the Local intranet and Trusted sites zones

However, if you are developing a web site you can’t expect your visitors to do this. It is better to fix the cause of the problem so that the warning is not displayed by default in IE 8. The only way to do this warning is to ensure that your HTTPS pages only access embedded resources using the HTTPS protocol. You can do this by following these steps:

  1. Use a sniffer like HttpWatch that supports HTTPS and shows files being read from the browser cache. The free Basic Edition is sufficient for this because you only need to see the URLs being accessed.
  2. Access the page causing the problem and click No when you see the security warning dialog.
  3. Any HTTP resources shown  in the HttpWatch window are the source of the problem; even if they loaded directly from the browser cache and didn’t cause a network round trip:Mixed Content in HttpWatch
  4. If you don’t initially see any HTTP based resources, try refreshing the page because a non-secure image may have been retrieved from the IE or Firefox image cache

EDIT #1: If you are a web developer trying to track down why your page causes this warning please also take a look  at where we cover some javascript snippets that can also trigger this warning. The comments section of both of these posts also contain useful information where people have found and solved related issues.

EDIT #2: Updated instructions to apply the change to all network zones

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