Questionable SNI Support in older IE

masen, 08 February 2013


ie8 logo In my current job, I tend to take on the maintenence responsibility for a large obsolete code base which is still being used, but doesn't currently have active maintainers on staff. It's usually low impact work, a few bug fixes here and there, and people stay happy. The software in question is a PHP survey app, which is still in frequent use by various departments at the University. It's one of those pieces of software that other folks would consider to be a lot more important that we do. So naturally, when it doesn't work we hear about it.

The Issue

This week we heard about a new issue, a Security Error relating to our certificate. Previously we had encountered mixed content issues where users would include external images which were served over HTTP, so without a sample URL or an error message, this assumption was where we started. There was a twist however, the initial report implicated 'some' surveys while viewed in IE. Later we realized that the issue was affecting all requests to the server in question from IE.

With that information, we reproduced the bug using Internet Explorer 8. Despite not being able to examine the 'bad' cert until accepting it and connecting to the site, upon examination it was obvious that the server was handing back a cert for a different domain!

In December we deployed two redundant reverse proxy servers which handled proper forwarding to our backend servers, we also offloaded SSL processing to these proxies. Because our public IP options were limited, this move also meant swallowing the SNI pill and going to Named SSL Virtual Hosts on nginx. Our testing showed us that this was not an issue, however our testing was mainly limited to Firefox and Chrome, not everyone's favorite browser, IE.

It's been published that all modern browsers come with SNI support, however the compatability of IE8's implementation is a bit questionable, while the proper page for the requested vhost was returned, the server didn't know which cert to send back to the client, hence the domain mismatch error.

The Solution

Since WWU uses a wildcard certificate, we decided to use Subject Alternate names to our advantage. We minted a proxy cert which includes the names of the SSL hosts that we serve from the proxies. This provides us with a few advantages:

  • only one cert and key to manage/serve/replace
  • SNI bugs like this are no problem, the cert works for all domains
  • simplifies the configuration and deployment of additional HA nodes

There are of course the problems such as increasing the value of our private key, and making it more difficult to add a new SSL host (although not too bad).

We deployed the new cert to our staging proxy and tested against an old XP/IE8 box. After a few snags, we were able to test the configuration and confirm that with the multi-name wildcard cert (consolicert), SNI behavior was working as expected in IE8 and presumably later versions of IE!

Side note about HOSTS and Internet Explorer

In the process of testing the solution on the staging server, I needed to add a HOSTS entry to the XP system to point our problem domain to the staging IP.

So I added a line to the HOSTS file:


After flushing the DNS with ipconfig /flushdns and clearing the temporary internet files in IE, I was able to ping and it returned the override address. In Firefox, when I havigated to it made a request to the staging server. But in IE, the problem persisted, in the staging server logs, there were no requests from IE! I thought that IE must not be respecting the hosts file, I tried a few different things to no prevail.

It turns out that IE doesn't do DNS lookups if it finds an autoconfigurable proxy! That's right, if a proxy is found, the request is sent directly to the proxy to lookup/deal with. If you rely on autoconfigure for network access, then you must disable 'Automatically Detect Settings' and instead directly enter the web proxy address. Finally, under the advanced box, enter the override hosts in the box labeled "Do not use proxy server for addresses beginning with". You must also ensure that these hosts are accessible behind the proxy. I'm not sure that HOSTS overrides will work if you must use the proxy to access the overridden address!

For those of us with a direct connection and an optional proxy, simply disabling the proxy seems to cause host overrides to work!