417 when accessing Visual Studio Extensions Gallery

If, while working behind a proxy, you click Tools > Extensions and Updates ... in VS 2012 and then select 'Online' on the left hand side you may get a 417 Error - this is caused by the proxy server not handling the 100 Continue response properly, luckily you can configure Visual Studio to not try the header-only check and get around the problem.

1. Find the VS config file (devenv.exe.config) - the simplest way is to use Task Manager (Open File Location on the process context menu)
2. Locate the section that configures network settings:
   

     <settings>
       <ipv6 enabled="true">
     </settings>
   

3. Add a line telling VS to skip the header-only check
   

     <settings>
       <ipv6 enabled="true">
       <serviceManager expect100Continue="false"/>
     </settings>
   

Then you should be up and running again