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.
- Find the VS config file (devenv.exe.config) - the simplest way is to use Task Manager (Open File Location on the process context menu)
- Locate the section that configures network settings:
<settings> <ipv6 enabled="true"> </settings>
- Add a line telling VS to skip the header-only check
<settings> <ipv6 enabled="true"> <serviceManager expect100Continue="false"/> </settings>