Just installed ArcGIS Server 10.3.1 on a windows server with no direct access to the internet.
Testing the 'Check for Updates' tool, have set the proxy connection in the settings for the tool. Tool always fails with 'Network Connection Error'.
Using the proxy to connect to the internet from this machine has been tested successfully with IE.
In fact, when I run the tool, I can see the request for the patches.json file from downloads.esri.com in the squid(proxy server) logs, and I can use that link to dl the patches.json file with IE on the server machine.
Anyone know what else I might check to get this working?