When you try to install plugins from the admin console, in the Plugins tab, the proxy server defined with the system properties update.proxy.host and update.proxy.port are not used and a connection is attempted to (from netstat):
jive.57274 63-246-20-125.contegix.com.80 0 0 65700 0 SYN_SENT
I would expect the system properties to be used. They are used to update the list of available plugins and auto-update.
I took a quick look in Jira and didn’'t see this as a bug.