I am trying to use the Slack desktop client for Ubuntu 16.04 behind a Websense Content Gateway (WCG) proxy. It will not connect. Notes:
- Windows workstations work with Slack client for Windows through the WCG proxy.
- Browser-based client works on Ubuntu 16.04, including the test site (https://slack.com/help/test), using Firefox
- SSL Decryption (purposeful MitM) is enabled on WCG but disabled for all slack sites. (*.slack.com *.slack-msgs.com *slack-files.com *slack-imgs.com *slack-edge.com *slack-core.com *slack-redir.net)
- All slack sites (listed above) are whitelisted in WCG and showing permitted in WCG console/logs.
- proxy enabled in Unity network settings, set profile (e.g. https_proxy), and in Firefox browser config.
- netstat -a shows all outbound connections heading to the proxy server and no direct attempts (although the network firewall is not blocking https anyway).
Notable error in Slack logs is:
warn: WebSocket connection to 'wss://mpmulti-s3to.slack-msgs.com/?lots_of_redacted_parameters' failed: WebSocket is closed before the connection is established.