Slack must have a persistent connection between our messaging server and members’ apps or browsers. To do so, Slack uses WebSockets over port 443.
Proxies and firewalls can sometimes interrupt this connection. If someone using Slack can’t connect from a specific location or if there’s a WebSocket failure in our Slack connection test, you’ll need to adjust your proxy or firewall to keep the connection to Slack open. Here’s how:
Visit https://my.slack.com/help/urls and whitelist all specified URLs. If your environment requires access to multiple workspaces and/or orgs, visit that page from each workspace.
Check whether your proxy is running SSL decryption. If it is, the proxy must either support WebSockets or you’ll need to exempt the following domains: *.slack-msgs.com wss-primary.slack.com wss-backup.slack.com wss-mobile.slack.com
Network administrators can restrict access to specific Slack workspaces using proxy header injection on their network. The header injection solution has the same network requirements as above.
Certain software, security settings and firewalls can interfere with Slack connections:
Avira Browser Safety
Spyware and adware
Visual Discovery by Superfish
Antivirus or ad-blocking software
Temporarily turning off your antivirus or ad-blocking software will let you troubleshoot your connection issue. If this fixes your issue, add *.slack-msgs.com wss-primary.slack.com wss-backup.slack.com wss-mobile.slack.com to your software’s exemption list before re-enabling.
Still having trouble? If you’re unable to resolve your connection issue, get in touch and include a net log if possible.