Forum Discussion
When using proxy on localhost teams fails to establish TCP connection for login
This is a strange problem, and it took us quite a while to get to the bottom of it. We tested on Windows 10 and Windows 11 with the latest Teams app. What is the problem? We are working on a...
Dinesh-MSFT
Microsoft
MicrosoftHi dakami,
To resolve the issue of Teams failing to establish a TCP connection for login when using a proxy on localhost, ensure that necessary endpoints are allowed by your firewall or proxy server. Specifically, allow traffic on TCP ports 80 and 443 for
*.microsoft.com*.office.com*.office.netlogin.microsoftonline.com
Verify that proxy settings on Windows devices do not block connections to login.microsoftonline.com and check your network configuration for any restrictive rules. Use Wireshark to analyze traffic and identify where connections are being blocked. If using a PAC file, ensure it correctly applies proxy settings without bypassing login.microsoftonline.com. Testing different proxy configurations can help isolate the issue. For further guidance, refer to Microsoft's documentation on proxy servers for Teams and Prepare your organization's network for Teams.
Dinesh-MSFTthanks for the answer! I still think it would be good for the Microsoft Teams Dev team to evaluate what's going on in teams when connecting to a proxy on local host.
What we are experiencing it not expected behavior, and it only happens on Windows when connecting to a proxy on localhost. We tested with multiple proxies and of course we made sure that the proxies are configured correctly, so that traffic is not blocked from the proxy side.
I posted a lengthy question that includes Wireshark traces on stackoverflow as well: https://stackoverflow.com/questions/78509456/windows-10-11-connection-to-localhost-proxy-fails-on-tcp-handshake
What we see is, that Teams sends different SYN packages when connecting to localhost -I don't think this is expected behaviour!
I read through Microsoft's recommendation for proxy servers - but to be frank - they are not very helpful for getting to the bottom of the specific problem we found. It would be great if we could connect to someone from the Microsoft Teams dev team to discuss our findings in depth. If there is a better place to raise this ticket please let me know.
- Dinesh-MSFT
Hi dakami,
We are mainly responsible for Microsoft Teams app development related issues.
For Microsoft Teams product issues/failures your tenant Admin can reach out to Microsoft 365 Product Support.
For general questions about Microsoft Teams please post your question on Microsoft Teams Community.
