Ensure your firewall allows outbound traffic on TCP/UDP 5938 .
TeamViewer setup usually adds rules automatically. To verify or manually add: teamviewer firewall
If you provide your and network topology (e.g., “Cisco ASA behind proxy”), I can give more specific CLI or GUI steps. Ensure your firewall allows outbound traffic on TCP/UDP 5938
Whether you’re using the built-in Windows Defender or a corporate-grade hardware appliance, the logic remains the same: Whether you’re using the built-in Windows Defender or
| Mistake | Solution | |---------|----------| | Blocking all outbound except HTTP/HTTPS | Allow port 5938 TCP/UDP explicitly. | | Allowing only master.teamviewer.com | TeamViewer uses many subdomains (router, relay, cdn). Use wildcard *.teamviewer.com . | | Allowing only TCP, not UDP | UDP 5938 is required for high-performance connections. | | Configuring inbound rules when not needed | In most cases, only outbound rules are required. | | Using IP whitelists with dynamic ranges | Switch to domain-based rules or update IP list weekly. |