How to solve the problem when rustdesk shows that it cannot connect to the relay server?

The immediate solution when RustDesk cannot connect to its relay server is to methodically diagnose the network path between your client and the server, as this error fundamentally indicates a failure in that communication channel. Begin by verifying the most basic elements: ensure your local machine has a functional internet connection and that no local firewall, such as Windows Defender Firewall or a third-party security suite, is blocking RustDesk's outgoing connections. You must explicitly permit both `rustdesk.exe` and `hbbs`/`hbbr` (if you are running your own server) through these firewall rules. Concurrently, confirm the correctness of the server address and ID server port (default 21116) entered in the RustDesk client settings; a single typographical error here is a common culprit. If you are relying on RustDesk's public servers, check their official status channels for any known outages, as the problem may be external. For those hosting a private relay server, the first step is to confirm the server process itself is running and that the necessary ports (21115-21119 TCP and UDP) are open not just on the server's local firewall but, critically, on any upstream network hardware like your router, which may require port forwarding rules to be correctly configured.

If basic checks do not resolve the issue, the investigation must deepen into network-level interference and configuration. Intermediate network devices are a frequent source of failure. Corporate or institutional networks often employ deep packet inspection, proxy servers, or restrictive outbound port rules that can silently drop RustDesk's traffic. Attempting a connection from a different network, such as a mobile hotspot, can quickly isolate whether the problem lies with your local network environment. For self-hosted servers, the requirement for a stable public IP address is paramount; if your internet service provider assigns a dynamic IP, it may have changed, invalidating your client settings. Utilizing a dynamic DNS (DDNS) service is the standard remedy for this. Furthermore, you must ensure your server's firewall (e.g., `ufw` on Linux or the Windows Firewall) is configured to allow traffic on the full suite of RustDesk ports, and that the server application is bound to the correct network interface (often `0.0.0.0` to accept connections on all interfaces).

Advanced troubleshooting involves systematic testing of the network path. Using command-line tools like `ping` to the server's IP address tests basic reachability, while `telnet <server_ip> 21116` tests if the specific TCP port is open and accepting connections; a failure here confirms a network or server-side blockage. For self-hosted setups, reviewing the server logs (`hbbs` and `hbbr` outputs) is indispensable, as they will often contain explicit error messages regarding failed connections or permission issues. If the server is behind a router with Network Address Translation (NAT), the port forwarding rules must be meticulously verified, and you may need to enable NAT loopback (hairpin NAT) to allow connections from within the same local network. In persistently complex environments, such as those with double NAT or restrictive ISP-grade firewalls, employing a tunneling service or setting up a VPN to bridge the networks may be the only viable mechanism to ensure a reliable relay path.

Ultimately, resolving RustDesk relay server connection failures is an exercise in network diagnostics, shifting the focus from the application itself to the infrastructure it depends upon. The implications of a failure are significant, as the relay server is essential for establishing direct peer-to-peer connections or, when a direct connection is impossible, for forwarding all session traffic. A methodical approach—progressing from client-side settings and local firewalls, through intermediate network policies, to server-side configuration and public IP management—will identify the constraint in the vast majority of cases. The process underscores that the reliability of a self-hosted RustDesk server is contingent on underlying network stability and correct administrative configuration, not merely on the software's operation.