What causes "login session server is down, try get new server." in XX-net/XX-Net?

Let the client retry — start() will resolve a new server via g.server_host = None and re-login

login session server is down, try get new server.

Error message

login session server is down, try get new server.

What it means

login_session received HTTP 521 from the X-Tunnel session API. 521 is Cloudflare's 'Web server is down' status, meaning the origin session server is offline. The code records g.last_api_error, clears g.server_host so a new server will be resolved, and returns False so the caller (start) retries with a different host.

Source

Thrown at code/default/x_tunnel/local/proxy_session.py:423

                                               self.session_id,
                                               g.config.max_payload, g.config.send_delay, g.config.windows_size,
                                               int(g.config.windows_ack), g.config.resend_timeout, g.config.ack_delay)
                upload_data_head += struct.pack("<H", len(g.config.login_account)) + utils.to_bytes(g.config.login_account)
                upload_data_head += struct.pack("<H", len(g.config.login_password)) + utils.to_bytes(g.config.login_password)
                extra_info = self.get_login_extra_info()
                upload_data_head += struct.pack("<H", len(extra_info)) + utils.to_bytes(extra_info)

                upload_post_data = encrypt_data(upload_data_head)

                content, status, response = g.http_client.request(method="POST", host=g.server_host, path="/data",
                                                                  data=upload_post_data,
                                                                  timeout=g.config.network_timeout)

                time_cost = time.time() - start_time

                if status == 521:
                    g.last_api_error = "session server is down."
                    xlog.warn("login session server is down, try get new server.")
                    g.server_host = None
                    return False

                if status != 200:
                    g.last_api_error = "session server login fail:%r" % status
                    xlog.warn("login session fail, status:%r", status)
                    continue

                if len(content) < 6:
                    g.last_api_error = "session server protocol fail, login res len:%d" % len(content)
                    xlog.error("login data len:%d fail", len(content))
                    continue

                info = decrypt_data(content)
                magic, protocol_version, pack_type, res, message_len = struct.unpack("<cBBBH", info[:6])
                message = info[6:]
                if isinstance(message, memoryview):
                    message = message.tobytes()

View on GitHub (pinned to cfa5bc17b6)

Solutions

Let the client retry — start() will resolve a new server via g.server_host = None and re-login
Check X-Tunnel status pages/community for known outages
If persistent, verify DNS/Cloudflare reachability from the client network; adjust available server list

Defensive patterns

Strategy: retry

Validate before calling

# rely on client rotation: g.server_host = None triggers new server resolution before next login

Try / catch

while not session.start():
    time.sleep(backoff); backoff = min(backoff*2, 300)  # rotates server each failure

Prevention

Let the client rotate session servers on 521 instead of pinning one host
Add backoff to avoid hammering a down server

When it happens

Trigger: POST to the login/session API returns 521 — the currently selected session server behind Cloudflare is unreachable.

Common situations: Transient outage of the session server, server blocked/taken down, regional Cloudflare issues; commonly resolves by rotating to another server host.

Related errors

AI-assisted analysis of XX-net/XX-Net@cfa5bc17b6 (2026-08-27). Data as JSON: /api/errors/bab41bbe42f6b724. Report an issue: GitHub.