What causes "write connection data err: %s ,retrying..." in snail007/goproxy?

Confirm the client key (cfg.Key) matches the server's key, since the server may drop conns on bad keys

write connection data err

Error message

write connection data err: %s ,retrying...

What it means

After GetConn succeeds, GetInConn writes a handshake packet (type, key length, key) to the new connection. This error is thrown when that write fails; the connection is closed immediately so the caller can retry, as the message says.

Solutions

Confirm the client key (cfg.Key) matches the server's key, since the server may drop conns on bad keys
Retry the operation — the code already closes the conn and the message indicates retrying; add backoff in your wrapper
Check server-side logs at the same timestamp for why the server closed the accepted connection
Stabilize the network path (keep-alives, TCP keepalive settings) if running over flaky links

Example fix

// before: fire-and-forget single attempt
conn, err := client.GetInConn(typ)
// after: retry with backoff
var conn net.Conn
for i := 0; i < 3; i++ {
    conn, err = client.GetInConn(typ)
    if err == nil { break }
    time.Sleep(time.Duration(1<<i) * time.Second)
}

Defensive patterns

Strategy: retry

Validate before calling

// verify key length is sane and matches server expectation before writing
if len(cfg.Key) == 0 || len(cfg.Key) > 65535 {
    return errors.New("tunnel key must be non-empty and <64KB")
}

Try / catch

conn, err := client.GetInConn(typ)
if err != nil && strings.Contains(err.Error(), "write connection data err") {
    time.Sleep(backoff) // conn already closed by lib; just retry
    conn, err = client.GetInConn(typ)
}

Prevention

Match the client key exactly with the server key config
Enable TCP keepalive and retry with exponential backoff on flaky links
Watch server logs for simultaneous close events to catch auth/version mismatch
Avoid restarting the server during active client handshakes; drain first

When it happens

Trigger: outConn.Write(pkg.Bytes()) fails after the connection was established — typically because the server closed the conn right after accept (auth/key rejected, TLS failure, server restarting) or the network dropped mid-write; GetInConn closes outConn and returns the wrapped error.

Common situations: Mismatched client key so the server drops the connection; server restarted between accept and read; intermediate proxy/firewall RSTs the connection; unstable mobile/VPN network dropping packets.

Related errors

AI-assisted analysis of snail007/goproxy@e6d6a821db (2026-09-03). Data as JSON: /api/errors/8fd8eecfc0075df6. Report an issue: GitHub.

Appendix: source

Thrown at services/tunnel_client.go:94

}
func (s *TunnelClient) Clean() {
	s.StopService()
}
func (s *TunnelClient) GetInConn(typ uint8) (outConn net.Conn, err error) {
	outConn, err = s.GetConn()
	if err != nil {
		err = fmt.Errorf("connection err: %s", err)
		return
	}
	keyBytes := []byte(*s.cfg.Key)
	keyLength := uint16(len(keyBytes))
	pkg := new(bytes.Buffer)
	binary.Write(pkg, binary.LittleEndian, typ)
	binary.Write(pkg, binary.LittleEndian, keyLength)
	binary.Write(pkg, binary.LittleEndian, keyBytes)
	_, err = outConn.Write(pkg.Bytes())
	if err != nil {
		err = fmt.Errorf("write connection data err: %s ,retrying...", err)
		utils.CloseConn(&outConn)
		return
	}
	return
}
func (s *TunnelClient) GetConn() (conn net.Conn, err error) {
	var _conn tls.Conn
	_conn, err = utils.TlsConnectHost(*s.cfg.Parent, *s.cfg.Timeout, s.cfg.CertBytes, s.cfg.KeyBytes)
	if err == nil {
		conn = net.Conn(&_conn)
	}
	return
}
func (s *TunnelClient) ServeUDP() {
	var inConn net.Conn
	var err error
	for {
		for {

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