shadow1ng/fscan · warning

socks5_success_response_failed: %w

Error message

socks5_success_response_failed: %w

What it means

The proxy built the SOCKS5 success reply (0x00) but the write to the client connection failed. The proxy closes the established target connection and aborts with socks5_success_response_failed wrapping the underlying write error.

Source

Thrown at plugins/local/socks5proxy.go:285

		return nil, 0, fmt.Errorf("%s", i18n.GetText("local_address_unavailable"))
	}
	localPort := localAddr.Port

	// 发送成功响应
	response := make([]byte, 10)
	response[0] = 0x05 // SOCKS版本
	response[1] = 0x00 // 成功
	response[2] = 0x00 // 保留
	response[3] = 0x01 // IPv4地址类型
	// 绑定地址和端口（使用127.0.0.1:localPort）
	copy(response[4:8], []byte{127, 0, 0, 1})
	response[8] = byte(localPort >> 8)
	response[9] = byte(localPort & 0xff)

	_, err = clientConn.Write(response)
	if err != nil {
		_ = targetConn.Close()
		return nil, 0, fmt.Errorf("%s: %w", i18n.GetText("socks5_success_response_failed"), err)
	}

	session.LogDebug(i18n.Tr("socks5_proxy_connection_established", targetAddr))
	return targetConn, localPort, nil
}

func containsByte(values []byte, target byte) bool {
	for _, value := range values {
		if value == target {
			return true
		}
	}
	return false
}

// relayData 双向数据转发
func (p *Socks5ProxyPlugin) relayData(clientConn, targetConn net.Conn) {
	done := make(chan struct{}, 2)

View on GitHub (pinned to 95cc12e753)

Solutions

1. Check the wrapped %w error: ECONNRESET/EOF means the client vanished — normal operational noise, log at debug
2. Ensure the client waits for the SOCKS5 reply before sending payload data
3. If frequent, reduce client handshake timeouts or verify intermediate proxies/firewalls between client and proxy

// caller
if _, _, err := handleSocks5Request(...); err != nil {
    if errors.Is(err, io.ErrClosedPipe) || errors.Is(err, syscall.ECONNRESET) || errors.Is(err, io.EOF) {
        log.Println("client disconnected during handshake") // benign
    } else {
        log.Printf("handshake write failed: %v", err)
    }
}

When it happens

Trigger: clientConn.Write(response) returns an error right after the target connection succeeded: client closed the connection, network reset, or client socket buffer issues.

Common situations: Impatient SOCKS5 client gives up and closes its socket while the proxy is dialing the target; NAT/firewall resets the client connection mid-handshake.

Related errors

• %s: %w [listen_port_failed]
• %s: %w [socks5_handshake_read_failed]
• %s [socks5_unsupported_version]
• socks5_target_connect_failed: %w
• ms17010_send_protocol_error: %w

AI-assisted analysis of shadow1ng/fscan@95cc12e753 (2026-09-06). Data as JSON: /api/errors/f517587827d9ded6. Report an issue: GitHub.