Serial Port Redirector

Serial ports are largely absent from modern personal computers, but can still be found on industrial devices and embedded systems. Since serial communication differs significantly from TCP/IP, debugging and analyzing serial-based protocols requires different approaches.

GOST added serial port redirector functionality after v3.0.0-rc8. This enables forwarding local serial port data to a TCP service, or TCP service data to a local serial port, or even forwarding between remote serial ports. Serial forwarding enables two use cases: remote serial communication and serial data monitoring.

Virtual Serial Ports

Virtual serial ports allow software to communicate via serial ports even without physical hardware.

On Windows, use Null-modem emulator.

On Linux, create virtual serial ports with socat:

socat -d -d pty,raw,echo=0 pty,raw,echo=0

Remote Serial Communication

Serial communication is point-to-point and requires physical proximity. Serial forwarding allows accessing serial ports like internet services. Two approaches: forward to a TCP service, or forward to a remote serial device.

Forwarding to a TCP Service

Since serial data transmission is similar to TCP (stream-based), serial data can be forwarded to a TCP service, effectively converting serial communication to network communication.

gost -L serial://COM1 -F tcp://192.168.1.1:8080

Forwarding to a Remote Serial Device

Normally, software communicates with a serial device through a local serial port. On Windows, this is typically COM1, COM2, etc. On Linux, it's /dev/ttyS* (e.g., /dev/ttyS0) or /dev/ttyUSB0 for USB-to-serial adapters.

When the device is not physically nearby, serial forwarding can relay the serial port from the connected host to any remote host.

Assume device-connected host A (192.168.1.1) has a physical serial port COM1. Remote host B (192.168.1.2) has virtual serial ports COM1 and COM2. We want software on host B to communicate with the device via virtual port COM2.

First, start a relay service on host A:

gost -L relay://:8420

Then, on host B, map COM1 to host A's COM1:

gost -L serial://COM1/COM1 -F relay://192.168.1.1:8420

Data sent through host B's COM2 is now forwarded via the relay service to host A's COM1, enabling remote serial communication.

Serial Data Monitoring

For TCP/IP networks, tools like Wireshark and tcpdump can capture and analyze data in real time, but these don't support serial ports. Serial monitoring typically requires specialized tools.

Serial ports have another peculiarity: a port can only be opened by one process at a time. This makes it difficult to capture communication data between software and a device. Serial Port Monitor is one of the few tools that can monitor an occupied serial port.

Instead of direct monitoring, serial forwarding can achieve the same goal indirectly. Using virtual serial ports, assume host A has a physical serial port COM1 connected to a device, and virtual ports COM3 and COM4. Configure the software to use COM4, and forward COM1 to COM3, capturing data during forwarding:

services:
- name: service-0
  addr: COM1
  recorders:
  - name: recorder-0
    record: recorder.service.handler.serial
    metadata:
      direction: true
      timestampFormat: '2006-01-02 15:04:05.000'
      hexdump: true
  handler:
    type: serial
  listener:
    type: serial
  forwarder:
    nodes:
    - name: target-0
      addr: COM3
recorders:
- name: recorder-0
  file:
    path: 'C:\serial.data'

The recorder logs all communication data to C:\serial.data. The output format:

>2023-09-18 10:16:25.117
00000000  60 02 a0 01 70 02 b0 01  c0 01 c0 01 40 02 30 01  |`...p.......@.0.|
...
<2023-09-18 10:16:25.120
00000000  d0 00 d0 00 10 01 10 02  50 01 e0 00 00 01 d0 01  |........P.......|
...

> indicates data sent from the source port (COM1 → COM3), < indicates data received by the source port (COM1 ← COM3).

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