tcp-multiplexer

Use it in front of a target server and let your client programs connect to it, if target server only allows you to create a limited number of TCP connections concurrently. While it has its limitation: increased latency as incoming request will block each other.

A common use case for tcp-multiplexer is to allow multiple modbus/TCP clients connect to solar inverters which often only support a single TCP connection.

Architecture

┌──────────┐
│ ┌────────┴──┐      ┌─────────────────┐      ┌───────────────┐
│ │           ├─────►│                 │      │               │
│ │ client(s) │      │ tcp-multiplexer ├─────►│ target server │
└─┤           ├─────►│                 │      │               │
  └───────────┘      └─────────────────┘      └───────────────┘

─────► TCP connection

drawn by https://asciiflow.com/

Unlike with a reverse proxy, the TCP connection between tcp-multiplexer and the target server will be reused for all clients' TCP connections.

Multiplexer is simple. For every TCP connection from clients, the handling logic:

for {
get lock...
data pipe:
    1. get request message from client
    2. forward request message to target server
    3. get response message from target server
    4. forward response message to client
release lock...
}

The lock makes sure that at any time, the TCP connection to the target server will be used in exactly one request-response loop. This way, all connections from clients share one TCP connection to the target server.

Next key point is how to detect message (e.g., HTTP) from the TCP data stream.

Supported application protocols

Every application protocol (request–response message exchange pattern) has its own message format. The following formats are supported currently:

1. echo: \n terminated
2. http1 (not including https, websocket): not fully supported
3. iso8583: with 2 bytes header of the length of iso8583 message
4. modbus-tcp

$ ./tcp-multiplexer list                                    
* iso8583
* echo
* http
* modbus

usage for example: ./tcp-multiplexer server -p echo

See detailed: https://github.com/ingmarstein/tcp-multiplexer/tree/master/example

Usage

$ ./tcp-multiplexer server -h
start multiplexer proxy server

Usage:
  tcp-multiplexer server [flags]

Flags:
  -p, --applicationProtocol string   multiplexer will parse to message echo/http/iso8583 (default "echo")
  -h, --help                         help for server
  -l, --listen string                multiplexer will listen on (default "8000")
  -t, --targetServer string          multiplexer will forward message to (default "127.0.0.1:1234")

Global Flags:
  -v, --verbose   verbose log

In a container

docker run ghcr.io/ingmarstein/tcp-multiplexer server -t 127.0.0.1:1234 -l 8000 -p modbus

Alternatively, use the included compose.yml file as a template if you prefer to use Docker Compose.

Testing

Start echo server (listen on port 1234)

$ go run example/echo-server/main.go
1: 127.0.0.1:1234 <-> 127.0.0.1:58088

Start TCP multiplexing (listen on port 8000)

$ ./tcp-multiplexer server -p echo -t 127.0.0.1:1234 -l 8000
INFO[2021-05-09T02:06:40+08:00] creating target connection
INFO[2021-05-09T02:06:40+08:00] new target connection: 127.0.0.1:58088 <-> 127.0.0.1:1234
INFO[2021-05-09T02:07:57+08:00] #1: 127.0.0.1:58342 <-> 127.0.0.1:8000
INFO[2021-05-09T02:08:16+08:00] closed: 127.0.0.1:58342 <-> 127.0.0.1:8000
INFO[2021-05-09T02:08:19+08:00] #2: 127.0.0.1:58402 <-> 127.0.0.1:8000

client test

$ nc 127.0.0.1 8000
kkk
kkk
^C
$ nc 127.0.0.1 8000
mmm
mmm