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https://github.com/willglynn/tcpserial_hook


https://github.com/willglynn/tcpserial_hook

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README 

        
# `tcpserial_hook`



This is a library which can be inserted into a closed-source application for the purpose of monitoring its serial port

activity over TCP/IP. It causes the application to listen for incoming connections (TCP port 7160), to which it

transmits a copy of every byte exchanged over the serial port.



Theory of operation

tcpserial_hook.so intercepts the read(), write(), and tcsetattr()

system calls. When inserted into a target process using LD_PRELOAD, the target process will call

tcpserial_hook.so instead of the operating system. These three functions all pass through to the operating

system, but with side effects.


The preloaded library contains a static initializer which listens on port 7160 and spawns a thread to accept

incoming connections prior to the application's main() function. When the application configures a serial

port using tcsetattr(), the library remembers its file descriptor, at which point any data exchanged by

future read() or write() calls on that file descriptor are broadcast to all connected

clients.


If a client's TCP send buffer is filled, the library must choose between a) allocating an unbounded amount of memory,

b) blocking the application, c) discarding data for that client, or d) terminating that connection. The library chooses

to terminate the connection. This alerts the consumer that it fell behind, and the consumer is of course free to

reconnect and try again.


This library supports a fixed maximum number of simultaneous clients (8). Additional connections are terminated

immediately. This is intended to provide another bound on resource consumption and prevent an accidental

denial-of-service condition.



It is intended for the [Tigo Cloud Connect Advanced (CCA)](https://www.tigoenergy.com/product/cloud-connect-advanced) or

similar, targeting the `meshdcd` process which communicates with a [Tigo Access Point

(TAP)](https://www.tigoenergy.com/product/tigo-access-point) using a serial port.



## Installation



Installation requires `root` access. Obtaining `root` access on a Tigo CCA is outside the scope of this document.



Start by downloading the shared library and ensure that the `wrap_mesh.sh` script does not exist:



```console

~ # curl -L --insecure -o /mnt/ffs/lib/tcpserial_hook.so https://github.com/willglynn/tcpserial_hook/releases/download/v1.0.0/tcpserial_hook.so

~ # ls -l /mnt/ffs/mgmtu/bin/wrap_mesh.sh

~ #

```



Assuming `ls` did not show an existing `wrap_mesh.sh` script, create one and make it executable:



```console

~ # echo -e '#!/bin/sh\nLD_PRELOAD=/mnt/ffs/lib/tcpserial_hook.so exec /mnt/ffs/mgmtu/bin/meshdcd' > /mnt/ffs/mgmtu/bin/wrap_mesh.sh

~ # chmod +x  /mnt/ffs/mgmtu/bin/wrap_mesh.sh

```



Finally, restart `meshdcd`:



```console

~ # /mnt/ffs/etc/rc.d/S040_MeshDCD

Shutting down meshdcd from meshdcd launcher

Killing program PID 3088

Shutting down wrap_mesh.sh from meshdcd launcher

Killing watcher PID 2815

Starting Tigo Mesh Data Collection service.

~ #

```



You can now connect to the CCA on TCP port 7160 to monitor the serial data.



## Uninstallation



Remove the files and restart `meshdcd`.



```console

~ # rm /mnt/ffs/mgmtu/bin/wrap_mesh.sh /mnt/ffs/lib/tcpserial_hook.so

~ # /mnt/ffs/etc/rc.d/S040_MeshDCD

Shutting down meshdcd from meshdcd launcher

Killing program PID 8158

Shutting down wrap_mesh.sh from meshdcd launcher

Killing watcher PID 7120

Starting Tigo Mesh Data Collection service.

~ #

```