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pipe-spy

<<<<<<< HEAD A C-program that can be injected between two processes to spy on their communcation over pipes.

Use case

I had a situation where one program exec:ed another and communicated bi-directionally over pipes. As I was interested in learning the details of this communication I wanted to spy in that conversation.

Alternatives

Actually I could not find any alternatives here. My question on stackoverflow did not get any answers and googling found none either.

I'm happy to add some alternatives here, if you'd like to share.

(Of course there is always strace but that just generates a lot of extra information that was in the way.)

So I implemented this little utility.

Usage

You have to find a way to get the caller to exec pipe-spy instead of the actual target. You compile this source and place it or soft-link in a place where your caller will find it, thinking it is the real program it calls.

The way I used it was to create a link to pipe-spy with the same name as the actual target but in a directory that was earlier in the PATH. This might not always be possible, so YMMV.

You can specify the real target either as a compile-time constant

$(CC) -DTARGET=<target> ...

or as an environment variable:

TARGET=<target> <command-that-invokes-pipe-spy>

If the caller tries to exec the actual target this utility will instead be run. It will in turn exec the target and spy and log the downstream and upstream communication in a file.

This file is by default /tmp/pipe-spyPID.log, but you can change where that logging goes using an environment variable:

LOGFILE=<logfile> <command-that-invokes-pipe-spy>

Implementation

pipe-spy forks three processes, two for the up- / down-stream communication and one that execs the actual target.

The complicated part is to set up the pipes correctly.

Improvements

  • The handling of closed pipes is probably not correct, so if your caller closes the communication pipe-spy will probably hang.

  • The path to the target now has to be compiled in. Any ideas on how to do avoid this are most welcome (note that we can't use command line arguments, since the requirement is that the caller cannot be changed).

PR:s are welcome.

A C-program that can be injected between two processes to spy on thier communcation over pipes.

You also have to tell pipe-spy which program it should invoke so that the communication can be established, creating a man-in-the-middle setup where pipe-spy is the man in the middle.

You can define the TARGET either through a compile-time constant: