netcat as a logging tcp proxy

I felt I needed to write an article about netcat, so here is it !
Netcat is an incredibly usefull tool, that allows you to play with tcp connection easily from the shell.
Basically, as it name implies, it’s just cat over the network, but what its name doesn’t tell you is that it also can act as a socket listener.
So let’s play with pipes, here is one of my favourite use of netcat:

mkfifo proxypipe
cat proxypipe | nc -l -p 80 | tee -a inflow | nc localhost 81 | tee -a outflow 1>proxypipe

This command will redirect traffic from localhost:80 to localhost:81, in the inflow file you while find the incoming http request, in the outfile, you will find the http response from the server.
Similarly, you can do this:

cat proxypipe | nc -l 80 | tee -a inflow | sed 's/^Host.*/Host: www.google.fr/' |  nc www.google.fr 80 | tee -a outflow >proxypipe

This will allow your browser to point to google using http://localhost .
Anyway, this is my favourite but netcat has thounds of other uses, have a look at it !
It can be usefull for file transfers (gzip|nc) , performance measurement (dd|gzip), protocol debugging (replaying requests), security testing (nc does port scan) …

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3 Comments »

  1. Gildas Le Nadan said

    Hi,

    Checking your blog since I saw you’ll come for devopsdays

    I recommend checking the “pv” command, Pipe Viewer. It is a very nice addition to any pipe combo, expecially when nc is involved 🙂

    More about it http://www.ivarch.com/programs/pv.shtml

    Cheers,
    Gildas

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