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Mohammad Hedayati
Asked: 2012-01-05 04:26:15 +0800 CST

echo server with netcat or socat

  • 772

I want a UDP echo server to get packets, and reply exactly what it has received. How can I simply do this using netcat or socat? It should stay alive forever and handle packets coming from several hosts.

netcat echoserver socat

6 Answers

  1. Best Answer

    Another netcat-like tool is the nmap version, ncat, that has lots of built in goodies to simplify things like this. This would work:

    ncat -e /bin/cat -k -u -l 1235

    -e means it executes /bin/cat (to echo back what you type)
    -k means keep-alive, that it keeps listening after each connection
    -u means udp
    -l 1235 means that it listens on port 1235

    • 27

  2. I used socat -v PIPE udp-recvfrom:5553,fork to run the server and socat - udp:localhost:5553 for clients. This was a great help!

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  3. You can also use socat (rather than using netcat) as echo server and netcat as client.

    Socat echo server (listens on TCP port 1234):

    socat -v tcp-l:1234,fork exec:'/bin/cat'

    Netcat client (connects to serverip on TCP port 1234):

    nc serverip 1234
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  4. You can write a C program that forks nc -u -l -p 4321 and then uses dup(2) to connect:

    • nc stdin with the parent's stdout
    • nc stdout with the parent's stdin

    Then in an endless loop the parent reads from stdin and writes in stdout whatever the parent reads.

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  5. netcat solution pre-installed in Ubunutu

    The netcat pre-installed in Ubuntu 16.04 comes from netcat-openbsd, and has no -c option, but the manual gives a solution:

    sudo mknod -m 777 fifo p
cat fifo | netcat -l -k localhost 8000 > fifo

    Then client example:

    echo abc | netcat localhost 8000

    TODO: how to modify the input string value? The following does not return any reply:

    cat fifo | tr 'a' 'b' | netcat -l -k localhost 8000 > fifo

    The remote shell example however works:

    cat fifo | /bin/sh -i 2>&1 | netcat -l -k localhost 8000 > fifo

    I don't know how to deal with concurrent requests simply however.

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  6. If you are running on Windows and use a unix like environment like cygwin, netcat migth not provide the -e parameter. This worked just fine for me.

    http://bansky.net/echotool/

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