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.
asked Jan 4, 2012 at 12:26
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8Please don't make this thing Internet-accessible. Since it's UDP-based it can be used to send a packet stream to arbitrary destinations using source-forged packets.– Evan AndersonJan 6, 2012 at 5:47
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6 Answers
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
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!
answered Jan 6, 2012 at 0:16
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5An alternative, coming from stackoverflow.com/a/35857095/222529 is
socat TCP4-LISTEN:2000,fork EXEC:cat– JirMar 10, 2017 at 17:05
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
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.
answered May 29, 2017 at 16:00
Ciro Santilli OurBigBook.comCiro Santilli OurBigBook.com
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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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I can use
socat -v PIPE UDP-LISTEN:5555,fork, but the second connection gets refused. By the way, it works fine with TCP-LISTEN. I am avoiding to code, otherwise I could write it all in C. Jan 4, 2012 at 13:16 -
You have to wrap the whole thing in an endless loop: while true; do socat stuff; done– adamoJan 4, 2012 at 18:53
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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.
