Send a binary file (line by line) to a socket server with Netcat

Question

As a spin off to this Stack Overflow question I want to archieve the same except for a couple of tweaks.

I want to connect to a host, send a binary file, line by line, and have a couple of seconds of delay between each line... and it must be in the same connection. The strings I want to send are mostly textbased but there are a couple of non-printable chars in the line.

This is an example of a string/line I want to send (thousands of lines per file):

<null>e<null><null>00326513,6598,no,8,,2z<null>

The <null> value is the HEX value 0x00.

I have rewitten my question from its first entry but @mzet answered me on my original question as my question was text-only then but I later on discovered that my string had these 0x00 chars inside which I could not see at first. I want to use his answer as a base as I think it almost works but it may need a tweak so this is his contribution, not mine.

If I have a file with 3 lines:

<null>e<null><null>00370513,6598,no,8,,2z<null>
<null>f<null><null>00891548,6598,yes,8,,3z<null>
<null>f<null><null>00129525,6598,yes,8,,2z<null>

I then thought I could tweak @mzet answer and replace the <null> values inside my file with \x00 and set -e on the echo command:

[root@srv]# mkfifo /tmp/fifoIn; cat /tmp/fifoIn | nc localhost 2222 &
[root@srv]# cat myfile | while read line; do echo -ne $line; sleep 2; done > /tmp/fifoIn

When I do that, I can see the x00 on the serverside:

[root@srv]# nc -l 2222
x00ex00x0000370513,6598,no,8,,2zx00
x00fx0000891548,6598,yes,8,,3zx00
x00fx00x0000129525,6598,yes,8,,2zx00

Is it possible to send a binary (text?) file as I want? And if it it not possible to send a file, line by line, is it then possible to send one string several thousands of times? Strictly necessary they don't need to be unique or in a file as I could manage with the same string repeating itself.

EDIT #1

Why is my script only sending one line to the server (I would expect 4 lines) afterwards it is just pausing(?) forever. The client (nor the server) closes its connection but it doesn't come with any errors:

rm -f /tmp/fifofile
mkfifo /tmp/fifofile
cat /tmp/fifofile | nc 192.168.20.6 5000 &
sleep 1

i="0"

while [ $i -lt 4 ]
do
  echo "$i"
  echo -ne "\x00e\x00\x00001212dsfdsfdsfsdfsdfsdfdsf\x00" | tee /tmp/fifofile
  sleep 1
  i=$[$i+1]
done

mzet · Accepted Answer · 2013-10-09 11:57:34Z

4

You can achieve it in two steps:

1) You need to start nc with a named pipe (fifo) as its input:

mkfifo /tmp/fifoIn; cat /tmp/fifoIn | nc localhost 2222 &

2) Send your data from file input.txt, line by line with 2 sec delay:

cat input.txt | while read line; do echo $line; sleep 2; done > /tmp/fifoIn

I've tested it with this "server" (I'm using openbsd netcat syntax):

nc -l localhost 2222

answered Oct 9, 2013 at 11:57

mzet

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2 Comments

Beauvais Over a year ago

This seems to work and I will accept your answer unless any better comes up within a couple of days. Do you also know if it is possible to avoid it sending the new-line after each line?

mzet Over a year ago

try echo -n instead of echo in while loop

Beauvais · Accepted Answer · 2013-10-10 18:41:56Z

After a lot of trying and pulling my hair I finally figured out that I could use NCat instead of Netcat as NCat can execute a command.

Start a connection with NCat to my socket server on port 5000 and execute the script ./sendlines.sh:

ncat --exec "./sendlines.sh" 192.168.1.10 5000

./sendlines.sh will send 4 lines with a delay of two seconds between each line:

#!/bin/bash
#
# sendlines.sh, v1.00, initial release
#
i="0"
while [ $i -lt 4 ]
do
  echo -ne "\x00e\x00\x0000370513,6598,no,8,,2z\x00"
  sleep 2
  i=$[$i+1]
done

I have not figured out how to send a binary file, line by line, but this is not strictly necessary as I can manage by sending the same string many times.

If you know a way to send a binary file, line by line, it would be great as it would be the best solution for me.

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