How to send a file over netcat after receiving a response?

Question

I basically want to do what is described in this question but instead of subsequent data coming from stdin, to come from another file.

If I do this:

cat file.txt | netcat 192.168.121.15

netcat (it seems) simply shuts down the connection after transmitting the file. What the above referenced post suggests is this:

cat file.txt - | netcat 192.168.121.15

However, this just keeps the connection open for input from stdin, whereas I want to be able to send a second file after receiving a response about the first file back from the server.

Therefore, I can't just send a bunch of files all at once, and so the answer to this question isn't what I'm looking for either because I can't alter the server to use tar.

Can netcat even do this or is there some other tool I should be using?

Answer 1

TL;DR: Yes, but you'll need something smarter than a piped cat

cat simply concatenates all files given to it and dumps them to stdout , - represents stdin so the way your referenced Q/A pair worked was by concatenating stdin to the file.

netcat is a fine tool for what you're trying to do as far as opening up a raw pipe to the other port. I think a better question for you would be, is piped output from cat the right tool? No.

The problem with just using cat ... | netcat ... cat ... | netcat ... is that its a one way deal. The output of cat is used as the input to netcat but the output will go to stdout , the pipe is not two way, you need interactivity.

If you want to interactively perform actions based on responses from the server over your netcat pipe there are a whole host of ways to programmatically interact with a pipe.

You could use read for instance, or expect .

If you're willing to expand your tool chest a little bit i suggest using ncat which is a more modern implementation of netcat and has a handy option -e which allows the attaching of a command or script to the netcat pipe. If you cannot get your hands on a netcat with -e you may need to learn a few things about named pipes and I/O Redirection to get the same effect.

I wanted to test some of this stuff before writing this answer, see below for testing/examples

I didn't write a listening server that handled multiple files sent via netcat , I'm just going to assume you have that working, but I can simulate some programmatic interaction from the client side and do the server by hand.

My dummy "client"/"server" interaction here is based on this fun activity of seting up 2 ncat sessions to talk to each other

1. I wrote this simple interactive script:

    #!/bin/bash i=0 while true; do read line if [[ $line == "goose" ]]; then echo "zoom" exit 0 else i=$(expr $i + 1) echo "$i ..." fi done 

2. I start my "server" ncat -l -p 1337

3. I start my "client" controlled by the interactive script ncat localhost 1337 -e ./interact.sh

4. And I operate the "server" by hand (since ncat doesn't clearly show I vs O I've notated Input with i: and output with o: ):

    i:duck o:1 ... i:duck o:2 ... i:duck o:3 ... i:goose o:zoom