Introduction

In this article we will see how to use cat, netcat and socat at least some basic examples and why do we have so many cats…

Also sorry for the awful recordings, but couldn’t figure out why it looks so bad with tmux.

cat

Cat as you might have guessed or know already is to con-cat-enate things, when used in conjunction with the shell redirections it can do a lot of powerful things but it’s often used when it’s not needed due to that, let’s see some examples.

So what happened there? Basically when you want to end the file or the input you send the keyword Ctrl+D, when typed at the start of a line on a terminal, signifies the end of the input. This is not a signal in the unix sense: when an application is reading from the terminal and the user presses Ctrl+D, the application is notified that the end of the file has been reached (just like if it was reading from a file and had passed the last byte). This can be used also to terminate ssh sessions or just log you out from a terminal.

If you want to copy and paste something there you go:

# Normal concatenation to stdout
cat test-1.txt test-2.txt 

# Creating a file (redirection)
cat > test-3.txt
Some content ctrl-d

# Appending to the same file (same redirection but in append mode)
cat >> test-3.txt
Some more content ctrl-d

# Reading the file (read and pipe to stdout)
cat test-3.txt

While cat is overly simplified here, it can do a lot of interesting things and it is usually misused see here

More info:

• Cat examples
• Bash redirections
• Zsh redirections

netcat

Netcat is a bit more interesting since it can use the network and it’s really simple also, so it let us use network sockets without too much complication, let’s see a couple of examples, first we spin up a server (listener), then connect from the other side and send some data, be aware that connections are bi-directional then Ctrl-C to finish the process. Then in the second example we spin up a server and wait for a compressed file to be sent from the client.

There are many more things that you can do with netcat and is usually really helpful to debug networking issues or to do a quick copy of files over the network.

If you want to copy and paste something there you go:

### Example one
# Server
# -l means listen, and -p to specify the port
nc -l -p 3000
Type here

# Client
# this one doesn't need a lot of explanation if it's not listening, 
# then it needs a host and a port to connect to,
nc localhost 3000
or type anything here

# Example two
# Server (Example copying a file, it can be used to copy anything that tar can send)
# First we create a tmp folder, move into it
# then we listen with netcat and pipe tar with xvf -
# that means that anything that comes from stdin
# will be treated as a tar compressed file and decompressed in place
mkdir tmp && cd tmp && nc -l -p 3000 | tar xvf - 

# Client (send the file)
# We create a file with some text
echo "Hello world!" > test.txt
# Then compress it with tar and print it to stdout 
# we also redirect that into nc so it will be sent over the network
# tar cvf is the opposite of tar xvf, x is extract, v is verbose
# and f is archive file, c is compress
tar cvf - test.txt | nc localhost 3000

Netcat is pretty good at it’s job and it’s always a good tool to have at hand, but there are other more complex tasks with sockets and for that we have socat.

More info:

• Many uses for netcat (with a cheatsheet)
• Several examples

socat

Socat is a command line based utility that establishes two bidirectional byte streams and transfers data between them. Because the streams can be constructed from a large set of different types of data sinks and sources (see address types), and because lots of address options may be applied to the streams, socat can be used for many different purposes. That bit was extracted from the man page, socat stands for SOcket CAT and it’s a multipurpose relay, we will see a few examples to clarify on what that means and some cool stuff that you can use socat for, at first it might look a bit intimidating, but trust me it worth learning to use it.

Something to have in mind when using socat it’s that it needs two addresses, sometimes you can skip them with a -. While socat has a gazillion more use cases than cat or netcat, I will just show you a few, but hand you a few links in case you are interested in learning more, what I find particularly useful it’s the ability to do a port-forward in just one line.

Basically with socat your imagination is the limit in what you can do.

If you want to copy and paste something there you go:

# Example one
# Redirect a port or port-forward
# since socat always need two addresses (it can be sockets, whatever)
# we need to define what we want to do, in this case
# we are telling it to listen in all interfaces in the port 2222 and fork
# that means that it can accept many connections (it's like a multiplexer) 
# then we tell it to send whatever comes from that socket into localhost and port 22
# with protocol TCP
socat TCP-LISTEN:2222,fork TCP:localhost:22

# Socat as a client 
# The client is simpler, we just ignore the first address with -
# and just use the remote to connect like with netcat or telnet
socat - tcp:localhost:2222

# Example two
# It can also be used as nc -l -p port
# In this example we see how we can simulate netcat basic behaviour
# by just specifying the local address and ignoring the remote
socat TCP-LISTEN:2222,fork -

# Same client
socat - tcp:localhost:2222

# Example three
# Poor's man remote session
# Here we listen locally in port 2222 and on any connection 
# we launch a bash shell with EXEC, Crazy right?
socat TCP-LISTEN:2223 EXEC:/bin/bash

# Same Client 
socat - tcp:localhost:2223

# Example four
# SSH tunnel
# In this one we listen in the port 2224 and send whatever comes in to 
# the address 192.168.1.50 port 22 and protocol TCP, in this case it's
# my raspberry ssh port, so it's just a tunnel.
socat TCP-LISTEN:2224,reuseaddr,fork TCP:192.168.1.50:22

# SSH client
# Then connect normally through the tunnel.
ssh pi@localhost -p 2224

More info:

• Socat Examples (great resource)
• More socat examples
• Linux unix TCP Port Forwarder

Closing notes

Be sure to check the links if you want to learn more about each different tool and I hope you enjoyed it, see you on twitter or github!

Errata

If you spot any error or have any suggestion, please send me a message so it gets fixed.

Also, you can check the source code and changes in the generated code and the sources here