How to open TCP/UDP sockets - BASH SHELL SCRIPT

How to open TCP/UDP sockets using a built-in feature in Bash

BASH - SHELL - SCRIPTING

Bash shell has a built-in feature that allows to open TCP/UDP sockets using a simple syntax. This is very useful when tools like netcat are not installed or we don’t have the permission to use it.

The syntax is: $ exec {file-descriptor}<>/dev/{protocol}/{host}/{port}

1. {file-descriptor} – 0, 1 and 2 are seserved for stdin, stout and stderr respectively. At least 3 must be used. The Bash manual suggest to be careful in using descriptors above 9 since there could be conflict with descriptors used internally by the shell.
2. <> – the file is open for both reading and writing
3. {protocol} – TCP or UDP
4. {host} – ip address or domain name of the host
5. {port} – logic port

Sockets can be closed using:

$ exec {file-descriptor}<>&-

To send a message through the socket

echo -e -n "$MSG_OUT" >&3

or

printf "$MSG_OUT" >&3

To read a message from the socket

read -r -u -n $MSG_IN <&3

Output can be printed recursively

while read LINE <&3
do
    echo $LINE >&1
done

Or read entirely in one variable

OUTPUT=$(dd bs=$BYTES count=1 <&3 2> /dev/null)

Example:

$ exec 3<>/dev/tcp/127.0.0.1/1234

We are opening a socket for reading and writing to the 1234 port in the loopback interface.

The /dev/tcp and /dev/udp files aren’t real devices but are keywords interpreted by the Bash shell. Being a “bashism” this solution is not portable even if seems that ksh and zsh shells have the same feature enabled.

In this example we fetch the Google main page:

$ exec 3<>/dev/tcp/www.google.com/80
$ echo -e "GET / HTTP/1.1\n\n" >&3
$ cat <&3

It’s good practice to always close file descriptors

$ exec 3<&-
$ exec 3>&-

Finally,  IRC server example:

#!/bin/bash

##########################################################
# Config

NICK="CyberPunk"
SERVER="irc.n0where.net"
PORT=6667
CHANNEL="#CyberPunk"

##########################################################
# Main

exec 3<>/dev/tcp/${SERVER}/${PORT}
echo "NICK ${NICK}" >&3
echo "USER ${NICK} 8 * : ${NICK}" >&3
echo "JOIN ${CHANNEL}" >&3
cat <&3

exit $?

Enable/disable net redirections

More the feature must be enabled in Bash at compile time. To enable it if you want to compile the Bash yourself include the flag

--enable-net-redirections

while to disable it explicitly use

--disable-net-redirections

Each distribution may or not have the feature enabled in their precompiled Bash.

This is a built-in feature that needs to be enabled in Bash at compile time but also ksh and zsh seem to have it. System administrators might want to disable this feature since could represent a security concern. In general the use of specific tools to create sockets like netcat and socat are preferable if possible.