The syntax is

$ exec {file-descriptor}<>/dev/{protocol}/{host}/{port}

• {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.
• <> – the file is open for both reading and writing
• {protocol} – TCP or UDP
• {host} – ip address or domain name of the host
• {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

done

    echo $LINE >&1

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

echo "USER ${NICK} 8 * : ${NICK}" >&3

exec 3<>/dev/tcp/${SERVER}/${PORT}
echo "NICK ${NICK}" >&3

exit $?

echo "JOIN ${CHANNEL}" >&3
cat <&3

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.