Saturday, July 2, 2016

How to open a TCP/UDP socket in a bash shell

Suppose you want to open a TCP/UDP socket on a Linux server for various reasons. For example, you want to check if a specific address/port is reachable. Or you want to fetch a remote web page or invoke a restful API for testing. Or you want to connect to a remote IRC server, etc. However, what if the Linux server you are on is very restrictive? On that server, none of standard tools such as netcat, curl or wget may be available, and you are pretty much left with the bash shell only.
In fact, one of built-in features of bash shell is to open TCP/UDP sockets via /dev/tcp (and /dev/udp) device file. In the rest of this tutorial, let's find out how to open a TCP/UDP socket, and read to and write from the socket in bash shell.

Open or Close a TCP/UDP Socket in Bash Shell

In a nutshell, you can open a TCP/UDP socket using the following syntax in bash shell.
$ exec {file-descriptor}<>/dev/{protocol}/{host}/{port}
The "file descriptor" is a unique non-negative integer associated with each socket. File descriptors 0, 1 and 2 are reserved for stdin, stdout and stderr, respectively. Thus you must specify 3 or higher (whichever is unused) as a file descriptor.
"<>" implies that the socket is open for both reading and writing. Depending on your need, you can open a socket for read-only (<) or write-only (>).
The "protocol" field can be either tcp or udp. The "host" and "port" fields are self-explanatory.
For example, to open a bi-directional TCP socket for with HTTP port and file descriptor 3:
$ exec 3<>/dev/tcp/
Once opened, a read/write socket can be closed using the following syntax. The first command close an input connection, while the latter closes an output connection.
$ exec {file-descriptor}<&-
$ exec {file-descriptor}>&-

Read from or Write to a TCP/UDP Socket in Bash Shell

Once a socket is opened, you can write a message to or read a message from the socket.
To write a message stored in $MESSSAGE to a socket:
$ echo -ne $MESSAGE >&3
$ printf $MESSAGE >&3
To read a message from a socket and store it in $MESSAGE:
$ read -r -u -n $MESSAGE <&3
$ MESSAGE=$(dd bs=$NUM_BYTES count=$COUNT <&3 2> /dev/null)

TCP/UDP Socket Examples in Bash Shell

Here I present several shell script examples that open and use a TCP socket.

1. Fetch a remote web page and print its content

exec 3<>/dev/tcp/
echo -e "GET / HTTP/1.1\r\nhost:\r\nConnection: close\r\n\r\n" >&3
cat <&3

2. Display a remote SSH server version

exec 3
In fact, the above script can be shortened to the following one-liner:
timeout 1 cat 

3. Display the current time from


4. Check the Internet connectivity

(echo >/dev/tcp/${HOST}/${PORT}) &>/dev/null
if [ $? -eq 0 ]; then
    echo "Connection successful"
    echo "Connection unsuccessful"

5. Perform TCP port scanning against a remote host

for ((port=$port_first; port<=$port_last; port++))
  (echo >/dev/tcp/$host/$port) >/dev/null 2>&1 && echo "$port open"

Final Notes

Opening a socket in bash requires that the bash shell have net-redirections enabled (i.e., compiled with "--enable-net-redirections"). Old distributions may have this feature disabled for bash, in which case you will encounter the following error.
/dev/tcp/ No such file or directory
Besides bash, socket support is known to be available in other shells such as ksh or zsh.

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