Testing Remote TCP Ports from Shell Scripts

Testing whether a remote TCP port is open can be crucial for various network-related tasks, such as checking if a server is accessible or if a specific service is running. This task can be accomplished directly from shell scripts using several approaches. In this tutorial, we will explore methods to test remote TCP ports using tools like nc (Netcat) and Bash’s built-in features.

Introduction to Netcat

Netcat (nc) is a powerful networking tool that can be used for a wide range of tasks, including port scanning, transferring files, and more. Its versatility makes it an excellent choice for testing TCP ports.

Using Netcat to Test Ports

To check if a remote TCP port is open using nc, you can use the following command:

nc -z -w 5 <host> <port>

• -z tells nc to only scan for listening daemons, without sending any data to them.
• -w 5 sets a timeout of 5 seconds. If no connection is established within this time frame, the command will exit.

If the port is open, nc will exit with status code 0; otherwise, it will exit with a non-zero status code.

Example

To check if port 80 on example.com is open:

if nc -z -w 5 example.com 80; then
    echo "Port 80 is open"
else
    echo "Port 80 is closed or not accessible"
fi

Using Bash’s Built-in Features

Bash, version 4 and later, supports a feature that allows you to use /dev/tcp to create TCP connections directly from the shell. This can be used as an alternative method for testing remote ports.

Basic Usage

The basic syntax for connecting to a host on a specific port using Bash is:

exec 3<>/dev/tcp/<host>/<port>

If this command succeeds, it means the connection was established (i.e., the port is open). You can then close the file descriptor 3 and check the exit status of the previous command.

Example

To test if port 80 on example.com is open:

#!/bin/bash
SERVER=example.com
PORT=80

if timeout 1 bash -c "exec 3<>/dev/tcp/$SERVER/$PORT"; then
    echo "Port $PORT on $SERVER is open"
else
    echo "Port $PORT on $SERVER is closed or not accessible"
fi

Note the use of timeout to avoid hanging indefinitely if the port is not open.

Choosing Between Netcat and Bash

• Netcat (nc): Offers more flexibility and options for customizing your port scans, including setting specific timeouts. It’s also widely available across different Linux distributions.
• Bash’s /dev/tcp: Provides a straightforward way to test ports directly from within Bash scripts without needing external tools. However, the availability of this feature depends on your Bash version.

Conclusion

Testing remote TCP ports is an essential task for network diagnostics and service monitoring. Both nc (Netcat) and Bash’s built-in /dev/tcp offer reliable methods for accomplishing this task directly from shell scripts. By choosing the right tool based on your specific needs, you can efficiently automate port testing as part of your system administration or development workflow.