In the world of networking, testing the availability and responsiveness of open ports is a crucial task. Traditionally, Telnet has been a go-to tool for such purposes, allowing users to connect to remote devices or servers. However, there are situations where Telnet might not be available or suitable for use. In those cases, several alternatives can help you achieve similar results. Let’s explore some of these alternatives, including Netcat, Telnet itself (in a different context), curl with the Telnet protocol, and a Bash TCP test.

Netcat

Netcat, often referred to as the “Swiss Army knife” of networking, is a versatile tool that can read and write data across network connections using the TCP/IP protocol. It’s particularly useful for testing open ports. Here’s a basic example of how to use Netcat to check if a port is open:

nc -zv <hostname> <port>

This command attempts to establish a connection to the specified hostname on the given port. The -z flag tells Netcat to

operate in zero-I/O mode (scanning), and the -v flag makes the operation more verbose, providing more information about the connection process.

For example, to check if port 80 is open on loredata.com.br:

nc -zv loredata.com.br 80

Using Telnet Itself

Even when Telnet isn’t your primary tool for connectivity, it can still be used for testing purposes if available on your system. The basic syntax to test an open port with Telnet is as follows:

telnet <hostname> <port>

This command attempts to open a Telnet session to the specified port on the given hostname. For instance, to check the availability of port 22 on loredata.com.br:

telnet loredata.com.br 22

If the port is open, you’ll get a response; otherwise, the connection will fail.

Curl with Telnet Protocol

Curl is a powerful tool primarily used for transferring data with URLs. Interestingly, it can also be used to test TCP connections by specifying the Telnet protocol. Here’s how you can use curl to check if a port is open:

curl telnet://<hostname>:<port>

This will attempt to establish a connection using the Telnet protocol. For example, to test port 443 on loredata.com.br:

curl telnet://loredata.com.br:443

If the port is open and accessible, Curl will attempt to establish a connection. Depending on the server’s response, you might not see much output, but the absence of an immediate error typically indicates that the connection was successful.

Bash TCP Test

For a pure Bash solution, you can use the /dev/tcp pseudo-device to test connectivity to a port. This method is particularly useful in environments where you have limited tools available but still have access to a Bash shell. Here’s a simple script to test if a port is open:

#!/bin/bash
if timeout 1 bash -c "echo > /dev/tcp/$1/$2"; then
  echo "Port $2 on $1 is open."
else
  echo "Port $2 on $1 is closed or unreachable."
fi

Save this script and make it executable. You can run it with two arguments: the hostname and the port you want to test. For example:

./testport.sh loredata.com.br 80

This command checks if port 80 on loredata.com.br is open. The timeout 1 command ensures the script does not hang for too long, attempting the connection for one second before giving up.

Each of these tools offers a unique set of features and advantages depending on your specific needs and the environment you’re working in. Whether you’re troubleshooting, performing routine checks, or just exploring your network, tools like Netcat, Telnet, Curl, and Bash provide flexible, powerful options for testing open ports.

I hope this helps. Thank you and see you on the next post,

Franky