Every now and then, you might encounter a scenario where you need to figure out which open TCP connections are in a listening state on a Mac. This could be helpful either when troubleshooting certain network problems or just to check which processes are monitoring specific ports on a Mac.

We’ll utilize the lsof command-line tool to discover what/who is monitoring open TCP connections on a Mac, making this ideal for experienced Mac users who are familiar with the Terminal and sudo.

How to Discover What Monitors TCP Ports on MacOS

The command we'll use here is designed for MacOS Ventura and MacOS Monterey, but it should also function on older MacOS versions:

1. Open the "Terminal" application from /Applications/Utilities/ or via Spotlight with Command Spacebar.

2. Enter the following command: sudo lsof -iTCP -sTCP:LISTEN -P -n

3. Press enter and input the admin password when prompted to run the command and view a list of what is actively monitoring which TCP ports on the Mac.

You'll get a list of commands/processes, the process ID (PID), type (IPv6 vs IPv4), device, node (TCP in this case), and the port.

Based on this data, you can decide your next actions, depending on your goals.

For those interested, let's break down the command and the different flags used here:

• sudo: Executes the command with superuser privileges, enabling it to show network connections opened by all users, including root.
• lsof: Lists all open files on the system, including open network connections.
• -iTCP: Limits the results to show only TCP connections.
• -sTCP:LISTEN: Limits the results to show only listening TCP connections.
• -P: Prevents port name resolution, showing only port numbers.
• -n: Prevents hostname resolution, showing only IP addresses.

The lsof command is highly versatile, and we've previously discussed its usage for determining what apps or processes interact with a particular file, identifying what processes are using internet connectivity, displaying open network connections, monitoring all open internet connections, and more.

By the way, if you prefer a graphical interface, you can try the Sloth app, which serves as a GUI wrapper for lsof and is available as a free download. Additionally, Activity Monitor offers some limited support for similar features.

Do you have any similar tricks, commands, or tools you use to find what or who is monitoring any given TCP port on a Mac? Share them in the comments below.

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