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TCP servers on macOS Catalina 10.15.7 not sending a SYN/ACK

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I have a macOS laptop running Catalina (10.15.7), using the built-in WiFi interface. I have a Thinkpad laptop running Ubuntu 20.04, and a Windows 11 Desktop. I'm trying to access a webserver on my macOS laptop but the browser returned ERR_CONNECTION_REFUSED. To dig deeper, I opened up Wireshark and noticed that the TCP SYN was being sent to the macOS server (and being received there), but there was no SYN/ACK packet being sent back.

To simplify the problem, I ran a simple TCP server with the command on the thinkpad echo thinkpad | nc -l 9000

And I ran a similar command on the MacBook: echo macbook | nc -l 9000

Then I connected to both using nc on my Windows Desktop (see screenshot)

enter image description here

The Thinkpad running Linux sends the SYN/ACK packet, finishes the TCP handshake, and returns the "thinkpad" string. The Mac never sends the SYN/ACK packet. This happens for everything I can run on it. The firewall on the mac is off, the VPN is off, and I can even see the incoming TCP packets on the mac (with Wireshark running there too).

What can I change on MacOS Catalina to get it to send the SYN/ACK packet and solve this problem?

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