Port 55555 in use on Mac OS Big Sur

I've noticed the port 55555 looks like it is already in use on Mac OS Big Sur.

If you run nc -l 55555 it shows an error. "Address already in use"

But nothing answers if you telnet it, and nothing shows up on lsof -i tcp:55555 too. Very odd behaviour.

Confirmed on several different machines.
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leandrotavares
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This is apparently a known problem: https://developer.apple.com/forums/thread/659864

The solution is to use some other port.
Posted by
Etresoft
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This is extremely irritating. It's not even documented, https://support.apple.com/en-gb/HT202944
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omg_it_burns
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I also met this problem after I upgraded to Big Sur. It seems the "well-known" problem is about iOS 14. I didn't find any official posts about this problem on Mac OS. I have started a question on stackoverflow.
Posted by
cosmozhang
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