macOS Big Sur 11.3 firewall blocking screen sharing

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showp1984 Author

Level 1

8 points

macOS Big Sur 11.3 firewall blocking screen sharing

Good day, denizens of this community. Or goodnight, depending on the rotation of your location. 😉

I have a weird issue with screen sharing on a brand new Macbook Air 2020 (M1).

My niece got this machine yesterday as a replacement for her old one and we proceeded to set it up as we had done on the old one. We set up screen sharing for remote sessions if help is required and everything was working. The new machine came with 11.2.3.

Tonight she ran into an issue (unrelated) and needed help. I logged on, fixed it for her and started the upgrade to 11.3. After the update went through I was no longer able to logon using screen share.

If the Mac is on the login screen it acts as if there is no screen sharing enabled.
If she is logged in it behaves like a black hole - as if the whole machine is not there.

Clarification: The screen share tool responds like that. The machine is still pingable on the network.

So I double checked everything. Is screen sharing enabled? Yep. Is the firewall set to allow incoming connections (by screen sharing)? Yep. Is the Mac pingable from the network? Yep.

After an hour of trying to find an issue in the logs (nothing presented) and even adding manual firewall rules (didn't help) I was growing desperate: I disabled screen sharing (just unticked the checkbox under "Sharing"), rebooted and enabled it again.

Voilà! It worked! One more reboot - same behaviour as above...

If I disable the firewall everything is working fine - but that is not a solution.

Additional information:

The issue persists on WiFi and a cable connection.

I am not connecting over the internet but through a VPN connection. For the Mac this is like a connection from a different subnet - that's it.

My subnet: 192.168.200.0/24

Macbook Air subnet: 192.168.1.0/24

Probably a bug - but I though to ask anyway: Does anybody know a solution?

-Dennis

Posted on May 1, 2021 1:06 PM

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Answer 1

Top-ranking reply

showp1984 Author

Level 1

8 points

May 2, 2021 2:26 PM in response to AnnieL2

Hi AnnieL2,

Thank you for your answer!

We are using the built-in screen sharing of macOS. There is no VPN or educational / corporate profile set up on that Mac.

I did try it with my own Admin account, but it didn't help.

I "solved" the problem just now by reinstalling and restoring from a TM backup.

Afterwards - when first clicking on screen sharing under "Sharing" in the system settings - the Mac suddenly showed a new dialog, prompting me to confirm 'screensharingd' receiving incoming connections. Now everything works as expected. 😌

Not sure why that dialog wasn't displayed before - or why it worked sometimes (as outlined above), or why screen sharing would have a green-ok-circle in Firewall settings if that dialog wasn't accepted / never appeared before.

We can close up here, the issue is now solved. :)

-Dennis

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Answer 2

AnnieL2

Community Specialist

May 2, 2021 12:17 PM in response to showp1984

Hi Dennis,

We understand that when attempting to connect with your niece's Mac via screen sharing, you're running in to an issue now that her Mac is running macOS 11.3.

To clarify, are you using the screen share feature of macOS or a third-party app? Does your niece use any type of VPN network or have an educational or corporate profile of any kind set up on her Mac?

During troubleshooting, did you have a chance to set up a new admin user and test screen sharing there? Set up users, guests, and groups on Mac That would let us know if what's happening is happening system-wide or is related to this specific user.

Thanks.

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Answer 3

AnnieL2

Community Specialist

May 2, 2021 2:35 PM in response to showp1984

Dennis,

We appreciate you letting us know and we're glad to hear you were able to get the situation resolved.

Thank you.

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macOS Big Sur 11.3 firewall blocking screen sharing

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