Emulator UI not serving UI when open in port 6000

[dutzi]

[REQUIRED] Environment info

firebase-tools: 8.4.0

Platform: macOS 10.15.4 (19E287)

[REQUIRED] Test case

I just initialized a new project, chose port 6000 for the UI and started the emulators, I got the following in the console:

┌───────────┬────────────────┬─────────────────────────────────┐
│ Emulator  │ Host:Port      │ View in Emulator UI             │
├───────────┼────────────────┼─────────────────────────────────┤
│ Functions │ localhost:5011 │ http://localhost:6000/functions │
├───────────┼────────────────┼─────────────────────────────────┤
│ Firestore │ localhost:8100 │ http://localhost:6000/firestore │
├───────────┼────────────────┼─────────────────────────────────┤
│ Hosting   │ localhost:5010 │ n/a                             │
└───────────┴────────────────┴─────────────────────────────────┘

But when accessing http://localhost:6000/ I got a message from Chrome saying the site is offline:

This site can’t be reachedThe webpage at http://localhost:6000/functions might be temporarily down or it may have moved permanently to a new web address.

I tried running lsof -i :6000 and found no process listening on that port.

But, when changing the port to 9292 the UI did load.

[REQUIRED] Steps to reproduce

See above.

[REQUIRED] Expected behavior

I see the UI

[REQUIRED] Actual behavior

UI isn't loaded

See logs:

firebase-debug.log
ui-debug.log
firestore-debug.log

[samtstern]

@dutzi you're right, I am able to reproduce this easily. I have to look into why this port would be special.

[samtstern]

I see some references online for these ports being used for X Window:
http://www.networksorcery.com/enp/protocol/ip/ports06000.htm

However I am able to get ports like 6001 and 6060 to work ... only exactly 6000 is a problem!

[dutzi]

This does not seem like an issue with the UI, running serve -p 6000 or python -m SimpleHTTPServer 6000 doesn't work as well...

[dutzi]

This helps: https://stackoverflow.com/questions/4313403/why-do-browsers-block-some-ports/22622633#22622633

[samtstern]

@dutzi good find! Maybe we can use that list to add a warning to the UI.