I plugged into ethernet (as wifi w/captive portal does not work for me). I think clearnet worked but I have no interest in that. Egress Tor traffic was blocked and so was VPN. I’m not interested in editing all my scripts and configs to use clearnet, so the library’s internet is useless to me (unless I bother to try a tor bridge).
I was packing my laptop and a librarian spotted me unplugging my ethernet cable and approached me with big wide open eyes and pannicked angry voice (as if to be addressing a child that did something naughty), and said “you can’t do that!”
I have a lot of reasons for favoring ethernet, like not carrying a mobile phone that can facilitate the SMS verify that the library’s captive portal imposes, not to mention I’m not eager to share my mobile number willy nilly. The reason I actually gave her was that that I run a free software based system and the wifi drivers or firmware are proprietary so my wifi card doesn’t work¹. She was also worried that I was stealing an ethernet cable and I had to explain that I carry an ethernet cable with me, which she struggled to believe for a moment. When I said it didn’t work, she was like “good, I’m not surprised”, or something like that.
¹ In reality, I have whatever proprietary garbage my wifi NIC needs, but have a principled objection to a service financed by public money forcing people to install and execute proprietary non-free software on their own hardware. But there’s little hope for getting through to a librarian in the situation at hand, whereby I might as well have been caught disassembling their PCs.
This sounds odd to me, unless you connected to an Ethernet port behind a desk or somehow forced open a network closet… They also might not like it if you disconnected one of the public computers to use its cable/port; otherwise if this was an open and public port, you used it as designed and the librarian probably has watched too many Hollywood hacking movies. I have to admit, I never thought of this as a way to bypass the captive portal (sorta just assumed everyone going through the public network would have to hit it, kinda of the equivalent to having everyone sign a liability waiver).
With that said, I can see some institutions not liking connections that aren’t part of the more traditional/commercial networking (but it doesn’t sound like the library took issue with your traffic, just the librarian didn’t like the PHY link you chose to use). For the SMS thing (I haven’t seen that used in a while, you might be able to use some sort of burner number app if they don’t filter them).
That's because if that library's network was properly configured it would work exactly like your expectation.