capnproto
build runs kj/compat/http-test which appears to reach out to the internet
I was building behind a firewall, so this eventually timed out and failed, and it looks like the build succeeded anyway, but it took a long time. The IP address it was trying to reach was the one shared by capnproto.org and sandstorm.io.
Dang, I carefully wrote the test such that if the DNS lookup or connection failed, it would skip the test, but I didn't account for a network that succeeds at DNS lookup but then silently drops packets. I suppose I could add a timeout.
(All it is doing is an HTTP GET to test the HTTP client code against a real server. In theory I should, like, bundle a real HTTP server into the test in order to do this hermetically, but...)