Another fun happy iOS story: we were launching our app a year ago, with a self-imposed deadline. As usual, tons of bugs were being fixed in the last moment.

And then our authentication stopped working on simulated iOS devices (while still working on the real devices!). After hours of frantic debugging and staring at Wireshark dumps, I found the issue: HTTP3 and QUIC. Apparently, the simulated stack was not tracking the MTU correctly and was trying to send 1506-byte UDP packets.

The "fix" was to add deny rules for UDP ports 80/443 to our firewall.