Hi! Please allow me to chime in since I'm also seeing an issue with macOS's builtin SSH when using a transparent proxy network extension, even without IPv6. Here's the steps: Set your network interface's IPv6 settings to link-local-only (effectively disabling IPv6) Start a network extension (this is a simple TCP forwarder for TCP and IPv4 only) that I've attached in thread/660163 and feedback FB8690357, also available here: https://github.com/ngorskikh/transparentproxyrepro Try connecting to some SSH server with Apple's default ssh, and OpenSSH from Homebrew. Here's my results: ➜ TransparentProxy git:(master) /usr/bin/ssh fa2.lamo.su    The authenticity of host 'fa2.lamo.su (0.0.0.0)' can't be established. ECDSA key fingerprint is SHA256:dZsSQrUwwPgm/EZ8wGkuijpIEslPTg3NcfIEQgWLqiY. Are you sure you want to continue connecting (yes/no/[fingerprint])? no Host key verification failed. ➜ TransparentProxy git:(master) /usr/local/Cellar/openssh/8.3p1/bin/ssh fa2.lamo.su The authenticity of host 'fa2.lamo.su (88.99.32.235)' can't be established. ECDSA key fingerprint is SHA256:dZsSQrUwwPgm/EZ8wGkuijpIEslPTg3NcfIEQgWLqiY. Are you sure you want to continue connecting (yes/no/[fingerprint])? no Host key verification failed. Apple's SSH sees the wrong remote address for some reason.

Greetings, @meaton! I've put the code on github: https://github.com/ngorskikh/transparentproxyrepro I've updated the feedback, too, but it looks like attachments don't work for some reason -- or at least I can't see my own attachments, even though the original feedback request contained some automatically collected logs, as well as a zip with an xcode project and a couple of screenshots. Kind regards, ngorskikh.