OpenConnect 8.00 from latest packages on FreeBSD 12.0-RELEASE-p2 cannot negotiate DTLS, and falls back to a TLS over TCP session. This is caused by compatibility issues with OpenSSL 1.1, which we select in our ports build (I'm not sure how, though). Fortunately, I found this has just been fixed (yesterday!) in OpenConnect master with commit 917f15ad8937a0e57602a9c79bd7cb167b4d76b4, and will presumably be in the next release (>8.01). We should take that patch and/or release, obviously. Not high priority for me, I can bump along with a local build.
OpenConnect 8.02 has been released. Works for me with './configure --with-openssl' (the --with-vpnc-script option is no longer needed for FreeBSD).
A commit references this bug: Author: zi Date: Wed Jan 16 20:08:11 UTC 2019 New revision: 490507 URL: https://svnweb.freebsd.org/changeset/ports/490507 Log: - Update to 8.02 PR: 234839 Reported by: John Hood <cgull@glup.org> Changes: head/security/openconnect/Makefile head/security/openconnect/distinfo