Created attachment 165549 [details] Fix DAD for stf(4) Modifications to DAD code - r287734 (MFC of r287094) and r290348 (MFC of r288600) - rendered 6to4 connectivity non-working. Address assigned to an stf(4) interface stays "tentative" indefinitely. Attached patch fixes that.
see https://lists.freebsd.org/pipermail/freebsd-stable/2015-December/083856.html and https://lists.freebsd.org/pipermail/freebsd-stable/2016-January/083951.html
The patch here has been confirmed to fix the problem and work fine by multiple people who were impacted by this issue. https://redmine.pfsense.org/issues/5748 (that's in reference to a 10-STABLE, now 10.3-PRERELEASE based version) Is there a reason it hasn't been committed yet?
AFAICT this PR can be closed. Updating affected hosts to recent 10-STABLE has fixed the issue, and 6to4 connectivity works there again, regardless stf(4) address being in "tentative" state. There were just couple of commits to sys/netinet6/nd6 after this bug was reported, r299014, r299145 and r303458, so I guess it was fixed by one of those. Thanks.