Mathieu,

As the original issue was reported for BIND 9.11, I prepared a patch
(attached) which applies cleanly on top of BIND 9.11.2, which is the
latest maintenance release in the 9.11 branch.

Could you please test it and tell me whether the modified BIND behavior
it causes is acceptable to you?  If it is, please do not pass the patch
on just yet; once it gets merged, I will refer you to the relevant git
commit.

Just applying the patch and recompiling BIND should be all that is
needed for FreeBSD to be handled in a bit more friendly manner.  As I
mentioned before, the new configure option (--disable-tcp-fastopen) is
only there for other systems that we currently do not know of which
might exhibit a similar behavior.

Thanks!