Ondřej Surý authored May 28, 2020 and Ondřej Surý committed Dec 10, 2020

BIND 9 uses a connect()ed UDP socket when sending recursive queries.
The connect()ed socket provides feedback on a variety of ICMP
errors (e.g.  port unreachable) which BIND 9 can then use to decide what
to do with errors (report them to the client, try again with a different
nameserver etc).

However, Linux's implementation does not report what it considers
"transient" conditions, which is defined as Destination Host
Unreachable, Destination Network Unreachable, Source Route Failed and
Message Too Big.

BIND 9 now uses set setsockopt(fd, SOL_IP(V6), IP(V6)_RECVERR, &one,
sizeof(one)); and receive a EPOLL_ERR, and recvmsg return value so that
it notices that the send has failed immediately rather than waiting for
a timeout.