Instead of disabling the fragmentation on the UDP sockets, we now
disable the Path MTU Discovery by setting IP(V6)_MTU_DISCOVER socket
option to IP_PMTUDISC_OMIT on Linux and disabling IP(V6)_DONTFRAG socket
option on FreeBSD. This option sets DF=0 in the IP header and also
ignores the Path MTU Discovery.
As additional mitigation on Linux, we recommend setting
net.ipv4.ip_no_pmtu_disc to Mode 3:
Mode 3 is a hardend pmtu discover mode. The kernel will only accept
fragmentation-needed errors if the underlying protocol can verify
them besides a plain socket lookup. Current protocols for which pmtu
events will be honored are TCP, SCTP and DCCP as they verify
e.g. the sequence number or the association. This mode should not be
enabled globally but is only intended to secure e.g. name servers in
namespaces where TCP path mtu must still work but path MTU
information of other protocols should be discarded. If enabled
globally this mode could break other protocols.
It was discovered that setting the thread affinity on both the netmgr
and netthread threads lead to inconsistent recursive performance because
sometimes the netmgr and netthread threads would compete over single
resource and sometimes not.
Removing setting the affinity causes a slight dip in the authoritative
performance around 5% (the measured range was from 3.8% to 7.8%), but
the recursive performance is now consistently good.
The isc/platform.h header was left empty which things either already
moved to config.h or to appropriate headers. This is just the final
cleanup commit.
The Windows support has been completely removed from the source tree
and BIND 9 now no longer supports native compilation on Windows.
We might consider reviewing mingw-w64 port if contributed by external
party, but no development efforts will be put into making BIND 9 compile
and run on Windows again.
