iflib_parse_header() pulls every IPv4 packet up to the end of the TCP or
UDP header, including fragments at a non-zero offset, which carry no L4
header at all. A trailing fragment shorter than that header (1-7 bytes
of UDP, 1-19 bytes of TCP) makes m_pullup() fail and the frame is freed
without a trace in the interface counters.
Only parse the L4 header on the first fragment. TSO never produces
fragments, so assert that.
With a 1500-byte MTU every UDP datagram of 1473-1479 bytes is lost
behind a forwarding host whose ingress NIC sets receive checksum flags,
or on any VLAN with hardware tagging. SIP over UDP is a common victim.
Reported by: Alex Roberts
Reviewed by: gallatin
Fixes: c224b2ce7d ("iflib: don't pullup UDP payloads to the TCP header size")
See also: https://redmine.pfsense.org/issues/17196
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")
R. Christian McDonald
f1726686cb