Via a new configuration attribute, the user can decide whether the router
should answer dropped fragmented IPv4 with an ICMP "destination unreachable"
packet and, if so, which value the ICMP code field of this packet should have.
The default is that the router doesn't send such responses (silently dropping
fragmented IPv4). The behavior is tested by the 'nic_router_ipv4_fragm' test.
Fixes#4236
If the new attribute 'dropped_fragm_ipv4' of the <report> tag in the NIC router
config is set "yes", the router will report the number of packets that were
dropped per interface respectively domain because fragmented IPv4 is not
supported. The default is not to report the counter. The behavior is tested by
the 'nic_router_ipv4_fragm' test.
Ref #4236
The NIC router used to ignore the IPv4 header fields "More fragments" and
"Fragment offset" completely. Therefore higher-level protocols of fragmented
IPv4 were interpreted wrong because each fragment was considered a self-
standing packet, expecting, for instance UDP/TCP headers somewhere inside of
the UDP/TCP data field. Normally, such packets were dropped as soon as the
UDP/TCP checksum check failed because of the misinterpretation. However,
it was also possible for fragmented IPv4 to pass the router although normally
only partially.
IPv4 fragmentation support in the router would introduce some potential
security risks and is presumably not an easy endeavor. So, for now, we settled
on not supporting IPv4 fragmentation. With this commit, the router simply drops
all fragmented IPv4. This is reflected to the log for each fragment as "drop
packet (fragmented IPv4 not supported)" when 'verbose_packet_drop="yes"' is
configured.
The new test 'run/nic_router_ipv4_fragm' is an automated test for this
behavior. The test is added to the autopilot list.
Ref #4236
