The ICMP RFC says that identifier and sequence number may be zero.
Having them zero for a Echo message, along with a data of zero's
as well will result in a Echo reply message with only zero's.
Some NAT implementations seem to get the checksum wrong on these
packages. Setting a checksum of 0x0 instead of 0xffff.
Through NAT:
Internet Control Message Protocol
Type: 0 (Echo (ping) reply)
Code: 0
Checksum: 0x0000 [incorrect, should be 0xffff]
Identifier (BE): 0 (0x0000)
Identifier (LE): 0 (0x0000)
Sequence number (BE): 0 (0x0000)
Sequence number (LE): 0 (0x0000)
Data (56 bytes)
Data: 000000000000000000000000000000000000000000000000...
[Length: 56]
Without NAT:
Internet Control Message Protocol
Type: 0 (Echo (ping) reply)
Code: 0
Checksum: 0xffff [correct]
Identifier (BE): 0 (0x0000)
Identifier (LE): 0 (0x0000)
Sequence number (BE): 0 (0x0000)
Sequence number (LE): 0 (0x0000)
[Request frame: 189]
[Response time: 0.024 ms]
Data (56 bytes)
Data: 000000000000000000000000000000000000000000000000...
[Length: 56]
And this in turn will make some hardware MAC checksum offloading
engines drop the packet.
(This was seen with a Synopsis MAC, the same one used in for instance the
stmmac Ethernet driver in the linux kernel.)
This change can be seen as a workaround for bugs in other layers.
But just setting an identifier for the Echo message packet will
avoid prodding the hornets nest.
function old new delta
common_ping_main 424 500 +76
Signed-off-by: Jonas Danielsson <jonasdn@axis.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
4d5acd2d42