Sam Mason wrote:
On Sat, Sep 02, 2006 at 08:19:01AM +0100, Laurence Skinner wrote:
I can see the packets coming in and going out with tcpdump, but they don't
reach the monitoring server:
[ ... ]
Your tcpdump log suggests that your network stack is generating invalid
UDP check-sums. This should cause the receiving system to discard the
packet, which would cause your box to appear unreachable.
If you run tcpdump on the host generating the traffic, tcpdump will see the
packets via BPF before they have actually been completely processed (ie, the
mbuf chain coalesced into a single wired-down buffer which can be used for DMA
and the checksums computed). This is especially true if you are using a NIC
with hardware Rx/Tx checksum capabilities.
In other words, seeing bad checksums in traffic sent by the sniffer host is
common and normal.
--
-Chuck
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