[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: bad cksum 0! repeat line in tcpdump in 3.5

> Hi Daniel,
>           I used ethereal on a separate box to sniff the packets on the
> LAN connected to the incoming interface where the checksum error was
> showing (em1) to see if there was some problem, but the packets had a
> good checksum. The only unknown was that I wasn't directly connected to
> the interface but a switch back. It looks like an extra line with a
> checksum error is being generated, either by tcpdump or by pf. From
> Mipam it may be a problem with the hardware. Has this showed up before
> with Intel pro/1000 adapters?
I didn't say it's a problem with the hardware. Seeing bad checksums is 
normal when checksum offloading is enabled. This is no problem at all.
However, in bridge mode checksum offloading cannot be enabled.
If you see in ifconfig em1 and checksum offloading is enabled, then it 
means you didnt configure the nics right for bridging.
But seeing checksum errors when checksum offloading is disabled is weird 
i'd say. If your machine is not generating traffic and you're just 
monitoring traffic which comes through the machine and then you see bad 
checksums, it might indicate a problem on the machines generating the 
traffic or tcpdump might not correctly show the packets. But the latter 
case seems unlikely to me. Though, their are packet generators for udp 
packets that do not calculate checksums. :-) Netwox can however. But then 
i'd be assuming you're seeing packets being generated with such tools. :-)