Re: TCPDUMP Filter don't work :(

[Jim Cliver <jim.c () aptalaska net>]

Hello MC,

I would try specifying the filter in a single statement something like
udp[8:2] != 0x8004. Or you could add another ! character before the
second udp statement so that both are negated.

Hope this helps.
clive

counterping () uk2 net wrote:
> Hiya,
>
> I have just started playing with filters within TCPDUMP and am a little
> confused ....
>
> I do NOT want to log RTP traffic on my network, but want to log everything else.
> RTP runs over UDP, The fist two bytes in the PAYLOAD are always the same (this
> is the RTP Header), and it has the hex value 80 and 04.
>
> I have used the follwing filter to look at the first 2 bytes AFTER the UDP
> packet (byte 8 and 9), UDP packets are always 8 Bytes. (so it's kinda fooling
> the app)
>
> "!udp[8] = 0x80 and udp[9] = 0x04"
>
> And it doesn't work .... BUT what's really weird.....
> if I remove the 'NOT' operator (!) it works just fine, capturing ALL the RTP
> traffic ONLY!
>
> Any help would be really appreciated, I must be doing something real stupid.
> Cheers
> MC
>
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