RE: TCPDUMP Filter don't work :(

["Wirth, Jeff" <WirthJe () DNB com>]

From: counterping () uk2 net [mailto:counterping () uk2 net]
> 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"

This would read packets that DO NOT have udp[8] = 0x80 and packets that DO
have udp[9] = 0x04.  If I understand what you are looking for, I think what
you want is:

!udp[8] = 0x08 and !udp[9] = 0x04   
or
not (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!

- Jeff


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