Wireshark · Wireshark-users: Re: [Wireshark-users] Wireshark Capture Filter Using Offset

["George E Burns" <geburns@xxxxxxxxxxx>]

Hello Guy,

I made a few changes to the filter and
this one is working:  udp port 53 and (udp[10] & 0xff) = 0x28

Thanks for your input!



George,



From:      
 Guy Harris <guy@xxxxxxxxxxxx>
To:      
 Community support list
for Wireshark <wireshark-users@xxxxxxxxxxxxx>
Date:      
 07/20/2010 02:55 AM
Subject:    
   Re: [Wireshark-users]
Wireshark Capture Filter Using Offset
Sent by:    
   wireshark-users-bounces@xxxxxxxxxxxxx

---

On Jul 19, 2010, at 11:37 PM, Guy Harris wrote:

> The UDP header is 16 bytes, so you have to add 16 to the offset from
the beginning of the DNS header.  The opcode is in the byte at an
offset of 3 from the beginning of the DNS header, so that's an offset of
19 (which is *NOT* 0x2C!), so the filter would be
> 
>                  udp
port domain and (udp[19] & 0x78) = 0x50

Sorry, that's

                
udp port domain and (udp[19] & 0x78) = 0x28

Caring about the query vs. response flag is left as an exercise for the
reader.
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