Re: Addition to my question: SMTP + 802.1q Virtual LAN

[Guy Harris <guy () alum mit edu>]

On Dec 17, 2003, at 6:22 AM, Cliff Clavin wrote:

> I hope I found the difference. Playing around with tcpdump and filters 
> I found out that tcpdump couldn't find tcp packets. After looking at a 
> piece with ethereal I recognized that between the IP and Ethernet 
> layer reside a 802.1q Virtual LAN layer.

Tcpdump should have no problem with 802.1q packets - it's supported 
that ever since tcpdump 3.5.

However, if you want to use a filter to find those packets, you have to 
use the "vlan" keyword, e.g. "vlan and tcp port 25".

Some other applications might not know about 802.1q at all.  Those 
applications would have to be changed to know about it, or you'd have 
to find some program that de-802.1q-ifies packets in libpcap-format 
captures (I don't know of any such program).

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