Moving ethernet VLAN tags into the mbuf packet header (from mtags)

Peter Jeremy peterjeremy at
Fri Sep 8 15:07:43 PDT 2006

On Thu, 2006-Sep-07 17:07:25 +0200, Andre Oppermann wrote:
>With the recent addition of a 16bit field for TSO into the mbuf packet
>header we've got 16bits left over.  I've reserved these bits for the
>ethernet VLAN tagging of packet to do away with the allocated mbuf mtag.

Sounds reasonable.  My only comment is that there doesn't appear to
be any way for bpf(4) to filter/capture VLAN information.

Before hardware tagging, you could run tcpdump on the vlan parent device
and filter on the VLAN tag as well as see the VLAN associated with each
packet.  I found this very useful for monitoring routed data as well as
finding cases where packets were appearing in the wrong VLAN.

With hardware tagging (with or without this patch), bpf doesn't have
access to the tag information.  This is a PITA.

Peter Jeremy
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