Sake Blok wrote:
While analysing a problem today I discovered that I had missed packets in
my session after capturing with a filter like "vlan and host x.x.x.x".
After making a trace with no filter, I could see that packets in one
direction were tagged, while they were not tagged in the other direction.
Then I triesd a filter like "(vlan or not vlan) and host x.x.x.x", but
that just doesn't work.
Does anyone know how to capture both vlan-tagges frames and untagged
frames with an ip-address filter?
Might this be a WinPcap bug?
Does it count as a bug if it's documented to work that way? :-)
To quote the OS X 10.4.9 tcpdump man page (this isn't WinPcap-specific -
it's common to all libpcap/WinPcap implementations):
vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN packet. If
[vlan_id] is specified, only true is the packet has the
specified vlan_id. Note that the first vlan keyword
encountered in expression changes the decoding offsets
for the remainder of expression on the assumption that
the packet is a VLAN packet.
As the second sentence says, "Note that the first vlan keyword
encountered in expression changes the decoding offsets for the
remainder of expression on the assumption that the packet is a VLAN
packet." The "vlan" in "vlan and host x.x.x.x" causes the "host
x.x.x.x" to check for VLAN-encapsulated packets with an IPv4 address of
x.x.x.x.
To check for both VLAN-encapsulated and non-VLAN-encapsulated packets
with that IPv4 address, try
host x.x.x.x or (vlan and host x.x.x.x)
(or use "ip host x.x.x.x" in both cases if you don't care about ARP
packets).
