Date: Tue, 22 Oct 2002 23:55:09 -0400 From: "Eric W. Bates" <ericx@vineyard.net> To: <freebsd-net@freebsd.org> Subject: debugging VLANs with tcpdump Message-ID: <009701c27a47$fb2d6c80$3ee380cc@alice>
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While using tcpdump to debug a new VLAN config, I noticed some problems and
wanted to ask whether I was simply doing something wrong.
I have 3 vlan interfaces attached to an fxp. The 3 vlan are bridged, but
the fxp is not included in the group. The fxp has no IP. vlan0 has the IP
for the bridge group.
fxp0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
ether 00:02:b3:5b:dd:98
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
vlan0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 192.168.10.1 netmask 0xffffff00 broadcast 192.168.10.255
ether 00:02:b3:5b:dd:98
vlan: 5 parent interface: fxp0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
vlan1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:02:b3:5b:dd:98
vlan: 10 parent interface: fxp0
vlan2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:02:b3:5b:dd:98
vlan: 20 parent interface: fxp0
I get the most useful information when I run tcpdump on the fxp. Doing so
shows all the packets passing by the NIC and it labels VLAN packets with
their tag.
tcpdump -i fxp0
20:00:54.747032 802.1Q vlan#10 P0 192.168.10.98 > 192.168.10.1: icmp: echo
request (ttl 32, id 232, len 60)
20:00:54.747210 802.1Q vlan#10 P0 192.168.10.1 > 192.168.10.98: icmp: echo
reply (ttl 64, id 22505, len 60)
As soon as I add an expression to the tcpdump, I lose the VLAN labels from
the output. I tried:
tcpdump -i fxp0 icmp
tcpdump -i fxp0 vlan
tcpdump -i fxp0 host fw.mvhost.com
I also tried increasing verbosity (-vvv).
Attaching tcpdump to a vlan was interesting. Presumably I was seeing only
packets tagged with the pseudo-interface's VLAN ID; but the labels were gone
again.
Is this a bug? If not, how do I display the explicit tag information under
more circumstances?
Thanks for your time.
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