netdev - tcpdump's capture filter: "vlan" doesn't match

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Message-ID: <DUB123-W23D5ECAD4869F9A799E1F6EDAA0@phx.gbl>
Date:	Thu, 16 Oct 2014 00:58:42 +0200
From:	Lukas Tribus <luky-37@...mail.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	John Fastabend <john.r.fastabend@...el.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Jiri Pirko <jpirko@...hat.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Atzm Watanabe <atzm@...atosphere.co.jp>,
	Patrick McHardy <kaber@...sh.net>,
	Jesse Gross <jesse@...ira.com>
Subject: tcpdump's capture filter: "vlan" doesn't match

Hi,

since 2.6.39 (including -rc1), tcpdump "vlan" capture filters don't match
anymore. All 2.6.38 and older kernels are fine.

I reproduced this specifically on a r8169 NIC on 2.6.39-rc1, but I found
this problem initially on bnx2 and e1000e nics.

Howto reproduce: just tcpdump with a "not vlan", "vlan" or "vlan <vlanid>"
capture filter on a passive eth interface (dot1q/vlan/ip config not necessary).

Actual behavior is that a "vlan [vlanid]" capture filter doesn't match the
(tagged) packet, and a "not vlan" capture filter matches everything.

Disabling rx-vlan-offloading via
ethtool -K eth0 rxvlan off

doesn't change anything.

Here we are filtering for "not vlan" and we can see that the matched frame
is vlan tagged:

# tcpdump -Uenc1 not vlan
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
22:03:39.077584 70:ca:9b:01:23:34> 00:18:f8:01:23:34, \
*ethertype 802.1Q (0x8100), length 70: vlan 7, p 0*, ethertype IPv4, \
192.168.47.9.443> 192.168.32.30.39436: Flags [.], ack 255248912, \
[...]
1 packet captured
169 packets received by filter
0 packets dropped by kernel
59 packets dropped by interface
#

As suggested here [1], we can pipe everything through another tcpdump
instance:
tcpdump -Uw - | tcpdump -en -r - vlan <vlanid>

But that is not something that works for my specific use-case (dedicated
sniffer box, dedicated interface connected to a Cisco SPAN/mirror port,
un/single/double-tagged packets, remotely accessible via remote-pcap [2]).

The sniffer should also be able to:
- maintain the frame as-is, including dot1q, dot1p (preferably
  without artificial recreation of header fields/values and including CFI/DEI)
- "direct" capture filter based on vlan (not through multiple userspace
  instances)

Kernel <= 2.6.38 perfectly satisfies those requirements.

Isn't disabling rx-vlan-offloading supposed to remedy those problems?

Thanks,

Lukas

[1] https://bugzilla.redhat.com/show_bug.cgi?id=498981
[2] https://github.com/frgtn/rpcapd-linux

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