Subject: Re: tcpdump's capture filter: "vlan" doesn't match

From: Daniel Borkmann <[email-masked]>
Date: Thu, 16 Oct 2014 08:10:52 +0200

On 10/16/2014 12:58 AM, Lukas Tribus wrote:
> 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?

There were some discussions on this in the past e.g. [1]. We have
SKF_AD_VLAN_TAG and SKF_AD_VLAN_TAG_PRESENT for the BPF filter on
this, but libpcap is currently not making use of any of them.

  [1] http://thread.gmane.org/gmane.linux.network/247947

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