netdev - Re: [Bonding-devel] ethernet bonding + VLAN: additional VLAN tag in tcpdump

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Message-ID: <CAAXf6LXOEpDNf-abqZ_FCJ4G85s-domOHLzH0ur+F77gOvO8EA@mail.gmail.com>
Date:	Mon, 5 Dec 2011 10:50:27 +0100
From:	Thomas De Schampheleire <patrickdepinguin@...il.com>
To:	Jiri Pirko <jpirko@...hat.com>
Cc:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>, bonding-devel@...ts.sourceforge.net,
	tcpdump-workers@...ts.tcpdump.org,
	Ronny Meeus <ronny.meeus@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Bonding-devel] ethernet bonding + VLAN: additional VLAN tag in tcpdump

Hi,

On Wed, Nov 30, 2011 at 10:06 AM, Thomas De Schampheleire
<patrickdepinguin@...il.com> wrote:
> On Wed, Nov 30, 2011 at 8:52 AM, Jiri Pirko <jpirko@...hat.com> wrote:
>> Tue, Nov 29, 2011 at 09:35:00PM CET, nicolas.2p.debian@...il.com wrote:
>>>Le 29/11/2011 14:38, Thomas De Schampheleire a écrit :
>>>>Hi,
>>>>
>>>>I'm seeing incorrect tcpdump output in the following scenario:
>>>>
>>>>* ethernet bonding enabled in the kernel, and a single network
>>>>interface (eth0) added as slave
>>>>* bonding mode was set to broadcast, but I don't think this matters
>>>>* VLAN added to the bond0 network interface
>>>>* ip address set on the vlan interface (bond0.1234)
>>>>* tcpdump capturing full packets (-xx or even -x) on the eth0 interface
>>>>
>>>>Then, when pinging from another machine to this ip address, the ping
>>>>reply packets shown by tcpdump incorrectly have a double VLAN tag.
>>>>However, what really appears on the wire is correct: a single VLAN
>>>>tag.
>>>
>>>Copied netdev, because bonding and vlan developers are there.
>>>
>>>Jiri, don't you think this might be related to the work you have done
>>>to make non-hw-accel rx path similar to hw-accel?
>>
>> I do not think so. The changes you are reffering to are unrelated to tx
>> path (where this issue has most probably roots in)
>>
>>>
>>>       Nicolas.
>>>
>>>>
>>>>Here is the output from tcpdump:
>>>># /tmp/tcpdump  -i eth0 -xx
>>
>> What hw is this?
>
> This is on a Freescale P4080 DPA mac (fsl,p4080-fman-1g-mac).
>
>>
>>>>tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
>>>>listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
>>>>01:04:04.607880 IP 192.168.1.2>  192.168.1.1: ICMP echo request, id 26933, seq 4
>>>>16, length 64
>>>>         0x0000:  0600 0000 0020 0600 0000 0020 8100 0ffe
>>>>         0x0010:  0800 4500 0054 0000 4000 4001 b755 c0a8
>>>>         0x0020:  0102 c0a8 0101 0800 98d7 6935 01a0 e528
>>>>         0x0030:  0f2a 0000 0000 0000 0000 0000 0000 0000
>>>>         0x0040:  0000 0000 0000 0000 0000 0000 0000 0000
>>>>         0x0050:  0000 0000 0000 0000 0000 0000 0000 0000
>>>>         0x0060:  0000 0000 0000
>>>>01:04:04.607889 IP 192.168.1.1>  192.168.1.2: ICMP echo reply, id 26933, seq 416
>>>>, length 64
>>>>         0x0000:  0600 0000 0020 0600 0000 0020 8100 0ffe
>>>>         0x0010:  8100 0ffe 0800 4500 0054 cc07 0000 4001<--------
>>>>extra VLAN header at 0x10
>>>>         0x0020:  2b4e c0a8 0101 c0a8 0102 0000 a0d7 6935
>>>>         0x0030:  01a0 e528 0f2a 0000 0000 0000 0000 0000
>>>>         0x0040:  0000 0000 0000 0000 0000 0000 0000 0000
>>>>         0x0050:  0000 0000 0000 0000 0000 0000 0000 0000
>>>>         0x0060:  0000 0000 0000 0000 0000
>>>>
>>>>
>>>>Initial debugging showed that the addition of the extra VLAN header
>>>>takes place in function pcap_read_linux_mmap() of libpcap, in the
>>>>following snippet:
>>>>
>>>>#ifdef HAVE_TPACKET2
>>>>                 if (handle->md.tp_version == TPACKET_V2&&  h.h2->tp_vlan_tci&&
>>>>                     tp_snaplen>= 2 * ETH_ALEN) {
>>>>                         struct vlan_tag *tag;
>>>>
>>>>                         bp -= VLAN_TAG_LEN;
>>>>                         memmove(bp, bp + VLAN_TAG_LEN, 2 * ETH_ALEN);
>>>>
>>>>                         tag = (struct vlan_tag *)(bp + 2 * ETH_ALEN);
>>>>                         tag->vlan_tpid = htons(ETH_P_8021Q);
>>>>                         tag->vlan_tci = htons(h.h2->tp_vlan_tci);
>>>>
>>>>                         pcaphdr.caplen += VLAN_TAG_LEN;
>>>>                         pcaphdr.len += VLAN_TAG_LEN;
>>>>                 }
>>>>#endif
>>
>> I haven't look into this code yet, but where's the code which does the
>> first header inclusion?
>
> I would assume this is done by the VLAN layer. This is a ping reply
> originating from the icmp code, passing down to the vlan layer, then
> to the ethernet bonding layer, and then to the hardware. But before
> this is passed to hardware, libpcap captures the packet.
>
> I haven't debugged that part, though, so I can't give you a direct
> pointer to the code that does it.
>
>>
>>
>>>>
>>>>Upon entry of this code, the packet in bp already contains a VLAN header.
>>>>
>>>>It's unclear to me where the problem lies exactly. I suspect it has
>>>>something to do with the ethernet bonding layer indicating it has
>>>>hardware vlan tagging support, while it does already fill in the vlan
>>>>header, and libpcap being confused by this.
>>>>
>>>>As mentioned previously, the packets on the wire are correct, and this
>>>>is purely a capturing problem.
>>>>
>

Does anyone have an idea on how this is supposed to work and why the
extra header gets inserted?

Thanks,
Thomas
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