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

[Jiri Pirko <jpirko () redhat com>]

Tue, Nov 29, 2011 at 09:35:00PM CET, nicolas.2p.debian () gmail 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?

> > 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?


> > 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.
> >
> > Best regards,
> > Thomas
> >
> > ------------------------------------------------------------------------------
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