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

From: Nicolas de Pesloüan<[email-masked]>
Date: Tue, 29 Nov 2011 21:35:00 +0100

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?

	Nicolas.

>
> Here is the output from tcpdump:
> # /tmp/tcpdump  -i eth0 -xx
> 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
>
> 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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