Les tags VLAN n'apparaissent pas dans la capture de paquets (Linux) via tcpdump

[Clippy]

Je suis adding a tagged VLAN onto eth0:

```
#ip link add link eth0 name eth0.20 type vlan id 20

```

This results in:

```
#ip link
2: eth0: mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 9c:c7:a6:95:65:1c brd ff:ff:ff:ff:ff:ff
....
12: eth0.20@eth0: mtu 1500 qdisc noqueue state UP
link/ether 9c:c7:a6:95:65:1c brd ff:ff:ff:ff:ff:ff

#ip -d link show eth0.20
70: eth0.20@eth0: mtu 1500 qdisc noqueue state UP
link/ether 9c:c7:a6:95:65:1c brd ff:ff:ff:ff:ff:ff
vlan id 20

#cat /proc/net/vlan/config
VLAN Dev name | VLAN ID
Name-Type: VLAN_NAME_TYPE_RAW_PLUS_VID_NO_PAD
eth0.234 | 234 | eth0
eth0.20 | 20 | eth0

```

Now I start dhclient:

```
#dhclient -d -v -1 eth0.20

```

What Je vois in tcpdump is an untagged DHCP discovery frame:

```
#tcpdump -i eth0 -XX
0x0000: ffff ffff ffff 9cc7 a695 651c 0800 4500
^^^^

```

Why is it pas tagged?

802.1q module seems to be used:

```
#lsmod | grep 8021q
8021q 28324 0
garp 14311 1 8021q

```

(OS: SLES11SP2 kernel 3.0.13-0.27-default)

BTW autre traffic is pas tagged soit (at least tcpdump ne show it)...

**Update 16 October**

```
# tcpdump -Uw - | tcpdump -i eth0 -en -r - &
[1] 7310
# tcpdump: WARNING: eth0: no IPv4 address assigned
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes

# dhclient -d -v -1 eth0.20
Internet Systems Consortium DHCP Client 4.2.3-P2
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0.20/9c:c7:a6:95:65:1c
Sending on LPF/eth0.20/9c:c7:a6:95:65:1c
Sending on Socket/fallback
DHCPDISCOVER on eth0.20 to 255.255.255.255 port 67 interval 3
reading from file -, link-type EN10MB (Ethernet)
18:49:14.437882 9c:c7:a6:95:65:1c > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 347: 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 9c:c7:a6:95:65:1c, length 305
^^^^^^

```

So, encore no tag shown here.

But indeed le transmit counter for eth0.20 in /proc/net/dev does increment quand running dhclient...

[Clippy]

You ne peut pas see VLAN tag depuis `tcpdump -i eth0` output on i686/x86_64 architecture parce que of VLAN acceleration. le VLAN layer will be filtered by kernel so it toujours looks untagged. Please refer to [Bug 498981 - tcpdump cannot deal with 802.1q vlan tag](https://bugzilla.redhat.com/show_bug.cgi?id=498981#c4)

According to votre case, you can get VLAN tags via:

```
tcpdump -i eth0 -Uw - | tcpdump -en -r - vlan 20

```

Vous devriez see le suivant output:

```
> Broadcast, ethertype 802.1Q (0x8100), length 346: vlan 20, p 0, ethertype IPv4, 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP

```