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Marki
Asked: 2013-10-09 08:34:55 +0800 CST

VLAN tags not shown in packet capture (Linux) via tcpdump

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I am 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: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> 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: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP
    link/ether 9c:c7:a6:95:65:1c brd ff:ff:ff:ff:ff:ff
    vlan id 20 <REORDER_HDR>

#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 I see in tcpdump is an untagged DHCP discovery frame:

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

Why is it not 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 other traffic is not tagged either (at least tcpdump doesn't 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, still no tag shown here.

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

dhcp

1 Answers

  1. You can't see VLAN tag from tcpdump -i eth0 output on i686/x86_64 architecture because of VLAN acceleration. the VLAN layer will be filtered by kernel so it always looks untagged. Please refer to Bug 498981 - tcpdump cannot deal with 802.1q vlan tag

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

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

    You should see the following output:

    <timestamp> <mac-addr-of-eth0> > 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
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