Most recent kernel where this bug did not occur: Unknown
Distribution: Several (see below)
Hardware Environment: Several (see below)
Software Environment: Several (see below)
Problem Description: Combining VLAN stacking with the REORDER_HDR flag on either the inner or outer VLAN interface does not work. The computer puts only one VLAN tag on the wire.

Steps to reproduce:

vconfig add eth0 12
ifconfig eth0.12 up
vconfig add eth0.12 2
ifconfig eth0.12.2 192.168.254.100
(default is REORDER_HDR 1, so they both have it set)
ping 192.168.254.101
(no response)

tcpdump -i eth0:

23:04:53.301156 vlan 2, p 0, arp who-has 192.168.254.101 tell tinygod2.local
23:04:54.301214 vlan 2, p 0, arp who-has 192.168.254.101 tell tinygod2.local
23:04:56.301326 vlan 2, p 0, arp who-has 192.168.254.101 tell tinygod2.local

But:

vconfig set_flag eth0.12 1 0
vconfig set_flag eth0.12.2 1 0
ping 192.168.254.101
(Responses)

tcpdump -i eth0:

23:10:44.932200 vlan 12, p 0, vlan 2, p 0, IP tinygod2.local > tinygod3.local: ICMP echo request, id 61054, seq 1, length 64
23:10:44.932364 vlan 12, p 0, vlan 2, p 0, IP tinygod3.local > tinygod2.local: ICMP echo reply, id 61054, seq 1, length 64
23:10:45.937238 vlan 12, p 0, vlan 2, p 0, IP tinygod2.local > tinygod3.local: ICMP echo request, id 61054, seq 2, length 64
23:10:45.937445 vlan 12, p 0, vlan 2, p 0, IP tinygod3.local > tinygod2.local: ICMP echo reply, id 61054, seq 2, length 64
23:10:46.937196 vlan 12, p 0, vlan 2, p 0, IP tinygod2.local > tinygod3.local: ICMP echo request, id 61054, seq 3, length 64
23:10:46.937401 vlan 12, p 0, vlan 2, p 0, IP tinygod3.local > tinygod2.local: ICMP echo reply, id 61054, seq 3, length 64

If REORDER_HDR is 1 on only one of the two VLAN interfaces, it's still broken. However, if it's 1 on the inner interface and 0 on the outer one, then it's the outer tag that shows up on the wire, instead of the inner tag as in the rest of the cases.

Note also that if the NIC card has VLAN acceleration then there's no problem with the above case. However, if you do 3 levels of stacked VLANs and put REORDER_HDR on one of the two inner ones, then it's broken in the analogous way.

I have tested this on 3 computers, which between them have the same number of distinct distributions, kernel versions, NIC cards, and architectures. They are:

(1) Kubuntu Feisty Fawn 2.6.20-15-generic on a Compaq AMD Athlon64 laptop with a Realtek RTL-8139 (used for the above output)
(2) Debian Lenny 2.6.18-4-amd64 on a Dell Intel Core 2 Duo laptop with a Broadcom NetXtreme BCM5752 (which has acceleration)
(3) uClinux 2.6.17-uc1 on a Nios II embedded system with the OpenCores MAC.

Note that this appears to be the same bug as was long ago discovered by another user and described here: http://www.candelatech.com/pipermail/vlan/2004-December/000190.html