I've hit a roadblock of sorts with an architectural challenge. I've got a server running KVM - which will feature a number of guests, some running virtualised firewalls and others, just plain web servers.

  • The web servers need a single untagged VLAN interface
  • The firewalls need a tagged VLAN trunk

Now typically, this would be straightforward, just add a bridge with eth0 in it, add a few VLANs on the bridge (vmbr0.1 etc.) - then allocate an interface with an untagged VLAN to each guest (or multiples thereof).

                             firewall1 (vlan 1,2,3,4)
switch ===== eth0  vmbr0     firewall2 (vlan 1,2,5,6)
                   (eth0)    server1 (vlan 7)
                             server2 (vlan 8)
     vlan trunk

Now, this works fine if you only have a few interfaces that you need to pass into a guest. But what happens when you need to put 500 VLANs into a firewall. Its impractical.

So what I can't figure out is how to create a number trunks (with common VLANs and different VLANs), and allocate those to a guest.

The closest solution I've found so far is to create the VLANs on the main bridge, then for the servers, just allocate a single VLAN from that.

Then for the firewalls, create a bridge for each, with just the guest's tap interface in it, then create the untagged vlan interface for that respective bridge, and add it to the main vmbr0 bridge.

The only problem with this is that the traffic exiting the untagged interface, is of course, untagged.

Is it possible to tag traffic exiting an interface?


Otherwise, how is it possible to achieve different trunks to KVM guests, that may have common VLANs and different VLANs (but never ALL vlans) - and be able to assign single untagged vlan interfaces

1 Answer 1


A lengthy solution ...

So I've been toying around with different concepts and I think I've made a workable solution. Basically, there is a main bridge trunk (bt) that eth0 belongs to. Then for each firewall VM has its own bridge.

bridge name     bridge id               STP enabled     interfaces
bt              8000.002618895a72       no              eth0
bt-fw1          8000.000000000000       no              tap100i0
bt-fw2          8000.000000000000       no              tap101i0

At this point the tagged traffic from the firewall VMs will enter their own bridge for traffic to enter.

I then created VLANs for each interface that needs tagged traffic - and the respective VLAN on the main bridge for untagged traffic.

Eg. VLANs 1, 4000-4005

bt-fw1.1       | 1  | bt-fw1
bt-fw1.4000    | 4000  | bt-fw1
bt-fw1.4001    | 4001  | bt-fw1
bt-fw1.4002    | 4002  | bt-fw1
bt-fw1.4003    | 4003  | bt-fw1
bt-fw1.4004    | 4004  | bt-fw1
bt-fw1.4005    | 4005  | bt-fw1
bt-fw2.1       | 1  | bt-fw2
bt-fw2.4000    | 4000  | bt-fw2
bt-fw2.4001    | 4001  | bt-fw2
bt-fw2.4002    | 4002  | bt-fw2
bt-fw2.4003    | 4003  | bt-fw2
bt-fw2.4004    | 4004  | bt-fw2
bt-fw2.4005    | 4005  | bt-fw2
bt.1           | 1  | bt
bt.4000        | 4000  | bt
bt.4001        | 4001  | bt
bt.4002        | 4002  | bt
bt.4003        | 4003  | bt
bt.4004        | 4004  | bt
bt.4005        | 4005  | bt

Then for each VLAN, a bridge is created, that combines all respective VLANs from each interface to allow untagged communication between the main bridge and VM bridges.

bt.v1                   8000.2a8c73ad057d       no      bt-fw1.1
bt.v4000                8000.2a8c73ad057d       no      bt-fw1.4000
bt.v4001                8000.2a8c73ad057d       no      bt-fw1.4001
bt.v4002                8000.2a8c73ad057d       no      bt-fw1.4002
bt.v4003                8000.2a8c73ad057d       no      bt-fw1.4003
bt.v4004                8000.2a8c73ad057d       no      bt-fw1.4004
bt.v4005                8000.2a8c73ad057d       no      bt-fw1.4005

This now allows trunked interfaces to the firewall VMs, with only the VLANs I want on them.

Any future guests that need a single untagged VLAN only, can merely be added to the respective bt.X bridge.

Adding host IP interfaces is as easy as adding the IP to the respective VLAN bridge


ip addr add dev bt.v4005

A helper script

As my /etc/network/interfaces file could rapidly end up being massive, I wrote a small script that allows for minimal configuration - but with the same desired end result.

In the /etc/network/interfaces file it contains

# bridge bt-c0-fw1 vlan 1 4000-4005 interfaces tap100i0
# bridge bt-c0-fw2 vlan 1 4000-4005 interfaces tap101i0
# bridge bt vlan 1 4000-4005 interfaces eth0
# bridge-vlan bt vlan 1 4000-4005 interfaces bt-fw1 bt-fw2 bt

auto eth0
iface eth0 inet manual
  post-up /scripts/build-bridges.sh || /bin/true

Then in the post-up script, /scripts/build-bridges.sh



# Tear down all interfaces
modprobe -r 8021q
modprobe 8021q

brctl show | awk 'NR>1{print $1}' | while read BRIDGE; do
  ifconfig "$BRIDGE" down
  brctl delbr "$BRIDGE"

function run()
  if [ $DEBUG -eq 1 ]; then
    echo "$@"
    eval "$@"

function get_vars()
  DATA=( $(echo "$2" | grep -Eoh "$1.+?" | sed -E "s/^$1 //g;s/($DEFINERS).+//g") )
  for VAL in ${DATA[@]}; do
    echo $VAL | grep -qE "[0-9]+-[0-9]+"
    if [ $? -eq 0 ]; then
      LOWER=$(echo $VAL | cut -f1 -d"-")
      UPPER=$(echo $VAL | cut -f2 -d"-")
      for i in $(seq $LOWER $UPPER); do
        echo $i
      echo $VAL

# Build bridges
while read LINE; do
  BRIDGE=$(get_vars bridge "$LINE")
  if [[ ! "$BRIDGE" == "" ]]; then
    run brctl addbr $BRIDGE
    run ifconfig $BRIDGE up
    for INTERFACE in $(get_vars interfaces "$LINE"); do
      ifconfig $INTERFACE >/dev/null 2>&1
      if [ $? -eq 0 ]; then
        run brctl addif $BRIDGE $INTERFACE
    run ifconfig $BRIDGE up
    for VLAN in $(get_vars vlan "$LINE"); do
      run vconfig add $BRIDGE $VLAN 2>&1 | grep -vE "(VLAN 1 does not work|consider another number)"
      run ifconfig $BRIDGE.$VLAN up
done < <(grep -E "^# bridge " $CONFIG)

# Build vlan bridges
while read LINE; do
  BRIDGE=$(get_vars "bridge-vlan" "$LINE")
  for VLAN in $(get_vars " vlan" "$LINE"); do
    run brctl addbr $BRIDGE.v$VLAN
    run ifconfig $BRIDGE.v$VLAN up
    for INTERFACE in $(get_vars interfaces "$LINE"); do
      ifconfig $INTERFACE.$VLAN >/dev/null 2>&1
      if [ $? -eq 0 ]; then
        run brctl addif $BRIDGE.v$VLAN $INTERFACE.$VLAN
done < <(grep -E "^# bridge-vlan " $CONFIG)

exit 0

I hope this helps someone else, I've spent DAYS reading and testing and this seems to be the most elegant, easy to manage solution.

There's some good reading here too http://blog.davidvassallo.me/2012/05/05/kvm-brctl-in-linux-bringing-vlans-to-the-guests/

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