Simple KVM Bridging

Following on from my post yesterday on "Basic KVM on CentOS 5", here's how to setup simple bridging to allow incoming network connections to your VM (and to get other standard network functionality like pings working). This is a simplified/tweaked version of Hadyn Solomon's bridging instructions.

Note this this is all done on your HOST machine, not your guest.

For CentOS:

# Install bridge-utils
yum install bridge-utils

# Add a bridge interface config file
vi /etc/sysconfig/network-scripts/ifcfg-br0
# DHCP version
# OR, static version

# Make your primary interface part of this bridge e.g.
vi /etc/sysconfig/network-scripts/ifcfg-eth0
# Add:
# Optional: comment out BOOTPROTO/IPADDR lines, since they're
# no longer being used (the br0 takes precedence)

# Add a script to connect your guest instance to the bridge on guest boot
vi /etc/qemu-ifup
BRIDGE=$(/sbin/ip route list | awk '/^default / { print $NF }')
/sbin/ifconfig $1 up
/usr/sbin/brctl addif $BRIDGE $1
# Silence a qemu warning by creating a noop qemu-ifdown script
vi /etc/qemu-ifdown
chmod +x /etc/qemu-if*

# Test - bridged networking uses a 'tap' networking device
qemu-kvm -hda $NAME.img -name $NAME -m ${MEM:-512} -net nic -net tap &

Done. This should give you VMs that are full network members, able to be pinged and accessed just like a regular host. Bear in mind that this means you'll want to setup firewalls etc. if you're not in a controlled environment.


  • If you want to run more than one VM on your LAN, you need to set the guest MAC address explicitly, since otherwise qemu uses a static default that will conflict with any other similar VM on the LAN. e.g. do something like:
# HOST_ID, identifying your host machine (2-digit hex)
# INSTANCE, identifying the guest on this host (2-digit hex)
# Startup, but with explicit macaddr
qemu-kvm -hda $NAME.img -name $NAME -m ${MEM:-512} \
  -net nic,macaddr=00:16:3e:${HOST_ID}:${INSTANCE}:00 -net tap &
  • This doesn't use the paravirtual ('virtio') drivers that Hadyn mentions, as these aren't available until kernel 2.6.25, so they're not available to CentOS linux guests without a kernel upgrade.

Simple dual upstream gateways in CentOS

Had to setup some simple policy-based routing on CentOS again recently, and had forgotten the exact steps. So here's the simplest recipe for CentOS that seems to work. This assumes you have two upstream gateways (gw1 and gw2), and that your default route is gw1, so all you're trying to do is have packets that come in on gw2 go back out gw2.

1) Define an extra routing table e.g.

$ cat /etc/iproute2/rt_tables
# reserved values
255     local
254     main
253     default
0       unspec
# local tables
102     gw2

2) Add a default route via gw2 (here to table gw2 on the appropriate interface (here eth1) e.g.

$ cat /etc/sysconfig/network-scripts/route-eth1
default table gw2 via

3) Add an ifup-local script to add a rule to use table gw2 for eth1 packets e.g.

$ cat /etc/sysconfig/network-scripts/ifup-local
# Script to add/delete routing rules for gw2 devices


if [ $(basename $0) = ifdown-local ]; then

if [ "$1" = "$GW2_DEVICE" ]; then
  ip rule $OP from $GW2_LOCAL_ADDR table gw2

4) Use the ifup-local script also as ifdown-local, to remove that rule

$ cd /etc/sysconfig/network-scripts
$ ln -s ifup-local ifdown-local

5) Restart networking, and you're done!

# service network restart

For more, see:

Finding/Delete Broken Symlinks

Find goodness (with a recent-ish find for the '-delete'):

find -L . -type l
find -L . -type l -delete

Using Network Driver Images on RedHat/CentOS Installs

I was building a shiny new CentOS 5.0 server today with a very nice 3ware 9650SE raid card.

Problem #1: the RedHat anaconda installer kernel doesn't support these cards yet, so no hard drives were detected.

If you are dealing with a clueful Linux vendor like 3ware, though, you can just go to their comprehensive download driver page, grab the right driver you need for your kernel, drop the files onto a floppy disk, and boot with a 'dd' (for 'driverdisk') kernel parameter i.e. type 'linux dd' at your boot prompt.

Problem #2: no floppy disks! So the choices were: actually exit the office and go and buy a floppy disk, or (since this was a kickstart anyway) figure out how to build and use a network driver image. Hmmm ...

Turns out the dd kernel parameter supports networked images out of the box. You just specify dd=http://..., dd=ftp://..., or dd=nfs://..., giving it the path to your driver image. So the only missing piece was putting the 3ware drivers onto a suitable disk image. I ended up doing the following:

# Decide what name you'll give to your image e.g.
mkdir /tmp/$DRIVER
cd /tmp/$DRIVER
# download your driver from wherever and save as $ (or whatever)
# e.g. wget -O $
#   though this doesn't work with 3ware, as you need to agree to their
#   licence agreement
# unpack your archive (assume zip here)
mkdir files
unzip -d files $
# download a suitable base image from somewhere
wget -O $DRIVER.img \
# mount your dos image
mkdir mnt
sudo mount $DRIVER.img mnt -o loop,rw
sudo cp files/* mnt
ls mnt
sudo umount mnt

Then you can just copy your $DRIVER.img somewhere web- or ftp- or nfs-accessible, and give it the appropriate url with your dd kernel parameter e.g.


Alternatives: here's an interesting post about how to this with USB keys as well, but I didn't end up going that way.

Fuzzy Displays and Dual NVIDIA 8xxx Cards

We've been chasing a problem recently with trying to use dual nvidia 8000-series cards with four displays. 7000-series cards work just fine (we're mostly using 7900GSs), but with 8000-series cards (mostly 8600GTs) we're seeing an intermittent problem with one of the displays (and only one) going badly 'fuzzy'. It's not a hardware problem because it moves displays and cables and cards.

Turns out it's an nvidia driver issue, and present on the latest 100.14.11 linux drivers. Lonni from nvidia got back to us saying:

This is a known bug ... it is specific to G8x GPUs ... The issue is still being investigated, and there is not currently a resolution timeframe.

So this is a heads-up for anyone trying to run dual 8000-series cards on linux and seeing this. And props to nvidia for getting back to us really quickly and acknowledging the problem. Hopefully there's a fix soonish so we can put these lovely cards to use.

Linux on Gigabyte GA-M59SLI-S5/S4 Motherboards

We've been having a bit of trouble with these motherboards under linux recently. The two S4/S5 variants are basically identical except that the S5 has two Gbit ethernet ports where the S4 has only one, and the S5 has a couple of extra SATA connections - we've been using both variants. We chose these boards primarily because we wanted AM2 boards with multiple PCIe 16x slots to use with multiple displays.

We're running on the latest BIOS, and have tested various kernels from 2.6.9 up to about 2.6.19 so far - all evidence the same the same problems. Note that these are much more likely to be BIOS bugs, we think, than kernel problems.

The problems we're seeing are:

  • kernel panics on boot due to apic problems - we can workaround by specifying a 'noapic' kernel parameter at boot time

  • problems with IRQ 7 - we get the following message in the messages log soon after boot:

    kernel: irq 7: nobody cared (try booting with the "irqpoll" option)
    kernel:  [<c044aacb>] __report_bad_irq+0x2b/0x69
    kernel:  [<c044acb8>] note_interrupt+0x1af/0x1e7
    kernel:  [<c05700ba>] usb_hcd_irq+0x23/0x50
    kernel:  [<c044a2ff>] handle_IRQ_event+0x23/0x49
    kernel:  [<c044a3d8>] __do_IRQ+0xb3/0xe8
    kernel:  [<c04063f4>] do_IRQ+0x93/0xae
    kernel:  [<c040492e>] common_interrupt+0x1a/0x20
    kernel:  [<c0402b98>] default_idle+0x0/0x59
    kernel:  [<c0402bc9>] default_idle+0x31/0x59
    kernel:  [<c0402c90>] cpu_idle+0x9f/0xb9
    kernel:  =======================
    kernel: handlers:
    kernel: [<c0570097>] (usb_hcd_irq+0x0/0x50)
    kernel: Disabling IRQ #7

    after which IRQ 7 is disabled and whatever device is using IRQ 7 seems to fail intermittently or just behave strangely (and "irqpoll" would just cause hangs early in the boot process).

This second problem has been pretty annoying, and hard to diagnose because it would affect different devices on different machines depending on what bios settings were on and what slots devices were in. I spent a lot of time chasing weird nvidia video card hangs which we were blaming on the binary nvidia kernel module, which turned out to be this interrupt problem.

Similarly, if it was the sound device that happened to get that interrupt, you'd just get choppy or garbled sound out of your sound device, when other machines would be working flawlessly.

So after much pain, we've even managed to come up with a workaround: it turns out that IRQ 7 is the traditional LPT port interrupt - if you ensure the parallel port is turned on in the bios (we were religiously turning it off as unused!) it will grab IRQ 7 for itself and all your IRQ problems just go away.

Hope that saves someone else some pain ...