I wanted to assign an external IP address to my KVM guest and went the bridged networking way. Unfortunately the guest has no network connectivity and I don't know why. After investigating it seems that it doesn't get any reply for it's ARP requests.
I have one physical interface: eth0, the bridged interface: br0 and the tap interface: tap0 brought up by qemu-ifup script by kvm. The host machine runs the latest Ubuntu Server. The guest machine runs GKRML live-cd (based on slackware).
What could be a potential problem is that the host machine and the guest machine are in the different networks. Unfortunately both IP addresses were assigned to me by the datacenter and I cannot change them.
Configuration details follow. Below xx.xx
are the same for both host and guest.
Host's /etc/network/interfaces:
# Loopback device:
auto lo
iface lo inet loopback
# Device: eth0
auto eth0
iface eth0 inet manual
# Device: br0
auto br0
iface br0 inet static
address xx.xx.110.69
netmask 255.255.255.224
network xx.xx.110.64
broadcast xx.xx.110.95
gateway xx.xx.110.65
bridge_ports eth0
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off
Host's routing table:
Destination Gateway Genmask Flags Metric Ref Use Iface
xx.xx.110.64 0.0.0.0 255.255.255.224 U 0 0 0 br0
0.0.0.0 xx.xx.110.65 0.0.0.0 UG 100 0 0 br0
Host's /etc/qemu-ifup scirpt (unmodified, shipped with qemu package):
#!/bin/sh
switch=$(/sbin/ip route list | awk '/^default / { print $5 }')
/sbin/ifconfig $1 0.0.0.0 up
/usr/sbin/brctl addif ${switch} $1
Output of brctl show
while the guest is running:
bridge name bridge id STP enabled interfaces
br0 8000.4061862b90d5 no eth0
tap0
KVM commandline:
kvm -cdrom grml_2009.10.iso -boot d -m 256 -vnc localhost:0 -net nic,macaddr=DE:AD:BE:EF:11:14 -net tap,script=/etc/qemu-ifup
Guest machine network setup (single eth0 interface):
$ ifconfig eth0 xx.xx.129.69/28 up
$ route add default gw xx.xx.129.65
Result of tcpdump -i tap0
while trying to ping anything from the guest:
tcpdump: WARNING: tap0: no IPv4 address assigned
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap0, link-type EN10MB (Ethernet), capture size 96 bytes
23:38:13.659655 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
23:38:14.659687 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
23:38:15.659655 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
23:38:16.666350 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
23:38:17.666319 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
23:38:18.666324 ARP, Request who-has xx.xx.129.65 tell xx.xx.129.69, length 28
… and so on without reply.
Thanks in advance for any help!
Best Answer
iptables -F FORWARD
iptables -I FORWARD -m physdev --physdev-is-bridged -j ACCEPT
iptables-save > /etc/sysconfig/iptables
net.ipv4.ip_forward = 1