Hi guys
To all interested parties, I think I have solved this issue. In the tap0 setup script that sets up a bridged connection so the KVM VM can access the network, I made two mistakes. My whole premise and idea of what I needed to do, and what I went and did, was wrong.
The first mistake was to setup the bridge based on the wrong IP address. !
The second mistake was to use the wrong netmask for the Class B network I was on.
Once I corrected both of the above, everything started working. In fact I had huge issues with the above Class C / Class B mixup, which I only discovered after extensively testing the setup. I got random connections to either the VM or the host when trying to SSH to 172.16.1.1, and the VNC instance also disconnected randomly. It was all because I had the wrong netmask in the ifup script, and I was bridging on the wrong IP.
First, the rationale. What I wanted was this:
172.16.1.1 - static IP of physical Centos 6 machine (host)
172.16.1.2 - static IP of virtual Centos 6 KVM qemu machine (e. g. the guest which runs "inside" a KVM on 172.16.1.1)
So here is my qemu-ifup.sh for the above:
Code:
#!/bin/sh
#
# script to bring up the tun device in QEMU in bridged mode
# first parameter is name of tap device (e.g. tap0)
#
# some constants specific to the local host - change to suit your host
#
ETH0IP=172.16.1.2
GATEWAY=172.16.1.9
BROADCAST=172.16.255.255
#
# First take eth0 down, then bring it up with IP 0.0.0.0
#
/sbin/ifdown eth0
/sbin/ifconfig eth0 0.0.0.0 promisc up
#
# Bring up the tap device (name specified as first argument, by QEMU)
#
/usr/local/sbin/openvpn --mktun --dev $1 --user `id -un`
/sbin/ifconfig $1 0.0.0.0 promisc up
#
# create the bridge between eth0 and the tap device
#
/usr/sbin/brctl addbr br0
/usr/sbin/brctl addif br0 eth0
/usr/sbin/brctl addif br0 $1
#
# only a single bridge so loops are not possible, turn off spanning tree protocol
#
/usr/sbin/brctl stp br0 off
#
# Bring up the bridge with ETH0IP and add the default route
#
/sbin/ifconfig br0 $ETH0IP netmask 255.255.0.0 broadcast $BROADCAST
/sbin/route add default gw $GATEWAY
The problem line at the top was
which I had as
which was WRONG, since the Centos host's IP was 172.16.1.2, the bridging script needs to bridge on 172.16.1.2 as well, NOT NOT NOT 172.16.1.1 !
The problem line at the bottom was
Code:
/sbin/ifconfig br0 $ETH0IP netmask 255.255.0.0 broadcast $BROADCAST
which I had as
Code:
/sbin/ifconfig br0 $ETH0IP netmask 255.255.255.0 broadcast $BROADCAST
255.255.255.0 was an invalid netmask for my class B network!
Once I fixed both of these everything started working - I can now ssh to 172.16.1.1 to get to the physical hosts's SSH daemon, and I can ssh to 172.16.1.2 to get to the KVM qemu guest's SSH daemon.
Also, both accesses are stable and I no longer get the "random ssh daemon" problem I had earlier. Clearly it was all my fault, I had let IPs clash in the bridging step and I was using the incorrect netmask for a class B network. Class C had nothing to do with this, this whole thread was simply ignorance, inexperience and a lack of understanding and attention to detail.
Once the above ifup script is done, I start my KVM Qemu instance - something like so:
Code:
sh -f /home/verisharepdc/Desktop/qemu-ifup.sh tap0
/usr/local/kvm/bin/qemu-system-x86_64 /home/verisharepdc/Desktop/vdisk.img -m 2048 -smp 2 -vnc 172.16.1.2:1 -net nic -net tap,ifname=tap0,script=no
sh -f /home/verisharepdc/Desktop/qemu-ifdown.sh tap0
in order to get it to use the tap0 device to be visible at 172.16.1.1 on the network.
Note that in the above, the VNC feed for the running QEmu KVM instance is available at 172.16.1.2:1 - if I pass this as is to TightVNC (for example) it works fine and I can see the XWindows instance running inside the qemu KVM instance.
For reference, here is my qemu-ifdown.sh:
Code:
#!/bin/sh
#
# Script to bring down and delete bridge br0 when QEMU exits
#
# Bring down eth0 and br0
#
/sbin/ifdown eth0
/sbin/ifdown br0
/sbin/ifconfig br0 down
#
# Delete the bridge
#
/usr/sbin/brctl delbr br0
#
# bring up eth0 in "normal" mode
#
/sbin/ifconfig eth0 -promisc
/sbin/ifup eth0
#
# delete the tap device
#
/usr/local/sbin/openvpn --rmtun --dev $1
For reference as well, here is my physical machine's /etc/rc.local:
Code:
#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.
touch /var/lock/subsys/local
/sbin/ifconfig eth0 172.16.1.2 netmask 255.255.0.0 broadcast 172.16.255.255 up
route add default gw 172.16.1.9 eth0
#service dhcpd start
#service named start
cp /etc/resolv.conf.bak /etc/resolv.conf
#smbd -D
#nmbd -D
modprobe kvm-intel
modprobe kvm
modprobe tun
sh -f /home/verisharepdc/Desktop/run_vm_unattended.sh &
- As you can see I assign it 172.16.1.2. Correlate with:
in my qemu-ifup,.sh
Here's my Qemu KVM instance's /etc/rc.local:
Code:
#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.
touch /var/lock/subsys/local
/sbin/ifconfig eth0 172.16.1.1 netmask 255.255.0.0 broadcast 172.16.255.255
route add default gw 172.16.1.6 eth0
#service dhcpd start
#service named start
cp /etc/resolv.conf.bak /etc/resolv.conf
#service nmb start
#service smb start
- As you can see I assign the VM 172.16.1.1
Hope this helps somebody...
