Qemu tap network script

In the previous posts on Qemu networking, we have seen a manual procedure to setup up network access using Qemu tap device. Its a good way to get started but ideally we would like all these to be automated to ease our life. Here is a script that will automatically bring up the tap device and logically assign an IP address to that device. Name the script as qemu-ifup and place it under /etc directory. Qemu automatically picks up this script. Here is the qemu command line

$ qemu-system-x86_64 -hda cirros-0.3.0-x86_64-disk.img -netdev tap,id=net0,downscript=no -device e1000,netdev=net0

Script

$ cat /etc/qemu-ifup

#!/bin/bash                                                           
                                                                      
tap=$1
num=`echo $tap | sed 's/[a-zA-Z]*\([0-9]*\)/\1/'`                     

device_num=$(( $num + 1 ))

echo "tap = $tap, device_num = $device_num, num = $num"

#now generate a IP address
ip_addr=10.1.70.$device_num

ip link set $tap up


ip addr add $ip_addr/24 broadcast + dev $tap

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

iptables -I FORWARD 1 -i $tap -j ACCEPT

iptables -I FORWARD 1 -o $tap -m state --state RELATED,ESTABLISHED -j ACCEPT

[links]
* http://tech-jnaan.blogspot.in/2013/08/configuring-qemu-tap-interface-for.html
* http://tech-jnaan.blogspot.in/2013/08/setting-up-qemu-with-nat.html

Attaching an additional hard disk to Qemu

I am using Cirros as my primary hard disk to boot the operating system through Qemu. It has been very handy and useful until I faced with the space shortage problem. So, I had to look for a way to increase the disk space to continue with my work. That's when I came across the Qemu option hdb which provides a means to add a new hard disk.

To start with create a disk image of your choice. I will use Qcow2 image format to create the disk.

$ qemu-img create -f qcow2 drive.qcow 4G

Once we have created the disk image, we should pass this as part of the Qemu command line.

$ qemu-system-x86_64 -hda cirros.img -hdb drive.qcow

Inside guest VM

When we login into the vm, before we can start using our new disk, we have to partition the disk and format the partitions with a file system. Before we format the disk, we must find the new block device that the kernel created as part of the boot process.
$ ls /dev/sd*

By default, the name of the block device will be sdb. It is a raw disk, we must create partitions underneath it. You can use the system tool that you are comfortable with. I will use cfdisk.

$ cfdisk /dev/sdb

Create partitions according to your requirement. Then, write the changes to the disk and exit.

Next, we just format the new partition with a file system. I will use ext4 file system format to format the new partition.

$ mkfs.ext4 /dev/sdb1

We must mount the new partition to access the disk space in it.

$ mkdir /drive
$ mount -t ext4 /dev/sdb1 /drive

$ df

Now, the new mount point will appear as part of the output of the df command.

[links]
* http://wiki.qemu.org/Main_Page
* http://en.wikipedia.org/wiki/Qcow
* https://launchpad.net/cirros/+download

Setting up QEMU with a NAT

This is a continuation of my previous post which deals with setting up network connection for Qemu between host and guest. In this post we will learn to configure Qemu to access outside world.

There are a number of ways to configure Qemu to access internet like bridging, proxy arp, etc. In this post I will use tap device that we configured in the previous post and then make some changes to host iptables to forward the packets over the internet.

Nat

$ echo 1 > /proc/sys/net/ipv4/ip_forward

# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# iptables -I FORWARD 1 -i tap0 -j ACCEPT

# iptables -I FORWARD 1 -o tap0 -m state --state RELATED,ESTABLISHED -j ACCEPT

Thats it. Verify now by pinging any of the outside connection in the guest. The simplest is,

$ ping google.com

Links
* http://tech-jnaan.blogspot.in/2013/08/configuring-qemu-tap-interface-for.html
* http://felipec.wordpress.com/2009/12/27/setting-up-qemu-with-a-nat/

Configuring Qemu tap interface for guest<-->host communication

I recently started playing with Qemu. One of the first things that I wanted to setup was a communication link between the host(physical machine) and guest(the vm). Qemu provides a number of techniques to achieve this. In this post I will explain about setting up network with tun/tap device.

It is easy to set it up when you know what it is. I spent couple of hours while I was fiddling with it. Read a lot of documents but dint find one which explained this completely and hence this post. I received a lot of help on the Qemu irc channel(#qemu on OFTC).

I assume that Qemu is already installed on your machine and you also have installed OS on the virtual disc. If not you can download the latest source code and install Qemu from it. Also, you can download a sample virtual disk. Once all these are ready, follow the steps below.

First, start the qemu emulator on the host machine. 

$ qemu-system-x86_64 -hda cirros-0.3.0-x86_64-disk.img -netdev tap,id=net0,script=no,downscript=no -device e1000,netdev=net0

The above command will start the Qemu with KVM. The most interesting parts of the command is the -netdev option. Here, we instruct the Qemu to use tap interface to provide networking capability. Once we have started the Qemu, lets configure the host to send/receive packets. On the host, check the name of the tap device that Qemu is using to communicate the packets. It picks up the first unused device. 

 $ ip addr show

Once we know the name of the tap device, we should bring the interface up and also assign an IP address to it. Here, we are assuming the interface name as tap0. 

$ ip link set tap0 up

Now assign IP address to the tap interface 

$ ip addr add 10.1.70.1/24 broadcast + dev tap0

Once we do this, we are all set to send/receive the packets from the host. Now, we must configure the guest to send/receive the packets over the tap device. Once the guest machine is up, assign an IP address to the guest machine from the same subnet. 

$ ip addr add 10.1.70.2/24 broadcast + dev eth0

Since we had already instructed Qemu to use the tap device, it will send the packets through it. Now, check for connectivity by running

$ ping 10.1.70.1

The above IP address is the IP that we assigned to the tap device. The interesting thing to note here is that when we try to access the IP address of your host machine, ping command will return with network unreachable error message. The reason being that the guest machine doesn't know the route to follow to reach the host when this IP address is given. Let us add the necessary rule which would be used to send/received the packets.

$ route add default gw 10.1.70.1 dev eth0

Now, we can use the IP address of tap or host to access the host machine.

Links:

* http://wiki.qemu.org/Main_Page

* http://en.wikipedia.org/wiki/TUN/TAP

* https://launchpad.net/cirros/+download
* https://people.gnome.org/~markmc/qemu-networking.html