[Qemu-devel] VDE HOWTO version 0.1

["Jim C. Brown" <jbrown106@phreaker.net>]

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Here is the next verion. Changes from Revision 1 added in. Added update on
Minix (nothing very big) and on ifconfig. Nothing major.

I didn't add in the use of tun0 because tun0 is for emulating the ip layer,
while tap0 is used to emulate ethernet frames. VDE emulates a full nic,
ethernet and all, at a lower level, so using vde_switch -tap tap0 is more
consistent.

-- 
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.

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Using VDE with Qemu HOWTO
by Jim Brown
26 June 2004
Version 0.1

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Introduction
	Copyright
	What is qemu?
	What is VDE?

Configuring and Installing VDE
	Installation
	vdeq & vdeqemu

User-mode networking
	How to enable user-mode networking
	Firewall configuration

Setting up qemu
	How to set up the guest OS

Credits

-----------------------------------------------------------------------------

Introduction

Copyright

Copyright (c) 2004 Jim Brown.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.  A copy of the license is available at http://www.gnu.org/licenses/fdl.txt

What is qemu?

	Qemu is a FAST! processor emulator by Fabrice Bellard, available at
http://fabrice.bellard.free.fr/qemu/. It is capable of emulationg the x86 and
PowerPC processors with support for other processors on the way. The original
purpose of qemu was to allow running x86-specific Linux applications, such as
WINE or DosEmu, on non-x86 systems. However, qemu has expanded into becoming
a full-fledged emulator. On the x86 side, it is capable of running Linux,
MS-DOS, Windows 95/98/Me, Windows NT/2k, Windows XP, Solaris, OpenBSD, and
FreeBSD. See http://fabrice.bellard.free.fr/qemu/ossupport.html for the full
listing.

	This howto assumes that you have already installed and set up qemu.

What is VDE?

	VDE is short for Virtual Distributed Ethernet. VDE, written by
Renzo Davoli, is based off of uml_switch by Jeff Dike. It is available at
http://sourceforge.net/projects/vde/. It has many uses, the main one providing
support for networking with emulated computers. (Not just qemu, but support
for user-mode linux and Bochs also exists). VDE must be set up and installed by
root, but the programs which use it do not need root privligies.

	This howto will walk you through the simple process of installing
VDE and setting up qemu to use it.

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Configuring and Installing VDE

Installation

	You may obtain the source code at http://sourceforge.net/projects/vde/.
The version of VDE which I used was 1.4.1, but this HOWTO should apply to all
versions.

	Once you have downloaded the source code, extract it. I assume you
will have extracted it to /space/vde. Go into that directory, and simply type
"make" followed by "make install". Now you should have vde_switch in /usr/bin.

vdeq & vdeqemu

	Now cd into the qemu directory. Type "make". This will build vdeq.
Qemu on its own only supports full networking with tuntap, which requires
root priviliges or an exposed /dev/net/tun. There is a -user-net option, but
that is not as useful as full networking. In order for qemu to use VDE, it must
be passed the file descriptor for a tun device. Futhermore the tun device itself
must already be configured to use VDE. vdeq sets this up and passes it to qemu
via the -tun-fd switch.

	There is no "make install". Instead, you just manually copy vdeq to
/usr/bin. It might also be helpful to copy or link vdeq to vdeqemu. vdeq
requires that the location of the qemu binary be passes to it as the first
command line parameter, but vdeqemu only needs the options you want to pass to
qemu. vdeqemu will locate the qemu binary itself (this requires that you install
qemu system-wide or have the qemu directory in your PATH).

For example if you have:
vdeq qemu -hda /mnt/myimage -m 64 -boot a

you can shorten this into

vdeqemu -hda /mnt/myimage -m 64 -boot a

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User-mode Networking

How to enable user-mode networking

	The following commands will need to be run as root:

# vde_switch -tap tap0 -daemon

If you need to run a sniffer, just in case you want to analyze the traffic,
you can also run it like this:

# vde_switch -hub -tap tap0 -daemon

(The -hub option is not available for version 1.4.1 of VDE, you will need a
later version. I don't know what the minimal version is but 1.5.1 does support
this option.)

Then you must run this:

# ifconfig tap0 <ip>
# chmod 755 /tmp/vde.ctl

	The vde_switch command will run VDE in the background. The -tap tap0
parameter tells VDE to set up the device tap0 using tuntap. -daemon runs
vde_switch in the background. -hub tells VDE to broadcast the message to all 
segment, just like real hub that you use on real network.

	<ip> is the ip address of the gateway you want to use for the guest
OS(es). For example:

# ifconfig tap0 192.168.254.254

	will make 192.168.254.254 the gateway between guest and host, and your 
guest OS(es) will belong to the subnet 192.168.254.0 with a netmask of 255.255.255.0 
and an ip address of 192.168.254.XXX (where you get to pick the XXX). You must have
the IP of the qemu guest and the IP of the gateway on the same subnet! While it
may be possible to have them on separate subnets, it will certainly be harder
to configure (and you won't like the way your routing tables will look either).

[Sidebar: The "gateway" is actually the host OS itself on the tap0 interface.
The host on the tap0 interface, aka 192.168.254.254, routes between the guest
OS and the host's eth0 interface (which on is the real network). The host on the
eth0 interface (ex. 192.168.0.2) can then route between the tap0 interface and
the real network / the internet.]

(Note that you might be required to do this:

# ifconfig tap0 192.168.254.254 netmask 255.255.255.0

Normally ifconfig should pick the correct netmask for you, but if it doesn't
for some reason then you will have to specify it manually. See ifconfig(8) for
details.

)

Note that you must run this before you run your firewall. I found it helpful
to put this into a script, and have the script load before the firewall does.

Firewall configuration

	You will need to enable masquerading between tap0 and your local area
network (for example, eth0). You will also need to enable masquerading between
tap0 and ppp0 if you use a dialup connection to the internet. The commands

# echo "1" > /proc/sys/net/ipv4/ip_forward
# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

	will allow you to enable this manually.

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Setting up qemu

How to set up the guest OS

	Set up the guest OS so that the default route is through the gateway
ip, <ip> (for example 192.168.254.254). Also set up the subnet and netmask
parameters as appropriate (for example 192.168.254.0 and 255.255.255.0).
The guest OS should see the ethernet device and be able to use it to access
the gateway. (Caveat: I haven't been able to do this for MS-DOS, and for Minix
2.0.4 I had to apply a patch to qemu since Minix is broken. Uodate: Minix 2.0.4
is still broken but a patch has been released to fix it. Using this patch,
Minix works on a vanilla qemu.) Also don't forget to set up the IP of the guest
OS itself (for example 192.168.254.1).

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Credits

	This HOWTO relied heavily on the documentation that Renzo wrote for
vde-1.4.1.
	Thanks to Mulyadi Santosa for helping with the first revision of
this document, and to Renzo for his input. (P.S. Will add info for ale4net
and slirpvde as soon as I figure out how to use it ;)