If I were doing the same thing using an iptables rule, I would simply do this:

iptables -t nat -I POSTROUTING -s 172.20.20.0/24 -j MASQUERADE


You do not have to specify which interface you’re masquerading (NAT-ing) to. It’ll figure that out based on the destination and the routing tables.

Alan

I suppose that you need to manually change the NAT configuration of vnet0 according to which interface is up, is that correct? Or does the NAT routing automatically switch to whichever is the “live” outgoing network? If it does, I suppose that’s a simpler solution. My setup does allow me to disconnect my guest machine from the internet while still keeping it available from my host. But that’s not something I ever remember doing…

My primary motivations were:

(1) I could start my VM’s whether my host’s eth0 or wlan0 were up or down. And I could move my laptop, unplug eth0, start using wlan0, and the VM’s would be unaffected. I would never need to change their bridge connection just because the host had changed the way it connected to the world.

(2) I could use dnsmasq to keep track of what winxp was doing, and I could also insert my own fake DNS entries, like “doubleclick.net = 127.0.0.1″, etc.

(3) Networking is cool.

A note about my example interfaces file above… the qlen line is not needed… that was an experiment. And you may have to get rid of the commented-out “bridge_ports none” line.

Alan

I was able to automate the creation of the interfaces. This is my /etc/network/interfaces file:


# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

# VirtualBox NAT bridge
auto vnet0
iface vnet0 inet static
address 172.20.20.1
netmask 255.255.255.0
#bridge_ports none
bridge_ports tap0
bridge_maxwait 0
bridge_fd 1
bridge_stp off 1
up ip link set vnet0 qlen 1000
up iptables -t nat -I POSTROUTING -s 172.20.20.0/24 -j MASQUERADE
down iptables -t nat -D POSTROUTING -s 172.20.20.0/24 -j MASQUERADE


I hope this helps others.

Alan

I am curious why you needed a tap interface on your bridge. What purpose does it serve?

The plot thickens. I am now trying to duplicate this setup on a Linux Mint Debian Edition box, and something is blocking some of my packets. For example, my winxp guest sends a DHCPDISCOVER to the vnet0 interface to ask for an IP address. My DHCP (running on the host) server sends a DHCPOFFER back onto vnet0, but the winxp guest never sees it. I am still debugging this. If you have ideas, I’d love to hear them.

Thanks for sharing your experience… I will experiment with the tun setup that you describe, and see if it helps.

Alan Porter

Now i am able to configure my host to guest networking..& able to access guest os from host .

How do I make the br0 and tap0 interfaces persistent on a Ubuntu Server 10.04 host?

And, as an aside, what is the virbr0 interface Ubuntu creates by default for, if not for this?

A major part of the problem for me was that the VirtualBox manual clearly says:

“when host-only networking is used, VirtualBox creates a new software interface on
the host which then appears next to your existing network interfaces.”

Equally clearly VirtualBox doesn’t create the interface on the host – the user has to do it, as per the above. Having mistakenly assumed that VB had created the virbr0 interface that appears in Ubuntu’s ifconfig I spent hours trying to connect to it!

Another little wrinkle that tripped me up – when creating the hostonlyadapter it has to carry the same number as the nic it is being associated with, ie nic3 hostonlyadapter3 even if it is the first or only hostonlyadapter.

The guest should then be able to fetch an IP from the hosts network and you can talk to the guest using that IP.

Worked easey peasey for me (Winblows host, Ubuntu10 guest)

Thanks for your contribution. I have a slight problem which is also replicated on the virtualbox documentation; on which machine do I type the above commands- ie the guest or the host? Please help.

Thanks for the follow-up

Do have the auto eth1 part? I realise I missed that from my howto. Or did you find a solution already?

I took a screenshot of the error message
http://files.tomasjancik.net/Screenshot.png
I tried to modprobe the modules, but still it didnt work…

can you help?

Your guide was concise and worked perfectly. You described how to make the changes persistent by defining the interface in the guest’s interfaces file in /etc/network. I did that and when I rebooted my guest OS, eth1 was not present when I did an ifconfig. I then did ifconfig eth1 up, but it did not have an ip address defined. I am running ubuntu server 9.10 as the guest and ubuntu desktop 9.10 as the host. I verified that my /etc/network/interfaces addition was the same as what you posted, but the changes still do not persist between GUEST restarts. (I attached eth1 to tap0 like you suggested)

I can create a script to do…

… easily enough, but it would be nice to persist it.

Also, is there a need to change any config on the host to make the creation of the bridge and the tap persistent?

Thanks again for the great guide.

Kevin