how to configure tap0/br0 correctly

[Petr Stehlík <pstehlik sophics cz>]

Hi there,

I am on Ubuntu 8.10, upgraded from previous release mainly to get the
shiny new Network Manager 0.7 that was supposed to honor
the /etc/network/interfaces settings.

I need to get tap0 bridged with eth0 so I have the following entries in
the 'interfaces' file:
======================
auto eth0
iface eth0 inet manual
	up ifconfig $IFACE 0.0.0.0 up
	down ifconfig $IFACE down

auto tap0
iface tap0 inet manual
	up ifconfig $IFACE 0.0.0.0 up
	down ifconfig $IFACE down
	tunctl_user username

auto br0
iface br0 inet dhcp
	bridge_ports eth0 tap0
======================

The goal is obviously to let the br0 replace the eth0 for all my
outgoing networking, including VPN connections etc.

Now I'd swear it worked perfectly for a couple of days but it stopped
working suddenly today and I am unable to get it working again. The
nm-applet does not appear until I restart the NM daemon and then the
nm-applet indicates that I have everything managed by
the /etc/network/interfaces so it will not allow me to launch VPN
connection on it.

I have read so many open bug reports in Ubuntu about "their" NM that I
decided I need to ask here for the right solution: how to set the br0 so
that NM will accept it and the nm-applet will allow me running VPN over
it...

When I tried removing the eth0 from /etc/network/interfaces to let NM
manage it the routing got messed up because both eth0 and br0 got their
address from DHCP.

Right now I have the default Ubuntu setting in nm-system-settings.conf,
that is [ifupdown] managed=false. I have tried enabling it but things
didn't look better so I restored the original setting.

Thanks for any advice. I am sure all Debian/Ubuntu/similar users of
VirtualBox and other emulators that use the tap interfaces would be more
than happy to know the right setting compatible with NM.

Petr