QEMU and tap problems

[RW]

On Fri, 21 Sep 2007 07:08:41 -0500 (CDT)
"Doug Poland" <doug at polands.org> wrote:

> On Thu, September 20, 2007 20:47, RW wrote:
> > On Thu, 20 Sep 2007 06:21:20 -0500
> > Doug Poland <doug at polands.org> wrote:
> >>
> >> I've been following the various instructions I've found on the web
> >> in an attempt to get tap networking with qemu-devel-0.9.0s.20070802
> >>
> >
> > Are you actually sure that you actually need tap?
> >
> Good question.  My reason for running Win2K server under QEMU is I'm
> working on a java app that speaks to SQLServer.  Initially, all I need
> is to communicate with the host/guest on the same machine.  After
> that, I'll need the QEMU guest to be on the network so I can connect
> to the java app from other computers.
> 
> >
> > A lot of the how-tos are out of date
> >
> I've noticed that :(
> 
> >
> > - recent versions of Qemu can give a guest network access without
> > it.
> >
> When I started QEMU with the -net nic -net user switches, then Windows
> gets a 10. address and the guest can see the network.  However, I
> cannot see open ports I'm interested in, 1433 and 3389, from the host.


I understand it when you run a guest without any networking switches,
it sees an emulated ethernet interface that behaves as if it's
connected to a basic NAT router. This emulation is running as an
ordinary user in the host, so it can't do anything that requires root
access - which is why you can't ping out. And because of the NAT you
can't make incoming connections to the guest (which prevents incoming
pings).

IIRC there is some kind of redirection switch that will allow you to
connect to guest ports via ports on the host (analogous to the port
forwarding on a NAT router).