[Xen-users] Re: network in general, when routed when bridged?

[Charles Duffy <cduffy@xxxxxxxxxxx>]

On Sun, 16 Oct 2005 14:24:19 +0200, Sueveges Gyoergy wrote:

>   I'm now a little bit confused with networking. When should be bridging
> used? And When should be the routed network used? When I have one public
> IP address, and for the domU-s I would have private ones (192.168....)
> then can I use bridged network? Unfortunetly in docu I cannot see enough
> info for the routed network settings (probably they assume that one is
> familiar with networking :))

Well, yes -- to set this all up and understand how it works, you *need* to
be familiar with networking. There are a lot of resources you can use to
learn, though, ranging from public documentation and published books to
University courses on the subject, and I strongly advise that you
look into them -- this is one of those topics that one really must know to
be a good sysadmin.

To give a very brief overview, though:

Bridging two devices acts like connecting them with an ethernet cable. If
you bridge your virtual instances to your outside network connection,
that's just like plugging your physical machine, your XenU instances and
your outside net connection all into the same Ethernet switch. Doing
firewalling on a bridge is possible (with ebtables) but is certainly
not beginners' functionality.

Routing puts a machine in the middle that can act like... well, a router.
It can do tricks like IP masquerading, (easier, iptables-based)
firewalling and whatnot. IP masquerading is probably what you want if you
wouldn't plug multiple machines directly into your outside network.

That's not to say you can't do routing *and* have a bridge! If you're
going to have multiple Xen instances and be starting or shutting them
down, I find it easiest to put them all on a bridge and then route between
that bridge and the outside world.


None of this is in any way Xen-specific, and so arguably this is the wrong
forum for both the question and its answers.


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