Re: network/bridging problems
On Thu, Oct 03, 2013 at 09:44:31AM -0700, Ross Boylan wrote:
> Greg, thanks for explaining this. I'm still puzzled about one point, below.
>
>
> On Wed, Oct 2, 2013 at 4:10 PM, Gregory Nowak <greg@gregn.net> wrote:
>
> > On Wed, Oct 02, 2013 at 12:40:26PM -0700, Ross Boylan wrote:
> > > Can anyone explain to me what difference between tap and the bridge is?
> > > They both seem to do the same thing*, but apparently tap needs to be
> > hooked
> > > in to a bridge. And for some reason the qemu/kvm docs seems to recommend
> > > tap.
> >
> > Ok, let me try to explain. If I'm wrong here, then someone who knows
> > better please jump in and correct me. Let's use a physical analogy. A
> > tap device is a virtual network device VS. a physical eth device which
> > is a card/chip in your computer. They both pass ethernet traffic, but
> > ethx is physical, and tapx is virtual.
> >
> > Now, sticking to physical analogies, pretend you have standard
> > ethernet cables, and an ethernet switch. Your ethx, and tapx devices
> > are ethernet cables. Your brx device is the network switch. When you
> > bridge ethernet interfaces like eth0 and tap0 into a single bridge
> > device like br0, you're doing the equivalent of plugging your ethernet
> > cables into an ethernet switch. When you do that, your bridge becomes
> > a single network device. Traffic from any of the eth/tap interfaces
> > bridged together is seen by all other eth/tap interfaces on that
> > bridge. So, maybe I should have compared the bridge to a network hub,
> > instead of a switch.
> >
> > Ok, here's where my analogy breaks down. When you bridge an ethx
> > interface, you don't use that interface anymore. So, if you bridge
> > your eth0 interface into an br0 bridge a line like iface eth0 in your
> > /etc/network/interfaces line becomes iface br0 instead.
> >
>
> I think this means eth2 will not have an IP address, but br0 will.
Correct.
> But how do packets reach the host machine? By analogy with hub, packets
> enter and go to all machines, which pick out the IP address meant for
> them. But if the IP address is that of the hub, how does the host machine
> know the packets are for it.
>
> I guess this is why you say the analogy breaks down. And I guess the
> bridge IP serves as the IP address of the host machine, whether it is being
> contacted by the guest or other physical machines on the network.
>
Correct again.
>
> > You have a line that says bridge_ports all tap0 in your interfaces
> > file, and that worries me. If I understand correctly, that line
> > combines all ethx interfaces on your system into a single bridge,
> >
> that's my understanding
>
> > which likely isn't what you want. You also seem to have eth0 through
> > eth2 on your system.
>
> At the moment there's only one cable going into the machine at eth2. But
> that won't always be the case,
> so I think I should change it as you suggest.
>
If you have only one of your ethx cards connected, then it won't
matter. You will probably want to change things though if you connect
cables to eth0/eth1.
> > Do you really want the virtual machine to be able
> > to communicate over all three interfaces?
>
> I might want to attach to the LAN and WAN in the future; I guess I should
> create 2 bridges in that case.
>
That's what I would do myself, since I want to keep my LAN and WAN
separated from each other.
Greg
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