Re: What happens if there are two interfaces?

[Nathanael Noblet <nathanael gnat ca>]

On 12/31/2012 05:44 AM, Timothy Murphy wrote:
> Nathanael Noblet wrote:
>
> > > "ip route" just tells me both interfaces are up,
> > > not which one is being used.
> > > Even "ifconfig", which gives the number of packets
> > > passing through each interface,
> > > is not completely conclusive.
>
> > ip route on my machine gives:
> > default via 192.168.4.1 dev wlan0  proto static
> > 192.168.4.0/24 dev wlan0  proto kernel  scope link  src 192.168.4.103
> >
> > Which means wlan0 is the default route. Everything goes out that way.
>
> Yes, thanks for that observation -
> I had overlooked the fact that "ip route" (and "route")
> specify a default interface.
>
> However, this doesn't completely answer the question.
> You appear to have only one interface active,
> so naturally all packets must go that way.
> I assume that is the situation with most people most of the time.

It does answer the question. Here's a more complex ip route output on 
one of my machines

10.8.0.2 dev tun0  proto kernel  scope link  src 10.8.0.1
192.168.4.0/24 via 10.8.0.2 dev tun0
10.0.0.0/24 dev eth1  proto kernel  scope link  src 10.0.0.1
10.8.0.0/24 via 10.8.0.2 dev tun0
24.72.122.0/24 dev eth0  proto kernel  scope link  src 24.72.122.184
169.254.0.0/16 dev eth1  scope link
default via 24.72.122.1 dev eth0


Two physical interfaces, one tunnel with multiple networks.

An internal network of 10.0.0.0/24 all traffic goes out eth1
The tunnel (tun0) will get traffic destined for 192.168.4.0/24, and 
10.8.0.0/24
The internal interface also has a link local / avahi / bonjour browsing 
connection.
Everything that doesn't match that goes out the default connection.

> My experience with 2 interfaces active, say WiFi and ethernet,
> does not entirely fit in with your theory,
> as ifconfig shows a large number of packets
> going through each interface.
> I'll experiment further and see more exactly what happens.

I would also expect that packets on a wireless network continuously rise 
for protocol / AP whatever reasons. Even if *data* wasn't going out.


The fact of the matter is ip route tells you exactly what you need to 
answer the question you asked.