Re: more than one default gw route

[Dan Williams <dcbw redhat com>]

On Tue, 2009-04-28 at 09:40 +0200, Nicolò Chieffo wrote:
> My situation is less complicated.
> 
> I have a laptop and a desktop. the laptop is connected only to
> wireless, but the desktop is connected also to wired and wireless.
> I have 2 routers placed in different rooms: I can't reach the wireless
> router with network cables, and I can't move the wireless router near
> the cables
> if I want all my rooms to be served by the wireless signal.
> 
> When the laptop is off, and I use the desktop, the wired router is on,
> and the wireless router is off (I don't want to use wireless if I can
> use wired).
> But when I need also my laptop, I switch on also the wireless routed,
> and the wired router loses the ADSL connection (which of course is
> taken by
> the wireless router).
> When this happens, the default gatway of my desktop pc is still set up
> to the wired router, even if it connects also to the wireless router.
> So the only thing I can do to get connected is either to unplug the
> cable or to switch off the wired router.

To be honest, you'd have to do manual twiddling no matter what.  Even if
NM supported per-interface 'disable' sort of thing, you'd need to do
that manually.

Since the cable is plugged in, and the wired router does DHCP, and the
wired connection still has a DHCP address, that's a valid route.  It's
just not a route to the internet anymore.  But that's quite hard to
determine in a way that's not horrible to some provider; you can't
really just pick a site to ping by default (see [1]).

Even if there was a mechanism to determine whether a specific route
could reach the internet or not, it would have to be quite good to be
used by default to move the default route around, otherwise it could
break perfectly good existing connections.

So if NM let you disable the wired interface, you'd have a few choices:

1) turn off the wired router
2) unplug the wired cable
3) manually disable the wired interface through some UI or something

I'm not opposed to adding some sort of MAC-detection or ping
functionality to NM to enhance the decision of what route actually gets
to the internet, but it shouldn't turned on by default due to security
issues (MAC spoofing) and DoS (ping).

Dan

[1] http://www.techworld.com/security/news/index.cfm?NewsID=409