[systemd-devel] Systemd-networkd, default route and multiple interfaces

[Dan Williams]

On Fri, 2014-06-06 at 10:52 -0400, Leonid Isaev wrote:
> Hi,
> 
> On 06/06, Lennart Poettering wrote:
> > Date: Fri, 6 Jun 2014 15:43:21 +0200
> > From: Lennart Poettering <lennart at poettering.net>
> > To: Mantas Mikulėnas <grawity at gmail.com>
> > Cc: systemd Mailing List <systemd-devel at lists.freedesktop.org>
> > Subject: Re: [systemd-devel] Systemd-networkd, default route and multiple
> >  interfaces
> > User-Agent: Leviathan/19.8.0 [zh] (Cray 3; I; Solaris 4.711; Console)
> > 
> > On Wed, 04.06.14 00:29, Mantas Mikulėnas (grawity at gmail.com) wrote:
> > 
> > > 
> > > On Tue, Jun 3, 2014 at 9:58 PM, Lennart Poettering
> > > <lennart at poettering.net> wrote:
> > > >
> > > > On Mon, 02.06.14 21:02, Tom Gundersen (teg at jklm.no) wrote:
> > > >
> > > > > On Mon, Jun 2, 2014 at 8:47 PM, Reventlov
> > > > > <contact+systemd+ml at volcanis.me> wrote:
> > > > > > Since network files are applied to links whenever the links
> > > > > > appear, how do systemd manage the "disappearance" of a default route ?
> > > > >
> > > > > You can have several default routes, so we just configure them all and
> > > > > let the kernel handle it for us. We probably should improve the config
> > > > > options to make it possible to override the priority of the routes.
> > > >
> > > > Windows initializes the route metric value for each route from the
> > > > "speed" of the interface. If there are multiple links and for one we
> > > > know it's a gigabit ethernet link, and the other is a 54mbit wlan link,
> > > > then the former would automatically get preference... it's a bit black
> > > > magic, but kinda cool black magic i think...
> > > >
> > > > that said, i am not sure we even have a sane API to determine the speed
> > > > of links... maybe ethtool reports it at least for wired ethernet? but
> > > > for wlan?
> > > 
> > > Both `iwconfig <dev>` and `iw <dev> link` report the link speed (one
> > > using the old wext and the other nl80211, I think?).
> > > 
> > > No idea about all the other link types that Linux supports though.
> > 
> > As long as we'd only have to check ethtool for wired ethernet and the
> > wireless stack for wireless ethernet and can cover 70% of all devices
> > with that, this sounds like the right thing to do for me...
> 
> Note, however, that wlan link speed can fluctuate in time. At least that's what
> happens with eduroam at our university (mostly cisco APs).

Wifi speeds are constantly recalculated by the driver/firmware based on
the RF environment, no matter when moving or standing still.  So the
farther away from the AP you are, or the more interference there is, the
slower your speed will be as your device attempts to increase
reliability of data transfer against interference or faint signal.

Dan