Re: Altering preferred connection

[Thomas Haller <thaller redhat com>]

On Thu, 2017-06-29 at 13:14 +0100, Colin Helliwell wrote:
> Is there a way to get NM to alter fully on-the-fly which connection
> is preferred/default for traffic? 
> I see that I can modify the route-metric parameter: if the connection
> is taken down and up again then I see the change in the output of
> 'route', and indeed this seems to change which connection is
> preferred. 
> But is there a way to poke the change through without the down/up?
> (ideally via NM, or in any other safe & sane way)
> Ta

Hi,

if you modify a connection profile that is currently active on an
interface, those modifications do not automatically apply on the
interface. You only modify the profile
  -- with the exception of connection.zone and connection.metered
     properties, these two apply immediately.

As you figured out, after you modified a profile, you can get the
connections working by doing a full down/up cycle. By just upping the
connection (again):

  nmcli connection modify "$CON" ipv4.route-metric 42
  nmcli connection up "$CON"



An alternative, a bit more graceful is

  nmcli connection modify "$CON" ipv4.route-metric 42
  nmcli device reapply "$DEV"

this takes the changes from the profile and applies them on the device
without going through a full down/up. Not all changes are re-applyable, 
so, if you change something that isn't, the command will just fail and
do nothing. Basically, IP settings are reapplyable.




There is also

  nmcli device set "$DEV" setting.property value [...]

for example

  nmcli device set "$DEV" ipv4.route-metric 42

the difference is, that this change is run-time only. The connection
profile.is never modified.




Another way of thinking of this is, when you activate a "settings-
connection" on a device, it copies the profile as "applied-connection".
Later modifications with
  nmcli connection modify "$NAME"
only modify the settings connection, not the applied connection (except
connection.zone and connection.metered).

Then
  nmcli device reapply
copies the settings connection over to the applied connection (and
applies them).

And 
  nmcli device set
only modifies the applied connection directly (and applies the
changes).


You can also see the applied connection on D-Bus:
  busctl --system call org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/Devices/19 
org.freedesktop.NetworkManager.Device GetAppliedConnection "u" 0


best,
Thomas

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