Following the NetworkManager description https://wiki.debian.org/NetworkManager the goal of NetworkManager is:
NetworkManager attempts to keep an active network connection available at all times.
I compared an Ubuntu 21.04 system with Debian 11 (bullseye), both as virtual guest in a Libvirt environment, using 2 "wired" network interfaces (different subnets) on each guest.
Procedure:
- Starting guest with one interface link active (second is down)
- Set active link down (via Virtual Manager)
- Set link from another network interface up (via Virtual Manager)
- Checking inside guest, if network is reconnected on the second interface
On Ubuntu 21.04 system everything works as desired, the guest is removing the interface with missing carrier (because link was set to down), and reconnects the net on the second interface automatically (which link was set to up).
The Debian guest is not reconnecting automatically, but it works if you switch on the interface inside guest manually.
If you change the procedure to ...
- Starting guest with one interface link active (second is down)
- Set active link down (via Virtual Manager)
- waiting to interface removed inside guest
- Set link from another network interface up (via Virtual Manager)
- Checking inside guest, if network is reconnected on the second interface
... network is reconnected automatically also in Debian guest.
So it seems, that NetworkManager in Debian systems has a serious problem. When second interface becomes a valid carrier if anoter interface is already active, the new interface is ignored for automatic connection now and in the future. If the active interface goes down, NetworkManager is not activating the interface on the second available interface, with the result guest is not longer connected to any network!
I try to compare versions, packets and config files and cannot see any differences.
My questions:
- Any idea why these systems react different on same procedure?
- What can be modified in Debian system to achieve the NetworkManager's goal (network active all the time) reliable?
