bond for wlan/lan failover: hook for dhcp changes?

[m1027 <m1027@xxxxxxxxxx>]

Hi,

With a working wlan/lan bond0 failover, how can we trigger other
local services whenever a failover happens?

Why: In our case, a local openvpn client service needs SIGUSR1
whenever the own IP address changes, after a failover. See man
openvpn(8), --ipchange.

The problem: After a failover, the openvpn client keeps sitting on
the wrong IP address/interface. Sending SIGUSR1 manually helps
indeed: openvpn then reconfigures itself and uses the correct IP
address. The man page recommends to write a hook script for the
local dhcpcd to send SIGUSR1 to the openvpn client, however we are
not using dhcpcd but systemd.

Some more information:

- Failover works in general here; pulling out the cable from lan1
  activates wlan0 and vice versa, and this triggers external DHCP
  servers to issue a new IP address. The external servers are
  triggered because of FailOverMACPolicy=active for bond0.

- Our bond0 is on top of wlan0 + lan1 and configured like this:

  [NetDev]
  Name=bond0
  Kind=bond
  [Bond]
  Mode=active-backup
  AdSelect=bandwidth
  FailOverMACPolicy=active 

- We know of networkmanager's dispatcher scripts but are looking for
  a solution within systemd. (It's also unclear whether our network
  setup could be configured entirely by networkmanager.)

- There is a workaround: There is the "inactive <sec>" option for
  the openvpn client to shutdown in case of no action. And we can
  additionally set the client to Restart=always. However, this
  interrupts even working openvpn client sessions every 60 sec of
  inactivity.

Thanks for any pointers...