[systemd-devel] Need help with DHCPv4 client

[Thomas Köller]

My host is on a dynamic IP address (IPv4 only), which it receives via 
DHCP from my provider. So far, I have been using ISC dhclient for this, 
the network interface being marked as unmanaged in its .network file. 
But since dhclient isn't maintained any more, I am exploring ways to 
replace it with systemd's DHCP client.

While address updates using systemd's DHCP client seem to work just 
fine, there is a problem, however. Whenever the host's IP address 
changes, some reconfiguration has to be done: running services listening 
on sockets opened before the address change have to be restarted or at 
least notified, and a dynamic DNS A record has to be updated. With ISC 
dhclient, this used to be unproblematic, as it can be configured to 
invoke external scripts when it receives a new address from the DHCP server.

As far as I can tell, systemd's DHCPv4 client does not provide any such 
hooks. I looked at the D-Bus interface of systemd-networkd 
(org.freedesktop.network1), but was unable to spot anything of that 
sort. However, as I am unfamiliar with Dbus in general, it is very 
possible that I missed something.

I even tried to write a program processing rtnetlink notifications 
resulting from address or route changes, however, this approach turned 
out to have it's own problems. At this level it is difficult to detect 
the point when all changes have been completed and the system has 
reached a new stable state, so that reconfiguration can safely commence.

I'd greatly appreciate any helpful comments!

Thomas