After some time I needed to do some kernel coding again. To seamlessly support dynamically created interfaces in Zorp, we need something I called "interface groups".
Each interface might belong to a single group that basically describes how the interface was created. For instance there's an interface group for each PPP profile, but an interface group can encapsulate interfaces created by PPTPD.
It is quite difficult to match dynamic interfaces by their nature: iptables sports wildcard interface name matching with the '+' character but it only works if interface names have some kind of prefix _AND_ if you don't want to differentiate between two groups.
If you have two sets of PPP devices (like in the example I described above), then you have no way to create a separate ruleset, unless you reload iptables everytime a new interface is added to the system.
Adding to the burden, in Zorp we want to be able to bind a service to these dynamically created interfaces, of course without listing the actual IP address in the configuration file.
The idea is simple, I added an "interface group ID" to the net_device struct, and an option to the "ip link" command to set/query this ID. Once an interface is created by some kind of program (for instance pppd), a script is executed in its /etc/ppp/ip-up.d directory and userspace can assign a group ID based on the PPP profile name. Then Zorp gets notified about the change through NETLINK and can react by binding to the IP address of the new interface. The configuration remains static, no reloading needs to be done when such a change happens, and you can create firewall policies for something like: please allow this set of services for everyone using this PPP profile, without entering one specific IP address to the configuration. Neat, eh?
I posted my work on netdev and netfilter-devel, I'm curious what the kernel maintainers will think about it.