calling net.ipv4.dhcp on tun device fails about badvpn HOT 7 CLOSED

I'm pretty sure you can't do DHCP on a TUN device (as opposed to TAP). Have you 
tried whether dhcpcd works?

I tried dhclient which didn't give any errors which is why I assumed it was OK 
(and the fact that dhcp works at the application layer). However, on trying 
dhcpcd I do get an error: "err, tun0: interface is not Ethernet, FireWire, 
InfiniBand or Token Ring"

Am currently writing a template to get the information from "ip route" output 
using regular expressions. If you can think of an easier way please tell me.

Well, the answer at this time is simply that it's not supported. Use TAP, if 
you can, or assign addresses some other way than DHCP.

Have you tried letting openvpn assign the address? I know that integrating such 
tools like VPN clients is painful, but it can be done. For inspiration you can 
check my PPOoE code, 
https://code.google.com/p/badvpn/source/browse/trunk/ncd/examples/router/pppoe.n
cdi

Yes openvpn automatically assigns the ip address & gateway to a tun device in 
the main routing table, but I need to know what they are so that I can update 
other routing tables. Anyway, I've done it now: I wrote a template to get the 
information from iproute2. I will send you a copy.

Not expecting this to be implemented anytime soon since it's a very rare 
configuration not even supported by dhcpcd.