[libvirt] RFC: automatic setting of ip_forwarding (or not)

Currently libvirt will turn on net.ipv4.ip_forward by writing "1\n" to /proc/sys/net/ipv4/ip_forward whenever a virtual network of with a forward mode of "nat" or "route" is started. This is problematic for two reasons: 1) /etc/sysctl.conf is not updated with this information, so any other process reprocessing /etc/sysctl.conf (with "sysctl -a -p") will potentially turn ip forward back to 0, leaving libvirt-created virtual networks in a non-working state, and 2) it's possible the administrator turned off ip forwarding on purpose for security reasons, and our silently turning it on leaves them mistakenly believing it is still off. We've discussed a few ways of remedying this situation lately, and I thought I should summarize all the mentioned ideas, and take a poll to try and determine which way we should fix this. 1) Leave it as is. The simplest solution, but has the problems outlines above. 2) Turn it on in the same place, but do it by writing net.ipv4.ip_forward = 1 to /etc/sysctl.conf and calling "sysctl -a -p". This gives us the same behavior as currently, but with the advantages that a) our change to the config is documented in /etc/sysctl.conf and b) virtual networked guests won't suddenly have their network fail when some other process runs "sysconfig -a -p". However, it seems rather drastic to be turning this on every time a virtual network is started, especially without alerting the admin that this has been done. 3) Whenever a virtual network that would require ip_forward = 1 to operate properly is started (ie at libvirtd start time, and when a network is newly defined), check if it's currently on, and if not log a warning message, informing the admin that they should turn on ip_forward in sysctl.conf and reload it in order to have properly working networking. This would assure that the admin was informed of the necessity for ip_forward, but eliminate the possibility of two processes fighting over the setting of ip_forward, leaving it up to the admin to make the decision and do the right thing. On the other hand, it would prevent libvirt's networking from "just working" on a new install. 4) Turn ip_forward on during libvirt install. This one doesn't make sense to me, because you don't know at the time of libvirt install whether or not the installation if going to end up with any virtual networks that need forwarding. 5) Make ip_forward a tunable in /etc/libvirt/libvirtd.conf, and set it accordingly every time libvirtd is started. I don't know if this makes sense either - if you have NATed or routed virtual networks, you will need ip_forward=1 for them to work properly, and if you don't have them, you don't care, so it's really redundant. **** I think the important things to accomplish are: 1) Avoid having networking magically stop working when someone else reloads sysctl.conf 2) Make sure that the admin realizes that ip_forward is being turned on (or needs to be turned on). 3) If we're going to turn it on, at least don't do it if it's not needed. 4) Something else we need to consider is the ability to provision a host for proper guest networking purely through the libvirt API, and if we were to stop turning on ip_forward automatically when a network was started, that wouldn't work anymore (unless ip_forward happened to be turned on already). So, what are your opinions? (BTW, the firewall rules added for virtual networks suffer from a similar problem - because they're loaded into the kernel directly with the iptables command, there is no external record of them, and some other process reloading the firewall will flush out all libvirt's rules, leaving the guests with nonworking networking. But that discussion is a bigger one, that probably needs to go outside just libvirt, so I'll avoid that here...)