[libvirt] ideas for custom iptables rules for libvirt networks.

We still periodically get requests to allow custom iptables rules for libvirt virtual networks (or, more commonly, a mode where libvirt simply leaves iptables alone, not adding or removing anything), and it's been a nagging item on my to-do list for a very long time. The problem is that, although the amount of code required to support *any* solution is very small, it's one of those things without a single obvious "only" way to do it. Anyway, I'm going to take one more stab at it. First, some background points: * For <forward mode='nat'> libvirt's iptables rules are essential to the operation of the forwarding, so we shouldn't mess with that. * For [no forward mode], libvirt's iptables rules are a part of what keeps the network isolated from the rest of the network, so we shouldn't mess with that either. * For <forward mode='route'> we currently allow all outgoing and incoming as long as it is to/from the IP address range defined for the network. So we really want something that can be used only for <forward mode='route'> I can see 3 different possibilities: 1) a new forward mode which is just like 'route', but doesn't add any iptables rules. (what to call it though? "filterless-route"? Too long and ugly :-/) 2) a new attribute to <forward> that takes effect only for mode='route'. Maybe call it "filter". We could have "filter='open'" (what it does currently, and will remain the default), "filter='outgoingOnly'", and "filter='none' (the most requested functionality - no iptables rules would be added for the network) 3) add a <filter> subelement to <forward> that allows specifying iptables rules for the network. Perhaps this could instead be a <filterref>, and use nwfilter to specify the rules? (that sounds really cool, and if it worked it would be a nice re-use of the nwfilter driver, but it may have undetermined pitfalls due to nwfilter being designed with guest traffic filtering in mind, would take a lot more work, and wouldn't address the most common request - "Don't mess with iptables! I want to do it myself!". Anyone have an opinion or alternate idea?