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

[Cole Robinson]

On 04/25/2016 01:48 PM, Laine Stump wrote:
> 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?

I would suggest adding the bare minimum necessary :) Which I think is probably
your second suggestion:

  <forward mode='route' filter='none'/>

or maybe filter=automatic|manual , which takes the qualitative aspect out of
the name. automatic=libvirt does whatever it wants with iptables, manual =
you're on your own.

I'd avoid adding anything extra like filter='outgoingOnly' until there's a
real legitimate request for it. But also since routed networking generally
requires external host config anyways it should be fine IMO to tell anyone
that wants it to use filter=automatic and add there own rules on top to
disable inbound traffic

- Cole