[libvirt] [PATCH] v2:Support for adding a static route to a bridge

[Gene Czarcinski]

On 04/02/2013 03:31 PM, Laine Stump wrote:
> On 03/15/2013 02:10 PM, Gene Czarcinski wrote:
>> This patch adds support for adding a static route for
>> a network.  The "via" specifies the gateway's IP
>> address.  Both IPv4 and IPv6 static routes are
>> supported although it is expected that this
>> functionality will have more use with IPv6.
>
> (First I want to make sure I correctly understand what you're wanting to
> do, so I'm going to try and explain it in my own words...)
>
> >From reading your earlier messages, my understanding is that the aim of
> this patch is to automatically setup a route to a virtual network whose
> bridge device has no IP address assigned, and is therefore reachable
> only via one of the guests, is this correct?
>
> In other words, let's say that I have the following two networks defined
> (using IPv4 and all static IPs for brevity, but the entire discussion is
> equally applicable to IPv6):
>
>
>     <network>
>       <name>netVisibleToHost</name>
>       <bridge name='virbr1'/>
>       <forward mode='route'/>
>       <ip address='192.168.200.1' prefix='24'/>
>     </network>
>
>     <network>
>       <name>netHiddenFromHost</name>
>       <bridge name='virbr2'/>
>     </network>
>
> and you have a guestDirect that has two interfaces:
>
>     <interface type='network'>  <!-- 192.168.200.2/24 -->
>       <source network='netVisibleToHost'/>
>     </interface>
>     <interface type='network'>  <!-- 192.168.201.1/24 -->
>       <source network='netHiddenFromHost'/>
>     </interface>
>
> and another guestIndirect that has only one interface:
>
>     <interface type='network'>  <!-- 192.168.201.2/24 -->
>       <source network='netHiddenFromHost'/>
>     </interface>
>
> Additionally, the default route on guestDirect points to 192.168.200.1,
> and the default route on guestIndirect points to 192.168.201.1.
>
> (Presumably you don't want to simply add an IP address to
> netHiddenFromHost because (while it would solve your routing problems)
> it would violate some security policy you've built into your network
> topology - namely that all traffic to and from netHiddenFromHost *must*
> go through guestDirect.)
>
> Traffic outbound from guestIndirect would have no problem reaching its
> destination - it would go across netHiddenFromHost to guestDirect
> (192.168.201.1), which would know to forward it to the host
> (192.168.200.1) via netVisibleToHost, and the host presumably knows how
> to get to anywhere. The problem comes when trying to route the
> *response* destined for 192.168.201.2 (guestIndirect) - the outside
> world may know that those packets have to be sent to the host, but the
> host doesn't have a route for 192.168.201.0/24 - only guestDirect does.
>
> So, the solution is to add a route on the *host* that points traffic
> destined for 192.168.201.0/24 to guestDirect, a.k.a. 192.168.200.2.
>
> Since there's no place in /etc/sysconfig/network-scripts/route-* to put
> static routes with destinations that are only reachable through a
> transient interface such as the bridge devices created by libvirt, the
> obvious solution is to cause libvirt to add such a route, and the way
> you propose to do that is to add an <ip> element in the network named
> "netUnreachable".
>
> Am I understanding the issue so far?

I believe you do understand.

I have not read the rest of your message yet but I did want to bring up 
one point.

With IPv4 I can get a lot of this functionality by the cleaver use of 
NAT but with IPv6 there is no NAT and everything is routed.  The big 
issue with IPv6 (and the reason I did this) is to provide the 
virtualization host with information needed to route response packets 
back to the "hidden" guest.  Without this, the can can send any packet 
it wants but will never get a reply.  That is the basic problem being 
addressed.
>
> Assuming that I am, then as far as I can see, the correct place to
> configure this route isn't in the setup for netHiddenFromHost, but
> rather in netVisibleToHost - this is more in line with the way static
> routes are configured in the standard host network config (route-*
> files), and eliminates the problem that would occur if netHiddenFromHost
> was started before netVisibleToHost existed (the route would be added
> pointing at the wrong interface, if at all)
You are correct.  The "route-via" must be defined in the "visible" 
network definition.  There is an additional constraint that the gateway 
address must be in the range of the visible network.  IIRC, tere are a 
couple examples in the code I added for tests.
>
> So, what I'm proposing is that, to automatically setup the route in the
> above example, netHiddenFromHost would remain unchanged, and
> netVisibleToHost would look something like this:
>
>     <network>
>       <name>netVisibleToHost</name>
>       <bridge name='virbr1'/>
>       <forward mode='route'/>
>       <ip address='192.168.200.1' prefix='24'/>
>       <route address='192.168.201.0' prefix='24' gateway='192.168.200.2'/>
>     </network>
>
> The route element could also have a family attribute just as <ip> does
> (although it's fairly simple to figure out the family just by trying to
> parse address and/or gateway). You might instead want to make <route> a
> subelement of <ip>, then validate that the gateway address is directly
> reachable from the given ip address (i.e. that it's on the same subnet).
The <route> above is similar to what I implemented.  Since I am defining 
a network which is "somehow" reachable via this network interface, I 
used the "regular" <ip> element to define the network and then added the 
"via=" (instead of "gateway=").  One reason I chose to use "via=" rather 
than "gateway=" is that when you run the
     ip  -6  route    or    ip  -4  route

you get:
    <ip-address> via <gateway-address>

Here is a part of a definition I have:
<network>
   <name>net7</name>
...
   <ip family='ipv6' address='fd00:beef:10:7::1' prefix='64'>
     <dhcp>
       <range start='fd00:beef:10:7::1:1' end='fd00:beef:10:7::1:f1' />
     </dhcp>
   </ip>
   <ip family='ipv6' address='fd00:beef:12::' prefix='48' 
via='fd00:beef:10:7::7:2'>
   </ip>
...
</network>

and here is what you get from  ip  -6  route:
fd00:beef:10:7::/64 dev virbr3  proto kernel  metric 256
fd00:beef:12::/48 via fd00:beef:10:7::7:2 dev virbr3  proto static metric 1

>
> By putting the configuration here, you could be assured that the
> interface that will be used for the route will always exist at the time
> the route is added. Also it is conceptually more similar to the way that
> the routes in /etc/sysconfig/route-ethX all have gateways that are
> directly reachable via "ethX".
Absolutely.  "Floating" routes make no sense to me.  This static route 
is closely coupled with the (main) address on that virtual network 
definition and needs to be included in the definition.
>
> *OR* (following is what I think is a bad idea, but maybe someone can
> tweak and salvage it :-)
I cringe here.  Since what you stated above is more or less what I 
implemented, I am of the opinion that this is the one that makes sense.

If you use NetworkManager to define some static routes, those 
definitions will need to be on the which of the network where the 
gateway is reachable.  Mmmm .. I just noticed that NetworkManager used 
"Gateway" and not "via" ... maybe I should change.
>
> Here is an alternate proposal that has the advantage of tying the
> existence of the static route to the existence of not just the bridge,
> but of even the guest interface will be the gateway: instead of putting
> the route in the configuration of the netVisibleToHost network, put it
> directly in the configuration of the guest interface itself. That way
> when the guest is started the route will be added, and when the guest is
> shutdown, the route (which will anyway now be useless) will be removed.
> That could be added to the <interface> definition something like this:
>
>
>     <interface type='network'>  <!-- 192.168.200.2/24 -->
>       <source network='netVisibleToHost'/>
>       <route address='192.168.201.0' prefix='24' gateway='192.168.200.2'/>
>     </interface>
>
> If this was done correctly, you could even hotplug a guest interface
> that would then be immediately used for a route, as well as updating an
> existing guest interface to add a route. The route added would use the
> information in the <route> element plus the bridge device the guest's
> tap device was connected to.
>
> I do have several reservations about this idea, though
>
> 1) Up until now there has been no "pollution" of the guest config with
> IP-related info, it has all been about hardware. (well, except for
> filterref parameters...)
>
> 2) Even worse than that is that the guest interface config *can't* know
> the IP address that the guest will use for the interface, yet we would
> now be hard-coding it into this new <route> element in the guest
> interface config. Ugh. (Now what would be *really* slick is if we could
> have routes in <interface> that learned their IP address from nwfilter's
> IP address snooping! :-)
>
> 3) If the interface happens to be connected to a bridge that has no IP
> address on the host (e.g. the bridge of netHiddenFromHost in the example
> above), the route will be a failure.
>
> 4) Likewise, if the interface is connected with macvtap, again the route
> would be a failure.
>
> So, in the end, the idea of adding <route> to <interface> is probably a
> no-go.
Yes, nogo with me also.

In many respects the route definition is not much different than other 
network definition included in a virtual network definition except that 
it is only for routing.
>
> (One upside - if the interface is a hostdev, we could simply add the
> route with no "dev X" appended, and it *would* work, as long as the host
> had an interface on the same subnet).
>
>
>> The command used is of the following form:
>>
>> ip route add <address>/<prefix> via <gateway> dev <virbr-bridge> \
>> proto static metric 1
>
> It seems to me that the "dev" given here is incorrect. If I'm not
> mistaken, the route you're adding with your current code will use the
> device name of the bridge on "netHiddenFromHost" in this command, while
> what you *really* want is the bridge used for "netVisibleToHost" (which
> isn't even known/available at the time you start netHiddenFromHost -
> another reason to do it in one of the ways I'm suggesting).
No, no.  The <virbr-bridge> is defininiton the visible virtual network 
interface.  The example I cited about is currently "running" with 
libvirt which includes the patch.  Here is the result for ip -6 addr:

3: virbr3: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500
     inet6 fd00:beef:10:7::1/64 scope global
        valid_lft forever preferred_lft forever
     inet6 fe80::5054:ff:fef8:9ca7/64 scope link
        valid_lft forever preferred_lft forever

So, gateway fd00:beef:10:7::7:2 is covered by network 
fd00:beef:10:7::/64 which is on device virbr3.  The network 
fd00:beef:12::/48 is routed via fd00:beef:10:7::7:2 on virbr3.

I do not know if you have given these patches a try but it does make 
more sense when you see it work.

BTW, one static route for IPv4 and one static route for IPv6 is 
supported by my "IPv6" updates to virt-manager.  I need to do a little 
more work to add that to the virtual-network-creation-wizard.

Gene