Multicast in OSPF with shared interface addresses

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Multicast in OSPF with shared interface addresses

Scott Nicholas
I joined a VPN network (dn42) to learn BGP and such and decided to do
so with OpenBSD, which I'm also learning. Most peers are Linux
machines and they re-use their address on each VPN tunnel as a /32. I
have been successful doing the same until I decided I needed ospf for
my internal routes.

openospfd sets the interface (identified only by its IP) as the
multicast source. Since several tunnels have that address, it sets it
incorrectly. A brief look at Linux headers show that their newer
ip_mreqn struct includes an interface index since Linux 2.2. Perhaps
this is a useful inclusion in the OpenBSD kernel so that userland can
pick the interface correctly?

For now I've worked around this by assigning /31 aliases in
192.168.0.0/16 to the interfaces. But I'm curious what others are
doing that use OpenBSD as a router, as it's all fairly new to me. I'm
reading that OSPF could also have unicast neighbors setup, but
OpenOSPFd doesn't have this feature.

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Re: Multicast in OSPF with shared interface addresses

Dimitris Papastamos
On Sun, Dec 03, 2017 at 04:01:35PM -0500, Scott Nicholas wrote:

> I joined a VPN network (dn42) to learn BGP and such and decided to do
> so with OpenBSD, which I'm also learning. Most peers are Linux
> machines and they re-use their address on each VPN tunnel as a /32. I
> have been successful doing the same until I decided I needed ospf for
> my internal routes.
>
> openospfd sets the interface (identified only by its IP) as the
> multicast source. Since several tunnels have that address, it sets it
> incorrectly. A brief look at Linux headers show that their newer
> ip_mreqn struct includes an interface index since Linux 2.2. Perhaps
> this is a useful inclusion in the OpenBSD kernel so that userland can
> pick the interface correctly?
>
> For now I've worked around this by assigning /31 aliases in
> 192.168.0.0/16 to the interfaces. But I'm curious what others are
> doing that use OpenBSD as a router, as it's all fairly new to me. I'm
> reading that OSPF could also have unicast neighbors setup, but
> OpenOSPFd doesn't have this feature.

I have also experienced the same thing and worked around it using /30s
but would be nice to know how to approach this properly.