Discussion:
Multicast in OSPF with shared interface addresses
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Scott Nicholas
2017-12-03 21:01:35 UTC
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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.
Dimitris Papastamos
2017-12-05 10:06:16 UTC
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Post by 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.
I have also experienced the same thing and worked around it using /30s
but would be nice to know how to approach this properly.

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