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Re: Thinking about "branes" for netbsd...

On 5 May, 2012, at 15:33 , Darren Reed wrote:
> Dennis Ferguson wrote:
>> The problem is that, no matter what the ioctl tells the socket
>> to do, the kernel cannot send a packet out an interface if it
>> does not have a route to the packet's destination pointing out
>> that interface.  If the destination is being routed with a
>> default route then there need to be two default routes (if it
>> is being routed with some other route there need to be two
>> of those), one for each interface, which means you need two
>> forwarding tables to store the different routes in.  It is a
>> multiple forwarding table problem even if it isn't one that
>> a "brane" works for.
> For reference, you might like to investigate the following
> IP socket options that are available on OpenSolaris:
> The latter of which is similar to Linux's SO_BINDTODEVICE.
> I don't know if Linux has an equivalent of IP_NEXTHOP but
> as an ioctl, it works in a similar way (for a particular
> socket) as does policy routing with ipfilter. I believe
> that either one or both of those are a solution to your
> problem without implementing virtual routing tables.

I'm aware of both of those.  I think you may be confused about
what IP_BOUND_IF does (hint: last I looked it only effects
where multicasts and broadcasts go).  IP_NEXTHOP does help if
you think this is solved by having each application do its
own routing (maybe the application could run DHCP to find out
the next hop for that interface's default route too); if all
applications did this then the kernel could get even simpler
by eliminating all forwarding tables.

Dennis Ferguson

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