Heart code [from another off-list discussion]

[Alan Robertson]

Tom and I were having another off-list discussion about the distinction
between SO_DONTROUTE (which is supported on Linux and other OSes), and
SO_BINDTODEVICE (which appears to be a Linux-only option).  We want the
code to be portable to other OSes, as much as possible.


Tom Vogt wrote:
> 
> Alan Robertson <alanr@bell-labs.com> wrote:
> > I guess in most cases, you can control which subnet you go out on just by
> > controlling the broadcast address.  Is that right?
> 
> I think so.
> 
> > I can still need to know
> > which interface, in order to look up the broadcast address.
> 
> the description of SO_DONTROUTE says that it'll bypass routing, taking the
> interface to use from the network portion of the address.

The only thing that SO_DONTROUTE won't do is select which interface
you're doing diagnostics on [which is a longer-term goal].  For example,
if I have two redundant NICs on a machine, and want to test one, I need
to be able to control which one gets a particular packet.  They will
both have the same network portion of the address, and you'll want to
know which one you're testing.  That's the only distinction I can see. 
And this distinction doesn't apply in heartbeats.

	-- Alan Robertson
	   alanr@bell-labs.com