[vox-tech] two networks, one app

[Ted Deppner]

Yes, possible, but in practice rather complex.  Basically a router can
only *ever* have ONE default route.  Nothing ever really changes that,
except, some times, in some routers, there are two equal cost default
routes.  Now something like this on an internet router is handled
"automatically" by a routing protocol like BGP, EIGRP, or OSPF.  While
linux routers can do those protocols, those protocols are all designed
to part of a mesh, and since your upstreams won't be part of your
mesh, that won't help.

And here enters the complexity.   Upstream "A" only expects it's ip
address they've given you to use, and no other, and the same for
upstream B.  So when you send a packet through your linux router, it
must NAT that packet onto the proper outbound interface with the
proper NATed IP for that interface.  The next challenge is what to
send where.  You really can't have one packet out A and one out B...
you must have a given interface used for all of a given "stream".  All
packets to site 1 stick to upstream A, etc.

The debian package "iproute" or "iproute2" in the past, contains the
tool "ip" which allows you to manipulate these settings.  You'll
probably also need to use iptables.  The outbound routing bit is "ip
rule".  the ip tool must be used rather than "route -n" once you start
doing these complex things.

I've done this, it works, it's complex, error prone, and hard to
maintain.  I've tried to give you some of the buzzwords and base stuff
as a starting point, which you can hone with Google.

On Wed, Sep 9, 2009 at 3:45 PM, Peter Jay Salzman <p at dirac.org> wrote:
> Is it possible to transparently get network connectivity from two network
> interfaces, say eth1 and wlan1, at the same time?  I guess which one is used
> depends on which one is busy at any given moment.