[PLUG] Multiple Internet Links...

[Daniel Pittman]

On Mon, Dec 6, 2010 at 16:49, Michael C. Robinson
<plug_1 at robinson-west.com> wrote:

> I'm set up via two gateways to route a lan onto the Internet via a DSL
> connection.  I'm adding Comcast temporarily to get a feel for the service
> and possibly be able to order a month or so of service in case of DSL
> outages or need.  I want to route through Comcast on demand.  To effect this
> change, I'm thinking of adding a physical nic on each gateway that will
> connect to the cable modem.
>
> First question, how do I set the default gateway properly?  Second question,
> how do I set the name servers I want, not Comcast's?

This is actually a *really* hard thing to do right, I fear.  Linux doesn't do
"link health" detection or anything, so if you have a gateway on the dead
device listed in the routing table it will get used even if a perfectly good
second path exists.

So, you actually need something on your router to sit there, keep an eye on
the state of the connection, and update the routing table.

> Do I really need to add another nic or can I alias a second address on an
> existing nic and use free ports on my DSL connected ethernet switch for the
> cable modem?

You should be able to configure all the devices into the same internal range,
and use the gateway server(s) to manage that appropriately.  You certainly can
run different network ranges as well, though that can sometimes require
additional work to make routing do the right thing.


Anyway, the basic list of things to do looks something like this:

1. Hook up both Internet routers so they can talk (somehow) to the gateways.
2. Configure the gateways so they can talk through those routers to the
   Internet successfully.
   - you may need to adjust the preferred source address on the link
   - you may need to do some source routing
3. Get both upstreams working when you route through the gateways
4. Configure something to monitor the two Internet services and verify they
   are working correctly
5. Have whatever monitors the service bring up and down the relevant routes
   when the service is healthy or dead.
   - since you want preferential routing, just assign the appropriate metric
     to them as default routes and it will all just work(tm) routing-wise.


With regards the question of using the right DNS servers: turn off the DHCP
services on all the Internet router devices (cable and DSL), I would suggest,
and then run DNS and DHCP service on your gateways.

That way you can supply your local resolver to client machines, configure
whatever forwarders you want, and be happily in control of the whole thing.


All of which should be reasonable straight-forward, but much of it will not
exactly be simple.

Regards,
        Daniel
-- 
  Daniel Pittman              daniel at rimspace.net              +61 401 155 707
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