Multiple Gateways


Jeniffer K

Dear NG, - I posted this to the RAS group i wanted to post it here

We have two internet connections, cable and DSL as a backup, we would like
to start hosting our own exchange server, the exchange server is a DC with
two nics and gateway'ing to the cable, we don't feel like hosting exchange
over cable because we cant get a static ip so we decided to use the second
nic with the DSL where we have a block of 5 usable IP's, my question is will
I have an issue if I register two ip's to the DNS considering that its a DC?
what will become the default route considering that I want to keep cable as
the default route? will exchange always route the DSL? here is an example on
how I plan do this...

NIC 1 - Currently registered with DNS
IP = - Nat'ed behind the cable router
SM =

NIC 2 - Currently not used
IP = - Nat'ed behind the DSL router
SM =
GW =

SMTP will be configured to listen to All Assigned ip's

is this ok?

Thanks for your help

Jaap de Koning


It shouldn't be a problem to have two NIC's connected to you DC, but
the configuration as you put it would most probably give you some
troubles (mainly I think the two default gateways will give you

I would configure the NIC 1 (which seems to be your internal network)
as following:

IP =
SM =
GW = empty (doesn't need a gateway because all it's traffic is

The second NIC, basicly your outside interface on DSL:

IP =
SM =
GW = (assuming that is your DSL router)
DNS = empty ( I assume your DNS server is configured to forward
internet requests)

Also you can now configure the SMTP server to listen only on the second
NIC. After this make sure you properly NAT your DSL connection so that
the mail server receives and sends mail on the same external IP address
- lots of spam protection will reject mail if the reverse DNS lookup
doesn't work.

Assuming you don't use a proxy, you can configure DHCP so that your
clients have their gateway set to (which I am assuming is
your cable router).

Hope this works for you,
Jaap de Koning

Jeniffer K

Good Morning Jaap, thanks for your reply - Our DNS server is configured to
forward all internet request to our ISP's DNS so I guess that means that all
DNS request from the entire network will go use the DSL which I'm too happy
about because we run VOIP on the DSL line and I would like to keep it as
clear as possible so now I will have exchange traffic and DNS traffic, any
workaround for the DNS - Once again thank you so much for your reply

Jaap de Koning

Well, an option you could use but which isn't such a "clean" resolution
is to add a permanent route to your routing table on the DNS server.
This way you can force IP to always use a certain path when going to a
certain address - in this case your ISP's DNS server.

if your ISP's DNS is (for example) and your cable broadband
router is, you can use the following setting:

route -p add mask

Another option would be to install DNS on a different server, which
would be the "clean" solution.

Jaap de Koning

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