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-   -   default gateway outside of the LAN (http://www.linux-archive.org/centos/694772-default-gateway-outside-lan.html)

Boris Epstein 08-16-2012 10:13 PM

default gateway outside of the LAN
 
Hello all,

We have a somewhat unique setup whereby our default router is outside of
the local network.

Let us say our network is 192.168.10.0/255.255.255.0

So we have a route-eth0 file that looks something like this:

10.1.1.1 via 192.168.10.1 dev eth0
default 10.1.1.1 dev eth0

The last definition would simply not take - but it has to for the setup to
work. And no, 192.168.10.1 does not operate as a full-fledge router due to
our setup.

When we attempt to add it manually we get the following error:

RTNETLINK answers: No such process

When we use ifup we get the following:

Error: either "to" is a duplicate, or "10.1.1.1" is a garbage.

A little discussion of this can be found here:

http://www.centos.org/docs/5/html/5.1/Deployment_Guide/s1-networkscripts-static-routes.html

and here:

http://www.linuxquestions.org/questions/linux-networking-3/etc-init-d-networking-restart-errors-637610/

It does not help us, however. Any tips much appreciated.

Cheers,

Boris.
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James Hogarth 08-17-2012 06:15 AM

default gateway outside of the LAN
 
> 10.1.1.1 via 192.168.10.1 dev eth0
> default 10.1.1.1 dev eth0
>

You need to revisit your networking knowledge ... specifically what a
gateway is...

You need a router (gateway) on 192.168.10.0/24 (presumably 192.168.10.1 in
your example). On this you have a default route to the next hop of
10.1.1.1...

In terms of routing systems on 192.168.10.0/24 have no knowledge of
10.1.1.1 and no ability to influence the route other than the next hop on
the same local link network as them (192.168.10.1 in this example).
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Peter Eckel 08-17-2012 12:01 PM

default gateway outside of the LAN
 
Hi Boris,

> We have a somewhat unique setup whereby our default router is outside of
> the local network.

'unique' is a very mild expression. The word you're actually looking for was probably 'wrong'.

Your gateway *must* be in the network your interfaces are in. You also would not want your living room be built without any doors and a sign on the wall 'the exit is in the kitchen'.

Best regards,

Peter.

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James Hogarth 08-17-2012 01:50 PM

default gateway outside of the LAN
 
> 'unique' is a very mild expression. The word you're actually looking for was probably 'wrong'.
>
> Your gateway *must* be in the network your interfaces are in. You also would not want your living room be built without any doors and a sign on the wall 'the exit is in the kitchen'.

There does exist one use case that was pointed out on IRC that might
be valid ... although the OP is somewhat abstract in his original
definition so this might not apply...

In the event an ISP provides new IPs and routes them to your host but
these are outside of the pre-existing subnet it is possible to use
them with no gateway in the subnet associated with these (it's
essentially a point to point link directing traffic at an
interface)...

This somewhat describes the situation:

http://www.adminsehow.com/2011/09/gateway-on-a-different-subnet-on-linux/

However it is a rather specific use case and may not apply here...
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Gordon Messmer 08-19-2012 06:40 AM

default gateway outside of the LAN
 
On 08/17/2012 06:50 AM, James Hogarth wrote:
> This somewhat describes the situation:
> http://www.adminsehow.com/2011/09/gateway-on-a-different-subnet-on-linux/

That still describes a configuration where the gateway is in a local
broadcast domain.

It's impossible to create a route via a host that can't be resolved by
ARP (for IPv4).

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James Hogarth 08-20-2012 09:14 AM

default gateway outside of the LAN
 
>
> It's impossible to create a route via a host that can't be resolved by
> ARP (for IPv4).
>

Very true in general - the assumption in IRC was that text is not
always the best medium for transmitting ideas - especially when
someone is specifically anonymising stuff or does not have english as
a first language...

The only thing we could think of was that sort of scenario that could
possibly be valid... but without the original poster coming back and
describing more who's to say?

You don't need to resolve like that in the event of a true point to
point link where you can direct traffic down it (think serial WAN) and
the network layer type of traffic (IP or otherwise... much less
specific subnets) doesn't actually matter... but that didn't seem to
be what the OP was talking about unless in his attempt to generalise
and disguise his network layout he hid that inadvertently.

For an IP packet to be routed though it does indeed require resolving
a gateway on the local link for that to be routed through...
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