Default Route question when there are two nic cards
David G. Miller
dave at davenjudy.org
Thu Oct 12 14:28:31 UTC 2006
John Austin <ja at jaa.org.uk> wrote:
>>I point people in this direction because their next question is usually,
>>> "How do I get the "other system" onto the internet?" Also, only one
>>> default gateway ends up defined in the routing table. The system does
>>> the right thing and uses the the default gateway specified for eth0 even
>>> though the gateway specified by eth1 comes "later:"
>>>
>>> Kernel IP routing table
>>> Destination Gateway Genmask Flags MSS Window irtt
>>> Iface
>>> 72.19.169.0 0.0.0.0 255.255.255.0 U 0 0 0
>>> eth0
>>> 169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0
>>> eth1
>>> 192.168.0.0 0.0.0.0 255.255.0.0 U 0 0 0
>>> eth1
>>> 0.0.0.0 72.19.169.1 0.0.0.0 UG 0 0 0
>>> eth0
>
>There is no gateway shown associated with eth1 !?
>
>So no notice has been taken of the GATEWAY=72.19.169.230
>in /etc/sysconfig/network-scripts/ifcfg-eth1
>
>I cannot see that this entry achieves anything
>
>John
>
There is a difference between a "gateway" and a route. 192.168.0.0/16
through eth1 does not need a gateway since all addresses on that subnet
are directly accessible. Likewise, the 72.19.169.0/24 subnet is
directly accessible through eth0. The default route shows up as a
gateway since addresses other than some subnet of 72.19.169.0/24 are
indirectly accessible (traffic has to go through other routers). I ran
across the following which puts all this a little more succinctly:
> Gateways are a type of router. /Routers/ connect two or more networks
> and provide the routing function. Some routers, for example, route at
> the network interface level or at the physical level. /Gateways/,
> however, route at the network level.
My approach may not work if I had multiple gateways or a more complex
network. I don't and it works quite well. I think I originally came up
with this approach after reading the O'Reilley book "Linux Network
Administration" probably when I was running RHL-5 or RHL-6.X.
Cheers,
Dave
--
Politics, n. Strife of interests masquerading as a contest of principles.
-- Ambrose Bierce
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