[CentOS] ip route and nexthop: the "CentOS" way

Wed Feb 1 21:06:05 UTC 2012
Les Mikesell <lesmikesell at gmail.com>

On Wed, Feb 1, 2012 at 1:35 PM, Steve Clark <sclark at netwolves.com> wrote:
>> I'm wanting to configure a CentOS 6 server to have a fall-back default route via
>> a second network interface.
>> Given:
>>   - eth0 with on subnet gateway
>>   - eth1 with on subnet gateway
>> Where eth0's network is a "back door" to the internet, and eth1's is the "front
>> door", I believe I can configure the routing table manually like this:
>>   ip route default scope global \
>>     nexthop via dev eth1 weight 1 \
>>     nexthop via dev eth0 weight 2
>> However, I've re-read the RHEL6 documents for configuring static routes here:
>> http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s1-networkscripts-static-routes.html
>> This kind of thing doesn't seem to fit into the scheme of
>> /etc/sysconfig/network-scripts/route-eth? described there, since the route isn't
>> "for" any single interface.  Is there a "RHEL/CentOS" way to do this, or do I
>> need to resort to some sort of script containing the above ip route command
>> inserted somewhere?
>> And how do I stop CentOS from trying to pick its own default gateway settings
>> (since /etc/sysconfig/network likely won't have a GATEWAY parameter)?
> Hmm...
> I just tried this and besides needing ip route "add" default
> It does not seem to work when I unplug the cable on my primary link.

I don't think CentOS is smart enough to automatically drop routes
associated with a NIC that is down like a Cisco would.  If you put
routes in /etc/sysconfig/network-scripts/routes-eth? to match the
device names, the ifup and ifdown scripts will add/remove routes when
you manually run time to enable/disable a particular NIC, but that
doesn't get you automatic failover.  And with ethernet type devices it
is pretty rare for the link to go away at the same time the packets
stop getting through anyway.

   Les Mikesell
     lesmikesell at gmail.com