routing tables on two NICs for network monitoring

[Shawn Iverson]

I have FC1 running nagios that monitors our network 24x7.  It has two NICs
installed:  one to the internal network (eth0) and another to a cable modem
via an ISP (eth1).  In case of a critical network failure after hours (i.e.
core router), I want nagios to send pages/email via eth1 to alert since it
will not be able to communicate information via the local network.

I am having trouble with the routing tables.  I have eth1 configured with
DHCP to get IP/DNS information.  NIC eth0 is static.  I need gateways set on
both so that nagios can monitor the internal network consisting of many
subnets and send alerts on eth1.

I can only have one default gateway, so how do I set up another gateway?  I
cheated and added another default gateway in rc.local for eth1, so the
routing table has two default gateways:

Destination     Gateway         Genmask         Flags Metric Ref    Use
Iface
172.16.1.0      *               255.255.255.0   U     0      0        0 eth1
10.10.0.0       *               255.255.0.0     U     0      0        0 eth0
169.254.0.0     *               255.255.0.0     U     0      0        0 eth1
127.0.0.0       *               255.0.0.0       U     0      0        0 lo
default         172.16.1.1      0.0.0.0         UG    0      0        0 eth1
default         10.10.0.254     0.0.0.0         UG    0      0        0 eth0

Well, all is well until one of the interfaces goes down or is unplugged.
Also, I have no control as to which interface traffic traverses.

I did man page reading and found that perhaps I could let eth1 have the
default gateway, set in ifcfg-eth1, and set up static network routes on eth0
perhaps as follows:

route add -net 10.0.0.0 netmask 255.0.0.0 gw 10.32.0.254 dev eth0
route add -net 192.168.0.0 netmask 255.255.0.0 gw 10.32.0.254 dev eth0

/etc/sysconfig/network-scripts/route-eth0 is my guess to add the above to
make the changes permanent

GATEWAY0=10.32.0.254
NETMASK0=255.0.0.0
ADDRESS0=10.0.0.0
GATEWAY1=10.32.0.254
NETMASK1=255.255.0.0
ADDRESS1=192.168.0.0

Would this be the best way do accomplish what I need?  If my reasoning is
correct, anything that is not bound for 10.x.x.x or 192.168.x.x will default
to eth1, so nagios will effectively send alerts via eth1 while still being
able to monitor all of our subnets via eth0.

Shawn Iverson
Technology Associate
New Castle Community School Corporation
765-593-6691
shawn at nccsc.k12.in.us