How to Add Custom Routing Rules for Multiple NICs on Linux

Besides the default route, you can configure additional routes on Linux.

For example, a server may have two interfaces (eth0 and eth1). By default, all traffic is routed through eth0 regardless of the IP address configured on eth1.

To route inbound and outbound traffic via eth1, you need to add extra routes for eth1.

eth0 is configured with IP 19.86.101.54, netmask 255.255.255.0, gateway 19.86.101.1

eth1 is configured with IP 19.86.100.176, netmask 255.255.255.0, gateway 19.86.100.1

You can view the current IP addresses with the ifconfig command:

# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:50:56:8E:0B:EC
          inet addr:19.86.101.54  Bcast:19.86.101.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3735 errors:0 dropped:0 overruns:0 frame:0
          TX packets:336 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:295679 (288.7 Kb)  TX bytes:50312 (49.1 Kb)

eth1      Link encap:Ethernet  HWaddr 00:50:56:8E:27:0D
          inet addr:19.86.100.176  Bcast:19.86.100.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:14 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:840 (840.0 b)  TX bytes:0 (0.0 b)

The netstat -rn output shows the default gateway points to eth0:

# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         19.86.101.1     0.0.0.0         UG        0 0          0 eth0
19.86.100.0     0.0.0.0         255.255.255.0   U         0 0          0 eth1
19.86.101.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0
127.0.0.0       0.0.0.0         255.0.0.0       U         0 0          0 lo
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0

To make eth1 reachable from external networks, create a new policy routing table in /etc/iproute2/rt_tables. The initial file may look like this:

# cat /etc/iproute2/rt_tables
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep
#

Show current rules with:

# ip rule show
0:      from all lookup local
32766:  from all lookup main
32767:  from all lookup default

First, back up the table file:

cd /etc/iproute2
cp rt_tables rt_tables.orig

Then add a new entry for a custom table named admin: echo "1 admin" >> /etc/iproute2/rt_tables Add routes to the admin table:

ip route add 19.86.100.0/24 dev eth1 src 19.86.100.176 table admin
ip route add default via 19.86.100.1 dev eth1 table admin

Explanation:

The first command adds the subnet 19.86.100.0/24 to the admin table with source IP 19.86.100.176 on device eth1.

The second command adds a default route via 19.86.100.1 to the admin table, ensuring all traffic defined in this table uses eth1.

Now tell the kernel to use this table for traffic to/from the eth1 address:

ip rule add from 19.86.100.176/24 table admin
ip rule add to 19.86.100.176/24 table admin
ip route flush cache

These rules place the new entries before the default main table (32766) because rules are evaluated in ascending order.

Verify the changes:

# ip rule show
0:      from all lookup local
32764:  from all to 19.86.100.176/24 lookup admin
32765:  from 19.86.100.176/24 lookup admin
32766:  from all lookup main
32767:  from all lookup default

At this point you should be able to ping 19.86.100.176 from an external network and see that traffic uses eth1 as expected.

To keep the configuration after a reboot, add the commands to /etc/init.d/boot.local (SUSE) or /etc/rc.d/rc.local (RedHat/CentOS).

If you need another IP on a different subnet, repeat the steps with a new table name (e.g., admin‑new).