You have two or more network cards (interfaces) in one Linux system and each of these interfaces has its own default gateway. By default, you can only have one default gateway on a system.

We assume that we have two interfaces:

eth0
eth1

Two networks that should be used are:

192.168.1.0/24
10.10.0.0/24

whereby the first IP address in each respective network should be the gateway. Config files for these interfaces are in

/etc/sysconfing/network-scripts/ifcfg-eth0

and

ifcfg-eth1

and it looks like this:

# cat /etc/sysconfig/network-scripts/ifcfg-eth0
TYPE=Ethernet
PROXY_METHOD=none
BROWSER_ONLY=no
BOOTPROTO=none
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=no
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
NAME=eth0
UUID=7f164bfd-1ae4-4062-aadd-e2ea8bc0121e
DEVICE=eth0
ONBOOT=yes
IPADDR=192.168.1.5
PREFIX=24
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=1.1.1.1

# cat /etc/sysconfig/network-scripts/ifcfg-eth1
TYPE=Ethernet
PROXY_METHOD=none
BROWSER_ONLY=no
BOOTPROTO=none
IPADDR=10.10.0.124
PREFIX=24
DNS1=8.8.8.8
DNS2=1.1.1.1
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=no
NAME=eth1
UUID=9320b706-e68e-3610-9fec-9988316bd478
DEVICE=eth1
ONBOOT=yes
GATEWAY=10.10.0.1
NETMASK=255.255.255.0
NM_CONTROLLED=yes

To use a second interface and address we need to add another routing table. To do this go to file:

# vim /etc/iproute2/rt_tables

and add at the end “1 rt2”:

#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep
1 rt2

Now we need to add routing rules and routes:

# ip route add default via 10.10.0.1 dev eth1 table rt2
# ip rule add from 10.10.0.0/24 table rt2

You can check these changes with commands:

# ip route show table rt2
# ip rule show

CentOS: Start custom script automatically after network startup Find the ifup-post under /etc/sysconfig/network-scripts. This scipt is called right after any network interface is brought up online. In this script, you will find the following code snippet toward the end.

if [ -x /sbin/ifup-local ]; then
    /sbin/ifup-local ${DEVICE}

In the code snippet above, if ifup-local script exists in /sbin location, then script gets executed with an interface name in argument. Usually no such ecript like ifup-local exists so in order to run a startup script automatically after a network interface is up. Create an executable script called ifup-local in /sbin and put in there any command or script you wish to run.

Here is an example:

if [[ "$1" == "eth0" ]]
then
  echo "this part will be executed right after eth0 is up."
  echo "so you can put any startup command for eth0 here"
else
  #DO_NOTHING
fi

when script is done, use command to get the script executable.

$ sudo chmod +x /sbin/ifup-local

[root@localhost ~]# cat /sbin/ifup-local
#!/bin/sh
 
#ip route add default via 10.5.23.254 dev eth1 table rt2;
#ip rule add from 10.5.23.0/24 table rt2
 
ip route add 10.5.23.0/24 dev eth1 src 10.5.23.10 table rt2;
ip route add default via 10.5.23.254 dev eth1 table rt2;
ip rule add from 10.5.23.10/32 table rt2;
ip rule add to 10.5.23.10/32 table rt2