Two default gateways on CentOS

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/sysconfig/network-scripts/.

/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

/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, add another routing table. In /etc/iproute2/rt_tables, add 1 rt2 at the end:

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

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

Verify the changes:

ip route show table rt2
ip rule show

To persist these changes across reboots, create a systemd service. First, create the script /usr/local/bin/default_routes.sh:

# !/bin/bash
ip route add default via 10.70.70.2 dev eth1 table int

Make it executable:

chmod +x /usr/local/bin/default_routes.sh

Create the systemd service file /etc/systemd/system/default_route.service:

[Unit]
Description=It adds default route for eth1
Wants=network-online.target
After=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/default_routes.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Two default gateways on CentOS

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