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--- linux:two_default_gateways [2021/04/08 14:44]
admin created
+++ linux:two_default_gateways [2021/04/08 15:17] (current)
admin
@@ Line 7: / Line 7: @@
 We assume that we have two interfaces:
-<color #00a2e8>eth0
-eth1</color>
+<code bash>
+eth0
+eth1
+</code>
 Two networks that should be used are:
 <code bash>
 .168.1.0/24
 .10.0.0/24
 </code>
@@ Line 28: / Line 31: @@
 ifcfg-eth1
 </code>
 and it looks like this:
@@ Line 111: / Line 115: @@
 # ip rule show
 </code>
+==== 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.
+<code bash>
+if [ -x /sbin/ifup-local ]; then
+    /sbin/ifup-local ${DEVICE}
+</code>
+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:
+<code bash>
+if [[ "$1" == "eth0" ]]
+then
+  echo "this part will be executed right after eth0 is up."
+  echo "so you can put any startup command for eth0 here"
+else
+  #DO_NOTHING
+fi
+</code>
+when script is done, use command to get the script executable.
+<code bash>
+$ sudo chmod +x /sbin/ifup-local
+</code>
+<code bash>
+[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
+</code>
+===== Two default gateways on Debian =====
+==== Initial Position ====
+We will assume that we have two interfaces: eth0 and eth1. The two networks that should be used are 192.168.0.0/24 and 10.10.0.0/24, whereby the first IP address in each respective network should be the gateway. Under Debian, the initial configuration would appear as follows. /etc/network/interfaces
+# This file describes the network interfaces available on your system
+# and how to activate them. For more information, see interfaces(5).
+# The loopback network interface
+auto lo
+iface lo inet loopback
+# The primary network interface
+<code bash>
+allow-hotplug eth0
+iface eth0 inet static
+    address 192.168.0.10
+    netmask 255.255.255.0
+    gateway 192.168.0.1
+</code>
+# The secondary network interface
+<code bash>
+allow-hotplug eth1
+iface eth1 inet static
+    address 10.10.0.10
+    netmask 255.255.255.0
+</code>
+Adding a Second Routing Table
+To add a new routing table, the file, /etc/iproute2/rt_tables must be edited. We will call the routing table “rt2” and set its preference to 1. The named file should then appear as follows.
+<code bash>
+#
+# reserved values
+#
+     local
+     main
+     default
+       unspec
+#
+# local
+#
+#1      inr.ruhep
+rt2
+</code>
+==== Configuring the New Routing Table ====
+From this point, four commands are needed to achieve our goal. First, the new routing table needs to be populated, which is done using the following command.
+<code bash>
+ip route add 10.10.0.0/24 dev eth1 src 10.10.0.10 table rt2
+ip route add default via 10.10.0.1 dev eth1 table rt2
+</code>
+The first command says that the network, 10.10.0.0/24, can be reached through the eth1 interface. The second command sets the default gateway.
+Routing Rules
+So that the system knows when to use our new routing table, two rules must be configured.
+<code bash>
+ip rule add from 10.10.0.10/32 table rt2
+ip rule add to 10.10.0.10/32 table rt2
+</code>
+These rules say that both traffic from the IP address, 10.10.0.10, as well as traffic directed to or through this IP address, should use the rt2 routing table.
+Making the Configuration permanent
+The ip rule and ip route commands will become invalid after a re-boot, for which reason they should become part of a script (for example, /etc/rc.local) that will be executed once the network has been started after booting. For Debian, these command can also be written directly into the /etc/network/interfaces file, which would then appear as follows.
+<code bash>
+iface eth1 inet static
+    address 10.10.0.10
+    netmask 255.255.255.0
+    post-up ip route add 10.10.0.0/24 dev eth1 src 10.10.0.10 table rt2
+    post-up ip route add default via 10.10.0.1 dev eth1 table rt2
+    post-up ip rule add from 10.10.0.10/32 table rt2
+    post-up ip rule add to 10.10.0.10/32 table rt2
+</code>
+==== More than Two Network Cards or Gateways ====
+If there are more than two networks, a routing table can be created for each additional network analogous to the example presented above.
+Testing the Configuration
+The following commands can be used to ensure that the rules as well as the routing entries are working as expected.
+<code bash>
+ip route list table rt2
+ip rule show
+</code>

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