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Tony Stark
Asked: 2017-11-25 03:49:33 +0800 CST

How do I set up routing for a second network interface in the same subnet?

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I have two EC2 instances on AWS. I attach a second network interface to one of the EC2 instances and configure Redhat to use the new interface and IP.

The problem is that I can't ping the EC2 instance over the second NIC.

I was reading that asymmetric routing has to be prevented for this but I did not manage to this correctly. My steps were as follows.

1) Setup the new NIC because it does not get the new IPv4 automatically.

cd /etc/sysconfig/network-scripts/
cat ifcfg-eth0 > ifcfg-eth1

The eth1 config looks as follows.

BOOTPROTO=dhcp
DEVICE=eth1
HWADDR=02:d9:f6:0e:09:00
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
IPADDR=192.168.125.232

ifdown eth1
ifup eth1

ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9001
        inet 192.168.125.247  netmask 255.255.255.224  broadcast 192.168.125.255
        inet6 fe80::3d:5cff:fef4:f5a8  prefixlen 64  scopeid 0x20<link>
        ether 02:3d:5c:f4:f5:a8  txqueuelen 1000  (Ethernet)


eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9001
        inet 192.168.125.232  netmask 255.255.255.224  broadcast 192.168.125.255
        inet6 fe80::d9:f6ff:fe0e:900  prefixlen 64  scopeid 0x20<link>
        ether 02:d9:f6:0e:09:00  txqueuelen 1000  (Ethernet)

[...]

route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.125.225 0.0.0.0         UG    100    0        0 eth0
0.0.0.0         192.168.125.225 0.0.0.0         UG    101    0        0 eth1
192.168.125.0   0.0.0.0         255.255.255.0   U     100    0        0 eth1
192.168.125.224 0.0.0.0         255.255.255.224 U     100    0        0 eth0
192.168.125.224 0.0.0.0         255.255.255.224 U     101    0        0 eth1

2) Trying to setup asymmetric routing like this.

ip route add default via 192.168.125.225 dev eth0 tab 1
ip route add default via 192.168.125.225 dev eth1 tab 2

ip rule add from 192.168.125.247/24 tab 1

As soon as I run the above command I get disconnected from the instance and I can't reconnect via SSH. After this I just re-create the instance.

I never manage to run the last command, which I think is needed.

ip rule add from 192.168.125.232/24 tab 2

What am I missing? How do I set up the routing correctly for my setup?

EDIT #1: New try with new IP addresses won't work.

ip route add 192.168.125.224/27 dev eth0 table t1
ip route add 192.168.125.224/27 dev eth1 table t2
ip route add default via 192.168.125.225 dev eth0 table t1
ip route add default via 192.168.125.225 dev eth1 table t2

ip rule add from 192.168.125.243/27 table t1 priority 100
ip rule add from 192.168.125.232/27 table t2 priority 200
ip route flush cache

The workaround to disable source/destination check works but is not really something I want.

EDIT #2: After even more googling and pulling hair out of my head. Another try (not working :/).

ip route add default via 192.168.125.225 dev eth0 table t1
ip route add default via 192.168.125.225 dev eth1 table t2

ip rule add from 192.168.125.243/32 table t1 priority 100
ip rule add from 192.168.125.232/32 table t2 priority 200
ip route flush cache
networking

1 Answers

  1. Best Answer

    The answer is to configure a route and a rule for both interfaces.

    echo 100 t1 >> /etc/iproute2/rt_tables
echo 101 t2 >> /etc/iproute2/rt_tables

ip route add default via 192.168.125.225 dev eth0 table t1
ip rule add from 192.168.125.243/32 table t1 priority 100

ip rule add from 192.168.125.232/32 table t2 priority 200
ip route add default via 192.168.125.225 dev eth1 table t2

    The ip commands can be added to /etc/rc.d/rc.local for persistent settings.

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