How to configure iptables rules for connecting 2 eth to the net (forwarding & masquerading)

Question

need some help on configuring iptables rule for forwarding and masquerading

Given I have these configuration:

switch1 ---- (eth1) ----- router ---- (eth2) ----- switch2
                                             |
                                             |
                                         (eth0)
                                             |
                                             |
                                      internet

where multiple PCs are connected though switch1 and switch2 as well

i'm running this network on a Ubuntu Server where I assume eth1 and eth2 have the same IP address since the default gateway for all the connected PCs is the IP address of this router.

so what i did are :

go to /etc/sysctl.conf and uncomment the net.ipv4.ip_forward = 1

go to /etc/rc.local and add rules for iptables such as :

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
iptables -A FORWARD -i eth2 -o eth0 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

and for the router network's interfaces configuration, go to /etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 10.151.74.30
netmask 255.255.255.252
gateway 10.151.74.29

auto eth1
iface eth1 inet static
address 172.16.0.1
netmask 255.255.255.0

auto eth2
iface eth2 inet static
address 172.16.0.1
netmask 255.255.255.0

So the problem is that all the PCs connected to the router through eth2 can't connect to the internet (tuntap) but all the PCs connected trough the eth1 can do.

Thus, is there anything I miss on configuring the iptables? Or did I misunderstood?
Help and suggestions are really appreciated. I just started on learning comp networking a month ago, so basically need a guidance here. Thanks a lot :)

Answer 1

You can not have 2 interfaces with the same IP address. Moreover it will not work like that even if you change one interface's IP address to another address in the same network (say 172.16.0.2).

Why do you need 2 interfaces in same network? Do switches switch1 and switch2 see each other or are they separated?

If they see each other then there is definitely something wrong with your configuration.

If they don't, you can set up another network on switch2, e.g. 172.16.1.0/24, and everything should be fine.

BTW: You don't need echo 1 > /proc/sys/net/ipv4/ip_forward if you have net.ipv4.ip_forward = 1 in /etc/sysctl.conf.