IP Forwarding and Routing RPi 2B Jessie

Question

I am struggling to forward packets from eth0 to eth1 (and back) on my RPi. I have enabled IP forwarding by adding net.ipv4.ip_forward=1 in /etc/sysctl.conf, and putting an ip_forward file containing 1 in /proc/sys/net/ipv4/. My IP Tables are set to accept all traffic.

The network is as follows:

PC#1(10.0.0.101 /24 gateway 10.0.0.201) hardwired to RPi eth0(10.0.0.201 /24) theoretically routed to RPi eth1(192.168.1.201 /24) and then hardwired to PC#2 (192.168.1.203 /24). PC#1 has his routing table set properly to send anything destined to the 192.168.1.x LAN through his 10.0.0.101 NIC.

If I ping either the eth0 or eth1 NIC from PC#1 I get good answers, but do not get answers from PC#2. If I take a cable directly from PC#1 to PC#2, PC#2 answers the ping which tells me that PC#1's routing tables are OK, and that PC#2 is capable of answering a ping request. Additionally, if I'm sitting at the RPi, I can successfully ping PC#1 and PC#2. This only leaves the routing piece of the RPi as the bad guy. I'm not an expert at Linux and am thinking I'm missing some simple, stupid thing, but I've crawled all over the internet without success. Any help is greatly appreciated.

Doc

Answer 1

You're doing stuff wrong.

1. You should only place net.ipv4.ip_forward=1 in /etc/sysctl.conf. Do NOT modify files in the /proc folder. It's okay, just be careful.

2. You are simply accepting traffic and you only allowed forwarding. It can forward traffic but you didn't tell it to forward traffic.

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Try this: First, put net.ipv4.ip_forward=1 in /etc/sysctl.conf. Then reboot.

An alternative, if you don't want to reboot, is to run (as root) echo 1> /proc/sys/net/ipv4/ip_forward

Then, flush your entire iptables.

iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

Then, masquerade the traffic, assuming eth1 is the output. Change it accordingly.

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

# Masquerade
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

Then, save it using iptables-save > /etc/iptables/rules.v4

To restore, simply run iptables-restore < /etc/iptables/rules.v4

As an added bonus, you can add the command on /etc/network/interfaces so you don't have to manually run the restore command every boot. Modify that file so it looks like

# ...

auto eth0
iface eth0 inet static
# your configuration and stuff
post-up iptables-restore < /etc/iptables/rules.v4     # The line you need to add

# ...