Packet Forwarding Interfaces (Linux/Raspberry Pi)

Enable IP forwarding by setting net.ipv4.ip_forward=1 in /etc/sysctl.conf and apply it with sysctl -p. Then, configure iptables to masquerade traffic from the sensor's Ethernet interface so packets are correctly routed through the Wi-Fi access point.

Understanding the Forwarding Requirement

When a Raspberry Pi acts as an access point, it typically bridges or routes wireless clients to the internet. However, devices connected via the Ethernet port are often treated as a separate network segment. If the Pi does not forward packets between the Ethernet interface (usually eth0) and the wireless interface (wlan0), the sensor cannot reach external networks or other devices on the Wi-Fi network. This is a common issue because Linux disables IP forwarding by default for security reasons. Enabling this feature tells the kernel to pass traffic between interfaces rather than dropping it.

Enabling IP Forwarding

The first step is to ensure the Linux kernel allows packet forwarding. You can check the current status by running:

cat /proc/sys/net/ipv4/ip_forward

If the output is 0, forwarding is disabled. To enable it temporarily, use:

sudo sysctl -w net.ipv4.ip_forward=1

This change persists only until the next reboot. To make it permanent, edit the configuration file:

sudo nano /etc/sysctl.conf

Find the line that says #net.ipv4.ip_forward=1 and remove the hash symbol to uncomment it. Save the file and apply the changes immediately with:

sudo sysctl -p

Verify the change again using cat /proc/sys/net/ipv4/ip_forward; it should now return 1. For more details on managing system settings, see our guide on Shutting Down Multiple Raspberry Pi Linux Systems.

Configuring NAT with iptables

Even with forwarding enabled, the Pi must perform Network Address Translation (NAT) to allow the sensor to communicate with the outside world. The sensor likely has a private IP address (e.g., 192.168.x.x) that is not routable on the public internet. You need to masquerade traffic leaving the Pi. Identify your outgoing interface. In most AP setups, this is wlan0. Your incoming interface for the sensor is likely eth0. Run the following command to set up masquerading:

sudo iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

This command tells the kernel to replace the source IP of packets coming from eth0 with the Pi’s own IP address when they exit via wlan0. You also need to ensure that the FORWARD chain allows traffic between these interfaces:

sudo iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT
sudo iptables -A FORWARD -i wlan0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT

To make these firewall rules persistent across reboots, you can install iptables-persistent:

sudo apt install iptables-persistent
sudo netfilter-persistent save

If you encounter issues with outbound connections after making these changes, refer to our article on Raspberry Pi: Outbound Connections Blocked.

Verifying Connectivity

Once configured, test the connection from the sensor. If the sensor supports ping, try pinging a public DNS server like 8.8.8.8. On the Raspberry Pi, you can monitor the traffic to ensure packets are being forwarded:

sudo tcpdump -i eth0

You should see packets arriving from the sensor. If they disappear without response, check the routing table on the sensor to ensure its gateway is set to the Pi’s Ethernet IP address. Also, verify that no local firewall on the Pi is blocking the traffic by checking iptables -L -n -v. For scenarios involving multiple internet sources or complex routing, consider reading about Configuring Routing for Multiple Internet Access on RPI4.

Frequently Asked Questions

Why can my Wi-Fi clients connect but the Ethernet device cannot?

Wi-Fi clients are usually handled by the hostapd service which manages bridging or routing automatically. The Ethernet port is often left unconfigured or isolated unless explicitly set up with iptables rules. You must manually enable forwarding and NAT for wired devices if they are not part of the same bridge as the Wi-Fi interface.

How do I find the correct interface names for iptables?

Use the command ip addr show to list all network interfaces. Look for the one with the IP address assigned to the Ethernet cable; it is typically named eth0 or usb0 depending on your setup. Similarly, identify your wireless interface, which is usually wlan0.

What if the sensor still cannot reach the internet after enabling forwarding?

Check the sensor’s subnet mask and gateway settings. The gateway must be the IP address of the Raspberry Pi on the Ethernet network. Additionally, ensure that the Pi’s DNS resolver is functioning correctly, as failing DNS resolution will appear as a connectivity issue.

Can I use nftables instead of iptables?

Yes, newer versions of Debian and Raspberry Pi OS support nftables. However, iptables remains the most documented and widely supported method for simple NAT configurations on older systems. If you prefer nftables, consult the official documentation for syntax differences, as the commands differ significantly from iptables.

Does this setup work for IPv6?

The steps above only cover IPv4. For IPv6, you would need to enable net.ipv6.conf.all.forwarding and configure ip6tables similarly. Ensure your router advertisement daemon is properly configured to announce the Pi as a default gateway for IPv6 on the Ethernet segment.