Bridging Wireless & Ethernet networks with a Raspberry Pi

We recently acquired a second hand Dell laser printer, which was timely: our inkjet printer is a little long in the tooth, and is starting to require more frequent head cleaning and nozzle alignment, both of which use that liquid gold we call “ink”.

Unfortunately, the laser printer only supported ethernet out of the box, and while a wireless adapter was available, it was retailing for over $150. Fortunately, however, I had a Raspberry Pi lying around, that I was able to turn into a working solution.

There are plenty of posts out there talking about bridging networks with Raspberry Pi and Linux, but most of them seemed to either assume I care about systemd (I don’t), want to deal with static IP addresses (I don’t), or want to bridge two ethernet networks (I don’t). So here’s what worked for me.

There were two inflection points in getting this to work: the first was understanding that I don’t really want a Bridge: I want an ARP¹ Proxy. What’s the difference? Simply put: a Proxy can inspect and modify traffic going through it, while a Bridge does not. Why do we care about that? Because wireless access points are fickle creatures, and will often discard traffic originating from a host they don’t know about. Additionally, there are restrictions built into the bridge utilities that prevent you from doing a direct bridge from ethernet to wireless. By running a proxy, we’re able to make the bridged traffic (from our printer) appear as if it’s originating from the Raspberry Pi, which the access point definitely knows about.

I knew that DHCP would be involved if I didn’t want to deal with static IP addresses. The second inflection point was realizing that in order to make that work, and to make it transparent-ish from either side of the bridge, I needed to make sure the ethernet (eth0) and wireless (wlan0) ports had the same IP address. More on how in a moment.

Here are the steps I followed, largely based on the Debian wiki:

Configure your Raspberry Pi with Raspbian and connect it to your wireless network. Getting this out of the way first will configure WPA supplicant, which is responsible for managing the wireless authentication and pre-shared keys. If you’re interested, /etc/wpa_supplicant/wpa_supplicant.conf will hold your configuration after you’re connected.
Install the proxy daemon and helpers we’re going to use: parprouted and dhcp-helper

$ sudo apt-get install parprouted dhcp-helper

Configure dhcp-helper to broadcast on wlan0
Add the following contents to /etc/default/dhcp-helper.
```
# relay dhcp requests as broadcast to wlan0
DHCPHELPER_OPTS="-b wlan0"
```
Configure mDNS: when your printer magically appears in the Add Printer list, that’s mDNS at work. And we definitely want it to just show up, otherwise we’ll be back in static IP land. To do this, we need to reflect messages from one network to the other. Luckily, Avahi, the Linux mDNS implementation installed with Raspbian, makes this a one-line change.
Enable the Reflector in /etc/avahi/avahi-daemon.conf.
```
[reflector]
enable-reflector=yes
```
Configure your network interfaces
While we’re most interested in our ethernet (eth0) and wireless (wlan0) interfaces, there are actually three interfaces for us to configure: loopback, ethernet, and wireless. I created a file for each in the /etc/network/interfaces.d/ directory.
lo
```
auto lo
iface lo inet loopback</code></pre>
```
eth0
```
auto eth0
allow-hotplug eth0
iface eth0 inet manual
```
wlan0
This is where the magic actually happens. I mentioned before that we need to bind the same IP address to both network interfaces. We also need to restart the DHCP Helper and proxy. We wire this up with post-up and post-down clauses in our wlan0 configuration: every post-up line is executed after the network interface comes up, and every post-down line is executed after it goes, ahem, down.
```
auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
  post-up /sbin/ip addr add $(/sbin/ip addr show wlan0 | perl -wne 'm|^\s+inet (.*)/| && print $1')/32 dev eth0
  post-up /sbin/ifup eth0
  post-up /usr/sbin/parprouted wlan0 eth0
  post-up /etc/init.d/dhcp-helper start
  post-down /usr/bin/killall /usr/sbin/parprouted
  post-down /etc/init.d/dhcp-helper stop
  post-down /sbin/ifdown eth0
```
Restart.
At this point your printer should be able to obtain an IP address using DHCP from your wireless network, and the printer should show up to your wireless clients.

Address Resolution Protocol, which is used to map IP addresses to MAC addresses. Yeah, I forget it exists, too. ↩︎