9

I want to bridge the wifi interface with the ethernet interface on a Raspberry Pi so any device on the ethernet port becomes a member of the wifi network. Usually this is done by a bridge (OSI layer 2) but on wifi you need WDS (wireless distribution system) with 4addr to do it. Unfortunately WDS isn't supported by the wifi on board chip of a Raspberry Pi so we have to workaround it with proxy arp. Bridging Network Connections with Proxy ARP shows how to do it on a Debian system. But it uses old style networking that isn't supported out of the box by Raspbian.

How can I make proxy arp available on a Raspberry Pi with Raspbian?

19

Proxy arp is a way to build a pseudo bridge that is working on OSI layer 3 but it behaves like a real layer 2 bridge. So it is a good way to workaround the lack of WDS support on Raspberry Pi. There are mainly two methods to use it.

You can use a subnet overlapping with the main local network. This is only a configuration issue and does not need additional helper programs. This makes it more stable and less error prone. But it requires some more know how about subnets to configure it and to understand how it works. And it has some restrictions in using ip address ranges, but this doesn't matter in most cases. You will see.

There are also helper programs available that makes usage of ip address ranges more flexible but it is more sensible with correct configuration and stability. If ip address ranges doesn't matter I would prefer the subnetting method.

#############################################################################

PROXY ARP WITH SUBNETTING (recommended)

Example Setup

                           ┌─proxy arp─┐     UPLINK
                wired      V     ║     V      wifi             wan
    laptop <────────────> (eth0)RPi(wlan0) <~.~.~.~.> hotspot <---> INTERNET
         \                  ╱    ║     ╲
    (dhcp from   192.168.50.241  ║  (dhcp from hotspot)
       RPi)                      ║
                                 ║
subnet:   192.168.50.240/28      ║      192.168.50.0/24

As you can see, we have two subnets:

the main subnet (local WiFi network): 192.168.50.0/24
                        ip addresses: 192.168.50.1 to 192.168.50.254 = 254 ip addresses
                   broadcast address: 192.168.50.255

                    the wired subnet: 192.168.50.240/28
                        ip addresses: 192.168.50.241 to 152.168.50.254 = 14 ip addresses
                   broadcast address: 192.168.50.255

Have in mind that you cannot use the first network address and the last broadcast address of each subnet. For details on subnets look at Wikipedia - Subnetwork.

For proxy arp it is important that the smaller wired subnet is a subset of the main subnet and that it fits to the correct boundaries of possible subnets in the main subnet. I have set it to the end of the main subnet. With this example we can address 14 devices on the wired subnet. If you need more or less simply use a bigger or smaller wired subnet. To calculate the subnet I use this IP Calculator but there are some others. Use your own favorite one.

So we have this overlapping:

                0                                     |240       255
main subnet:   |N------------------------------------------------B|
wired subnet:                                         |N---------B|

Here you can see that a device on the main subnet can also broadcast for ip addresses 241 to 254. If you ensure - and that is important - that there are no devices on the main subnet having these addresses, the RasPi can use proxy arp to reply to the broadcast for an ip address instead of the device on the wired subnet, which isn't direct addressable from the main subnet. The RasPi works as proxy for the arp request. Please note that this has nothing to do with routing using ip addresses. It only works with mac addresses. That is also the reason why this only works on local area networks with one main subnet (broadcast domain).

Again, you must ensure that there are no devices on the main subnet using the ip addresses from the wired subnet. If using a DHCP server you must exclude this ip range from its address pool.

Now with this background information let's configure the RasPi. To simplify things I will use systemd-networkd.

Tested with
Raspberry Pi OS (32-bit) Lite 2020-05-27 on a Raspberry Pi 4B updated at 2020-08-19.
Updates done with sudo apt update && sudo apt full-upgrade && sudo reboot.

Switch over to systemd-networkd

Just follow to Use systemd-networkd for general networking. You can use section ♦ Quick Step. Then come back here.

Configure the WiFi client connection

Create this file for wpa_supplicant with your settings for country=, ssid= and psk=:

rpi ~$ sudo -Es    # if not already done
rpi ~# cat > /etc/wpa_supplicant/wpa_supplicant-wlan0.conf <<EOF
country=DE
ctrl_interface=DIR=/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="TestNet"
    psk="testingPassword"
}
EOF

rpi ~# chmod 600 /etc/wpa_supplicant/wpa_supplicant-wlan0.conf
rpi ~# systemctl disable wpa_supplicant.service
rpi ~# systemctl enable wpa_supplicant@wlan0.service
rpi ~# rfkill unblock 0

We have to enable promiscuous mode on wlan0 to see broadcasts on both subnets. Edit the wpa_supplicant.service with:

rpi ~# sudo systemctl edit wpa_supplicant@wlan0.service

In the empty editor insert these statements, save them and quit the editor:

[Service]
ExecStartPre=/sbin/ip link set %i promisc on
ExecStopPost=/sbin/ip link set %i promisc off

Configure interfaces

Create these two files.

rpi ~# cat > /etc/systemd/network/04-wired.network <<EOF
[Match]
Name=e*

[Network]
# Have attention to the bit mask at the end of the address
Address=192.168.50.241/28
# or just the smallest possible subnet with 2 ip addresses
#Address=192.168.50.253/30
# or the half of the main subnet with 126 ip addresses
#Address=192.168.50.129/25
DHCPServer=yes

[DHCPServer]
DNS=84.200.69.80 1.1.1.1
EOF

rpi ~# cat > /etc/systemd/network/08-wifi.network <<EOF
[Match]
Name=wl*

[Network]
DHCP=yes
IPForward=ipv4
IPv4ProxyARP=yes
EOF

Reboot.
That's it.

#############################################################################

PROXY ARP WITH HELPER PROGRAMS

In general I followed the tutorial (2). Have a look at it for the background.

Example Setup

                    ┌─proxy arp─┐     UPLINK
          wired     V           V      wifi             wan
laptop <─────────> (eth0)RPi(wlan0) <~.~.~.~.> hotspot <---> INTERNET
     \                           \
  (dhcp from hotspot)         (dhcp from hotspot)

I will present two setups. One that is only static but simpler. It is configured one time on startup and does not respect changes on its interfaces. You have to reboot then. It is good for static use cases, for example to connect a printer. The other setup is a bit more complex but it will monitor connection changes on its interfaces. It is usable on mobile RasPis that will connect to different WiFi networks and wired connections on the fly.

Tested with
Raspbian Buster Lite 2019-09-26 on a Raspberry Pi 4B updated at 2020-02-05. Updates done with sudo apt update && sudo apt full-upgrade && sudo reboot.
Here you can find the last tested revision for previous Raspbian versions.


♦ Static configuration of proxy arp

First do ♦ General Setup (look at the end).

Then follow this setup. parprouted runs as a daemon but has no systemd unit installed. So we will make it and enable also promiscous mode:

rpi ~# systemctl edit --full --force parprouted.service

In the empty editor insert these statements, save them and quit the editor:

[Unit]
Description=proxy arp routing service
Documentation=https://raspberrypi.stackexchange.com/q/88954/79866

[Service]
Type=forking
PIDFile=/run/parprouted.pid
# Restart until wlan0 gained carrier
Restart=on-failure
RestartSec=5
TimeoutStartSec=30
ExecStartPre=/lib/systemd/systemd-networkd-wait-online --interface=wlan0 --timeout=6 --quiet
ExecStartPre=/bin/echo 'systemd-networkd-wait-online: wlan0 is online'
# clone the dhcp-allocated IP to eth0 so dhcp-helper will relay for the correct subnet
ExecStartPre=/bin/bash -c '/sbin/ip addr add $(/sbin/ip -4 -br addr show wlan0 | /bin/grep -Po "\\d+\\.\\d+\\.\\d+\\.\\d+")/32 dev eth0'
ExecStartPre=/sbin/ip link set dev eth0 up
ExecStartPre=/sbin/ip link set wlan0 promisc on

#         v minus sign
ExecStart=-/usr/sbin/parprouted eth0 wlan0

ExecStopPost=/sbin/ip link set wlan0 promisc off
ExecStopPost=/sbin/ip link set dev eth0 down
ExecStopPost=/bin/bash -c '/sbin/ip addr del $(/sbin/ip -4 -br addr show eth0 | /bin/grep -Po "\\d+\\.\\d+\\.\\d+\\.\\d+")/32 dev eth0'

[Install]
WantedBy=wpa_supplicant@wlan0.service

Enable the service:

rpi ~# systemctl enable parprouted.service

Reboot.
That's it.


♦ Dynamic configuration of proxy arp with monitoring interfaces

Because we will monitor the state of the inerfaces wlan0 and eth0 we have to use a program that will report changes. For this I have made the ifplug.service.

So first Make ifplugd available again since Raspbian Stretch.

If you have tested the ifplug.service and it works,

then do ♦ General Setup.

After that configure the eth0 interface with this file:

rpi ~# cat > /etc/systemd/network/04-eth.network <<EOF
[Match]
Name=eth0
EOF

As action file for the ifplug.service use this /etc/ifplugs/ifplugs.action script:

#!/bin/bash
# redirect all output into a logfile for debug
#exec 1>> /tmp/ifplug-debug.log 2>&1

INTERFACE="$1"
EVENT="$2"
IPADDR=''

get_ipaddr () {
    if [ "$EVENT" = "down" ]; then
        IPADDR=$(/sbin/ip -4 -br addr show $INTERFACE | /bin/grep -Po "\d+\.\d+\.\d+\.\d+")
        return 0
    fi
    # check 10 times with 1 sec delay if ip address is available
    for ((i=10; i>0; i--)); do
        IPADDR=$(/sbin/ip -4 -br addr show $INTERFACE | /bin/grep -Po "\d+\.\d+\.\d+\.\d+")
        [ $? -eq 0 ] && break
        /bin/sleep 1
    done
}

case "$INTERFACE" in
eth0)
    case "$EVENT" in
    up)
        # clone ip address from wlan0 and start parprouted
        IPADDR=$(/sbin/ip -4 -br addr show wlan0 | /bin/grep -Po "\d+\.\d+\.\d+\.\d+")
        if [ -n "$IPADDR" ]; then
            /sbin/ip addr add $IPADDR/32 dev eth0
            /usr/sbin/parprouted eth0 wlan0
        fi
        ;;
    down)
        # stop parprouted
        /usr/bin/killall -q parprouted
        /sbin/ip -4 addr flush dev eth0
        ;;
    *)
        >&2 echo empty or undefined event for "$INTERFACE": \""$EVENT"\"
        exit 1
        ;;
    esac
    ;;

wlan0)
    case "$EVENT" in
    up)
        # clone ip address from wlan0 and start parprouted
        get_ipaddr
        if [ -n "$IPADDR" ]; then
            /sbin/ip addr add $IPADDR/32 dev eth0
            /sbin/ip link set wlan0 promisc on
            /usr/sbin/parprouted eth0 wlan0
        fi
        ;;
    down)
        # stop parprouted
        /usr/bin/killall -q parprouted
        /sbin/ip link set wlan0 promisc off
        /sbin/ip -4 addr flush dev eth0
        ;;
    *)
        >&2 echo empty or undefined event for "$INTERFACE": \""$EVENT"\"
        exit 1
        ;;
    esac
    ;;

*)
    >&2 echo empty or unknown interface: \""$INTERFACE"\"
    exit 1
    ;;
esac

Reboot.
That's it.

You can follow actions with:

rpi ~$ journalctl --follow | grep "ifplugd\|parprouted"

♦ General Setup

Install helpers and setup systemd-networkd

I will use systemd-networkd for reasons so first we have to switch over to it. For detailed information look at (1). Here only in short. Execute these commands:

# install helpers
rpi ~$ sudo -Es
rpi ~# apt install parprouted dhcp-helper
rpi ~# systemctl stop dhcp-helper
rpi ~# systemctl enable dhcp-helper

# deinstall classic networking
rpi ~# apt --autoremove purge ifupdown dhcpcd5 isc-dhcp-client isc-dhcp-common
rpi ~# rm -r /etc/network /etc/dhcp

# setup systemd-resolved
rpi ~# apt --autoremove purge avahi-daemon
rpi ~# apt install libnss-resolve
rpi ~# systemctl enable systemd-resolved.service
rpi ~# ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

# enable systemd-networkd
rpi ~# systemctl enable systemd-networkd.service

Configure wpa_supplicant

To configure wpa_supplicant create this file with your settings for country=, ssid= and psk=. You can just copy and paste this in one block to your command line beginning with cat and including EOF (delimiter EOF will not get part of the file):

rpi ~# cat > /etc/wpa_supplicant/wpa_supplicant-wlan0.conf <<EOF
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country=DE

network={
    ssid="TestNet"
    key_mgmt=WPA-PSK
    proto=RSN WPA
    psk="verySecretPassword"
}
EOF

rpi ~# chmod 600 /etc/wpa_supplicant/wpa_supplicant-wlan0.conf
rpi ~# systemctl disable wpa_supplicant.service
rpi ~# systemctl enable wpa_supplicant@wlan0.service

To configure the wlan0 interface create this file:

rpi ~# cat > /etc/systemd/network/08-wlan0.network <<EOF
[Match]
Name=wlan0
[Network]
IPForward=yes
DHCP=yes
# for a static ip address comment DHCP=yes
# and uncomment next 3 lines with your settings
#Address=192.168.50.2/24
#Gateway=192.168.50.1
#DNS=84.200.69.80 1.1.1.1
EOF

Setup helpers

We have installed dhcp-helper, a proxy to get ip addresses from the wifi network, and parprouted that manages proxy arp.

Enable DHCP relay in /etc/default/dhcp-helper:

# relay dhcp requests as broadcast to wlan0
DHCPHELPER_OPTS="-b wlan0"

End of General Setup. Go back.


references:
[1] Howto migrate from networking to systemd-networkd with dynamic failover
[2] Bridging Network Connections with Proxy ARP
[3] ProxyARP Subnetting HOWTO

| improve this answer | |
  • I have followed your tutorial, but I get this error in syslog when inserting a network cable "parprouted[426]: error: ioctl SIOCGIFADDR for eth0: Cannot assign requested address". What can be wrong? – The87Boy Sep 11 '18 at 8:40
  • @The87Boy I guess it is the first error message on startup I have pointed to in the last sentence of my answer. If you look with journalctl -b -e you should also find the message "systemd[1]: Started proxy arp routing service" after your error message. Distinguish it from Starting .... Your error message should not occur more than one or two times. – Ingo Sep 11 '18 at 9:08
  • It starts occuring, when I insert the network cable and continues, until I remove it again – The87Boy Sep 11 '18 at 9:32
  • Ah, ok. Haven't tested autoplug. I will look at it. Just a moment please. Try to use it first with cable plugged in. – Ingo Sep 11 '18 at 10:03
  • 1
    @SebiF But the interface wlan0 will get an ip address from the uplink router as shown in the drawing and defined in /etc/systemd/network/08-wlan0.network with DHCP=yes. So just use this ip address to connect to the RasPi. – Ingo Sep 18 '19 at 16:33
1

The above answer is great and helped me to save a lot of time with my setup, but I chose to make a few changes I thought I'll share.

The setup is this: I have a Denon amplituner that has an Ethernet jack, but no wifi; it also doesn't have Spotify. Since it accepts HDMI input, I chose to use Volumio on a Raspberry Pi 1B - audio via HDMI, Raspberry gets WiFi uplink, and Denon is connected to RPi Ethernet so I can control it from Android app.

My Denon is using static IP, so dhcp-helper is not needed, but if you use dynamic IP then just install it as described above. Also, due to appliance-like nature of Volumio I did not want to move from networking to systemd-networkd (actually I tried, made a lot of mess and had to re-burn Volumio). Also, wpa_supplicant service is not running. parprouted.service file is updated to reflect this.

What is needed:

  1. After burning Volumio image to SD card, connect Raspberry Ethernet jack to your router for initial network configuration - this is needed since it's also updating some Volumio config files, you'll just lose time if you start to configure interfaces from CLI.

  2. Update line net.ipv4.ip_forward in /etc/sysctl.conf to net.ipv4.ip_forward=1.

  3. Install parprouted: sudo apt install parprouted.

  4. Create systemd unit file - unfortunately, on current (September 2019) Volumio image systemctl edit doesn't work. Instead, run sudo touch /etc/systemd/system/parprouted.service and then use your editor of choice to put this inside:

parprouted.service

[Unit]
Description=proxy arp routing service
Documentation=https://raspberrypi.stackexchange.com/q/88954/79866

[Service]
Type=forking
# Restart until wlan0 gained carrier
Restart=on-failure
RestartSec=5
TimeoutStartSec=30
ExecStartPre=/usr/bin/perl -e 'sleep 1 until -e "/sys/class/net/wlan0"'
ExecStartPre=/bin/echo 'parprouted: wlan0 is online'
# clone the dhcp-allocated IP to eth0 so dhcp-helper will relay for the correct subnet
ExecStartPre=/bin/bash -c '/sbin/ip addr add $(/sbin/ip -4 addr show wlan0 | /bin/grep -Po "\\d+\\.\\d+\\.\\d+\\.\\d+\/")32 dev eth0'
ExecStartPre=/sbin/ip link set dev eth0 up
ExecStartPre=/sbin/ip link set wlan0 promisc on

#         v minus sign
ExecStart=-/usr/sbin/parprouted eth0 wlan0

ExecStopPost=/sbin/ip link set wlan0 promisc off
ExecStopPost=/sbin/ip link set dev eth0 down
ExecStopPost=/bin/bash -c '/sbin/ip addr del $(/sbin/ip -4 addr show eth0 | /bin/grep -c1 -Po "\\d+\\.\\d+\\.\\d+\\.\\d+")/32 dev eth0'

[Install]
WantedBy=multi-user.target
  1. Finish by running sudo systemctl enable parprouted.
| improve this answer | |

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