Using your Raspberry Pi Zero’s USB wifi adapter as both Wifi client and access point

The Raspberry Pi Zero captivates with its small dimensions. This comes at a cost, however, with only one micro USB port available for peripherals of any kind. In this scenario you’ll probably think twice about what you connect to that port. “A USB hub” may sound like a natural choice but if you’re like me, you’ll want to carry the gadget around a bit and minimize the number of accessories.

Now there are solutions to stack a USB hub onto the Pi Zero, eg. Circuitbeard’s one or Richard Hawthorn’s one, but actually I don’t want to carry around a USB keyboard, especially if I have no HDMI-capable display around at all times. Instead I want to login onto the Pi via Wifi while still having Internet connectivity even when not at home. Thus I want the Pi to be an access point AND maintain a Wifi client connection at the same time. This is rather easy to do with two USB wifi adapters — but with the Pi Zero we’ll have to do with a single one!

Turns out that there are two obstacles to achieve this goal:

  1. finding the right hardware
  2. getting the configuration right

Hardware issues

Running both an access point and a client configuration on the same network adapter requires support for so-called virtual interfaces. Whether a certain Wifi hardware and driver combo supports virtual interfaces and in which configurations can be found out by looking at iw list output. The following examples were not conducted on the Pi itself but on my Notebook with openSUSE Tumbleweed and Linux kernel 4.4.

Here’s the relevant iw list output for the iwlwifi driver and the Intel Wireless 7265 adapter built into my Notebook (PCI vendor ID 8086, PCI product ID 095a):

[...]
        valid interface combinations:
                 * #{ managed } < = 1, #{ AP, P2P-client, P2P-GO } <= 1, #{ P2P-device } <= 1,
                   total <= 3, #channels <= 2
[...]

Here's by contrast the output of the rtl8192cu driver and a RealTek RTL8188CUS-based USB wifi adapter, the Edimax EW-7811Un (USB vendor ID 7392, USB product ID 7811):

[...]
       interface combinations are not supported
[...]

As you can see, this driver does not support any virtual interfaces at all. The linux-wireless wiki says that it will be replaced by rtl8xxxu.

Here’s another example for the rt2800usb driver and the Ralink RT5370-based USB wifi adapter shipped in thepihut.com’s Mega Zero 8GB WiFi Pack and also available on its own (USB vendor id 148f, USB product ID 5370, recognized as “Ralink Technology, Corp. RT5370 Wireless Adapter”):

[...]
        valid interface combinations:
                 * #{ AP, mesh point } < = 8,
                   total <= 8, #channels <= 1
[...]

To correctly understand this output, compare it to the iwlwifi output above: because managed is not explicitly mentioned here, virtual interfaces are only supported in access point mode, ie. you could create two APs with different security settings, but that's not what we're looking for here.

Doing a bit of research on the Web, you'll quickly learn that what we're looking for is an adapter with an Atheros chipset. But, as I outlined earlier, these are actually rather hard to come by. I eventually got a FRITZ!WLAN USB Stick N (carl9170 driver) via eBay. So far I didn't run into any lockup issues but then again I haven't done much testing yet.

Configuration

There are a number of guides on the Web but they all are either targeted at setups with two USB sticks or describe unnecessarily intricate configurations.

Key to our desired setup is having two network interfaces, one that we can run hostapd on and one that we can run wpa_supplicant on. Because the Atheros driver requires us to create access point interfaces before we create ordinary client interfaces, we need to delete the automatically created wlan0 interface first. And, by contrast to eg. the rt2800usb driver, all added virtual interfaces will by default share the same MAC address which would cause problems, so we need to set a different MAC address on the second interface.

Some guides suggest doing the following steps in the pre step of /etc/network/interfaces but that would require ifuping the interfaces in a certain order for the reasons described above which is hard to achieve. Also it's such a low-level thing that I believe it to be better placed upon udev.

Insert the following into /etc/udev/rules.d/70-persistent-net.rules:

SUBSYSTEM=="ieee80211", ACTION=="add|change", ATTR{macaddress}=="00:11:22:33:44:55", KERNEL=="phy0", \
    RUN+="/sbin/iw dev wlan0 del", \
    RUN+="/sbin/iw phy phy0 interface add ap0 type __ap", \
    RUN+="/sbin/iw phy phy0 interface add sta0 type station", \
    RUN+="/bin/ip link set sta0 address 00:11:22:33:44:56"

where you replace 00:11:22:33:44:55 with the mac address of your Wifi adapter as shown on the out-of-the-box interface wlan0. Also replace 00:11:22:33:44:56 with the same mac address but with some characters modified. You can pick almost any mac address here as long as it is not otherwise used on your local network. Again, sta0 must not use the same mac address as ap0!

Raspbian’s udev will add its own rule to the file similar to this one:

SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:11:22:33:44:55", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="sta*", NAME="sta0"

This rule is generated by /lib/udev/write_net_rules and while I don’t know how to keep it from doing so, it doesn’t really hurt unless you try to ifup interfaces yourself, then you might get a client interface named as sta1 and things won’t work. So remember this rule if you have to debug things and try deleting it if it causes trouble.

Now for the actual IP setup in /etc/network/interfaces:

auto lo
iface lo inet loopback

allow-hotplug ap0
iface ap0 inet static
    address 192.168.200.1
    netmask 255.255.255.0
    hostapd /etc/hostapd/hostapd.conf

allow-hotplug sta0
iface sta0 inet dhcp
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

This takes advantage of the hostapd and wpa-conf directives provided through hostapd’s /etc/network/if-pre-up.d/hostapd and wpa_supplicant’s /etc/network/if-pre-up.d/wpasupplicant and /etc/network/if-up.d/wpasupplicant helpers. This is much nicer than doing stuff such as starting wpa_supplicant yourself.

Now we will do need the two referenced configuration files, of course. Here’s a minimal sample /etc/hostapd/hostapd.conf that you should tweak for your own requirements:

ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
interface=ap0
ssid2="Pi Zero"
country_code=DE # Change this (see the crda(8) and regulatory.bin(5) man pages)
ieee80211d=1
ieee80211h=1
ieee80211n=1
hw_mode=g
channel=1 # Change this to a low-traffic channel in your neighbourhood
macaddr_acl=0
wmm_enabled=1
wpa=1
wpa_passphrase=yoursecret # Change this
wpa_key_mgmt=WPA-PSK
wpa_pairwise=CCMP

Likewise edit /etc/wpa_supplicant/wpa_supplicant.conf, adding information for each Wifi network you might want to visit with the Pi zero:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

# Repeat this section for other Wifi networks
network={
  ssid="MyNetwork" # Change this
  psk="ItsSecret" # Change this
}

When you reboot, you might notice that the boot sequence appears to be hanging for a bit because dhcpcd appears to be waiting for IP address assignment. Running raspi-config, select Wait for Network at boot and Fast Boot without waiting for network connection will fix this. Since dhcpcd will continue running in the background, it is likely to have assigned sta0 an IP address by the time you’re logging onto the Pi through ap0.

Certainly you will be likely to connect to your Pi Zero over Wifi using NetworkManager. In that case, to keep things simple I’d recommend to edit the connection profile created for your configured SSID to use a fixed IP address. That way you don’t have to run a DHCP sever on the Pi.

Of course, if you’d like the Pi zero to act as a router you’ll really need one, eg. dnsmasq. And you’ll need to setup firewall rules accordingly (eg. masquerading). Don’t forget to enable IP routing by editing /etc/sysctl.conf:

[...]
net.ipv4.ip_forward=1
[...]

Happy hacking!

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