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Toradex’s Linux Images - How to setup the Wi-Fi interface for Access Point Mode

 

Article updated at 14 Sep 2020
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Introduction

This article describes how to set up a Wi-Fi Access Point, sometimes mentioned as AP mode, using open-source software Hostapd in Embedded Linux.

Other Wi-Fi related articles

If this is not the information you are looking for, please, refer to the Wi-Fi Connectivity with Toradex’s Computer on Module (CoM) article for more Wi-Fi related information.

Hostapd Example

Toradex provides a package in the BSP demo images called hostapd-example which provides basically what is written in this article.

To start the AP configured in that demo image on a Toradex demo image once, enter:

systemctl start hostapd-example

If the AP should be started upon boot, enter:

systemctl enable hostapd-example

You can find the related openembedded recipe for this package in:

http://git.toradex.com/cgit/meta-toradex-demos.git/tree/recipes-connectivity/hostapd-example

Prerequisites

  • A Toradex SoM with on Board Wi-Fi or
  • An external Wi-Fi adapter with support for Access Point Mode

How to find out if my external Wi-Fi adapter supports Access Point mode

You can check a list of tested modules in the 3rd Party Modules Tested with Toradex COMs section of Wi-Fi Connectivity on Toradex’s Computer on Module (CoM) article.

This setup was tested using the following hardware and software

  • Embedded Linux BSP release 2.8
  • Colibri iMX6ULL 512MB WB IT V1.1A.

Instructions may vary when using other Wi-Fi adapters.

Configure AP Mode

Steps are provided for configuring AP mode.

Get the Correct Interface Name

Check current mode and available interfaces, you should see a single interface in AP mode, like this:

$ iw dev
phy#0
        Interface <IF_NAME>
                ifindex 5
                wdev 0x2
                addr d0:c5:d3:33:cd:31
                type **AP**
                txpower 0.00 dBm

Considering the test environment from Prerequisites section, the name of the AP-interface that is defined by the mwifiex kernel driver is uap0. This name can be different depending on userspace.

Configure Hostapd

Set the Access Point interface IP and SSID in Hostapd configuration file. Change the following variables in /etc/hostapd.conf:

/etc/hostapd.conf
interface=uap0 ssid=access-point hw_mode=g channel=1 own_ip_addr=192.168.8.1

An alternate hostapd.conf file that creates a WLan in the 5GHz range, protected with WPA2:

/etc/hostapd.conf
interface=uap0 ssid=testwifi hw_mode=a channel=40 ieee80211n=1 own_ip_addr=192.168.8.1 wpa=2 wpa_passphrase=MyNotSoSecretPassword1234

The hostapd.conf is documented in the default hostapd.conf file that is deployed, as well as here: https://w1.fi/cgit/hostap/plain/hostapd/hostapd.conf

To provide the Access Point interface with a static IP address and DHCP, it is necessary to create the file /etc/systemd/network/hostapd.network:

/etc/systemd/network/hostapd.network
[Match] Name=<IF_NAME> [Network] Address=192.168.8.1/24 DHCPServer=yes

The Hostapd service starts by default after the network.target. This target is only loosely defined and means just "start after the network stack is up". This will fail by default because the network device (uap0 by default) is not up yet, or not every time.

To fix this we can tell systemd that the hostapd.service file is depending on a device. First step is to find out how the "*.device" file is named that is associated with the network device. This can be done with:

systemctl --all --full -t device

In our demo-image case this is named sys-subsystem-net-devices-uap0.device Edit the properties BindsTo and After from file /lib/systemd/system/hostapd.service:

/lib/systemd/system/hostapd.service
BindsTo=sys-subsystem-net-devices-uap0.device After=sys-subsystem-net-devices-uap0.device

Both BindsTo and After are necessary in order to tell systemd the strong dependence that hostapd.service has on our case uap0.

Reload the systemd configuration:

systemctl --system daemon-reload

Enble Hostapd

At last, activate the following services:

$ connmanctl enable wifi
$ systemctl enable hostapd
$ systemctl start hostapd

Check the Access Point interface:

$ ip a s dev <IF_NAME>

Now you can connect to this Access Point from other Wi-Fi devices. Access Point will automatically start after reboot.

Automate Enabling Wifi

To make sure wifi is enabled at every boot you can also do a service for it. Create a service for example like this one:

/etc/systemd/system/enable-wifi.service
[Unit] Description=Enable wifi BindsTo=sys-subsystem-net-devices-uap0.device After=sys-subsystem-net-devices-uap0.device [Service] Type=oneshot ExecStart=/usr/bin/connmanctl enable wifi ExecStop= RemainAfterExit=yes [Install] WantedBy=multi-user.target

The hostapd.service can then be started After this, so change After= of hostapd.service to:

/lib/systemd/system/hostapd.service
After=enable-wifi.service

Don't forget to also enable this new service:

systemctl daemon-reload
systemctl enable enable-wifi.service

Configure Internet Sharing

Once AP Mode is configured, it's a common scenario to share an internet connection from another interface. This section relies on the Internet Sharing article from the Arch Wiki.

Enable Packet Forwarding

Create a file to add the packet forwarding rules:

/etc/sysctl.d/30-ipforward.conf
net.ipv4.ip_forward=1 net.ipv6.conf.default.forwarding=1 net.ipv6.conf.all.forwarding=1

The changes will take effect after a reboot.

Enable NAT

NAT relies on kernel configuration and userspace tools. This section is thus split into two.

Kernel Configuration

For the example from this article, we have to make sure that the following kernel parameters are enabled. You have to evaluate if additional parameters are required for your use case:

CONFIG_IP_NF_IPTABLES=y
CONFIG_NETFILTER_XT_MATCH_CONNTRACK=y

You will find those configs in Networking support > Networking options > Network packet filtering framework (Netfilter). See an illustration (you may click it to see in higher resolution):


  • Core Netfilter Configuration

    Core Netfilter Configuration Menu

It may be possible to compile those configs as modules - check the kernel documentation. For instructions on how to build the kernel or modules, read the article Build U-Boot and Linux Kernel from Source Code.

After the new kernel is deployed, you can verify if it has the correct configs:

zcat /proc/config.gz | grep IP_NF_IPTABLES
zcat /proc/config.gz | grep NETFILTER_XT_MATCH_CONNTRACK

iptables Configuration and Setup

You might have to enable some kernel modules, depending on your configuration.

You can use iptables to enable NAT and a systemd service to make it start on reboot. First, enable the rules for the current session:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i uap0 -o eth0 -j ACCEPT

Save the current config to /etc/iptables/iptables.rules:

mkdir /etc/iptables/
iptables-save > /etc/iptables/iptables.rules

Create a systemd service file iptables.service. The example is modified from the iptables package from Arch Linux:

/lib/systemd/system/iptables.service
[Unit] Description=IPv4 Packet Filtering Framework Before=network-pre.target Wants=network-pre.target [Service] Type=oneshot ExecStart=/usr/sbin/iptables-restore /etc/iptables/iptables.rules ExecReload=/usr/sbin/iptables-restore /etc/iptables/iptables.rules RemainAfterExit=yes [Install] WantedBy=multi-user.target

Notice that in the example above we don't have the rule to stop the service. Carefully evaluate if you need it and the best method for stopping the service, where a good starting point is the iptables package aforementioned.

Reload the systemd services and enable:

systemctl --system daemon-reload
systemctl enable iptables

Enable DHCP and DNS

Some utils can enable both DHCP and DNS servers. You might want to use dhcpd or dnsmasq.