OpenWRT under hood

Posted on July 8, 2024 • 3 minutes • 543 words

Table of contents

Wireless configuration

Wireless configuration

Behind the scenes, UCI (Unified Configuration Interface) in OpenWRT uses various command-line utilities and services to apply the wireless configurations defined in /etc/config/wireless. Here are some key components and commands involved in this process:

netifd:

The netifd (network interface daemon) is responsible for managing network interfaces in OpenWRT. It handles the configuration of interfaces based on the UCI configuration files.

hostapd and wpa_supplicant:

OpenWRT typically uses hostapd for managing Access Point (AP) interfaces and wpa_supplicant for client interfaces. These services are controlled and configured by netifd based on the UCI settings.

iw and ip commands:

netifd uses iw to configure wireless devices and interfaces. It also uses ip to manage network interfaces at a lower level.

Example Workflow

When you configure an SSID in /etc/config/wireless and run wifi reload, the following steps typically occur:

netifd reads the UCI configuration:

netifd parses the /etc/config/wireless file to understand the desired configuration for wireless interfaces.

netifd interacts with hostapd or wpa_supplicant:

If you are configuring an AP, netifd will generate a hostapd configuration file based on the UCI settings and start/reload hostapd with this configuration.
For client interfaces, it will do the same with wpa_supplicant.

Use of iw and ip commands:

netifd uses iw commands to set up the physical and virtual wireless interfaces. For example, it uses iw dev to list interfaces, iw phy to configure physical wireless settings, and iw dev interface set to configure specific settings for a virtual interface.
ip commands are used to bring interfaces up or down, set IP addresses, and configure other network parameters.

Example Commands Used by `netifd`

For a given wireless interface setup, the following types of commands might be used:

Bringing up an interface:
```
ip link set dev phy0-ap2 up
```

Setting SSID and other wireless parameters (via iw):

iw dev phy0-ap2 set type __ap
iw dev phy0-ap2 set ssid YourNewSSID
iw dev phy0-ap2 set channel 6

Starting hostapd with a generated configuration:
```
hostapd /var/run/hostapd-phy0.conf
```

Example Configuration and Commands

Here is a more detailed look at how you might see these processes play out:

Configuration in /etc/config/wireless:

config wifi-device 'radio0'
    option type 'mac80211'
    option channel '6'
    option hwmode '11g'

config wifi-iface
    option device 'radio0'
    option network 'lan'
    option mode 'ap'
    option ssid 'YourSSID'
    option encryption 'psk2'
    option key 'YourPassphrase'
    option ifname 'phy0-ap2'

netifd generates hostapd configuration:

interface=phy0-ap2
driver=nl80211
ssid=YourSSID
hw_mode=g
channel=6
auth_algs=1
wpa=2
wpa_passphrase=YourPassphrase
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP

Commands executed by netifd:

ip link set dev phy0-ap2 up
iw dev phy0-ap2 set ssid YourSSID
iw dev phy0-ap2 set channel 6
hostapd /var/run/hostapd-phy0.conf

An example of real wifi config

cat /etc/config/wireless

config wifi-device 'radio0'
	option type 'mac80211'
	option path 'platform/soc/18000000.wifi'
	option channel '1'
	option band '2g'
	option htmode 'HE20'
	option disabled '0'
	option country 'US'

config wifi-iface 'default_radio0'
	option device 'radio0'
	option network 'lan'
	option mode 'ap'
	option ssid 'tuf-ax4200-2g-0-0'
	option encryption 'psk2'
	option key '12345678'
	option ifname 'vif0'

config wifi-iface 'vif1_radio0'
  option device 'radio0'
  option network 'lan'
  option mode 'ap'
	option ssid 'tuf-ax4200-2g-0-1'
	option encryption 'psk2'
	option key '12345678'
	option ifname 'vif1'

config wifi-device 'radio1'
	option type 'mac80211'
	option path 'platform/soc/18000000.wifi+1'
	option channel '36'
	option band '5g'
	option htmode 'HE80'
	option disabled '0'
	option country 'US'

config wifi-iface 'default_radio1'
	option device 'radio1'
	option network 'lan'
	option mode 'ap'
	option ssid 'tuf-ax4200-5g-1'
	option encryption 'psk2'
	option key '12345678'