+++
title = "Access Point on a Raspberry Pi"
date = 2020-09-26
tags = ["admin"]
+++
The Raspberry Pi family who have wireless capabilities can all serve as wifi
Access Points - meaning some other devices connect to it, perhaps accessing
internet through it. We'll leave the "forward internet" part for another
time, and concentrate on serving wifi connections as a hotspot.

If what you need is a "rescue" Access Point to be able to log back into the
machine, you might rather check my other article on [RPi basics][arch-on-rpi],
where you will see how to let `wpa_supplicant` fall back to being an Access
Point if it can't connect to any.

And for now let's use `hostapd`, a package to implement and advertise a wifi
access point. It might be a better option for production use since it's a
dedicated tool, whereas `wpa_supplicant` is a bit out of its league here.

`hostapd` needs the interface to be up and configured with an IP.  So we'll
configure `systemd-networkd` to do that, and by the way propose an IP address to
clients that get connected to our AP. Change
`/etc/systemd/network/05-wlan.network` so that it reads :
``` systemd
[Network]
Address=192.168.1.1
DHCPServer=true
IPMasquerade=true
IPForward=true

[DHCPServer]
PoolOffset=100
PoolSize=20
EmitDNS=true
```
Now `hostapd` won't work (at least in this configuration) if you have
`wpa_supplicant` installed. Go on and 
```
# pacman -Rs wpa_supplicant
```
And last, you will need to tweak `/etc/hostapd/hostapd.conf`. This one was taken
from [Archwiki's][aw-ap], and curated for RPi zero W :
```
interface=wlan0
#bridge=br0
# Country code (ISO/IEC 3166-1)
country_code=FR

# SSID to be used in IEEE 802.11 management frames
ssid=zero
wpa_passphrase=YOUR_PASSWORD_HERE

# Driver interface type (hostap/wired/none/nl80211/bsd) - default hostapd
#driver=rtl871xdrv
#driver=nl80211

# Operation mode (a = IEEE 802.11a (5 GHz), b = IEEE 802.11b (2.4 GHz)
# RPi zero W only supports b
hw_mode=b
# Channel number
channel=5
# Maximum number of stations allowed
max_num_sta=5

# Bit field: bit0 = WPA, bit1 = WPA2
wpa=2
# Bit field: 1=wpa, 2=wep, 3=both
auth_algs=1

# Set of accepted cipher suites; disabling insecure TKIP
wpa_pairwise=CCMP
# Set of accepted key management algorithms
wpa_key_mgmt=WPA-PSK

# hostapd event logger configuration
logger_stdout=-1
#  0 = verbose debugging
#  1 = debugging
#  2 = informational messages
#  3 = notification
#  4 = warning
logger_stdout_level=2

## QoS support
#wmm_enabled=1
## Use "iw list" to show device capabilities and modify ht_capab accordingly
#ht_capab=[HT40+][SHORT-GI-40][TX-STBC][RX-STBC1][DSSS_CCK-40]
ht_capab=[HT20][TX-STBC1][DSSS_CCK-40]
```

If you use a Realtek-based wifi adapter, there is a specific hostapd package for
hosts using rtl871 hardware. You will have to install it from the AUR ; you will
have to
```
$ yay -S hostapd-rtl871xrdv
```
_And_ modify `/etc/hostapd/hostapd.conf` to mention `driver=rtl871xdrv`.

[arch-on-rpi]: {% post_url computing/2020-09-26-Arch-on-RPi %}
[aw-ap]: https://wiki.archlinux.org/index.php/Software_access_point