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Create a file /etc/wpa_supplicant/c-base-crew.conf with this content (change $crewname and $password to your crew name and password):
network={ ssid="c-base-crew" key_mgmt=WPA-EAP eap=PEAP identity="$crewname" password="$password" ca_cert="/etc/ssl/certs/cacert.org.pem" subject_match="C=DE, ST=Berlin, L=Berlin, O=c-base e.V., CN=wlan.cbrp3.c-base.org" }
And start it with:
wpa_supplicant -D wext -i wlan0 -c /etc/wpa_supplicant/c-base-crew.conf