screen_cast/scripts/pi-hotspot.sh

#!/usr/bin/env bash
#
# pi-hotspot.sh — turn the receiver into a Wi-Fi access point so a sender
# can connect directly, without any router in between.
#
# Usage (on the Pi):
#
#   sudo scripts/pi-hotspot.sh on [--ssid NAME] [--psk KEY] [--band bg|a]
#   sudo scripts/pi-hotspot.sh off
#   sudo scripts/pi-hotspot.sh status
#
# The hotspot takes over the Wi-Fi interface: the Pi leaves your router's
# network while it is active. Connect to the SSID from the sending machine;
# the Pi is reachable at 10.42.0.1 (SSH, screencast) and the sender gets an
# address via the built-in DHCP. Discovery and streaming then run over the
# direct link.
#
# The passphrase is stored in /etc/screencast-hotspot.conf (root only) so
# `status` can show it again.

set -euo pipefail

CONNECTION_NAME="screencast-hotspot"
INTERFACE="${SC_HOTSPOT_INTERFACE:-wlan0}"
CONFIG_FILE="/etc/screencast-hotspot.conf"

log() { printf '\n=== %s\n' "$*"; }
die() { printf 'pi-hotspot: error: %s\n' "$*" >&2; exit 1; }

require_root() {
    [ "$(id -u)" = 0 ] || die "run with sudo"
}

random_passphrase() {
    if command -v openssl >/dev/null 2>&1; then
        openssl rand -base64 12 | tr -d '=+/' | head -c 16
    else
        tr -dc 'A-Za-z0-9' </dev/urandom | head -c 16
    fi
}

cmd_on() {
    local ssid="screencast" psk="" band="bg"
    while [ $# -gt 0 ]; do
        case "$1" in
            --ssid) ssid="$2"; shift 2 ;;
            --psk) psk="$2"; shift 2 ;;
            --band) band="$2"; shift 2 ;;
            *) die "unknown option: $1" ;;
        esac
    done

    command -v nmcli >/dev/null 2>&1 || die "NetworkManager (nmcli) not found"
    systemctl is-active --quiet NetworkManager || die "NetworkManager is not running"
    ip link show "$INTERFACE" >/dev/null 2>&1 || die "interface ${INTERFACE} does not exist"

    if [ -z "$psk" ]; then
        psk="$(random_passphrase)"
    fi

    log "warning: this takes over ${INTERFACE}; the Pi leaves any router Wi-Fi while the hotspot is active"
    log "creating hotspot '${ssid}' (WPA2, band ${band})"

    apt-get install -y --no-install-recommends network-manager dnsmasq-base >/dev/null 2>&1 || true

    nmcli connection delete "$CONNECTION_NAME" >/dev/null 2>&1 || true
    nmcli connection add \
        type wifi \
        ifname "$INTERFACE" \
        con-name "$CONNECTION_NAME" \
        autoconnect yes \
        connection.autoconnect-priority 100 \
        802-11-wireless.ssid "$ssid" \
        802-11-wireless.mode ap \
        802-11-wireless.band "$band" \
        802-11-wireless-security.key-mgmt wpa-psk \
        802-11-wireless-security.psk "$psk" \
        ipv4.method shared \
        ipv6.method disabled >/dev/null

    nmcli connection up "$CONNECTION_NAME" >/dev/null

    umask 077
    printf 'ssid=%s\npsk=%s\n' "$ssid" "$psk" >"$CONFIG_FILE"

    log "hotspot is up"
    printf '    SSID:      %s\n' "$ssid"
    printf '    Password:  %s\n' "$psk"
    printf '    Pi address: 10.42.0.1 (also announced via mDNS)\n'
    printf '\nOn the sending machine: connect to the Wi-Fi above, then\n'
    printf '    screencast --send\n'
}

cmd_off() {
    log "removing the hotspot (Wi-Fi returns to normal client mode)"
    nmcli connection delete "$CONNECTION_NAME" >/dev/null 2>&1 || {
        log "no hotspot configured"
        exit 0
    }
    rm -f "$CONFIG_FILE"
    log "done; the Pi will reconnect to any known router Wi-Fi"
}

cmd_status() {
    if [ -f "$CONFIG_FILE" ]; then
        log "hotspot configuration (${CONFIG_FILE})"
        grep -E '^(ssid|psk)=' "$CONFIG_FILE" | sed -e 's/^/    /'
    else
        log "no hotspot configured (run: sudo scripts/pi-hotspot.sh on)"
    fi
    nmcli --get-values NAME,TYPE,DEVICE connection show --active 2>/dev/null | grep wifi | sed 's/^/    /' || true
}

case "${1:-}" in
    on) require_root; shift; cmd_on "$@" ;;
    off) require_root; shift; cmd_off "$@" ;;
    status) cmd_status ;;
    *)
        cat <<'EOF'
usage: sudo scripts/pi-hotspot.sh on [--ssid NAME] [--psk KEY] [--band bg|a]
       sudo scripts/pi-hotspot.sh off
       sudo scripts/pi-hotspot.sh status
EOF
        exit 1
        ;;
esac