Update install.sh

Reviewed-on: #15

fallback-ap.sh

@@ -4,8 +4,9 @@ set -e
 
 # Configuration =======================================================================================================
 #
-# Connection settings (SSID, passphrase, subnet, DHCP range, interface) can
-# be supplied three ways, highest priority first:
+# Connection settings (SSID, passphrase, subnet, DHCP range, interface,
+# monitor interfaces, check mode) can be supplied three ways, highest
+# priority first:
 #   1. Environment variables (FALLBACK_AP_*) - used for headless installs.
 #   2. The persisted config file below, written by `install`.
 #   3. The hard-coded defaults in this block.
@@ -16,21 +17,59 @@ set -e
 CONFIG_FILE="/etc/fallback-ap.conf"
 if [ -f "$CONFIG_FILE" ]; then source "$CONFIG_FILE"; fi
 
+# The interface the AP is created on. The AP is NEVER created on any other
+# interface, regardless of what is being monitored.
 WLAN0_IFACE="${FALLBACK_AP_IFACE:-${SAVED_IFACE:-wlan0}}"
 
+# Which interfaces to watch for connectivity. If any of them is online, the
+# AP stays off. Comma-separated list (e.g. "eth0,wlan1,wwan0").
+# Leave BLANK to monitor all REAL interfaces: physical ethernet, wifi and
+# cellular modems. Virtual interfaces (docker0, br-*, veth*, virbr*, tun/tap,
+# wireguard, vlan, bridges, bonds, ...) are detected automatically and never
+# count, so Docker/LXC/VM networking cannot keep the AP off.
+# Note the missing colon in the expansion: an explicitly empty
+# FALLBACK_AP_MONITOR_IFACE="" overrides a value saved in the config file.
+MONITOR_IFACE="${FALLBACK_AP_MONITOR_IFACE-${SAVED_MONITOR_IFACE-}}"
+MONITOR_IFACE="${MONITOR_IFACE// /}"
+
+# Extra interfaces to ignore when monitoring all (comma-separated, globs
+# allowed, e.g. "enx*,usb0"). Only needed for physical NICs you do not want
+# to count; virtual interfaces are already excluded. Has no effect when
+# MONITOR_IFACE names interfaces explicitly.
+IGNORE_IFACE="${FALLBACK_AP_IGNORE_IFACE-${SAVED_IGNORE_IFACE-}}"
+IGNORE_IFACE="${IGNORE_IFACE// /}"
+
+# What "online" means:
+#   link     - (default) a monitored interface is connected. An outage here is
+#              local and fixable; an upstream outage is not our problem, so
+#              the AP stays off. Fast and never flaps.
+#   internet - a monitored interface is connected AND can ping a target
+#              out of that specific interface (also detects dead uplinks)
+CHECK_MODE="${FALLBACK_AP_CHECK_MODE:-${SAVED_CHECK_MODE:-link}}"
+
+# Hosts pinged (space-separated, any one answering counts) in internet mode.
+CHECK_TARGETS="${FALLBACK_AP_CHECK_TARGETS:-${SAVED_CHECK_TARGETS:-1.1.1.1 8.8.8.8}}"
+
 AP_NAME="${FALLBACK_AP_SSID:-${SAVED_SSID:-$HOSTNAME}}"
-AP_PASSWORD="${FALLBACK_AP_PASSWORD:-${SAVED_PASSWORD:-guesswhatitis}}"
+AP_PASSWORD="${FALLBACK_AP_PASSWORD:-${SAVED_PASSWORD:-abcd1234}}"
 AP_ENCRYPTION="${FALLBACK_AP_ENCRYPTION:-${SAVED_ENCRYPTION:-wpa-psk}}" # none | wpa-psk | sae
 
 FALLBACK_AP_CIDR="${FALLBACK_AP_CIDR:-${SAVED_CIDR:-10.10.10.1/24}}"
 FALLBACK_AP_DHCP_START_OCTET="${FALLBACK_AP_DHCP_START_OCTET:-${SAVED_DHCP_START_OCTET:-100}}"
 FALLBACK_AP_DHCP_END_OCTET="${FALLBACK_AP_DHCP_END_OCTET:-${SAVED_DHCP_END_OCTET:-250}}"
 
-AP_CIDR="$FALLBACK_AP_CIDR"
-AP_NETWORK_BASE="${AP_CIDR%%/*}"   # e.g. "10.10.10.1" out of "10.10.10.1/24"
-AP_NETWORK_BASE="${AP_NETWORK_BASE%.*}" # e.g. "10.10.10"
-DHCP_START="${AP_NETWORK_BASE}.${FALLBACK_AP_DHCP_START_OCTET}"
-DHCP_END="${AP_NETWORK_BASE}.${FALLBACK_AP_DHCP_END_OCTET}"
+# Values derived from the CIDR / DHCP settings. A function, because the
+# `install` command-line flags can change those settings and need to
+# recompute these.
+compute_derived() {
+    AP_CIDR="$FALLBACK_AP_CIDR"
+    AP_NETWORK_BASE="${AP_CIDR%%/*}"   # e.g. "10.10.10.1" out of "10.10.10.1/24"
+    AP_NETWORK_BASE="${AP_NETWORK_BASE%.*}" # e.g. "10.10.10"
+    DHCP_START="${AP_NETWORK_BASE}.${FALLBACK_AP_DHCP_START_OCTET}"
+    DHCP_END="${AP_NETWORK_BASE}.${FALLBACK_AP_DHCP_END_OCTET}"
+}
+
+compute_derived
 
 # How often, while the AP has no clients, to stop the AP and give
 # NetworkManager a chance to reconnect to a known Wi-Fi network.
@@ -44,6 +83,13 @@ RECOVERY_WINDOW=20
 # we query it again.
 STATE_SETTLE_DELAY=1
 
+# Internet mode only: how often to run the ping checks, how many consecutive
+# failed checks are needed before declaring "offline" (prevents the AP from
+# flapping on a single dropped ping), and the per-ping timeout in seconds.
+ONLINE_CHECK_INTERVAL=5
+FAIL_THRESHOLD=3
+PING_TIMEOUT=2
+
 CONNECTION_NAME="Fallback AP"
 SERVICE_NAME="fallback-ap"
 SERVICE_FILE="/etc/systemd/system/$SERVICE_NAME.service"
@@ -56,6 +102,18 @@ STATE_FILE="$STATE_DIR/state"
 # End of Configuration ================================================================================================
 
 
+validate_check_mode() {
+    case "$CHECK_MODE" in
+        link|internet) ;;
+        *)
+            echo "Invalid check mode '$CHECK_MODE' (use: link | internet)"
+            exit 1
+            ;;
+    esac
+}
+
+validate_check_mode
+
 # Resolve the actual location of this script.
 SCRIPT_PATH="$(readlink -f "$0")"
 
@@ -86,17 +144,210 @@ is_ap_on() {
 }
 
 
-is_wifi_connected() {
-    nmcli -t -f DEVICE,TYPE,STATE,CONNECTION device status 2>/dev/null |
-        awk -F: -v iface="$WLAN0_IFACE" -v ap="$CONNECTION_NAME" \
-        '$1 == iface && $2 == "wifi" && $3 == "connected" && $4 != ap {found=1} END {exit !found}'
+# --- Connectivity monitoring -----------------------------------------------------------------------------------------
+
+# True if no monitor interface is configured (= monitor everything).
+monitor_all() {
+    [ -z "$MONITOR_IFACE" ]
 }
 
 
+# True if the AP interface is among the monitored interfaces. If it is not,
+# a "recovery attempt" (taking the AP down to let wlan0 reconnect to Wi-Fi)
+# could never change the result, so it is skipped.
+monitors_wlan0() {
+    if monitor_all; then
+        return 0
+    fi
+
+    case ",${MONITOR_IFACE}," in
+        *",${WLAN0_IFACE},"*) return 0 ;;
+    esac
+
+    return 1
+}
+
+
+# A "real" interface is backed by hardware. Linux exposes this as a `device`
+# link under /sys/class/net/<iface>/, which physical NICs (PCI, USB, SDIO,
+# virtio, ...) have and virtual ones (docker0, veth*, br-*, tun*, wg*, vlan,
+# bridges, bonds, dummy) do not. Cellular modems are the exception: NetworkManager
+# names them after their control port (e.g. cdc-wdm0), which is not in
+# /sys/class/net, so gsm/cdma types are accepted as-is.
+is_real_interface() {
+    local dev="$1"
+    local type="$2"
+
+    case "$type" in
+        gsm|cdma) return 0 ;;
+    esac
+
+    [ -e "/sys/class/net/$dev/device" ]
+}
+
+
+# True if the device matches an entry of IGNORE_IFACE (globs allowed).
+is_ignored_interface() {
+    local dev="$1"
+    local pattern
+    local -a patterns=()
+
+    IFS=',' read -r -a patterns <<< "$IGNORE_IFACE" || true
+
+    for pattern in "${patterns[@]}"; do
+        if [ -n "$pattern" ] && [[ "$dev" == $pattern ]]; then
+            return 0
+        fi
+    done
+
+    return 1
+}
+
+
+# Prints (one per line) the monitored devices that NetworkManager reports as
+# connected, excluding the fallback AP connection itself.
+#   - Blank MONITOR_IFACE: every real (hardware-backed) ethernet, wifi or
+#     cellular device that is not in IGNORE_IFACE.
+#   - Otherwise: only the devices named in MONITOR_IFACE (any type).
+# The state is matched with a prefix because nmcli reports values such as
+# "connected (externally)" or "connected (site only)".
+get_monitored_devices() {
+    local dev type
+    local all=0
+
+    if monitor_all; then
+        all=1
+    fi
+
+    while IFS=: read -r dev type; do
+        if [ -z "$dev" ]; then
+            continue
+        fi
+
+        if [ "$all" -eq 1 ]; then
+            case "$type" in
+                ethernet|wifi|gsm|cdma) ;;
+                *) continue ;;
+            esac
+
+            if ! is_real_interface "$dev" "$type"; then
+                continue
+            fi
+
+            if is_ignored_interface "$dev"; then
+                continue
+            fi
+        else
+            case ",${MONITOR_IFACE}," in
+                *",${dev},"*) ;;
+                *) continue ;;
+            esac
+        fi
+
+        echo "$dev"
+    done < <(
+        nmcli -t -f DEVICE,TYPE,STATE,CONNECTION device status 2>/dev/null |
+            awk -F: -v ap="$CONNECTION_NAME" \
+                '$3 ~ /^connected/ && $4 != ap {print $1 ":" $2}'
+    )
+}
+
+
+# Can this specific interface reach the internet? Pings are forced out of the
+# given interface (-I) so another interface cannot answer on its behalf.
+iface_has_internet() {
+    local dev="$1"
+    local target
+
+    for target in $CHECK_TARGETS; do
+        if ping -I "$dev" -c 1 -W "$PING_TIMEOUT" "$target" >/dev/null 2>&1; then
+            return 0
+        fi
+    done
+
+    return 1
+}
+
+
+# Is ANY monitored interface online right now (according to CHECK_MODE)?
+check_online() {
+    local dev
+
+    while read -r dev; do
+        if [ -z "$dev" ]; then
+            continue
+        fi
+
+        if [ "$CHECK_MODE" = "link" ]; then
+            return 0
+        fi
+
+        if iface_has_internet "$dev"; then
+            return 0
+        fi
+    done < <(get_monitored_devices)
+
+    return 1
+}
+
+
+# Debounced wrapper around check_online, used by the main loop. Updates the
+# `online`, `fail_count` and `last_online_check` variables of run().
+#   link mode:     checked every tick, no debounce (cheap).
+#   internet mode: checked every ONLINE_CHECK_INTERVAL seconds, and only
+#                  declared offline after FAIL_THRESHOLD consecutive failures.
+#                  Coming back online is detected on the first success.
+update_online() {
+    local now="$1"
+    local interval=0
+    local threshold=1
+
+    if [ "$CHECK_MODE" = "internet" ]; then
+        interval="$ONLINE_CHECK_INTERVAL"
+        threshold="$FAIL_THRESHOLD"
+    fi
+
+    if (( now - last_online_check < interval )); then
+        return 0
+    fi
+
+    last_online_check="$now"
+
+    if check_online; then
+        fail_count=0
+        online=1
+    else
+        fail_count=$((fail_count + 1))
+
+        if (( fail_count >= threshold )); then
+            online=0
+        fi
+    fi
+}
+
+
+wait_for_online() {
+    local timeout="$1"
+    local i
+
+    for ((i=0; i<timeout; i++)); do
+        if check_online; then
+            return 0
+        fi
+
+        sleep 1
+    done
+
+    return 1
+}
+
+
+# --- Wi-Fi bookkeeping -----------------------------------------------------------------------------------------------
+
 get_wifi_connection() {
     nmcli -t -f DEVICE,TYPE,STATE,CONNECTION device status 2>/dev/null |
         awk -F: -v iface="$WLAN0_IFACE" \
-        '$1 == iface && $2 == "wifi" && $3 == "connected" {print $4; exit}'
+        '$1 == iface && $2 == "wifi" && $3 ~ /^connected/ {print $4; exit}'
 }
 
 
@@ -148,27 +399,14 @@ wait_for_ap_state() {
 }
 
 
-wait_for_wifi() {
-    local timeout="$1"
-
-    for ((i=0; i<timeout*10; i++)); do
-        if is_wifi_connected; then
-            return 0
-        fi
-
-        sleep 0.1
-    done
-
-    return 1
-}
-
+# --- AP control (always on $WLAN0_IFACE only) ------------------------------------------------------------------------
 
 start_ap() {
     if is_ap_on; then
         return 0
     fi
 
-    log "Enabling AP: $AP_NAME"
+    log "Enabling AP: $AP_NAME (on $WLAN0_IFACE)"
 
     nmcli connection up "$CONNECTION_NAME" ifname "$WLAN0_IFACE" >/dev/null 2>&1 || {
         log "Failed to start AP"
@@ -207,23 +445,32 @@ stop_ap() {
     return 0
 }
 
+
+# --- Main loop logic -------------------------------------------------------------------------------------------------
+
+# Called once per second from run(). Relies on run()'s local variables
+# (last_attempt, online, fail_count, last_online_check) via dynamic scoping.
 tick() {
     local now="$1"
 
-    if is_wifi_connected; then
+    update_online "$now"
+
+    if (( online )); then
         remember_wifi
 
         if is_ap_on; then
-            stop_ap
+            log "Connectivity available, shutting down fallback AP"
+            stop_ap || true
         fi
 
-        set_state "wifi"
+        set_state "online"
         last_attempt="$now"
         return
     fi
 
     if ! is_ap_on; then
-        start_ap
+        log "No connectivity on monitored interface(s), starting fallback AP"
+        start_ap || true
         set_state "fallback"
         last_attempt="$now"
         return
@@ -239,14 +486,20 @@ tick() {
         return
     fi
 
+    # Recovery only makes sense if wlan0 is one of the monitored interfaces:
+    # it works by briefly freeing wlan0 so it can reconnect to known Wi-Fi.
+    if ! monitors_wlan0; then
+        return
+    fi
+
     if (( now - last_attempt >= RECOVERY_CHECK_INTERVAL )); then
         last_attempt="$now"
-        attempt_recovery
+        attempt_recovery || true
     fi
 }
 
 attempt_recovery() {
-    log "No AP clients, checking for known Wi-Fi (AP down for up to ${RECOVERY_WINDOW}s)"
+    log "No AP clients, checking for connectivity (AP down for up to ${RECOVERY_WINDOW}s)"
 
     set_state "checking"
 
@@ -254,25 +507,27 @@ attempt_recovery() {
 
     if is_ap_on; then
         log "AP did not go down, aborting recovery attempt"
-        start_ap
+        start_ap || true
         set_state "fallback"
         return 1
     fi
 
-    if wait_for_wifi "$RECOVERY_WINDOW"; then
+    if wait_for_online "$RECOVERY_WINDOW"; then
         sleep "$STATE_SETTLE_DELAY"
 
-        if is_wifi_connected; then
-            log "Wi-Fi reconnected"
+        if check_online; then
+            log "Connectivity restored"
             remember_wifi
-            set_state "wifi"
+            online=1
+            fail_count=0
+            set_state "online"
             return 0
         fi
     fi
 
-    log "No Wi-Fi available, restoring AP"
+    log "No connectivity available, restoring AP"
 
-    start_ap
+    start_ap || true
     set_state "fallback"
 
     return 1
@@ -283,6 +538,10 @@ save_config() {
     mkdir -p "$(dirname "$CONFIG_FILE")"
     cat > "$CONFIG_FILE" <<EOF
 SAVED_IFACE="$WLAN0_IFACE"
+SAVED_MONITOR_IFACE="$MONITOR_IFACE"
+SAVED_IGNORE_IFACE="$IGNORE_IFACE"
+SAVED_CHECK_MODE="$CHECK_MODE"
+SAVED_CHECK_TARGETS="$CHECK_TARGETS"
 SAVED_SSID="$AP_NAME"
 SAVED_PASSWORD="$AP_PASSWORD"
 SAVED_ENCRYPTION="$AP_ENCRYPTION"
@@ -294,25 +553,115 @@ EOF
 }
 
 
+usage_install() {
+    cat <<EOF
+Usage: $SCRIPT_PATH install [options]
+
+Options (each overrides the matching FALLBACK_AP_* env var and any saved value):
+  --iface NAME            Interface the AP is created on         (default: wlan0)
+  --monitor-iface LIST    Comma-separated interfaces to watch for connectivity.
+                          Blank ("") monitors all REAL interfaces; virtual ones
+                          (docker, veth, bridges, ...) are skipped (default: blank)
+  --ignore-iface LIST     Comma-separated physical interfaces to ignore when
+                          monitoring all; globs allowed, e.g. "enx*"
+                                                                  (default: none)
+  --check-mode MODE       link | internet                         (default: link)
+  --check-targets "H.."   Space-separated hosts to ping in internet mode
+                                                                  (default: "1.1.1.1 8.8.8.8")
+  --ssid NAME             AP name                                 (default: hostname)
+  --password PASS         AP passphrase                           (default: abcd1234)
+  --encryption TYPE       none | wpa-psk | sae                    (default: wpa-psk)
+  --cidr ADDR/MASK        AP address and subnet                   (default: 10.10.10.1/24)
+  --dhcp-start OCTET      First DHCP address, last octet          (default: 100)
+  --dhcp-end OCTET        Last DHCP address, last octet           (default: 250)
+  -h, --help              Show this help
+
+Both "--opt value" and "--opt=value" are accepted. Precedence, highest first:
+flags, FALLBACK_AP_* env vars, saved config file, built-in defaults.
+
+Examples:
+  $SCRIPT_PATH install
+  $SCRIPT_PATH install --monitor-iface eth0,wwan0
+  $SCRIPT_PATH install --iface wlan1 --monitor-iface "" --check-mode internet
+EOF
+}
+
+
+parse_install_args() {
+    local opt val
+
+    while [ $# -gt 0 ]; do
+        opt="$1"
+
+        case "$opt" in
+            -h|--help)
+                usage_install
+                exit 0
+                ;;
+            --*=*)
+                val="${opt#*=}"
+                opt="${opt%%=*}"
+                shift
+                ;;
+            --*)
+                if [ $# -lt 2 ]; then
+                    echo "Option $opt requires a value."
+                    exit 1
+                fi
+                val="$2"
+                shift 2
+                ;;
+            *)
+                echo "Unknown argument: $opt"
+                usage_install
+                exit 1
+                ;;
+        esac
+
+        case "$opt" in
+            --iface)          WLAN0_IFACE="$val" ;;
+            --monitor-iface)  MONITOR_IFACE="${val// /}" ;;
+            --ignore-iface)   IGNORE_IFACE="${val// /}" ;;
+            --check-mode)     CHECK_MODE="$val" ;;
+            --check-targets)  CHECK_TARGETS="$val" ;;
+            --ssid)           AP_NAME="$val" ;;
+            --password)       AP_PASSWORD="$val" ;;
+            --encryption)     AP_ENCRYPTION="$val" ;;
+            --cidr)           FALLBACK_AP_CIDR="$val" ;;
+            --dhcp-start)     FALLBACK_AP_DHCP_START_OCTET="$val" ;;
+            --dhcp-end)       FALLBACK_AP_DHCP_END_OCTET="$val" ;;
+            *)
+                echo "Unknown option: $opt"
+                usage_install
+                exit 1
+                ;;
+        esac
+    done
+
+    validate_check_mode
+    compute_derived
+}
+
+
 install() {
     if [ "$EUID" -ne 0 ]; then
         echo "Run this script as root."
         exit 1
     fi
 
+    parse_install_args "$@"
+
     log "Installing NetworkManager fallback AP"
 
     apt-get update
-    apt-get install -y network-manager iw
+    apt-get install -y network-manager iw iputils-ping
 
     systemctl enable NetworkManager
     systemctl start NetworkManager
 
     mkdir -p "$STATE_DIR"
 
-    if is_wifi_connected; then
-        remember_wifi
-    fi
+    remember_wifi
 
     create_connection
     save_config
@@ -345,7 +694,15 @@ EOF
     log "Script: $SCRIPT_PATH"
     log "Config file: $CONFIG_FILE"
     log "SSID: $AP_NAME"
-    log "Interface: $WLAN0_IFACE"
+    log "AP interface: $WLAN0_IFACE"
+    log "Monitored interfaces: ${MONITOR_IFACE:-all real interfaces}"
+    if monitor_all && [ -n "$IGNORE_IFACE" ]; then
+        log "Ignored interfaces: $IGNORE_IFACE"
+    fi
+    log "Check mode: $CHECK_MODE"
+    if [ "$CHECK_MODE" = "internet" ]; then
+        log "Ping targets: $CHECK_TARGETS"
+    fi
     log "Network: $AP_CIDR"
     log "DHCP: $DHCP_START - $DHCP_END"
     log "Encryption: $AP_ENCRYPTION"
@@ -484,10 +841,56 @@ status() {
     echo
     echo "=== Config ($CONFIG_FILE) ==="
     echo "SSID: $AP_NAME"
-    echo "Interface: $WLAN0_IFACE"
+    echo "AP interface: $WLAN0_IFACE"
+    echo "Monitored interfaces: ${MONITOR_IFACE:-all real interfaces}"
+    if monitor_all && [ -n "$IGNORE_IFACE" ]; then
+        echo "Ignored interfaces: $IGNORE_IFACE"
+    fi
+    echo "Check mode: $CHECK_MODE"
+    if [ "$CHECK_MODE" = "internet" ]; then
+        echo "Ping targets: $CHECK_TARGETS"
+    fi
     echo "Network: $AP_CIDR"
     echo "DHCP: $DHCP_START - $DHCP_END"
 
+    echo
+    echo "=== Connectivity ($CHECK_MODE) ==="
+    local dev found=0
+    while read -r dev; do
+        if [ -z "$dev" ]; then
+            continue
+        fi
+
+        found=1
+
+        if [ "$CHECK_MODE" = "internet" ]; then
+            if iface_has_internet "$dev"; then
+                echo "$dev: internet OK"
+            else
+                echo "$dev: connected, NO internet"
+            fi
+        else
+            echo "$dev: connected"
+        fi
+    done < <(get_monitored_devices)
+
+    if [ "$found" -eq 0 ]; then
+        echo "No monitored interface is connected"
+    fi
+
+    echo
+    echo "=== Connected but NOT counted (virtual / ignored / unmonitored) ==="
+    local monitored
+    monitored="$(get_monitored_devices)"
+    nmcli -t -f DEVICE,STATE,CONNECTION device status 2>/dev/null |
+        awk -F: -v ap="$CONNECTION_NAME" \
+            '$2 ~ /^connected/ && $3 != ap {print $1}' |
+        while read -r dev; do
+            if ! grep -Fxq "$dev" <<< "$monitored"; then
+                echo "$dev"
+            fi
+        done
+
     echo
     echo "=== Last Known Wi-Fi ==="
     get_last_wifi || echo "None"
@@ -507,27 +910,36 @@ status() {
 
 
 run() {
+    # These are shared with tick/update_online/attempt_recovery through
+    # dynamic scoping.
     local last_attempt=0
+    local last_online_check=0
+    local fail_count=0
+    local online=0
     local now
 
     mkdir -p "$STATE_DIR"
 
     log "Fallback AP service started"
     log "Script: $SCRIPT_PATH"
+    log "AP interface: $WLAN0_IFACE | monitoring: ${MONITOR_IFACE:-all real interfaces} | mode: $CHECK_MODE"
     exec 9>"$LOCK_FILE"
 
     flock 9
 
-    if is_wifi_connected; then
+    if check_online; then
+        online=1
         remember_wifi
-        set_state "wifi"
-        log "Wi-Fi already connected"
+        set_state "online"
+        log "Connectivity already available"
     else
-        log "No Wi-Fi connection, starting fallback AP"
-        start_ap
+        log "No connectivity on monitored interface(s), starting fallback AP"
+        start_ap || true
         set_state "fallback"
     fi
 
+    last_online_check="$(date +%s)"
+
     flock -u 9
 
     while true; do
@@ -551,7 +963,8 @@ fi
 
 case "${1:-}" in
     install)
-        install
+        shift
+        install "$@"
         ;;
 
     uninstall)
@@ -575,7 +988,8 @@ case "${1:-}" in
         ;;
 
     *)
-        echo "Usage: $SCRIPT_PATH {install|uninstall|start|stop|status|run}"
+        echo "Usage: $SCRIPT_PATH {install [options]|uninstall|start|stop|status|run}"
+        echo "       $SCRIPT_PATH install --help   for install options"
         exit 1
         ;;
 esac

install.sh

@@ -11,22 +11,137 @@ if [ "$EUID" -ne 0 ]; then
     exit 1
 fi
 
+# Ask a question; Enter accepts the default.
 prompt() {
     local __var="$1" __question="$2" __default="$3" __answer=""
-    if [ -e /dev/tty ]; then
-        read -r -p "$__question [$__default]: " __answer < /dev/tty || __answer=""
-    fi
+    read -r -p "$__question [$__default]: " __answer < /dev/tty || __answer=""
     printf -v "$__var" '%s' "${__answer:-$__default}"
 }
 
-[ -z "${FALLBACK_AP_SSID:-}" ]              && prompt FALLBACK_AP_SSID "Access point SSID" "$(hostname)"
-[ -z "${FALLBACK_AP_PASSWORD:-}" ]          && prompt FALLBACK_AP_PASSWORD "Access point passphrase" "guesswhatitis"
-[ -z "${FALLBACK_AP_CIDR:-}" ]              && prompt FALLBACK_AP_CIDR "Access point subnet (CIDR)" "10.10.10.1/24"
-[ -z "${FALLBACK_AP_DHCP_START_OCTET:-}" ]  && prompt FALLBACK_AP_DHCP_START_OCTET "DHCP range start (last octet)" "100"
-[ -z "${FALLBACK_AP_DHCP_END_OCTET:-}" ]    && prompt FALLBACK_AP_DHCP_END_OCTET "DHCP range end (last octet)" "250"
+# Like prompt, but "blank" is a legitimate value. Enter accepts the default;
+# typing the clear word (e.g. "all" or "none") sets the value to empty.
+prompt_blankable() {
+    local __var="$1" __question="$2" __default="$3" __clear="$4" __answer=""
+    read -r -p "$__question [${__default:-$__clear}]: " __answer < /dev/tty || __answer=""
 
-export FALLBACK_AP_SSID FALLBACK_AP_PASSWORD FALLBACK_AP_CIDR FALLBACK_AP_DHCP_START_OCTET FALLBACK_AP_DHCP_END_OCTET
+    case "$__answer" in
+        "")          __answer="$__default" ;;
+        "$__clear")  __answer="" ;;
+    esac
 
+    printf -v "$__var" '%s' "${__answer// /}"
+}
+
+# Ask until the answer is one of the allowed choices. Enter accepts the default.
+prompt_choice() {
+    local __var="$1" __question="$2" __default="$3" __answer="" __opt
+    shift 3
+
+    while true; do
+        read -r -p "$__question [$__default]: " __answer < /dev/tty || __answer=""
+        __answer="${__answer:-$__default}"
+
+        for __opt in "$@"; do
+            if [ "$__answer" = "$__opt" ]; then
+                printf -v "$__var" '%s' "$__answer"
+                return 0
+            fi
+        done
+
+        echo "  Please enter one of: $*"
+    done
+}
+
+# Lists wireless interfaces (those with a /sys/class/net/<iface>/wireless dir).
+detect_wireless() {
+    local dir
+    for dir in /sys/class/net/*; do
+        if [ -d "$dir/wireless" ]; then
+            basename "$dir"
+        fi
+    done
+}
+
+EXISTING_CONFIG="/etc/fallback-ap.conf"
+
+if [ -f "$EXISTING_CONFIG" ]; then
+    # shellcheck disable=SC1090
+    source "$EXISTING_CONFIG"
+fi
+
+DETECTED_WIFI="$(detect_wireless | tr '\n' ' ')"
+DETECTED_WIFI="${DETECTED_WIFI% }"
+
+DEFAULT_SSID="${SAVED_SSID:-$(hostname)}"
+DEFAULT_PASSWORD="${SAVED_PASSWORD:-abcd1234}"
+DEFAULT_IFACE="${SAVED_IFACE:-${DETECTED_WIFI%% *}}"
+DEFAULT_IFACE="${DEFAULT_IFACE:-wlan0}"
+DEFAULT_CIDR="${SAVED_CIDR:-10.10.10.1/24}"
+DEFAULT_DHCP_START_OCTET="${SAVED_DHCP_START_OCTET:-100}"
+DEFAULT_DHCP_END_OCTET="${SAVED_DHCP_END_OCTET:-250}"
+DEFAULT_MONITOR_IFACE="${SAVED_MONITOR_IFACE-}"
+DEFAULT_IGNORE_IFACE="${SAVED_IGNORE_IFACE-}"
+DEFAULT_CHECK_MODE="${SAVED_CHECK_MODE:-link}"
+DEFAULT_CHECK_TARGETS="${SAVED_CHECK_TARGETS:-1.1.1.1 8.8.8.8}"
+
+# --- Access point ----------------------------------------------------------------------------------------------------
+
+[ -z "${FALLBACK_AP_SSID:-}" ]              && prompt FALLBACK_AP_SSID "Access point SSID" "$DEFAULT_SSID"
+[ -z "${FALLBACK_AP_PASSWORD:-}" ]          && prompt FALLBACK_AP_PASSWORD "Access point passphrase" "$DEFAULT_PASSWORD"
+
+if [ -z "${FALLBACK_AP_IFACE:-}" ]; then
+    if [ -n "$DETECTED_WIFI" ]; then
+        echo "Detected Wi-Fi interfaces: $DETECTED_WIFI"
+    fi
+    prompt FALLBACK_AP_IFACE "Interface to create the access point on" "$DEFAULT_IFACE"
+fi
+
+[ -z "${FALLBACK_AP_CIDR:-}" ]              && prompt FALLBACK_AP_CIDR "Access point subnet (CIDR)" "$DEFAULT_CIDR"
+[ -z "${FALLBACK_AP_DHCP_START_OCTET:-}" ]  && prompt FALLBACK_AP_DHCP_START_OCTET "DHCP range start (last octet)" "$DEFAULT_DHCP_START_OCTET"
+[ -z "${FALLBACK_AP_DHCP_END_OCTET:-}" ]    && prompt FALLBACK_AP_DHCP_END_OCTET "DHCP range end   (last octet)" "$DEFAULT_DHCP_END_OCTET"
+
+# --- Connectivity monitoring -----------------------------------------------------------------------------------------
+#
+# The AP only starts when none of the monitored interfaces are online, and it
+# is only ever created on the interface chosen above.
+#
+# These use "unset" (not "empty") to decide whether to ask, because an empty
+# FALLBACK_AP_MONITOR_IFACE / FALLBACK_AP_IGNORE_IFACE is a valid answer
+# meaning "all real interfaces" / "ignore nothing".
+
+if [ -z "${FALLBACK_AP_MONITOR_IFACE+x}" ]; then
+    echo
+    echo "Which interfaces should count as 'online'? Enter a comma-separated list"
+    echo "(e.g. eth0,wlan0), or 'all' for every real interface. Virtual interfaces"
+    echo "(docker, veth, bridges, VPNs) are skipped automatically."
+    prompt_blankable FALLBACK_AP_MONITOR_IFACE "Interfaces to monitor" "$DEFAULT_MONITOR_IFACE" "all"
+fi
+
+if [ -z "$FALLBACK_AP_MONITOR_IFACE" ] && [ -z "${FALLBACK_AP_IGNORE_IFACE+x}" ]; then
+    echo
+    echo "Any physical interfaces to ignore (globs allowed, e.g. enx*,usb0)?"
+    prompt_blankable FALLBACK_AP_IGNORE_IFACE "Interfaces to ignore" "$DEFAULT_IGNORE_IFACE" "none"
+fi
+
+if [ -z "${FALLBACK_AP_CHECK_MODE:-}" ]; then
+    echo
+    echo "How should 'online' be determined?"
+    echo "  link     - the interface is connected (an upstream outage keeps the AP off)"
+    echo "  internet - the interface is connected AND can ping out"
+    prompt_choice FALLBACK_AP_CHECK_MODE "Check mode" "$DEFAULT_CHECK_MODE" link internet
+fi
+
+if [ "$FALLBACK_AP_CHECK_MODE" = "internet" ] && [ -z "${FALLBACK_AP_CHECK_TARGETS:-}" ]; then
+    prompt FALLBACK_AP_CHECK_TARGETS "Hosts to ping (space-separated)" "$DEFAULT_CHECK_TARGETS"
+fi
+
+export FALLBACK_AP_SSID FALLBACK_AP_PASSWORD FALLBACK_AP_IFACE FALLBACK_AP_CIDR FALLBACK_AP_DHCP_START_OCTET FALLBACK_AP_DHCP_END_OCTET
+export FALLBACK_AP_CHECK_MODE
+[ -n "${FALLBACK_AP_MONITOR_IFACE+x}" ] && export FALLBACK_AP_MONITOR_IFACE
+[ -n "${FALLBACK_AP_IGNORE_IFACE+x}" ]  && export FALLBACK_AP_IGNORE_IFACE
+[ -n "${FALLBACK_AP_CHECK_TARGETS+x}" ] && export FALLBACK_AP_CHECK_TARGETS
+
+echo
 echo "Downloading fallback-ap..."
 curl -fsSL "$SCRIPT_URL" -o "$INSTALL_PATH"
 chmod +x "$INSTALL_PATH"