#!/bin/sh
# host-probe <host> [port]              — one-shot: print state and exit
# host-probe --watch <host> [port]      — loop, emit only on state change
#
# Distinguishes three states by probing layers independently:
#   up          ICMP + TCP accept + SSH banner exchange all succeed
#   wedged      ICMP + TCP accept succeed, banner exchange times out
#               (kernel networking alive, userspace frozen — classic
#                D-state / OOM / disk hang signature)
#   down        no ICMP or no TCP accept
#
# Suitable both as a standalone check and as the command body for the
# Monitor tool (one stdout line per state change).
#
# Env:
#   HOST_PROBE_INTERVAL    seconds between polls in --watch mode (default 30)
#   HOST_PROBE_TIMEOUT     per-probe timeout in seconds (default 3)

set -eu

interval=${HOST_PROBE_INTERVAL:-30}
timeout=${HOST_PROBE_TIMEOUT:-3}

usage() {
    sed -n '2,/^$/p' "$0" | sed 's/^# \{0,1\}//'
    exit 2
}

watch=false
case "${1:-}" in
    ''|-h|--help|help) usage ;;
    --watch) watch=true; shift ;;
esac
[ $# -ge 1 ] && [ $# -le 2 ] || usage

host=$1
port=${2:-22}

probe_icmp() {
    ping -c1 -W"$timeout" "$host" >/dev/null 2>&1
}

probe_tcp() {
    # -G is the BSD/macOS connect timeout flag; falls back to -w on Linux nc.
    nc -z -G"$timeout" "$host" "$port" >/dev/null 2>&1 \
        || nc -z -w"$timeout" "$host" "$port" >/dev/null 2>&1
}

probe_banner() {
    # SSH banner arrives unsolicited within milliseconds on a healthy sshd.
    # Frozen userspace: TCP accepts but no banner ever lands.
    banner=$(
        ( nc -G"$timeout" "$host" "$port" </dev/null &
          nc_pid=$!
          ( sleep "$timeout"; kill "$nc_pid" 2>/dev/null ) &
          wait "$nc_pid" 2>/dev/null ) 2>/dev/null | head -c 100
    )
    [ -n "$banner" ]
}

classify() {
    if ! probe_icmp; then echo down; return; fi
    if ! probe_tcp;  then echo down; return; fi
    if ! probe_banner; then echo wedged; return; fi
    echo up
}

stamp() { date -u +%H:%M:%SZ; }

if [ "$watch" = false ]; then
    classify
    exit 0
fi

prev=""
while :; do
    state=$(classify)
    if [ "$state" != "$prev" ]; then
        echo "[$(stamp)] $host:$port $state"
        prev=$state
    fi
    sleep "$interval"
done