#!/usr/bin/env bash
# Debian 13: switch system name resolution from systemd-resolved to local BIND9.
# No package installation, BIND configuration or networking changes.
set -Eeuo pipefail
umask 077

readonly STATE_DIR=/var/backups/oopen-bind9-direct-resolver
readonly CURRENT_FILE="$STATE_DIR/current"
readonly RESOLV=/etc/resolv.conf
readonly NSS=/etc/nsswitch.conf
readonly EXPECTED_NSS='hosts:          files myhostname dns'

MODE="${1:---check}"
case "$MODE" in --check|--apply|--rollback|--help) ;; *) echo "Usage: $0 [--check|--apply|--rollback|--help]" >&2; exit 2;; esac

# Colorize status labels on terminals; respect NO_COLOR and redirected output.
if [[ -t 1 && ! -v NO_COLOR && "${TERM:-}" != dumb ]]; then
  GREEN=$'\033[32m'
  GREY=$'\033[90m'
  RED=$'\033[31m'
  YELLOW=$'\033[33m'
  RESET=$'\033[0m'
else
  GREEN='' GREY='' RED='' YELLOW='' RESET=''
fi

info()    { printf '[ Info    ] %s\n' "$*"; }
ok()      { printf '%s[ ok      ]%s %s\n' "$GREEN" "$RESET" "$*"; }
skipped() { printf '%s[ skipped ]%s %s\n' "$GREY" "$RESET" "$*"; }
failed()  { printf '%s[ failed  ]%s %s\n' "$RED" "$RESET" "$*" >&2; }
# Delimited summary at the end of --check / before an interactive --apply.
status_banner(){
  local color="$1" message="$2" detail="${3:-}"
  printf '\n%s------------------------------------------------------------%s\n' "$color" "$RESET"
  printf '%s%s%s\n' "$color" "$message" "$RESET"
  [[ -z "$detail" ]] || printf '%s%s%s\n' "$color" "$detail" "$RESET"
  printf '%s------------------------------------------------------------%s\n' "$color" "$RESET"
}
# Every invocation finishes with one additional empty output line.
trap 'printf "\n"' EXIT
fatal()   { failed "$*"; exit 1; }
echononl(){ printf '%-56s' "$*"; }
require_root(){ [[ "${EUID}" -eq 0 ]] || fatal 'Please run as root.'; }
need(){ command -v "$1" >/dev/null 2>&1 || fatal "Missing command: $1"; }
confirm(){ local answer; read -r -p 'Einverstanden [yes/no, default: no]? ' answer || true; [[ "$answer" == yes ]]; }

# Only this script's state file controls rollback. Older manual backups are untouched.
# Each run's backup retains the exact file or symlink, along with active/enabled state.
state_dir_from_marker(){
  [[ -f "$CURRENT_FILE" ]] || return 1
  local p
  IFS= read -r p < "$CURRENT_FILE" || true
  [[ "$p" == "$STATE_DIR"/run-* && -d "$p" && -f "$p/metadata" ]] || fatal 'Invalid resolver rollback marker.'
  printf '%s\n' "$p"
}

is_direct(){
  [[ -f "$RESOLV" && ! -L "$RESOLV" ]] &&
    [[ "$(awk '$1=="nameserver" {print $2}' "$RESOLV")" == '127.0.0.1' ]] &&
    grep -Eq '^[[:space:]]*hosts:[[:space:]]*files[[:space:]]+myhostname[[:space:]]+dns[[:space:]]*$' "$NSS"
}

is_fully_switched(){
  is_direct && ! systemctl is-active --quiet systemd-resolved &&
    [[ "$(systemctl is-enabled systemd-resolved 2>/dev/null || true)" == disabled ]]
}

# BIND must reject a known bogus signature, not merely answer normal queries.
preflight(){
  echononl 'BIND9 on 127.0.0.1:53'
  if dig @127.0.0.1 oopen.de A +time=3 +tries=1 +short | grep -Eq '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'; then ok 'reachable'; else echo; failed 'Local BIND9 does not resolve oopen.de'; return 1; fi
  echononl 'DNSSEC positive test (AD flag)'
  local reply
  reply=$(dig @127.0.0.1 cloudflare.com A +dnssec +adflag +time=5 +tries=1)
  if grep -q 'status: NOERROR' <<< "$reply" && grep -Eq 'flags:.*[[:space:]]ad[;[:space:]]' <<< "$reply"; then ok 'validated'; else echo; failed 'BIND9 DNSSEC positive test failed'; return 1; fi
  echononl 'DNSSEC negative test (SERVFAIL)'
  reply=$(dig @127.0.0.1 www.dnssec-failed.org A +time=5 +tries=1)
  if grep -q 'status: SERVFAIL' <<< "$reply"; then ok 'rejected as expected'; else echo; failed 'BIND9 does not reject invalid DNSSEC test zone'; return 1; fi
}

show_check(){
  require_root
  need dig; need systemctl; need getent
  printf '\n=== DNS resolver status ===\n'
  printf 'resolv.conf: '; if [[ -L "$RESOLV" ]]; then readlink "$RESOLV"; else echo 'regular file'; fi
  grep -E '^[[:space:]]*(nameserver|search|options)[[:space:]]' "$RESOLV" || true
  grep -E '^hosts:' "$NSS" || true
  printf 'systemd-resolved active: %s\n' "$(systemctl is-active systemd-resolved 2>/dev/null || true)"
  printf 'systemd-resolved enabled: %s\n' "$(systemctl is-enabled systemd-resolved 2>/dev/null || true)"
  local readiness=0
  if preflight; then readiness=1; fi
  if is_fully_switched; then
    if (( readiness )); then
      status_banner "$GREY" 'Umstellung auf BIND9 bereits durchgeführt.'
    else
      status_banner "$YELLOW" 'Umstellung bereits durchgeführt, aber DNS-Prüfung fehlgeschlagen.'
    fi
  elif [[ -f "$CURRENT_FILE" ]]; then
    status_banner "$YELLOW" 'Umstellung kann nicht ausgeführt werden.' \
      'Ein vorheriger Rollback-Stand ist vorhanden; zunächst prüfen oder zurückbauen.'
  elif (( readiness )); then
    status_banner "$GREEN" 'Umstellung kann durchgeführt werden.'
  else
    status_banner "$YELLOW" 'Umstellung kann nicht ausgeführt werden.' \
      'BIND9-/DNSSEC-Voraussetzungen nicht erfüllt; bitte Fehler prüfen.'
  fi
  if [[ -f "$CURRENT_FILE" ]]; then
    printf '\n=== Rollback information ===\n'
    info 'Backup from before the last --apply is available'
    info "Backup: $(state_dir_from_marker)"
    info "Restore with: $0 --rollback"
  fi
}

restore_files(){
  local path="$1"
  [[ -d "$path" && -e "$path/nsswitch.conf" ]] || fatal 'Incomplete backup: NSS'
  [[ -e "$path/resolv.conf" || -L "$path/resolv.conf" ]] || fatal 'Incomplete backup: resolv.conf'
  cp -a --remove-destination "$path/nsswitch.conf" "$NSS"
  cp -a --remove-destination "$path/resolv.conf" "$RESOLV"
}

restore_services(){
  local path="$1" enabled active
  enabled=$(sed -n 's/^resolved_enabled=//p' "$path/metadata")
  active=$(sed -n 's/^resolved_active=//p' "$path/metadata")
  case "$enabled" in
    enabled|enabled-runtime) systemctl enable systemd-resolved >/dev/null ;;
    disabled) systemctl disable systemd-resolved >/dev/null ;;
    static|indirect|masked|alias|'') info "Previous enabled state '$enabled'; no enable-state change" ;;
    *) info "Unrecognized previous enabled state '$enabled'; no enable-state change" ;;
  esac
  case "$active" in
    active) systemctl start systemd-resolved ;;
    inactive|failed) systemctl stop systemd-resolved ;;
    *) info "Previous active state '$active'; no service-state change" ;;
  esac
}

rollback_to(){
  local path="$1"
  info "Restoring config from $path"
  # Stop resolved first only when it was previously not active; otherwise start after files.
  restore_files "$path"
  restore_services "$path"
  ok 'Configuration and resolved service state restored'
}

apply(){
  require_root
  for cmd in dig getent systemctl cp mv mktemp grep awk sed date; do need "$cmd"; done
  [[ -f "$NSS" ]] || fatal "$NSS missing"
  [[ -e "$RESOLV" || -L "$RESOLV" ]] || fatal "$RESOLV missing"
  [[ ! -f "$CURRENT_FILE" ]] || {
    if is_fully_switched; then status_banner "$GREY" 'Umstellung auf BIND9 bereits durchgeführt.'; skipped 'No changes'; exit 0; fi
    status_banner "$YELLOW" 'Umstellung kann nicht ausgeführt werden.' 'Vorheriger Rollback-Stand vorhanden.'
    fatal 'An earlier script run has rollback state. Use --check or --rollback first.';
  }
  if is_fully_switched; then status_banner "$GREY" 'Umstellung auf BIND9 bereits durchgeführt.'; skipped 'No changes'; exit 0; fi
  info 'Preflight checks (no changes)'
  if ! preflight; then
    status_banner "$YELLOW" 'Umstellung kann nicht ausgeführt werden.' 'BIND9-/DNSSEC-Voraussetzungen nicht erfüllt.'
    return 1
  fi
  status_banner "$GREEN" 'Umstellung kann durchgeführt werden.'
  printf '\nPlanned changes:\n  hosts: files myhostname dns\n  resolv.conf: nameserver 127.0.0.1\n  stop and disable systemd-resolved\n'
  info 'Network interfaces, routes, BIND9 and MTA are not changed.'
  confirm || { skipped 'Abbruch durch User'; exit 0; }

  mkdir -p "$STATE_DIR"
  chmod 700 "$STATE_DIR"
  local path timestamp
  timestamp=$(date +%Y%m%d-%H%M%S)
  path=$(mktemp -d "$STATE_DIR/run-${timestamp}-XXXXXX")
  cp -a "$RESOLV" "$path/resolv.conf"
  cp -a "$NSS" "$path/nsswitch.conf"
  {
    printf 'resolved_active=%s\n' "$(systemctl is-active systemd-resolved 2>/dev/null || true)"
    printf 'resolved_enabled=%s\n' "$(systemctl is-enabled systemd-resolved 2>/dev/null || true)"
  } > "$path/metadata"
  printf '%s\n' "$path" > "$CURRENT_FILE"
  info "Backup: $path"

  # Roll back this transaction automatically if a command or signal fails.
  trap 'rc=$?; trap - ERR INT TERM; failed "Apply failed (exit $rc). Rolling back"; rollback_to "$path" || failed "Automatic rollback failed; backup: $path"; exit "$rc"' ERR
  trap 'exit 130' INT
  trap 'exit 143' TERM

  if ! grep -Eq '^[[:space:]]*hosts:[[:space:]]*files[[:space:]]+myhostname[[:space:]]+dns[[:space:]]*$' "$NSS"; then
    sed -i -E '/^[[:space:]]*hosts:/c\hosts:          files myhostname dns' "$NSS"
  else skipped 'NSS already configured'; fi

  if [[ ! -f "$RESOLV" || -L "$RESOLV" ]] || ! grep -Eq '^nameserver[[:space:]]+127\.0\.0\.1$' "$RESOLV"; then
    local search_domain=""
    search_domain=$(awk '$1=="search" { $1=""; sub(/^[[:space:]]+/, ""); print; exit }' "$RESOLV" 2>/dev/null || true)
    rm -f "$RESOLV"
    {
      printf '# Local BIND9 recursive resolver; DNSSEC validated by BIND9.\n'
      printf 'nameserver 127.0.0.1\noptions edns0 trust-ad\n'
      [[ -z "$search_domain" ]] || printf 'search %s\n' "$search_domain"
    } > "$RESOLV"
    chmod 644 "$RESOLV"
  else skipped 'resolv.conf already uses local BIND9'; fi

  echononl 'NSS forward resolution'
  getent ahostsv4 oopen.de >/dev/null && ok 'works' || { echo; false; }
  echononl 'NSS reverse resolution'
  getent hosts 2607:f8b0:4864:20::72e >/dev/null && ok 'works' || { echo; false; }
  echononl 'Resolver configuration'
  is_direct && ok 'direct BIND9' || { echo; false; }

  systemctl stop systemd-resolved
  systemctl disable systemd-resolved >/dev/null
  echononl 'systemd-resolved disabled'
  is_fully_switched && ok 'yes' || { echo; false; }
  preflight
  trap - ERR INT TERM
  ok 'Switch completed successfully'
  info "Rollback available: $0 --rollback"
}

rollback(){
  require_root
  local path
  path=$(state_dir_from_marker) || fatal 'No rollback state from this script exists'
  info "Rollback backup: $path"
  confirm || { skipped 'Abbruch durch User'; exit 0; }
  rollback_to "$path"
  rm -f "$CURRENT_FILE"
  ok 'Rollback completed; preserved backup remains in place'
}

case "$MODE" in
  --help)
    cat <<'HELP'
Usage: switch-resolver-to-bind9.sh [--check|--apply|--rollback|--help]
  --check     read-only checks (default)
  --apply     verify BIND9 DNSSEC; back up; switch DNS/NSS; stop+disable resolved
  --rollback  restore files and resolved service state saved before last --apply
  --help      this help
Notes: run as root; requires BIND9 already working; no package changes.
HELP
    ;;
  --check) show_check ;;
  --apply) apply ;;
  --rollback) rollback ;;
esac