#!/usr/bin/env bash
#
# Rebuild dynamic IPtables chain with DNS lookups of named hosts
# Required utils: getent, grep, cut, iptables, logger, stat
#
# SPDX-License-Identifier: MIT

_VERSION="0.0.4"
_NAME=$(basename "${0}")

# Server prep and usage:
#  1. Place script in /usr/local/sbin/dyniptables.sh (root:root, 0744)
#
#  2. Add to system on-boot iptables rules a new filter chain and (j)ump:
#       :DYNAMIC - [0:0]
#       -A INPUT -j DYNAMIC
#     ...where DYNAMIC is the name of the $DCHAIN below
#
#  3. Add to root's crontab a refresh every 6 hours:
#       5 */6 * * * /usr/local/sbin/dyniptables.sh
#
#  4. Add an override.conf to systemd iptables startup:
#         DEB clones: `systemctl edit netfilter-persistent.service`
#          or
#         RPM clones: `systemctl edit iptables.service`              (IPv4)
#                     `systemctl edit ip6tables.service`             (IPv6)
#
#       [Service]
#       ExecStartPost=/usr/local/sbin/dyniptables.sh     # (DEB, all rules)
#        or
#       ExecStartPost=/usr/local/sbin/dyniptables.sh -4  # (RPM, IPv4 only)
#       ExecStartPost=/usr/local/sbin/dyniptables.sh -6  # (RPM, IPv6 only)

# List of hosts, declare emtpy array (do not edit)
declare -a DHOSTS

# Name of chain, declare empty variable
declare DCHAIN=""

# Our config for DHOSTS and DCHAIN
_CONFIG="/etc/dyniptables.conf"

##############################################################################
## Regular Expressions to match IPv4/IPv6
## https://stackoverflow.com/a/17871737/150649
## https://gist.github.com/syzdek/6086792

RE_IPV4="((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])\.)"
RE_IPV4="${RE_IPV4}{3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])"

# [1:2:3:4:5:6:7:8]
RE_IPV6="([0-9a-fA-F]{1,4}:){7,7}[0-9a-fA-F]{1,4}|"
# [1::]                       1:2:3:4:5:6:7::
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,7}:|"
# [1::8]                      1:2:3:4:5:6::8  1:2:3:4:5:6::8
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|"
# [1::7:8]                    1:2:3:4:5::7:8  1:2:3:4:5::8
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|"
# [1::6:7:8]                  1:2:3:4::6:7:8  1:2:3:4::8
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|"
# [1::5:6:7:8]                1:2:3::5:6:7:8  1:2:3::8
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|"
# [1::4:5:6:7:8]              1:2::4:5:6:7:8  1:2::8
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|"
# [1::3:4:5:6:7:8]            1::3:4:5:6:7:8  1::8
RE_IPV6="${RE_IPV6}[0-9a-fA-F]{1,4}:((:[0-9a-fA-F]{1,4}){1,6})|"
# [::2:3:4:5:6:7:8]           ::2:3:4:5:6:7:8  ::8  ::
RE_IPV6="${RE_IPV6}:((:[0-9a-fA-F]{1,4}){1,7}|:)|"
# [fe08::7:8%eth0]            fe08::7:8%1
RE_IPV6="${RE_IPV6}fe08:(:[0-9a-fA-F]{1,4}){2,2}%[0-9a-zA-Z]{1,}|"
# [::255.255.255.255]         ::ffff:255.255.255.255  ::ffff:0:255.255.255.255
RE_IPV6="${RE_IPV6}::(ffff(:0{1,4}){0,1}:){0,1}${RE_IPV4}|"
# [2001:db8:3:4::192.0.2.33]  64:ff9b::192.0.2.33
RE_IPV6="${RE_IPV6}([0-9a-fA-F]{1,4}:){1,4}:${RE_IPV4}"

##
##############################################################################

# Internal variables
declare _QUIET=0 _RULESV4=1 _RULESV6=1 _VALID4=1 _VALID6=1

# Allow Ctrl+C, etc.
function error_exit() {
  echo "Trapped a kill signal, exiting."
  exit 99
}
trap error_exit SIGHUP SIGINT SIGTERM

# Log to SYSLOG
function logme() {
  if [[ $_QUIET -eq 0 ]]; then
    echo "$1" | logger -t dyniptables
  fi
}

# Converting input to int for port check
function to_int() {
  local -i _num="10#${1}"
  echo "${_num}"
}

# Config
function create_config() {
  if [[ -f "${_CONFIG}" ]]; then
    echo "Config file ${_CONFIG} already exists, not creating."
    return 1
  fi
  echo "Creating config ${_CONFIG}"
  cat << 'EOCONF' >> "${_CONFIG}"
# dyniptables config file
#  permissions on this file must be root:root, 0644

# The DHOSTS array is already declared; uncomment and add a line for
# each host to allow, notice '+=' to ADD to the array each line.
#
#  Format: 'TYPE,HOST,PORT' (comma seperated, no spaces!)
#           add as many lines as needed using += as shown
#
#  TYPE is one of: ipv4, ipv6
#  HOST is the remote host to look up in DNS
#  PORT is the local incoming port to open
#
# Examples:
#DHOSTS+=('ipv4,host1.domain.com,10051')
#DHOSTS+=('ipv6,host1.domain.com,10051')
#DHOSTS+=('ipv4,host2.domain.com,10051')
#DHOSTS+=('ipv6,host3.domain.com,10051')

# Name of dyname chain for both IPv4 and IPv6 - IT WILL BE FLUSHED
#
#  The named chain must be created in your iptables chains FIRST manually
#    - this script does not create/delete the basic (empty) chain!
#    - example:
#
#       :DYNAMIC - [0:0]
#       -A INPUT -j DYNAMIC
#
#  - you must use a unique name (not INPUT, etc.)
#  - iptables has a 30char limit on chain names
#
DCHAIN="DYNAMIC"

EOCONF

  # check our work
  if [[ -f "${_CONFIG}" ]]; then
    chown root:root "${_CONFIG}"
    chmod 0644 "${_CONFIG}"
    if [[ $(stat -c "%u:%g:%a" "${_CONFIG}") != "0:0:644" ]]; then
      echo "ERROR: incorrect permissions on ${_CONFIG}"
      return 1
    fi
  else
    echo "ERROR: failed to create ${_CONFIG}"
    return 1
  fi
  return 0
}

# Usage
function usage() {
  echo "$_NAME version $_VERSION"
  echo
  echo "Rebuild dynamic IP chain with DNS lookups of named hosts"
  echo
  echo " Usage: $_NAME [-q] [-4|-6] || [-C|-V|-h]"
  echo " Default behavior: both IPv4 and IPv6 rules processed."
  echo
  echo "  -C    Create default config and exit"
  echo "  -4    Process IPv4 rules only"
  echo "  -6    Process IPv6 rules only"
  echo "  -q    Suppress logging (quiet mode)"
  echo "  -V    Version of program"
  echo "  -h    This help text"
  echo
  echo " Note: -4 and -6 together results in no rules processed."
}

# Commandline options
while getopts ":C46qVh" opt; do
  case "$opt" in
    C)
      create_config
      exit $? ;;
    4)
      _RULESV6=0 ;;
    6)
      _RULESV4=0 ;;
    q)
      _QUIET=1 ;;
    V)
      echo "$_NAME version $_VERSION"
      exit 0 ;;
    h)
      usage
      exit 0 ;;
    *)
      echo "Unrecognized option: $OPTARG (Run '$_NAME -h' for help)"
      exit 1 ;;
  esac
done
shift $((OPTIND-1))

# Attempt to read in our config
if [[ -r "${_CONFIG}" ]]; then
  if [[ $(stat -c "%u:%g:%a" "${_CONFIG}") != "0:0:644" ]]; then
    echo "ERROR: Config file security - ${_CONFIG}"
    echo "ERROR: Config is not 'chown root:root', 'chmod 644' - exiting."
    exit 1
  else
    # shellcheck source=/dev/null
    source "${_CONFIG}"
    if [[ ${#DHOSTS[@]} -eq 0 ]]; then
      echo "ERROR: no hosts configred in DHOSTS, exiting."
      exit 1
    fi
    if [[ -z "${DCHAIN}" ]]; then
      echo "ERROR: no chain configured in DCHAIN, exiting."
      exit 1
    fi
  fi
else
  echo "ERROR: Missing config ${_CONFIG}"
  echo "ERROR: Run '$_NAME -C' to create default, then edit."
  exit 1
fi

# Networking may not be up; running a bad iptable rule can cause the
# service to fail the ExecStartPost in systemd and leave the system
# open. A lot of error checking is required for safety.

# Flush the existing chains; if the IP changed, we don't know what the
# old host->IP was without maintaining our own database
if [[ $_RULESV4 -eq 1 ]]; then
  logme "Flushing $DCHAIN IPv4 chain"
  /sbin/iptables -F "${DCHAIN}"
  # shellcheck disable=SC2181
  if [[ $? -ne 0 ]]; then
    logme "IPv4 chain ${DCHAIN} flush error, skipping IPv4 rules"
    _VALID4=0
  fi
fi
if [[ $_RULESV6 -eq 1 ]]; then
  logme "Flushing $DCHAIN IPv6 chain"
  /sbin/ip6tables -F "${DCHAIN}"
  # shellcheck disable=SC2181
  if [[ $? -ne 0 ]]; then
    logme "IPv6 chain ${DCHAIN} flush error, skipping IPv6 rules"
    _VALID6=0
  fi
fi

# Loop through the entries, take action
# for each element:
#  convert comma to space to make new child array
#  _tmparray[0] = TYPE
#  _tmparray[1] = HOST
#  _tmparray[2] = PORT

# shellcheck disable=SC2068
for iprule in ${DHOSTS[@]}; do
  # shellcheck disable=SC2206
  _tmparray=(${iprule//,/ })

  # ensure 3 elements in the array
  if [[ ${#_tmparray[@]} -ne 3 ]]; then
    logme "Malformed entry, skipping: $iprule"
    continue
  fi

  # port check
  # https://docwhat.org/bash-checking-a-port-number
  _portnum=$(to_int "${_tmparray[2]}" 2>/dev/null)
  # shellcheck disable=SC2004
  if (( $_portnum < 1 || $_portnum > 65535 )) ; then
    logme "Invalid port, skipping: $iprule"
    continue
  fi

  # IPv4 host
  if [[ "${_tmparray[0]}" == "ipv4" ]]; then

    # Silently skip if IPv4 ruls are disabled
    if [[ $_RULESV4 -ne 1 ]]; then
      continue
    fi

    # If we couldn't flush the chain, skip the rule
    if [[ $_VALID4 -ne 1 ]]; then
      logme "IPv4 chain error, skipping: $iprule"
      continue
    fi

    # networking may not be up
    _hostip=$(getent ahostsv4 "${_tmparray[1]}" | \
              grep STREAM | \
              cut -d' ' -f1 | \
              grep -iE "${RE_IPV4}")
    if [[ -z "$_hostip" ]]; then
      logme "Unknown host, skipping: $iprule"
      continue
    fi

    # add the rule if all checks pass
    logme "Adding $_hostip to ${_tmparray[0]} chain, port ${_tmparray[2]}"
    /sbin/iptables -I "$DCHAIN" -p tcp -m tcp \
      -s i"${_hostip}" --dport "${_tmparray[2]}" -j ACCEPT

  # IPv6 host
  elif [[ "${_tmparray[0]}" == "ipv6" ]]; then

    # Silently skip if IPv6 ruls are disabled
    if [[ $_RULESV6 -ne 1 ]]; then
      continue
    fi

    # If we couldn't flush the chain, skip the rule
    if [[ $_VALID6 -ne 1 ]]; then
      logme "IPv6 chain error, skipping: $iprule"
      continue
    fi

    # networking may not be up
    _hostip=$(getent ahostsv6 "${_tmparray[1]}" | \
              grep STREAM | \
              cut -d' ' -f1 | \
              grep -iE "${RE_IPV6}")
    if [[ -z "$_hostip" ]]; then
      logme "Unknown host, skipping: $iprule"
      continue
    fi

    # add the rule if all checks pass
    logme "Adding $_hostip to ${_tmparray[0]} chain, port ${_tmparray[2]}"
    /sbin/ip6tables -I "$DCHAIN" -p tcp -m tcp \
      -s "${_hostip}" --dport "${_tmparray[2]}" -j ACCEPT

  # Oops
  else
    logme "Invalid type, skipping: $iprule"
    continue
  fi

done;

# Clean exit
exit 0