Add support for /etc/wake-on-lan/special-wakeup.d

Server systems, like the big boxes, can't be woken up using
traditional wake-on-lan magic packets. For those systems, we plan to
use IPMI. The idea is to have an executable file, probably a simple
bash script, in /etc/wake-on-lan/special-wakeup.d/<hostname> that
triggers the IPMI wakeup for the particular host.

wol.py

@@ -1,12 +1,15 @@
 #!/usr/bin/env python3
 
+from pathlib import Path
 from pyparsing import *
 from typing import Dict, Optional
 import socketserver
 import argparse
 import logging
 import psutil
+import stat
 import socket
+import subprocess
 import sys
 import os
 import re
@@ -74,9 +77,16 @@ def sendMagicPacket(macAddress: str, iface: str) -> bool:
 			logging.exception('Sending magic packet to {} (on {}) failed'.format(macAddress, iface))
 	return False
 
+SPECIAL_WAKEUP_DIR = Path("/etc/wake-on-lan/special-wakeup.d")
+
 def wake(hostname: str) -> bool:
 	global hosts
-	if hostname in hosts:
+	special_wakeup_file = SPECIAL_WAKEUP_DIR / hostname
+	if special_wakeup_file.is_file() and os.access(special_wakeup_file, os.X_OK):
+		with subprocess.Popen(special_wakeup_file) as proc:
+			returncode = proc.wait()
+			return returncode == 0
+	elif hostname in hosts:
 		logging.info('Waking up {}...'.format(hostname))
 		host = hosts[hostname]
 		if not 'iface' in host:
@@ -89,6 +99,8 @@ def wake(hostname: str) -> bool:
 		logging.warning('Unknown host "{}"'.format(hostname))
 		return False
 
+LDH_RE = re.compile(r"[0-9A-Za-z\-]+")
+
 class WakeRequestHandler(socketserver.StreamRequestHandler):
 	def handle(self):
 		self.connection.settimeout(6)
@@ -97,12 +109,20 @@ def handle(self):
 		try:
 			while self.rfile:
 				hostname = self.rfile.readline().decode('ascii').strip()
-				if hostname:
+				if not hostname:
+					break
+
+				# Check if nostname matches the letter-digits-hyphen rule of DNS.
+				# This also prevents remote code execution in case hostname is
+				# e.g., "../../../bin/sh".
+				if LDH_RE.match(hostname):
 					logging.info('Request WoL at "{}" from {}'.format(hostname, client))
 					success = wake(hostname)
-					self.wfile.write(b"success\n" if success else b"failed\n")
 				else:
-					break
+					success = False
+					logging.warning(f"Invalid hostname: {hostname}")
+				self.wfile.write(b"success\n" if success else b"failed\n")
+
 		except socket.timeout:
 			logging.debug('Timeout of {}'.format(client))