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freb/check_openvpn.py

Last active Sep 25, 2020
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#!/usr/bin/env python
#
# Check if an OpenVPN server runs on a given UDP or TCP port.
#
import os
import sys
import time
import hmac
import hashlib
import struct
import socket
import argparse
import binascii
P_CONTROL_HARD_RESET_CLIENT_V2 = 7
P_CONTROL_HARD_RESET_SERVER_V2 = 8
MAX_P_CONTROL_SIZE = 128 # TODO: adopt
MAX_CIPHER_KEY_LENGTH = 64
MAX_HMAC_KEY_LENGTH = 64
BUFFER_SIZE = 1024
ALGORITHMS_AVAILABLE = hashlib.algorithms_available if hasattr(hashlib, "algorithms_available") else hashlib.algorithms
def ok(msg):
print('OK: %s' % msg)
return 0
def warning(msg):
print('WARN: %s' % msg)
return 1
def critical(msg):
print('CRIT: %s' % msg)
return 2
def build_p_control_hard_reset_client_v2(sid, digest, key):
# see openvpn source code "src/openvpn/crypto.c" function openvpn_decrypt_v1
pid = 1 # packet id
ts = int(time.time()) # net time
if key:
# generate hmac
h = hmac.new(key, digestmod=digest)
h.update(struct.pack('>I', pid)) # packet id
h.update(struct.pack('>I', ts)) # net time
h.update(struct.pack('>B', P_CONTROL_HARD_RESET_CLIENT_V2 << 3)) # packet type
h.update(sid) # session id
h.update(struct.pack('>B', 0)) # message packet id array length
h.update(struct.pack('>I', 0)) # message packet id
# build packet
result = b''
result += struct.pack('>B', P_CONTROL_HARD_RESET_CLIENT_V2 << 3) # packet type
result += sid # session id
if key: result += h.digest() # hmac
if key: result += struct.pack('>I', pid) # packet id
if key: result += struct.pack('>I', ts) # net time
result += struct.pack('>B', 0) # message packet id array length
result += struct.pack('>I', 0) # message packet id
return result
def validate_p_control_hard_reset_server_v2(packet, query_sid, digest, digest_size, key):
# see openvpn source code "src/openvpn/crypto.c" function openvpn_decrypt_v1
# identify packet
if False: pass
elif len(packet) - struct.unpack('>B', packet[9:10])[0] * 4 == 14: plen = 0 # type sid mpida mpid
elif len(packet) - struct.unpack('>B', packet[9:10])[0] * 4 == 22: plen = 1 # type sid mpida rsid mpid
elif len(packet) - struct.unpack('>B', packet[17+digest_size:18+digest_size])[0] * 4 == 30+digest_size: plen = 2 # type sid hmac pid ts mpida rsid mpid
else: return 20
# parse packet
ptype = struct.unpack('>B', packet[:1])[0] # packet type
packet = packet[1:]
sid = packet[:8] # session id
packet = packet[8:]
if plen >= 2:
phmac = packet[:digest_size] # hmac
packet = packet[digest_size:]
pid = struct.unpack('>I', packet[:4])[0] # packet id
packet = packet[4:]
ts = struct.unpack('>I', packet[:4])[0] # net time
packet = packet[4:]
mpidlen = struct.unpack('>B', packet[:1])[0] # message packet id array length
packet = packet[1:]
mpidarray = [] # packet id array
for i in range(mpidlen):
mpidarray.append(struct.unpack('>I', packet[:4])[0]) # message packet id array element
packet = packet[4:]
if plen >= 1:
rsid = packet[:8] # remote session id
packet = packet[8:]
mpid = struct.unpack('>I', packet[:4])[0] # message packet id
# validate packet
if ptype != P_CONTROL_HARD_RESET_SERVER_V2 << 3: return 20
if mpid != 0: return 20
if plen >= 1 and rsid != query_sid: return 20
if plen >= 2 and key:
if pid != 1: return 20
# generate hmac
h = hmac.new(key, digestmod=digest)
h.update(struct.pack('>I', pid)) # packet id
h.update(struct.pack('>I', ts)) # net time
h.update(struct.pack('>B', ptype)) # packet type
h.update(sid) # session id
h.update(struct.pack('>B', mpidlen)) # message packet id array length
for e in mpidarray: h.update(struct.pack('>I', e)) # message packet id array element
h.update(rsid) # remote session id
h.update(struct.pack('>I', mpid)) # message packet id
if phmac != h.digest(): return 10
return 1
return 0
def check(host, port, tcp, timeout, digest, digest_size, client_key, server_key, retrycount, validate):
sid = os.urandom(8) # session id
packet = build_p_control_hard_reset_client_v2(sid, digest, client_key)
query_server = query_tcp_server if tcp else query_udp_server
try:
s = create_socket(host, port, tcp, timeout)
except socket.error:
return critical('Unable to create socket')
try:
response = query_server(s, host, port, packet, retrycount)
except RuntimeError:
return critical('Invalid response')
except:
return critical('Not responding')
finally:
s.close()
if response is None:
return critical('Not responding')
# for debugging purpose
# response = binascii.hexlify(response)
if not validate: return ok('Responded')
valid = validate_p_control_hard_reset_server_v2(response, sid, digest, digest_size, server_key)
if valid == 0: return ok('Response validated')
if valid == 1: return ok('Response validated, checked HMAC')
if valid == 10: return warning('Invalid HMAC')
return critical('Invalid response')
def create_socket(host, port, tcp, timeout):
# thanks to glucas for the idea
sock_type = socket.SOCK_STREAM if tcp else socket.SOCK_DGRAM
af, socktype, proto, canonname, sa = socket.getaddrinfo(host, port, socket.AF_UNSPEC, sock_type)[0]
s = socket.socket(af, socktype, proto)
s.settimeout(timeout)
return s
def query_udp_server(s, host, port, packet, retrycount):
# Send up to 'retrycount' UDP packets with 'timeout' secs between each.
# Return data after receiving first UDP packet.
for i in range(retrycount):
try:
s.sendto(packet, (host, port))
# TODO: check response address
data, _ = s.recvfrom(BUFFER_SIZE)
return data
except socket.timeout:
pass
except:
return None
return None
def query_tcp_server(s, host, port, packet, retrycount):
# ignore retrycount
s.connect((host, port))
s.send(struct.pack('>H', len(packet)) + packet)
length = struct.unpack('>H', s.recv(2))[0]
if length > MAX_P_CONTROL_SIZE: raise RuntimeError
data = s.recv(length)
if len(data) != length: raise RuntimeError
return data
def readkey(path):
key = None
try:
with open(path, 'r') as myfile: key = myfile.read()
except:
return None
index_start = key.find('-\n');
index_end = key.find('\n-', index_start);
if index_start < 0 or index_end < 0 or index_end <= index_start:
return None
index_start += 2
key = key[index_start:index_end].replace('\n', '').replace('\r', '')
key = binascii.unhexlify(key)
return key
def optionsparser(argv=None):
parser = argparse.ArgumentParser()
parser.add_argument('-p', '--port', help='set port number (default is %(default)d)', type=int, default=1194)
parser.add_argument('-t', '--tcp', help='use tcp instead of udp', action='store_true')
parser.add_argument('--timeout', help='set timeout in seconds, for udp counted per packet (default is %(default)d)', type=int, default=2)
parser.add_argument('--digest', help='set digest algorithm (default is "%(default)s")', default='sha1')
parser.add_argument('--digest-size', help='set HMAC digest size', type=int)
parser.add_argument('--digest-key-client', help='set client HMAC key')
parser.add_argument('--digest-key-server', help='set server HMAC key for packet validation')
parser.add_argument('--tls-auth', help='set tls-auth file')
# TODO: direction argument (normal, inverse)
parser.add_argument('--tls-auth-inverse', help='set tls-auth file direction to inverse (1)', action='store_true')
parser.add_argument('--retrycount', help='number of udp retries before giving up (default is %(default)d)', type=int, default=3)
parser.add_argument('--no-validation', help='do not validate response', action='store_true')
parser.add_argument('host', help='the OpenVPN host name or IP')
return parser.parse_args(argv)
def main(argv=None):
args = optionsparser(argv)
if args.digest_size and args.digest_size < 0:
critical('digest size must be positive')
if args.retrycount < 1:
critical('retry count must be positive')
if args.tls_auth and (args.digest_key_client or args.digest_key_server):
critical('--tls-auth cannot go with --digest-key')
client_key = binascii.unhexlify(args.digest_key_client) if args.digest_key_client else None
server_key = binascii.unhexlify(args.digest_key_server) if args.digest_key_server else None
digest = args.digest
digest_size = args.digest_size
digest = digest.lower()
if digest not in ALGORITHMS_AVAILABLE:
return critical('digest not available')
try:
digest = getattr(hashlib, digest)
if not digest_size: digest_size = digest().digest_size
except:
return critical('digest creation failed')
if args.tls_auth:
# see openvpn source code "src/openvpn/crypto.h", "src/openvpn/crypto.c" and "src/openvpn/crypto_backend.h"
# 64 byte cipher direction 0, 64 byte hmac direction 0, 64 byte cipher direction 1, 64 byte hmac direction 1 (=> 2048 bit)
key = readkey(args.tls_auth)
if key == None: return critical('cannot read tls auth file')
index_start = MAX_CIPHER_KEY_LENGTH + MAX_HMAC_KEY_LENGTH + MAX_CIPHER_KEY_LENGTH
index_end = index_start + MAX_HMAC_KEY_LENGTH
client_key = key[index_start:index_end]
index_start = MAX_CIPHER_KEY_LENGTH
index_end = index_start + MAX_HMAC_KEY_LENGTH
server_key = key[index_start:index_end]
if args.tls_auth_inverse: client_key, server_key = server_key, client_key
# reduce key size to required size
client_key = client_key[:digest_size]
server_key = server_key[:digest_size]
return check(args.host, args.port, args.tcp, args.timeout, digest, digest_size, client_key, server_key, args.retrycount, not args.no_validation)
if __name__ == '__main__':
code = main()
sys.exit(code)
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