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tls-sniff-ciphers
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tls-sniff-ciphers
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#!/usr/bin/env python
# tls-sniff-ciphers (part of ossobv/vcutil) // wdoekes/2016,2023
# // Public Domain
#
# Sniff TLS traffic off the wire and show which TLS/SSL cipher is being
# negotiated. Useful to debug TLS cipher selection issues, commonly
# caused by old clients or bad configuration.
#
# Sure, wireshark does that too, and more. But this is more convenient
# to run on the server directly.
#
# Uses scapy and scpya-ssl_tls to capture and decode packets. Prints a
# very coarse listing of TLS Client Hello and TLS Server Hello packets.
# The client lists what it accepts and the server chooses one, so the
# Server Hello will contain the selected cipher.
#
# Example:
#
# $ tls-sniff-ciphers host 1.2.3.4 and not port 22
#
# 2016-09-30 13:34:16.924273: 1.2.3.4:1782 -> 5.5.5.5:25: TLS Client Hello:
# VERSION 0x303: TLS_1_2
# CIPHER 0xc02f: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
# CIPHER 0xc02b: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
# CIPHER 0x009e: TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
# ...
#
# 2016-09-30 13:34:16.927713: 5.5.5.5:25 -> 1.2.3.4:1782: TLS Server Hello:
# VERSION 0x303: TLS_1_2
# CIPHER 0xc030: TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
#
# Here we see that the TLS setup between 1.2.3.4 and 5.5.5.5 selected
# TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384. That's fine.
#
# At other times, you may see that the client sends ciphers that are not
# in your cipherlist. For instance, you have a cipherlist set to HIGH,
# and the client shows up with cipher 0x000a DES-CBC3-SHA. You can check
# that it's not compatible with your server cipherlist using "openssl
# ciphers":
#
# $ openssl ciphers -V HIGH | grep 0x00,0x0A
# (empty)
#
# $ openssl ciphers -V HIGH:DES-CBC3-SHA | grep 0x00,0x0A
# 0x00,0x0A - DES-CBC3-SHA SSLv3 Kx=RSA Au=RSA Enc=3DES(168) Mac=SHA1
#
# Or, you may see that the server has selected an unexpected cipher
# like 0xc019 TLS_ECDH_anon_WITH_AES_256_CBC_SHA (AECDH-AES256-SHA)
# after which the client reports "unable to get issuer certificate".
# (Well, maybe it shouldn't have provided anonymous ciphers in the if
# it wanted to do authentication.) You can then consider dropping the
# aNULL ciphers from the list:
#
# $ openssl ciphers -V 'HIGH:!aNULL' | grep 0xC0,x19
# (empty)
#
# Installing tls-sniff-ciphers:
#
# apt-get install python-scapy python-pip python-crypto
# pip install scapy-ssl_tls
# install tls-sniff-ciphers /usr/local/bin/
#
# Further reading:
#
# A mapping between the common and OpenSSL cipher names, and a
# description of how the cipher groups work can be found in the
# ciphers(1ssl) man-page.
#
from __future__ import print_function
import sys
from datetime import datetime
from scapy.all import IP, TCP, sniff # python3-scapy (no-install-recommends)
from scapy.main import load_layer
from scapy.layers.tls.crypto.suites import (
_tls_cipher_suites as TLS_CIPHER_SUITES)
assert TLS_CIPHER_SUITES[5].endswith('RSA_WITH_RC4_128_SHA'), TLS_CIPHER_SUITES
TLS_VERSIONS = {
# SSL
0x0002: "SSL_2_0",
0x0300: "SSL_3_0",
# TLS:
0x0301: "TLS_1_0",
0x0302: "TLS_1_1",
0x0303: "TLS_1_2",
0x0304: "TLS_1_3",
# DTLS
0x0100: "PROTOCOL_DTLS_1_0_OPENSSL_PRE_0_9_8f",
0x7f10: "TLS_1_3_DRAFT_16",
0x7f12: "TLS_1_3_DRAFT_18",
0xfeff: "DTLS_1_0",
0xfefd: "DTLS_1_1",
}
# Places lots of stuff in __builtins__. Like:
# TLS, TLSClientHello, TLSServerHello
load_layer('tls')
# Avoid flake8 complaining about us using them out of the blue.
TLS = __builtins__.TLS
TLSClientHello = __builtins__.TLSClientHello
TLSServerHello = __builtins__.TLSServerHello
def traffic_callback(pkt):
if TCP not in pkt:
print(pkt.show(), file=sys.stderr)
return
try:
pkt[TCP].decode_payload_as(TLS)
except TypeError:
pass
else:
if TLS in pkt:
tls_callback(pkt)
def tls_callback(pkt):
# pkt[TLS].show()
if TLSClientHello in pkt or TLSServerHello in pkt:
us = '%s:%s' % (pkt[IP].src, pkt[TCP].sport)
them = '%s:%s' % (pkt[IP].dst, pkt[TCP].dport)
tuple4 = '%s -> %s' % (us, them)
when = datetime.now()
if TLSClientHello in pkt:
hello = pkt[TLSClientHello]
print('%s: %s: TLS Client Hello:' % (when, tuple4))
print(' VERSION 0x%x: %s' % (
hello.version, TLS_VERSIONS.get(hello.version)))
for cipher in hello.ciphers:
print(' CIPHER 0x%04x: %s' % (
cipher, TLS_CIPHER_SUITES.get(cipher)))
print()
if TLSServerHello in pkt:
hello = pkt[TLSServerHello]
print('%s: %s: TLS Server Hello:' % (when, tuple4))
print(' VERSION 0x%x: %s' % (
hello.version, TLS_VERSIONS.get(hello.version)))
print(' CIPHER 0x%04x: %s' % (
hello.cipher, TLS_CIPHER_SUITES.get(hello.cipher)))
print()
def main():
if len(sys.argv) <= 1:
print(
'Listen for TCP packets and report TLS Hello packets.\n'
'Use this to debug SSL/TLS setup problems.\n'
'\n'
'Please supply a pcap-filter expression on the command line,\n'
'like: port 25 and host 1.2.3.4\n',
file=sys.stderr)
sys.exit(1)
pcap_filter = ' '.join(sys.argv[1:])
real_filter = 'tcp and (%s)' % (pcap_filter,)
print('Listening for TCP packets on: %s' % (pcap_filter,))
sniff(prn=traffic_callback, filter=real_filter, store=0)
if __name__ == '__main__':
main()
# vim: set ts=8 sw=4 sts=4 et ai: