tls-sniff-ciphers

#!/usr/bin/sh-c exec "$(which python3 python false|head -n1)" "$0" "$@"
# tls-sniff-ciphers (part of ossobv/vcutil) // wdoekes/2016 // 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: ECDHE_RSA_WITH_AES_128_GCM_SHA256
#       CIPHER 0xc02b: ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
#       CIPHER 0x009e: 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: 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
# 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.
#
import sys
from datetime import datetime
from scapy.all import IP, TCP, sniff
from scapy_ssl_tls.ssl_tls import (
    TLS_VERSIONS, TLS_CIPHER_SUITES,
    TLS, TLSClientHello, TLSServerHello)


def traffic_callback(pkt):
    if TCP not in pkt:
        print >>sys.stderr, pkt.show()
        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.cipher_suites:
                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)
            cipher = hello.cipher_suite
            print '  VERSION 0x%x: %s' % (
                hello.version, TLS_VERSIONS.get(hello.version))
            print '  CIPHER 0x%04x: %s' % (
                cipher, TLS_CIPHER_SUITES.get(cipher))
            print


def main():
    if len(sys.argv) <= 1:
        print >>sys.stderr, (
            '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')
        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: