[tor-bugs] #4185 [Tor Bridge]: Bridge easily detected by GFW
Tor Bug Tracker & Wiki
torproject-admin at torproject.org
Tue Jan 3 10:48:54 UTC 2012
#4185: Bridge easily detected by GFW
------------------------+---------------------------------------------------
Reporter: hrimfaxi | Owner:
Type: defect | Status: new
Priority: normal | Milestone: Tor: unspecified
Component: Tor Bridge | Version: Tor: 0.2.3.5-alpha
Keywords: blocking | Parent:
Points: | Actualpoints:
------------------------+---------------------------------------------------
Comment(by asn):
> Checked several IP of them, it seems that most of them are dynamic IP
addresses of DSL and PPP running variety of OS, from Windows 2003 with
Terminal Service, Linux with Mysql, cheap home router.
>
Seems so, indeed. Fucked up, isn't it?
It seems like none of their probers is an actual dedicated server probing
box...
> Some questions:
> a) After the SSL negotiation, does the GFW probes also send an HTTP
request or just finish the SSL handshake and close it?
>
There are two kinds of probes: 'Garbage probing' and 'Tor probing'.
In 'Garbage probing', the prober connects and spits a "random" blob of
data to the host, a la: http://www.nsc.liu.se/~nixon/sshprobes.html
In 'Tor probing', the prober connects, does an SSL handshake, does an
SSL renegotiation and then speaks the Tor protocol. Specifically, the
Tor probers we managed to capture, build an one-hop circuit and send a
BEGIN_DIR cell, like Tor clients are supposed to do. When the bridge
replies with its descriptor, the prober hangs up.
> b) Does the prober announce a specific/detectable set of SSL/TLS
version/ciphers?
>
The SSL handshakes of probers seem similar to the SSL handshake of Tor
clients.
The cipher list (also, see #4744) is the same, and they both have the
same fake 'server_name' extension. What is different is that probers
don't seem to have the ECC TLS extensions, which I '''think''' are
added automatically by new versions of OpenSSL (still, probers seem to
have the RI ciphersuite, which is also added automatically by new
versions of OpenSSL).
> c) Does anyone checked actively with OS fingerprinting tools if the
"prober's OS" can be recognized?
We did some checks. There seems to be a mixture of Windows and Linux
machines.
I've noticed that some of the machines had IIS running.
Also, some probers [1] are using a source port that is not in the
ephemeral port range of modern OSes (usually higher than TCP port
20000), which hints to Windows XP [2].
> Would it possible that the detection come from some predictable guessing
of the Common Name and Certificate Chain?
>
> Connecting via OpenSSL to a Tor Server i see:
> Certificate chain
>
> 0 s:/CN=www.zb42yfn4kbhtd.net
>
> i:/CN=www.noyz3wih.com
>
> Are the way those fields get generated guessable by Active Probing
and/or passive analysis?
>
Sure. Although, it seems like the current probes are caused by the
cipher list (see #4744). Also, see proposal 179.
[0]:
This is an example prober SSL handshake:
{{{
TLSv1 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)
Version: TLS 1.0 (0x0301)
Length: 139
Handshake Protocol: Client Hello
Handshake Type: Client Hello (1)
Length: 135
Version: TLS 1.0 (0x0301)
Random
gmt_unix_time: Dec 9, 2011 07:18:12.000000000 CET
random_bytes:
2ade076693d5ecf217d0fc3cfb0e6d1c9cb03f81358b2534...
Session ID Length: 0
Cipher Suites Length: 58
Cipher Suites (29 suites)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
(0xc00a)
Cipher Suite: TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (0xc014)
Cipher Suite: TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x0039)
Cipher Suite: TLS_DHE_DSS_WITH_AES_256_CBC_SHA (0x0038)
Cipher Suite: TLS_ECDH_RSA_WITH_AES_256_CBC_SHA (0xc00f)
Cipher Suite: TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA (0xc005)
Cipher Suite: TLS_RSA_WITH_AES_256_CBC_SHA (0x0035)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_RC4_128_SHA (0xc007)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
(0xc009)
Cipher Suite: TLS_ECDHE_RSA_WITH_RC4_128_SHA (0xc011)
Cipher Suite: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013)
Cipher Suite: TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x0033)
Cipher Suite: TLS_DHE_DSS_WITH_AES_128_CBC_SHA (0x0032)
Cipher Suite: TLS_ECDH_RSA_WITH_RC4_128_SHA (0xc00c)
Cipher Suite: TLS_ECDH_RSA_WITH_AES_128_CBC_SHA (0xc00e)
Cipher Suite: TLS_ECDH_ECDSA_WITH_RC4_128_SHA (0xc002)
Cipher Suite: TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA (0xc004)
Cipher Suite: TLS_RSA_WITH_RC4_128_MD5 (0x0004)
Cipher Suite: TLS_RSA_WITH_RC4_128_SHA (0x0005)
Cipher Suite: TLS_RSA_WITH_AES_128_CBC_SHA (0x002f)
Cipher Suite: TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
(0xc008)
Cipher Suite: TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA (0xc012)
Cipher Suite: TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA (0x0016)
Cipher Suite: TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA (0x0013)
Cipher Suite: TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA (0xc00d)
Cipher Suite: TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA
(0xc003)
Cipher Suite: SSL_RSA_FIPS_WITH_3DES_EDE_CBC_SHA (0xfeff)
Cipher Suite: TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a)
Cipher Suite: TLS_EMPTY_RENEGOTIATION_INFO_SCSV (0x00ff)
Compression Methods Length: 1
Compression Methods (1 method)
Compression Method: null (0)
Extensions Length: 36
Extension: server_name
Type: server_name (0x0000)
Length: 28
Data (28 bytes)
Extension: SessionTicket TLS
Type: SessionTicket TLS (0x0023)
Length: 0
Data (0 bytes)
}}}
[1]:
2011-12-05 18:26:58.839414 : 218.10.51.83:1629
2011-12-05 18:26:58.839469 |
'\xd7s\x10\x05\x95\x86\xe6\x89ok\x07\xd2\x95\xab}\x16\x10\xc4\xc8F\xac|ax\x86d\xc6\x03\x19_=\xd1\x13W\x14\x8dG\x0e;Y.\'\xd2\x90\xa3\xab
X\xb1w\x97\xe4e\xd8A=\xc7\xd0\xe4S\xc46\xcb\x86\xea9\x9bIz\xe0\xa8\xe7\xb7\x1b\x8c\xa8H\x1aK\xfdl\t\xdf\xa0\xb4\xd3\xcd\xe8r~\xd5\\u\xfa\xdb\xab\xad\xf5s\'U\x9a\x0e\x0c4\x993|3\xfc\xf8\x1e\x85\xd8\xbd9\xab\n!\x9d\x87vx\xfbp\xd2&\x9c\xc7\x18\xc2\x1c\xb1O\xa6eg\xd8`\x99UX\xb6\xd90s\x12[|\xb2\xf7\x83(\xee~\x17\xc0$\xb2\x87;\xf3"lB\xc7\xd0\xa8\x1f\xaf\xc1s\x07wL\xb6j\xdd\x11e\x8f\x87\xe76u\xe5\xcb5\t};C\xef\\\xae1\xa2\xfdX\xc0\xac\x19\xb33\x90\xfe\xe8y[\xf8]\xe9\xfeE\x9es\xa9i\'\xb1easT<\xa1\x84]\x9e\xdc\x9c\xcatO\xfc\x04\xcdd\xfb(\xdbY\x91\xd9\x1d/L\xc5\x98\xf1\xf5|Rh\xd0\r\t\xd3i\'/\x84p\xa3\xd3l&\xa0\xcf"G*\xf9\xd7\x03\x16\x07\xcdZ\x1e'
[2]:
http://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html#Windows
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/4185#comment:27>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
More information about the tor-bugs
mailing list