Ключевые слова:security, stack, ping, (найти похожие документы) Subject: [Phrack] Remote OS detection via TCP/IP Stack FingerPrinting
---[ Phrack Magazine Volume 8, Issue 54 Dec 25th, 1998, article 09 of 12
-------------------------[ Remote OS detection via TCP/IP Stack FingerPrinting
--------[ Fyodor <firstname.lastname@example.org> (www.insecure.org) October 18, 1998
This paper discusses how to glean precious information about a host by querying
its TCP/IP stack. I first present some of the "classical" methods of
determining host OS which do not involve stack fingerprinting. Then I
describe the current "state of the art" in stack fingerprinting tools. Next
comes a description of many techniques for causing the remote host to leak
information about itself. Finally I detail my (nmap) implementation of this,
followed by a snapshot gained from nmap which discloses what OS is running on
many popular Internet sites.
I think the usefulness of determining what OS a system is running is pretty
obvious, so I'll make this section short. One of the strongest examples of
this usefulness is that many security holes are dependent on OS version. Let's
say you are doing a penetration test and you find port 53 open. If this is a
vulnerable version of Bind, you only get one chance to exploit it since a
failed attempt will crash the daemon. With a good TCP/IP fingerprinter, you
will quickly find that this machine is running 'Solaris 2.51' or 'Linux 2.0.35'
and you can adjust your shellcode accordingly.
A worse possibility is someone scanning 500,000 hosts in advance to see what
OS is running and what ports are open. Then when someone posts (say) a root
hole in Sun's comsat daemon, our little cracker could grep his list for
'UDP/512' and 'Solaris 2.6' and he immediately has pages and pages of rootable
boxes. It should be noted that this is SCRIPT KIDDIE behavior. You have
demonstrated no skill and nobody is even remotely impressed that you were able
to find some vulnerable .edu that had not patched the hole in time. Also,
people will be even _less_ impressed if you use your newfound access to deface
the department's web site with a self-aggrandizing rant about how damn good
you are and how stupid the sysadmins must be.
Another possible use is for social engineering. Lets say that you are scanning
your target company and nmap reports a 'Datavoice TxPORT PRISM 3000 T1
CSU/DSU 6.22/2.06'. The hacker might now call up as 'Datavoice support' and
discuss some issues about their PRISM 3000. "We are going to announce a
security hole soon, but first we want all our current customers to install the
patch -- I just mailed it to you..." Some naive administrators might assume
that only an authorized engineer from Datavoice would know so much about their
Another potential use of this capability is evaluation of companies you may
want to do business with. Before you choose a new ISP, scan them and see what
equipment is in use. Those "$99/year" deals don't sound nearly so good when
you find out they have crappy routers and offer PPP services off a bunch of
----[ CLASSICAL TECHNIQUES
Stack fingerprinting solves the problem of OS identification in a unique way.
I think this technique holds the most promise, but there are currently many
other solutions. Sadly, this is still one the most effective of those
playground~> telnet hpux.u-aizu.ac.jp
Connected to hpux.u-aizu.ac.jp.
Escape character is '^]'.
HP-UX hpux B.10.01 A 9000/715 (ttyp2)
There is no point going to all this trouble of fingerprinting if the machine
will blatantly announce to the world exactly what it is running! Sadly, many
vendors ship _current_ systems with these kind of banners and many admins do
not turn them off. Just because there are other ways to figure out what OS is
running (such as fingerprinting), does not mean we should just announce our OS
and architecture to every schmuck who tries to connect.
The problems with relying on this technique are that an increasing number of
people are turning banners off, many systems don't give much information, and
it is trivial for someone to "lie" in their banners. Nevertheless, banner
reading is all you get for OS and OS Version checking if you spend thousands of
dollars on the commercial ISS scanner. Download nmap or queso instead and
save your money :).
Even if you turn off the banners, many applications will happily give away
this kind of information when asked. For example lets look at an FTP server:
payfonez> telnet ftp.netscape.com 21
Connected to ftp.netscape.com.
Escape character is '^]'.
220 ftp29 FTP server (UNIX(r) System V Release 4.0) ready.
215 UNIX Type: L8 Version: SUNOS
First of all, it gives us system details in its default banner. Then if we
give the 'SYST' command it happily feeds back even more information.
If anon FTP is supported, we can often download /bin/ls or other binaries and
determine what architecture it was built for.
Many other applications are too free with information. Take web servers for
playground> echo 'GET / HTTP/1.0\n' | nc hotbot.com 80 | egrep '^Server:'
Hmmm ... I wonder what OS those lamers are running.
Other classic techniques include DNS host info records (rarely effective) and
social engineering. If the machine is listening on 161/udp (snmp), you are
almost guaranteed a bunch of detailed info using 'snmpwalk' from the CMU SNMP
tools distribution and the 'public' community name.
----[ CURRENT FINGERPRINTING PROGRAMS
Nmap is not the first OS recognition program to use TCP/IP fingerprinting.
The common IRC spoofer sirc by Johan has included very rudimentary
fingerprinting techniques since version 3 (or earlier). It attempts to place
a host in the classes "Linux", "4.4BSD", "Win95", or "Unknown" using a few
simple TCP flag tests.
Another such program is checkos, released publicly in January of this year by
Shok of Team CodeZero in Confidence Remains High Issue #7. The fingerprinting
techniques are exactly the same as SIRC, and even the _code_ is identical in
many places. Checkos was privately available for a long time prior to the
public release, so I have no idea who swiped code from whom. But neither
seems to credit the other. One thing checkos does add is telnet banner
checking, which is useful but has the problems described earlier.
Su1d also wrote an OS checking program. His is called SS and as of Version
3.11 it can identify 12 different OS types. I am somewhat partial to this one
since he credits my nmap program for some of the networking code :).
Then there is queso. This program is the newest and it is a huge leap forward
from the other programs. Not only do they introduce a couple new tests, but
they were the first (that I have seen) to move the OS fingerprints _out_ of
the code. The other scanners included code like:
/* from ss */
if ((flagsfour & TH_RST) && (flagsfour & TH_ACK) && (winfour == 0) &&
(flagsthree & TH_ACK))
reportos(argv,argv,"Livingston Portmaster ComOS");
Instead, queso moves this into a configuration file which obviously scales
much better and makes adding an OS as easy as appending a few lines to a
Queso was written by Savage, one of the fine folks at Apostols.org.
One problem with all the programs describe above is that they are very limited
in the number of fingerprinting tests which limits the granularity of answers.
I want to know more than just 'this machine is OpenBSD, FreeBSD, or NetBSD', I
wish to know exactly which of those it is as well as some idea of the release
version number. In the same way, I would rather see 'Solaris 2.6' than simply
'Solaris'. To achieve this response granularity, I worked on a number of
fingerprinting techniques which are described in the next section.
----[ FINGERPRINTING METHODOLOGY
There are many, many techniques which can be used to fingerprint networking
stacks. Basically, you just look for things that differ among operating
systems and write a probe for the difference. If you combine enough of these,
you can narrow down the OS very tightly. For example nmap can reliably
distinguish Solaris 2.4 vs. Solaris 2.5-2.51 vs Solaris 2.6. It can also tell
Linux kernel 2.0.30 from 2.0.31-34 or 2.0.35. Here are some techniques:
The FIN probe -- Here we send a FIN packet (or any packet without an
ACK or SYN flag) to an open port and wait for a response. The
correct RFC793 behavior is to NOT respond, but many broken
implementations such as MS Windows, BSDI, CISCO, HP/UX, MVS, and
IRIX send a RESET back. Most current tools utilize this
The BOGUS flag probe -- Queso is the first scanner I have seen to use
this clever test. The idea is to set an undefined TCP "flag" ( 64
or 128) in the TCP header of a SYN packet. Linux boxes prior to
2.0.35 keep the flag set in their response. I have not found any
other OS to have this bug. However, some operating systems seem
to reset the connection when they get a SYN+BOGUS packet. This
behavior could be useful in identifying them.
TCP ISN Sampling -- The idea here is to find patterns in the initial
sequence numbers chosen by TCP implementations when responding to
a connection request. These can be categorized in to many groups
such as the traditional 64K (many old UNIX boxes), Random
increments (newer versions of Solaris, IRIX, FreeBSD, Digital
UNIX, Cray, and many others), True "random" (Linux 2.0.*, OpenVMS,
newer AIX, etc). Windows boxes (and a few others) use a "time
dependent" model where the ISN is incremented by a small fixed
amount each time period. Needless to say, this is almost as
easily defeated as the old 64K behavior. Of course my favorite
technique is "constant". The machines ALWAYS use the exact same
ISN :). I've seen this on some 3Com hubs (uses 0x803) and Apple
LaserWriter printers (uses 0xC7001).
You can also subclass groups such as random incremental by
computing variances, greatest common divisors, and other functions
on the set of sequence numbers and the differences between the
It should be noted that ISN generation has important security
implications. For more information on this, contact "security
expert" Tsutomu "Shimmy" Shimomura at SDSC and ask him how he was
owned. Nmap is the first program I have seen to use this for OS
Don't Fragment bit -- Many operating systems are starting to set the
IP "Don't Fragment" bit on some of the packets they send. This
gives various performance benefits (though it can also be annoying
-- this is why nmap fragmentation scans do not work from Solaris
boxes). In any case, not all OS's do this and some do it in
different cases, so by paying attention to this bit we can glean
even more information about the target OS. I haven't seen this
one before either.
TCP Initial Window -- This simply involves checking the window size on
returned packets. Older scanners simply used a non-zero window on
a RST packet to mean "BSD 4.4 derived". Newer scanners such as
queso and nmap keep track of the exact window since it is actually
pretty constant by OS type. This test actually gives us a lot of
information, since some operating systems can be uniquely
identified by the window alone (for example, AIX is the only OS I
have seen which uses 0x3F25). In their "completely rewritten"
TCP stack for NT5, Microsoft uses 0x402E. Interestingly, that is
exactly the number used by OpenBSD and FreeBSD.
ACK Value -- Although you would think this would be completely
standard, implementations differ in what value they use for the
ACK field in some cases. For example, lets say you send a
FIN|PSH|URG to a closed TCP port. Most implementations will set
the ACK to be the same as your initial sequence number, though
Windows and some stupid printers will send your seq + 1. If you
send a SYN|FIN|URG|PSH to an open port, Windows is very
inconsistent. Sometimes it sends back your seq, other times it
sends S++, and still other times is sends back a seemingly random
value. One has to wonder what kind of code MS is writing that
changes its mind like this.
ICMP Error Message Quenching -- Some (smart) operating systems follow
the RFC 1812 suggestion to limit the rate at which various error
messages are sent. For example, the Linux kernel (in
net/ipv4/icmp.h) limits destination unreachable message generation
to 80 per 4 seconds, with a 1/4 second penalty if that is
exceeded. One way to test this is to send a bunch of packets to
some random high UDP port and count the number of unreachables
received. I have not seen this used before, and in fact I have
not added this to nmap (except for use in UDP port scanning).
This test would make the OS detection take a bit longer since you
need to send a bunch of packets and wait for them to return. Also
dealing with the possibility of packets dropped on the network
would be a pain.
ICMP Message Quoting -- The RFCs specify that ICMP error messages
quote some small amount of an ICMP message that causes various
errors. For a port unreachable message, almost all
implementations send only the required IP header + 8 bytes back.
However, Solaris sends back a bit more and Linux sends back even
more than that. The beauty with this is it allows nmap to
recognize Linux and Solaris hosts even if they don't have any
ICMP Error message echoing integrity -- I got this idea from something
Theo De Raadt (lead OpenBSD developer) posted to
comp.security.unix. As mentioned before, machines have to send
back part of your original message along with a port unreachable
error. Yet some machines tend to use your headers as 'scratch
space' during initial processing and so they are a bit warped by
the time you get them back. For example, AIX and BSDI send back an
IP 'total length' field that is 20 bytes too high. Some BSDI,
FreeBSD, OpenBSD, ULTRIX, and VAXen fuck up the IP ID that you sent
them. While the checksum is going to change due to the changed
TTL anyway, there are some machines (AIX, FreeBSD, etc.) which send
back an inconsistent or 0 checksum. Same thing goes with the UDP
checksum. All in all, nmap does nine different tests on the ICMP
errors to sniff out subtle differences like these.
Type of Service -- For the ICMP port unreachable messages I look at
the type of service (TOS) value of the packet sent back. Almost
all implementations use 0 for this ICMP error although Linux uses
0xC0. This does not indicate one of the standard TOS values, but instead is
part of the unused (AFAIK) precedence field. I do not know why
this is set, but if they change to 0 we will be able to keep
identifying the old versions _and_ we will be able to identify
between old and new.
Fragmentation Handling -- This is a favorite technique of Thomas
H. Ptacek of Secure Networks, Inc (now owned by a bunch of Windows
users at NAI). This takes advantage of the fact that different
implementations often handle overlapping IP fragments differently.
Some will overwrite the old portions with the new, and in other
cases the old stuff has precedence. There are many different
probes you can use to determine how the packet was reassembled. I
did not add this capability since I know of no portable way to send
IP fragments (in particular, it is a bitch on Solaris). For more
information on overlapping fragments, you can read their IDS paper
TCP Options -- These are truly a gold mine in terms of leaking
information. The beauty of these options is that:
1) They are generally optional (duh!) :) so not all hosts implement
2) You know if a host implements them by sending a query with an
option set. The target generally show support of the option by
setting it on the reply.
3) You can stuff a whole bunch of options on one packet to test
everything at once.
Nmap sends these options along with almost every probe packet:
Window Scale=10; NOP; Max Segment Size = 265; Timestamp; End of Ops;
When you get your response, you take a look at which options were
returned and thus are supported. Some operating systems such as
recent FreeBSD boxes support all of the above, while others, such
as Linux 2.0.X support very few. The latest Linux 2.1.x kernels
do support all of the above. On the other hand, they are more
vulnerable to TCP sequence prediction. Go figure.
Even if several operating systems support the same set of options,
you can sometimes distinguish them by the _values_ of the options.
For example, if you send a small MSS value to a Linux box, it will
generally echo that MSS back to you. Other hosts will give you
And even if you get the same set of supported options AND the same
values, you can still differentiate via the _order_ that the
options are given, and where padding is applied. For example
Solaris returns 'NNTNWME' which means:
<no op><no op><timestamp><no op><window scale><echoed MSS>
While Linux 2.1.122 returns MENNTNW. Same options, same values,
but different order!
I have not seen any other OS detection tools utilizes TCP options,
but it is very useful.
There are a few other useful options I might probe for at some
point, such as those that support T/TCP and selective
Exploit Chronology -- Even with all the tests above, nmap is unable to
distinguish between the TCP stacks of Win95, WinNT, or Win98.
This is rather surprising, especially since Win98 came out about 4
years after Win95. You would think they would have bothered to
improve the stack in some way (like supporting more TCP options)
and so we would be able to detect the change and distinguish the
operating systems. Unfortunately, this is not the case. The NT
stack is apparently the same crappy stack they put into '95. And
they didn't bother to upgrade it for '98.
But do not give up hope, for there is a solution. You can simply
start with early Windows DOS attacks (Ping of Death, Winnuke, etc)
and move up a little further to attacks such as Teardrop and Land.
After each attack, ping them to see whether they have crashed.
When you finally crash them, you will likely have narrowed what
they are running down to one service pack or hotfix.
I have not added this functionality to nmap, although I must admit
it is very tempting :).
SYN Flood Resistance -- Some operating systems will stop accepting new
connections if you send too many forged SYN packets at them
(forging the packets avoids trouble with your kernel resetting the
connections). Many operating systems can only handle 8 packets.
Recent Linux kernels (among other operating systems) allow
various methods such as SYN cookies to prevent this from being a
serious problem. Thus you can learn something about your target
OS by sending 8 packets from a forged source to an open port and
then testing whether you can establish a connection to that port
yourself. This was not implemented in nmap since some people get
upset when you SYN flood them. Even explaining that you were
simply trying to determine what OS they are running might not help
----[ NMAP IMPLEMENTATION AND RESULTS
I have created a reference implementation of the OS detection techniques
mentioned above (except those I said were excluded). I have added this to my
Nmap scanner which has the advantage that it already _knows_ what ports are
open and closed for fingerprinting so you do not have to tell it. It is also
portable among Linux, *BSD, and Solaris 2.51 and 2.6, and some other operating
The new version of nmap reads a file filled with Fingerprint templates that
follow a simple grammar. Here is an example:
FingerPrint IRIX 6.2 - 6.4 # Thanks to Lamont Granquist
Lets look at the first line (I'm adding '>' quote markers):
> FingerPrint IRIX 6.2 - 6.3 # Thanks to Lamont Granquist
This simply says that the fingerprint covers IRIX versions 6.2 through 6.3 and
the comment states that Lamont Granquist kindly sent me the IP addresses or
fingerprints of the IRIX boxes tested.
This means that ISN sampling put it in the "i800 class". This means that each
new sequence number is a multiple of 800 greater than the last one.
The test is named T1 (for test1, clever eh?). In this test we send a SYN
packet with a bunch of TCP options to an open port. DF=N means that the
"Don't fragment" bit of the response must not be set. W=C000|EF2A means that
the window advertisement we received must be 0xC000 or EF2A. ACK=S++ means
the acknowledgement we receive must be our initial sequence number plus 1.
Flags = AS means the ACK and SYN flags were sent in the response.
Ops = MNWNNT means the options in the response must be (in this order):
<MSS (not echoed)><NOP><Window scale><NOP><NOP><Timestamp>
Test 2 involves a NULL with the same options to an open port. Resp=Y means we
must get a response. Ops= means that there must not be any options included
in the response packet. If we took out '%Ops=' entirely then any options sent
Test 3 is a SYN|FIN|URG|PSH w/options to an open port.
This is an ACK to an open port. Note that we do not have a Resp= here. This
means that lack of a response (such as the packet being dropped on the network
or an evil firewall) will not disqualify a match as long as all the other
tests match. We do this because virtually any OS will send a response, so a
lack of response is generally an attribute of the network conditions and not
the OS itself. We put the Resp tag in tests 2 and 3 because some operating
systems _do_ drop those without responding.
These tests are a SYN, ACK, and FIN|PSH|URG, respectively, to a closed port.
The same options as always are set. Of course this is all probably obvious
given the descriptive names 'T5', 'T6', and 'T7' :).
This big sucker is the 'port unreachable' message test. You should recognize
the DF=N by now. TOS=0 means that IP type of service field was 0. The next
two fields give the (hex) values of the IP total length field of the message
IP header and the total length given in the IP header they are echoing back to
us. RID=E means the RID value we got back in the copy of our original UDP
packet was expected (ie the same as we sent). RIPCK=E means they didn't fuck
up the checksum (if they did, it would say RIPCK=F). UCK=E means the UDP
checksum is also correct. Next comes the UDP length which was 0x134 and DAT=E
means they echoed our UDP data correctly. Since most implementations
(including this one) do not send any of our UDP data back, they get DAT=E by
The version of nmap with this functionality is currently in the 6th private
beta cycle. It may be out by the time you read this in Phrack. Then again,
it might not. See http://www.insecure.org/nmap/ for the latest version.
----[ POPULAR SITE SNAPSHOTS
Here is the fun result of all our effort. We can now take random Internet
sites and determine what OS they are using. A lot of these people have
eliminated telnet banners, etc. to keep this information private. But this is
of no use with our new fingerprinter! Also this is a good way to expose the
<your favorite crap OS> users as the lamers that they are :)!
The command used in these examples was: nmap -sS -p 80 -O -v <host>
Also note that most of these scans were done on 10/18/98. Some of these folks
may have upgraded/changed servers since then.
Note that I do not like every site on here.
# "Hacker" sites or (in a couple cases) sites that think they are
www.l0pht.com => OpenBSD 2.2 - 2.4
www.insecure.org => Linux 2.0.31-34
www.rhino9.ml.org => Windows 95/NT # No comment :)
www.technotronic.com => Linux 2.0.31-34
www.nmrc.org => FreeBSD 2.2.6 - 3.0
www.cultdeadcow.com => OpenBSD 2.2 - 2.4
www.kevinmitnick.com => Linux 2.0.31-34 # Free Kevin!
www.2600.com => FreeBSD 2.2.6 - 3.0 Beta
www.antionline.com => FreeBSD 2.2.6 - 3.0 Beta
www.rootshell.com => Linux 2.0.35 # Changed to OpenBSD after
# they got owned.
# Security vendors, consultants, etc.
www.repsec.com => Linux 2.0.35
www.iss.net => Linux 2.0.31-34
www.checkpoint.com => Solaris 2.5 - 2.51
www.infowar.com => Win95/NT
# Vendor loyalty to their OS
www.li.org => Linux 2.0.35 # Linux International
www.redhat.com => Linux 2.0.31-34 # I wonder what distribution :)
www.debian.org => Linux 2.0.35
www.linux.org => Linux 2.1.122 - 2.1.126
www.sgi.com => IRIX 6.2 - 6.4
www.netbsd.org => NetBSD 1.3X
www.openbsd.org => Solaris 2.6 # Ahem :)
www.freebsd.org => FreeBSD 2.2.6-3.0 Beta
# Ivy league
www.harvard.edu => Solaris 2.6
www.yale.edu => Solaris 2.5 - 2.51
www.caltech.edu => SunOS 4.1.2-4.1.4 # Hello! This is the 90's :)
www.stanford.edu => Solaris 2.6
www.mit.edu => Solaris 2.5 - 2.51 # Coincidence that so many good
# schools seem to like Sun?
# Perhaps it is the 40%
# .edu discount :)
www.berkeley.edu => UNIX OSF1 V 4.0,4.0B,4.0D
www.oxford.edu => Linux 2.0.33-34 # Rock on!
# Lamer sites
www.aol.com => IRIX 6.2 - 6.4 # No wonder they are so insecure :)
www.happyhacker.org => OpenBSD 2.2-2.4 # Sick of being owned, Carolyn?
# Even the most secure OS is
# useless in the hands of an
# incompetent admin.
www.lwn.net => Linux 2.0.31-34 # This Linux news site rocks!
www.slashdot.org => Linux 2.1.122 - 2.1.126
www.whitehouse.gov => IRIX 5.3
sunsite.unc.edu => Solaris 2.6
Notes: In their security white paper, Microsoft said about their lax security:
"this assumption has changed over the years as Windows NT gains popularity
largely because of its security features.". Hmm, from where I stand it
doesn't look like Windows is very popular among the security community :).
I only see 2 Windows boxes from the whole group, and Windows is _easy_ for
nmap to distinguish since it is so broken (standards wise).
And of course, there is one more site we must check. This is the web site of
the ultra-secret Transmeta corporation. Interestingly the company was funded
largely by Paul Allen of Microsoft, but it employs Linus Torvalds. So do they
stick with Paul and run NT or do they side with the rebels and join the Linux
revolution? Let us see:
We use the command:
nmap -sS -F -o transmeta.log -v -O www.transmeta.com/24
This says SYN scan for known ports (from /etc/services), log the results to
'transmeta.log', be verbose about it, do an OS scan, and scan the class 'C'
where www.transmeta.com resides. Here is the gist of the results:
neon-best.transmeta.com (22.214.171.124) => Linux 2.0.33-34
www.transmeta.com (126.96.36.199) => Linux 2.0.30
neosilicon.transmeta.com (188.8.131.52) => Linux 2.0.33-34
ssl.transmeta.com (184.108.40.206) => Linux unknown version
linux.kernel.org (220.127.116.11) => Linux 2.0.35
www.linuxbase.org (18.104.22.168) => Linux 2.0.35 ( possibly the same
machine as above )
Well, I think this answers our question pretty clearly :).
The only reason Nmap is currently able to detect so many different operating
systems is that many people on the private beta team went to a lot of effort
to search out new and exciting boxes to fingerprint! In particular, Jan Koum,
van Hauser, Dmess0r, David O'Brien, James W. Abendschan, Solar Designer, Chris
Wilson, Stuart Stock, Mea Culpa, Lamont Granquist, Dr. Who, Jordan Ritter,
Brett Eldridge, and Pluvius sent in tons of IP addresses of wacky boxes and/or
fingerprints of machines not reachable through the Internet.
Thanks to Richard Stallman for writing GNU Emacs. This article would not be
so well word-wrapped if I was using vi or cat and ^D.
Questions and comments can be sent to fyodor@DHP.com (if that doesn't work for
some reason, use email@example.com). Nmap can be obtained from