Minggu, 26 April 2009

Identifying remote hosts

Identifying remote hosts

One of the challenges of network security is learning about the bad guys. To understand your threats and better
protect against them, you have to Know Your Enemy. Passive Fingerprinting is a method to learn more about the
enemy, without them knowing it. Specifically, you can determine the operating system and other characteristics of
the remote host using nothing more then sniffer traces. Though not 100% accurate, you can get surprisingly good
results. The subterrain crew has developed siphon, a passive network and system mapping and OS fingerprinting
tool. Also, Michael Zalewski (Poland's finest) and Bill Stearns are maintaining p0f. Both of these tools
demonstrate the functionality we are about to discuss.
Traditionally, Operating System fingerprinting has been done using active tools, such as queso or nmap. These
tools operate on the principle that every operating system's IP stack has its own idiosyncrasies. Specifically, each
operating system responds differently to a variety of malformed packets. All one has to do is build a database on
how different operating systems respond to different packets. Then, to determine the operating system of a
remote host, send it a variety of malformed packets, determine how it responds, then compare these responses to
a database. Fyodor's nmap is tool of choice when using this methodology. He has also written a detailed paper on
Passive fingerprinting follows the same concept, but is implemented differently. Passive fingerprinting is based on
sniffer traces from the remote system. Instead of actively querying the remote system, all you need to do is
capture packets sent from the remote system. Based on the sniffer traces of these packets, you can determine
the operating system of the remote host. Just like in active fingerprinting, passive fingerprinting is based on the
principle that every operating system's IP stack has its own idiosyncrasies. By analyzing sniffer traces and
identifying these differences, you may be able determine the operating system of the remote host.
The Signatures
There are four TCP areas that we will look at to determine the operating system (however there are other
signatures that can be used). These signatures are:
 TTL - What the operating system sets the Time To Live on the outbound packet
 Window Size - What the operating system sets the Window Size at.
 DF - Does the operating system set the Don't Fragment bit.
 TOS - Does the operating system set the Type of Service, and if so, at what.
By analyzing these factors of a packet, you may be able to determine the remote operating system. This system
is not 100% accurate, and works better for some operating systems then others. No single signature can reliably
determine the remote operating system. However, by looking at several signatures and combining the
information, you increase the accuracy of identifying the remote host. An example would be the easiest way to
explain. Below is the sniffer trace of a system sending a packet. This system launched a mountd exploit against
us, so we want to learn more about it. We do not want to finger or nmap the box, that could give us away. Rather,
we want to study the information passively. This signature was captured using snort, our passive weapon of

TCP TTL:45 TOS:0x0 ID:56257
***F**A* Seq: 0x9DD90553
Ack: 0xE3C65D7 Win: 0x7D78
Based on our 4 criteria, we identify the following:
 TTL: 45
 Window Size: 0x7D78 (or 32120 in decimal)
 DF: The Don't Fragment bit is set
 TOS: 0x0
We then compare this information to a database of signatures. First, we look at the TTL used by the remote host.
From our sniffer trace above, you can see the TTL is set at 45. This most likely means it went through 19 hops to
get to us, so the original TTL was set at 64. Based on this TTL, it appears this packet was sent from a Linux or
FreeBSD box, (however, more system signatures need to be added to the database). This TTL is confirmed by
doing a traceroute to the remote host. If you are concerend about the remote host detecting your traceroute, you
can set your traceroute time-to-live (default 30 hops), to be one or two hops less then the remote host (-m option).
For example, in this case we would do a traceroute to the remote host, but using only 18 hops (traceroute -m 18).
This gives you the path information (including their upstream provider) without actually touching the remote host.
For more information on TTLs, check out this Research Paper on Default TTL values.
The next step is too compare the Window size. We have found the Window Size to be another effective tool,
specifically what Window Size is used and how often the size changes. In the above signature, we see it set at
0x7D78, a default Window Size commonly used by Linux. Also, Linux, FreeBSD, and Solaris tend to maintain the
same Window Size throughout a session (as this one did). However, Cisco routers (at least my 2514) and
Microsoft Windows/NT Window Sizes are constantly changing. We have found that Window Size is more accurate
if measured after the initial three-way handshake (due to TCP slow start). For more information on Window Size,
see Stevens, "TCP/IP Illustrated, Volume 1" Chapter 20.
Most systems use the DF bit set, so this is of limited value. However, this does make it easier to identify the few
systems that do not use the DF flag (such as SCO or OpenBSD). After further testing, we feel that TOS is also of
limited value. This seems to be more session based then operating system. In other words, its not so much the
operating system that determines the TOS, but the protocol used. TOS defintely requires some more testing. So,
based on the information above, specifcally TTL and Window size, you can compare the results to the database
of signatures and with a degree of confidence determine the OS (in our case, Linux kernel 2.2.x).
Keep in mind, just as with Active Fingerprinting, Passive Fingerprinting has some limitations. First, applications
that build their own packets (such as nmap, hunt, nemesis, etc) will not use the same signatures as the operating
system. Second, it is relatively simple for a remote host to adjust the TTL, Window Size, DF, or TOS setting on
packets. For example, to change the default TTL value:
Solaris: ndd -set /dev/ip ip_def_ttl 'number'
Linux: echo 'number' > /proc/sys/net/ipv4/ip_default_ttl
NT: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters
However, by combining a variety of different packets and signatures, in this case TTL and Window Size, you can
reliably approximate the remote system.
Other Signatures and Uses
We are not limited to the four signatures discussed so far. There are other areas that can be tracked, such as
initial sequence numbers, IP Identification numbers, TCP or IP options. For example, Cisco routers tend to start
IP Identification numbers at 0, instead of randomly assigning them. For TCP Options, the option Selective
Acknowledgement SackOK is commonly used by Windows and Linux, but not commonly used by FreeBSD or
Solaris. With Maximum Segment Size (MSS), most operating systems use a MSS of 1460, however Novell
commonly uses 1368, and some FreeBSD variants may use a MSS of 512. Also, ICMP packets can be used.
Honeynet member Ofir Arkin has done extensive research in using ICMP for fingerprinting, publishing the paper

ICMP Usage in Scanning. The ICMP signatures he discusses in this paper can be used for passively
fingerprinting systems based on their ICMP signatures. For example, Microsoft ICMP REQUEST payloads
contain the alphabet, while most Unix systems, such as Solaris or Linux, ICMP REQUEST payloads have number
and symbols.
Passive fingerprinting can be used for several other purposes. It can be used by the bad guys as 'stealthy'
fingerprinting. For example, to determine the Operating System of a 'potential victim', such as a webserver, one
only needs to request a webpage from the server, then analyze the sniffer traces. This bypasses the need for
using an active tool that can be detected by various IDS systems. Also, Passive Fingerprinting may be used to
identify remote proxy firewalls. Since proxy firewalls rebuild connection for clients, it may be possible to ID the
proxy firewalls based on the signatures we have discussed. Organizations can use Passive Fingerprinting to
identify 'rogue' systems on their network. These would be systems that are not authorized on the network. For
example, a Microsoft or Sun shop can quickly identify 'rogue' Linux or FreeBSD systems that mysteriously
appeared on their network. Passive Fingerprinting can be used to quickly inventory an organizations operating
systems without touching or imapcting any systems or network performance. You would be surprised how may
organizations do not know what systems they have on their internal network. For individuals conducting security
assessment, Passive Fingerprinting also allows one to quickly identify critical systems (such as Unisys
Mainframe). This method can also be used to identify rogue or unautorized systetms or OS types within an
organization, a possible indication of activity.
The Project has developed a test database to demonstrate these concepts of passive fingerprinting. The
database was built by testing a variety of systems with the Telnet, FTP, HTTP, and SSH protocol.

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