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Fun with IP Identification Field Values (Identifying Older MS Based OSs)
From: Ofir Arkin <ofir () SYS-SECURITY COM>
Date: Sat, 5 May 2001 23:21:55 -0700

RFC 791 gives a description about the IP Identification field.

The identification field value is used to uniquely identify the fragments of
a particular datagram. Fragments of a particular datagram are assembled if
they have the same source, destination, protocol, and Identifier. The
identifier is being chosen to be unique for this  "this source, destination
pair and protocol for the time the datagram (or any fragment of it) could be
alive in the internet"[1].

The IP identifier field can have 65,536 different values. It is important
for an operating system to have some sort of a mechanism in order to control
the identification numbers correctly.

Since every operating system should have its own mechanism in order to deal
with this field numbering we might find some patterns different from one
operating system to another.


The Gap between one IP ID field value to the next
With the implementation in many operating systems, the Kernel is increasing
the IP ID field value by 1, from one packet to the next.

However, there are operating systems that will increase the value of the IP
ID field value with a value different than 1, from one packet to the next.

In the next example I have sent two ICMP Echo requests from a Windows NT 4
Server with SP6a based machine targeting a LINUX machine based on Kernel
2.2.14:

08/10-16:55:06.638539 10.0.0.117 -> 10.0.0.105
ICMP TTL:32 TOS:0x0 ID:28416
ID:256   Seq:768  ECHO
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:55:06.638592 10.0.0.105 -> 10.0.0.117
ICMP TTL:255 TOS:0x0 ID:1452
ID:256   Seq:768  ECHO REPLY
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:55:07.639784 10.0.0.117 -> 10.0.0.105
ICMP TTL:32 TOS:0x0 ID:28672
ID:256   Seq:1024  ECHO
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:55:07.639841 10.0.0.105 -> 10.0.0.117
ICMP TTL:255 TOS:0x0 ID:1453
ID:256   Seq:1024  ECHO REPLY
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi


The first ICMP Echo request sent from the Microsoft NT 4 based machine was
sent with IP ID of 28416. The second ICMP Echo request was sent with IP ID
value of 28672. Simple calculation will show a gap of 256 between the IP ID
field values.

Looking at the replies the LINUX based machine produced, we see a gap of 1
between one IP ID to the next.


Other OSs that act the same
The other operating systems that act the same are the older Microsoft based
operating systems. They include – Windows 95, Windows 98, Windows 98 SE,
Windows NT 4 family (regardless of the Service Pack installed).

With newer versions of their operating systems (MS Windows ME, MS Windows
2000 family), Microsoft has changed this behavior, and now acts as most
operating systems do.


How Can We Use This?
We can use this information as another parameter for Active OS
fingerprinting and for Passive OS fingerprinting.

One example might be when we need another parameter to differentiate between
a Windows NT 4 based machine to a Windows 2000 based machine.


In The Real World
In the real world when we wish to use this information for fingerprinting
operating systems we will see something a bit different that we should be
aware of. Since the machines we try to fingerprint are hosts available on
the Internet they might communicate with other hosts on the Internet while
we query them. Therefore the gap we will have from one IP ID to the next
might be higher than 256 (in the older MS based OSs case). With the older
implementations of Microsoft based operating systems identifying these OSs
is quite simple. We will extract the first IP ID from the second IP ID and
divide the result with 256. The result should be a complete number.

With the operating systems that use a gap of 1 between one IP ID field
number to the next, we might have a gap a bit higher than 1, usually between
2-8 (but it can be more than that as well).

In the next example a Microsoft ME based machine sent two ICMP Echo requests
targeting a LINUX based on kernel 2.2.14 machine. The gap between the first
IP ID field value to the next is 5 with the LINUX machine:

08/10-16:49:45.633417 10.0.0.117 -> 10.0.0.105
ICMP TTL:32 TOS:0x0 ID:134
ID:768   Seq:256  ECHO
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:49:45.633465 10.0.0.105 -> 10.0.0.117
ICMP TTL:255 TOS:0x0 ID:810
ID:768   Seq:256  ECHO REPLY
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:49:46.635971 10.0.0.117 -> 10.0.0.105
ICMP TTL:32 TOS:0x0 ID:135
ID:768   Seq:512  ECHO
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi

08/10-16:49:46.636018 10.0.0.105 -> 10.0.0.117
ICMP TTL:255 TOS:0x0 ID:815
ID:768   Seq:512  ECHO REPLY
61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70  abcdefghijklmnop
71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69  qrstuvwabcdefghi


[1] RFC 791: Internet Protocol. http://www.ietf.org/rfc/rfc0791.txt


Ofir Arkin [ofir () sys-security com]
Founder
The Sys-Security Group
http://www.sys-security.com
PGP CC2C BE53 12C6 C9F2 87B1 B8C6 0DFA CF2D D360 43FA


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