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Several Misbehaviors with the ICMP implementation (and the 'ping' utility) with MS based operating systems
From: Ofir Arkin <ofir () SYS-SECURITY COM>
Date: Thu, 3 May 2001 06:51:26 -0700

RFC 792 (Internet Control Message Protocol) suggests how the ICMP Identifier
field and the ICMP Sequence Number field should be used:

"The identifier and sequence number may be used by the echo sender to aid in
matching the replies with the echo requests.  For example, the identifier
might be used like a port in TCP or UDP to identify a session, and the
sequence number might be incremented on each echo request sent.  The echoer
returns these same values in the echo reply".

It literally suggests that the ICMP Identifier field will be used to
differentiate between ICMP Query messages sent to different hosts. It also
suggests that the ICMP Sequence Number field will be used to differentiate
between the ICMP query messages sent to the same host.

The 'ping' utility with UNIX and UNIX-like operating systems has adopted
this suggestion.

When examining the behavior of the 'ping' utility with Microsoft Windows
based operating systems I have encountered a different behavioral pattern.

The next example is a trace (using the windump program -
http://netgroup-serv.polito.it/windump/install/Default.htm) of ICMP Echo
requests initiated by lunching two ping commands at the same time from a
Microsoft Windows 2000 SP1 operating system based machine. One instant was
aimed at the host 172.18.1.2, and the other at the host 172.18.1.134:

E:\>windump -xnvv -s 1600 icmp
windump: listening on\Device\Packet_{79C233F1-6CD7-49EB-8FA2-FA825CB1C9C3}
11:31:21.848025 172.18.1.179 > 172.18.1.2: icmp: echo request (ttl 128, id
11071)
                         4500 003c 2b3f 0000 8001 b4a8 ac12 01b3
                         ac12 0102 0800 265c 0300 2400 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

11:31:22.221772 172.18.1.179 > 172.18.1.134: icmp: echo request (ttl 128, id
11075)
                         4500 003c 2b43 0000 8001 b420 ac12 01b3
                         ac12 0186 0800 255c 0300 2500 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

11:31:22.844726 172.18.1.179 > 172.18.1.2: icmp: echo request (ttl 128, id
11077)
                         4500 003c 2b45 0000 8001 b4a2 ac12 01b3
                         ac12 0102 0800 245c 0300 2600 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

11:31:23.215222 172.18.1.179 > 172.18.1.134: icmp: echo request (ttl 128, id
11078)
                         4500 003c 2b46 0000 8001 b41d ac12 01b3
                         ac12 0186 0800 235c 0300 2700 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

11:31:23.846116 172.18.1.179 > 172.18.1.2: icmp: echo request (ttl 128, id
11079)
                         4500 003c 2b47 0000 8001 b4a0 ac12 01b3
                         ac12 0102 0800 225c 0300 2800 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

11:31:24.216645 172.18.1.179 > 172.18.1.134: icmp: echo request (ttl 128, id
11080)
                         4500 003c 2b48 0000 8001 b41b ac12 01b3
                         ac12 0186 0800 215c 0300 2900 6162 6364
                         6566 6768 696a 6b6c 6d6e 6f70 7172 7374
                         7576 7761 6263 6465 6667 6869

As it can be seen, the ICMP Identifier field value is the same with both
instances. This is regardless the fact we are using the 'ping' utility to
send ICMP Echo requests to two separate hosts. The number assigned to this
field is 768 decimal.

So how does the 'ping' utility with Microsoft based operating systems
differentiate between the different ICMP Queries?
The 'ping' utility is using the Sequence Number field. For each ICMP Echo
Request the ICMP Sequence Number is a unique number. The gap between one
ICMP Sequence Number field value to another is 100 hex/256 decimal.

This raises another interesting question.

If the ICMP Identifier field has a constant value, can we identify the
different Microsoft operating systems passively when someone is using the
'ping' utility to query our machines?

Yes.

Microsoft Windows NT - 256

Microsoft Windows 98/98SE - 512
Microsoft Windows 2000 - 512

Microsoft Windows ME – 768
Microsoft Windows 2000 Family with SP1 - 768


With the 'ping' utility with Microsoft based operating systems the values
assigned for the different ICMP datagram fields are OS based (in contrast
with the 'ping' utility on UNIX and UNIX-like operating systems which uses
the application own values for the different ICMP datagram fields). When
using other applications with Microsoft based operating systems to generate
ICMP Query messages the ICMP Identifier field values will still be the same
as it was with the 'ping' utility, if these applications will be using the
Microsoft MFC.

Therefore when ever we see an ICMP Query datagram with an ICMP Identifier
field value of 256/512/768 it will indicate that the underlying operating
system to be used is an MS based.

We can also look at the ICMP Sequence Number field value for extra
information. The 'ping' utility with MS based operating systems will issue
its first ICMP Query message with the ICMP Sequence Number field set to a
value of 256 (the 'ping' utility with UNIX and UNIX-like operating systems
will have this field value set to 0 on its first query to a Host). This
field value will increase with 256 decimal each time we send an ICMP Query
message (with the UNIX and UNIX-like 'ping' utility the field value will
increase only if we are sending sequential Queries. Each time we issue the
'ping' command this field value will be set to 0 on the first query to be
sent).


We can even calculate the number of ICMP Query messages a Windows based OS
have issued since the last boot time. All we need to do is divide the ICMP
Sequence number field value with 256.


Microsoft can argue that their ICMP implementation is not in contrast with
RFC 792, since the term that was used in order to describe the usage of the
ICMP Identifier field was "may be used". But if we use common sense, than
what role, in the Microsoft case, the ICMP Identifier field has?



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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