26th Jun 2001 [SBWID-145]
COMMAND
kernel
SYSTEMS AFFECTED
OpenBSD 2.6...2.9
PROBLEM
Ofir Arkin found following. When a host generates an ICMP Error
message it will echo some parts of the offending packet. The
ICMP Error message will include the IP Header and at least 8
bytes of data from the offending packet.
There are some fingerprinting methods which relay on the fact
that some operating systems do not echo correctly the offending
packet's data.
It is known that some operating systems do not echo correctly the
IP Total Length field value. The problem is that some operating
systems echo a value which is 20 bytes bigger than the original
value carried with the offending packet.
During the research on X Ofir has found a new fingerprinting
method that involves the same field value. With this method the
IP Total Length field value being echoed (with an ICMP Port
Unreachable Error Message) is 20 bytes less than the original
value. With the next example I have used hping2 to generate a
UDP datagram with 80 bytes of data querying UDP port 50 on
OpenBSD 2.9 i386 based machine:
[root@godfather /root]# hping2 -2 -y -p 50 -d 80 172.18.2.145
eth0 default routing interface selected (according to /proc)
HPING 172.18.2.145 (eth0 172.18.2.145): udp mode set, 28 headers + 80 data bytes
ICMP Port Unreachable from 172.18.2.145 (unknown host name)
ICMP Port Unreachable from 172.18.2.145 (unknown host name)
ICMP Port Unreachable from 172.18.2.145 (unknown host name)
...
The tcpdump trace:
09:52:33.989622 eth0 > 172.18.2.201.2999 > 172.18.2.145.re-mail-ck: udp 80
(DF) (ttl 64, id 5207)
4500 006c 1457 4000 4011 c8ab ac12 02c9
ac12 0291 0bb7 0032 0058 c808 5858 5858
5858 5858 5858 5858 5858 5858 5858 5858
5858 5858 5858 5858 5858 5858 5858 5858
5858 5858 5858 5858 5858 5858 5858 5858
5858 5858 5858 5858 5858 5858 5858 5858
5858 5858 5858 5858 5858 5858
09:52:33.989622 eth0 < 172.18.2.145 > 172.18.2.201: icmp: 172.18.2.145 udp
port re-mail-ck unreachable Offending pkt: 172.18.2.201.2999 >
172.18.2.145.re-mail-ck: udp 80 (DF) (ttl 64, id 5207) (ttl 255, id 41822)
4500 0038 a35e 0000 ff01 bae7 ac12 0291
ac12 02c9 0303 28b3 0000 0000 4500 0058
1457 4000 4011 c8bf ac12 02c9 ac12 0291
0bb7 0032 0058 c808
The snort trace:
06/20-09:53:07.989622 172.18.2.201:3033 -> 172.18.2.145:50
UDP TTL:64 TOS:0x0 ID:10872 IpLen:20 DgmLen:108 DF
Len: 88
58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
06/20-09:53:07.989622 172.18.2.145 -> 172.18.2.201
ICMP TTL:255 TOS:0x0 ID:51307 IpLen:20 DgmLen:56
Type:3 Code:3 DESTINATION UNREACHABLE: PORT UNREACHABLE
** ORIGINAL DATAGRAM DUMP:
172.18.2.201:3033 -> 172.18.2.145:50
UDP TTL:64 TOS:0x0 ID:10872 IpLen:20 DgmLen:88
Len: 88
** END OF DUMP
00 00 00 00 45 00 00 58 2A 78 40 00 40 11 B2 9E ....E..X*x@.@...
AC 12 02 C9 AC 12 02 91 0B D9 00 32 00 58 C7 E6 ...........2.X..
Looking at the traces you can see that the datagram length
reported by the ICMP Port Unreachable error message is 88 bytes,
while the offending packet was 108 bytes long.
The same behavior is produced with OpenBSD 2.6-2.8 as well
(checked on the i386 platform).
But OpenBSD 2.6-2.9 is not the only operating system that acts
like this. Cross referencing this fingerprinting method with
nmap's fingerprinting database (www.insecure.org) reveals that
Apollo Domain/OS SR10.4, NFR IDS Appliance, Extreme Networks
Black Diamond Switch, Extreme Networks Gigabit Switch, Network
Systems router NS6614 (NSC 6600 series), and Cabletron Systems
SSR 8000 System Software version 3.1.B.16 does the same.
Since all the other operating systems and networking devices
listed above have other echoing integrity problems with their
ICMP Port Unreachable error messages, while OpenBSD 2.6-2.9
correctly echoes all the other data fields carried with the ICMP
Port Unreachable error message, it enables us to fingerprint
OpenBSD 2.6-2.9 based systems initiating one crafted query
receiving one ICMP Port Unreachable error message.
SOLUTION
Nothing Yet.