03 How to detect a sniffer running.
Description
This article is from the Sniffer FAQ, by Christopher Klaus cklaus@iss.net with numerous contributions by others.
03 How to detect a sniffer running.
To detect a sniffing device that only collects data and does not respond to
any of the information, requires physically checking all your ethernet
connections by walking around and checking the ethernet connections
individually.
It is also impossible to remotely check by sending a packet or ping if a
machine is sniffing.
A sniffer running on a machine puts the interface into promiscuous mode,
which accepts all the packets. On some Unix boxes, it is possible to detect
a promiscuous interface. It is possible to run a sniffer in non-promiscuous
mode, but it will only capture sessions from the machine it is running on.
It is also possible for the intruder to do similiar capture of sessions by
trojaning many programs such as sh, telnet, rlogin, in.telnetd, and so on to
write a log file of what the user did. They can easily watch the tty and
kmem devices as well. These attacks will only compromise sessions coming
from that one machine, while promiscuous sniffing compromises all sessions
on the ethernet.
For SunOs, NetBSD, and other possible BSD derived Unix systems, there is a
command
"ifconfig -a"
that will tell you information about all the interfaces and if they are in
promiscuous mode. DEC OSF/1 and IRIX and possible other OSes require the
device to be specified. One way to find out what interface is on the system,
you can execute:
# netstat -r
Routing tables
Internet:
Destination Gateway Flags Refs Use Interface
default iss.net UG 1 24949 le0
localhost localhost UH 2 83 lo0
Then you can test for each interface by doing the following command:
#ifconfig le0
le0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,PROMISC,MULTICAST>
inet 127.0.0.1 netmask 0xffffff00 broadcast 255.0.0.1
Intruders often replace commands such as ifconfig to avoid detection. Make
sure you verify its checksum.
There is a program called cpm available on ftp://ftp.cert.org/pub/tools/ that
only works on Sunos and is suppose to check the interface for promiscuous
flag.
Ultrix can possibly detect someone running a sniffer by using the commands
pfstat and pfconfig.
pfconfig allows you to set who can run a sniffer
pfstat shows you if the interface is in promiscuous mode.
These commands only work if sniffing is enabled by linking it into the
kernel. by default, the sniffer is not linked into the kernel. Most other
Unix systems, such as Irix, Solaris, SCO, etc, do not have any flags
indication whether they are in promiscuous mode or not, therefore an
intruder could be sniffing your whole network and there is no way to detect
it.
Often a sniffer log becomes so large that the file space is all used up. On
a high volume network, a sniffer will create a large load on the machine.
These sometimes trigger enough alarms that the administrator will discover a
sniffer. I highly suggest using lsof (LiSt Open Files) available from
coast.cs.purdue.edu:/pub/Purdue/lsof for finding log files and finding
programs that are accessing the packet device such as /dev/nit on SunOs.
There is no commands I know of to detect a promiscuous IBM PC compatible
machine, but they atleast usually do not allow command execution unless from
the console, therefore remote intruders can not turn a PC machine into a
sniffer without inside assistance.
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