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This Sniffer FAQ will hopefully give administrators a clear understanding of sniffing problems and hopefully possible solutions to follow up with. Sniffers is one of the main causes of mass break-ins on the Internet today.
This FAQ will be broken down into:
Unlike telephone circuits, computer networks are shared communication channels. It is simply too expensive to dedicate local loops to the switch (hub) for each pair of communicating computers. Sharing means that computers can receive information that was intended for other machines. To capture the information going over the network is called sniffing.
Most popular way of connecting computers is through ethernet. Ethernet protocol works by sending packet information to all the hosts on the same circuit. The packet header contains the proper address of the destination machine. Only the machine with the matching address is suppose to accept the packet. A machine that is accepting all packets, no matter what the packet header says, is said to be in promiscuous mode.
Because, in a normal networking environment, account and password information is passed along ethernet in clear-text, it is not hard for an intruder once they obtain root to put a machine into promiscuous mode and by sniffing, compromise all the machines on the net.
Sniffing is one of the most popular forms of attacks used by hackers. One special sniffer, called Esniff.c, is very small, designed to work on Sunos, and only captures the first 300 bytes of all telnet, ftp, and rlogin sessions. It was published inPhrack, one of the most widely read freely available underground hacking magazines. You can find Phrack on many FTP sites. Esniff.c is also available on many FTP sites such as coombs.anu.edu.au:/pub/net/log.
You may want to run Esniff.c on an authorized network to quickly see how effective it is in compromising local machines.
Other sniffers that are widely available which are intended to debug network problems are:
Commercial Sniffers are available at:
Network General produces a number of products. The most important are the Expert Sniffer, which not only sniffs on the wire, but also runs the packet through a high-performance expert system, diagnosing problems for you. There is an extension onto this called the "Distributed Sniffer System" that allows you to put the console to the expert sniffer on you Unix workstation and to distribute the collection agents at remote sites.
" My commercial site runs many protocols on one wire - NetBeui, IPX/SPX, TCP/IP, 802.3 protocols of various flavors, most notably SNA. This posed a big problem when trying to find a sniffer to examine the network problems we were having, since I found that some sniffers that understood Ethernet II parse out some 802.3 traffic as bad packets, and vice versa. I found that the best protocol parser was in Microsoft's Net Monitor product, also known as Bloodhound in its earlier incarnations. It is able to correctly identify such oddities as NetWare control packets, NT NetBios name service broadcasts, etc, which etherfind on a Sun simply registered as type 0000 packet broadcasts. It requires MS Windows 3.1 and runs quite fast on a HP XP60 Pentium box. Top level monitoring provides network statistics and information on conversations by mac address (or hostname, if you bother with an ethers file). Looking at tcpdump style details is as simple as clicking on a conversation. The filter setup is also one of the easiest to implement that I've seen, just click in a dialog box on the hosts you want to monitor. The number of bad packets it reports on my network is a tiny fraction of that reported by other sniffers I've used. One of these other sniffers in particular was reporting a large number of bad packets with src mac addresses of aa:aa:aa:aa:aa:aa but I don't see them at all using the MS product. - Anonymous
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
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 onftp.cert.org:/pub/tools/cpm 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.
Active hubs send to each system only packets intended for it rendering promiscuous sniffing useless. This is only effective for 10-Base T.
The following vendors have available active hubs:
There are several packages out there that allow encryption between connections therefore an intruder could capture the data, but could not decypher it to make any use of it.
Some packages available are:
The product is built by Hughes Aircraft and they can be reached at 800-825-LOCK or email at email@example.com.
Kerberos is another package that encrypts account information going over the network. Some of its draw backs are that all the account information is held on one host and if that machine is compromised, the whole network is vulnerable. It is has been reported a major difficulty to set up. Kerberos comes with a stream-encrypting rlogind, and stream-encrypting telnetd is available. This prevents intruders from capturing what you did after you logged in.
There is a Kerberos FAQ atftp at rtfm.mit.edu in /pub/usenet/comp.protocols/kerberos/Kerberos_Users__Frequently_Asked_Questions_1.11
S/key and other one time password technology makes sniffing account information almost useless. S/key concept is having your remote host already know a password that is not going to go over insecure channels and when you connect, you get a challenge. You take the challenge information and password and plug it into an algorithm which generates the response that should get the same answer if the password is the same on the both sides. Therefore the password never goes over the network, nor is the same challenge used twice. Unlike SecureID or SNK, with S/key you do not share a secret with the host. S/key is available onftp:thumper.bellcore.com:/pub/nmh/skey
Other one time password technology is card systems where each user gets a card that generates numbers that allow access to their account. Without the card, it is improbable to guess the numbers.
The following are companies that offer solutions that are provide better password
authenication (ie, handheld password devices):
Digital Pathways, Inc.
201 Ravendale Dr. Mountainview, Ca.
Phone: 415-964-0707 Fax: (415) 961-7487
One Alewife Center
Cambridge, MA 02140-2312
USA Phone: 617-547-7820
Fax: (617) 354-8836
Secure ID uses time slots as authenication rather than challenge/response.
PO Box 1480
Hudson, OH 44236
Tel:US+216-686-0090 Fax: US+216-686-0092
OneTime Pass (OTP):
This program provides unrestricted one-time pass codes on a user by user basis without any need for cryptographic protocols or hardware devices. The user takes a list of usable pass codes and scratches out each one as it is used. The system tracks usage, removing each passcode from the available list when it is used. Comes with a very small and fast password tester and password and pass phrase generation systems.
This is the original Argued Key system that mutually authenticates users and systems to each other based on their common knowledge. No hardware necessary. Comes with a very small and fast password tester and password and pass phrase generation systems.
480 Spring Park Place
Herndon, VA 22070
1-800-521-6261 ext 217
Arnold Consulting, Inc.
2530 Targhee Street, Madison, Wisconsin
Phone : 608-278-7700 Fax: 608-278-7701
CRYPTOCard is a modern, SecureID-sized, SNK-compatible device.
Enigma Logic, Inc.
2151 Salvio #301
Concord, CA 94520
510-827-5707 Fax: (510)827-2593
For information about Enigma ftp to: ftp.netcom.com in directory /pub/sa/safeword
2675 Long Lake Road
Roseville, MN 55113
Tel: (612) 628-2700
Fax: (612) 628-2701
You can try to make sure that most IBM DOS compatible machines have interfaces that will not allow sniffing. Here is a list of cards that do not support promiscuous mode:
Test the interface for promiscuous mode by using the Gobbler. If you find a interface that does do promiscuous mode and it is listed here, please e-mail
I would like to thank the following people for the contribution to this FAQ that has helped to update and shape it:
This paper is Copyright (c) 1994, 1995 by Christopher Klaus of Internet Security Systems, Inc.
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Please send suggestions, updates, and comments to:Christopher Klaus <firstname.lastname@example.org> of Internet Security Systems, Inc. <email@example.com>
Internet Security Systems, Inc, located in Atlanta, Ga., specializes in the developement of security scanning software tools. Its flagship product, Internet Scanner, is software that learns an organization's network and probes every device on that network for security holes. It is the most comprehensive "attack simulator" available, checking for over 100 security vulnerabilities.