Sniffer FAQ

Version: 1.7
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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:

   *  What a sniffer is and how it works
   *  Where are sniffers available
   *  How to detect if a machine is being sniffed
   *  Stopping sniffing attacks:
        o  Active hubs
        o  Encryption
        o  Kerberos
        o  One-time password technology
        o  Non-promiscuous interfaces

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What a sniffer is and how it works

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.

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Where are sniffers available

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 in Phrack, 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:

   *  Etherfind on SunOs4.1.x
   *  Snoop on Solaris 2.x and SunOs 4.1 (on ftp playground.sun.com)
   *  Tcpdump 3.0 uses bpf for a multitude of platforms.
   *  Packetman, Interman, Etherman, Loadman works on the following platforms:
     SunOS, Dec-Mips, SGI, Alpha, and Solaris. It is available on
     ftp.cs.curtin.edu.au:/pub/netman/[sun4c|dec-mips|sgi|alpha|solaris2]/
     [etherman-1.1a|interman-1.1|loadman-1.0|packetman-1.1].tar.Z
     Packetman was designed to capture packets, while Interman, Etherman, and
     Loadman monitor traffic of various kinds.

     DOS based sniffers

   *  Gobbler for IBM DOS Machines
   *  ethdump v1.03
     Available on ftp
     ftp.germany.eu.net:/pub/networking/inet/ethernet/ethdp103.zip
   *  ethload v1.04
     Companion utility to a ethernet monitor. Available on ftp
     ftp.germany.eu.net:/pub/networking/monitoring/ethload/ethld104.zip

Commercial Sniffers are available at:

   *  Network General.

          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.

   *  Microsoft's Net Monitor

          " 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

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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
             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.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.

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Stopping sniffing attacks

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:

   *  3Com
   *  HP

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Encryption

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:

   *  deslogin is one package available at ftp
     coast.cs.purdue.edu:/pub/tools/unix/deslogin .

   * swIPe is another package available at
     ftp.csua.berkeley.edu:/pub/cypherpunks/swIPe/

   * Netlock encrypts all (tcp, udp, and raw ip based) communications
     transparently. It has automatic (authenticated Diffie-Helman) distibuted
     key management mechanism for each host and runs on the SUN 4.1 and HP 9.x
     systems. The product comes with a Certification Authority Management
     application which generates host certificates (X.509) used for
     authentication between the hosts. and provides centralized control of each
     Hosts communications rules.

     The product is built by Hughes Aircraft and they can be reached at
     800-825-LOCK or email at netlock@mls.hac.com.

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Kerberos

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 at ftp at rtfm.mit.edu in
/pub/usenet/comp.protocols/kerberos/Kerberos_Users__Frequently_Asked_Questions_1.11

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One time password technology

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 on ftp: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):

Secure Net Key (SNK)

Digital Pathways, Inc.
201 Ravendale Dr. Mountainview, Ca.
97703-5216 USA

Phone: 415-964-0707 Fax: (415) 961-7487

Secure ID

Security Dynamics,
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.

ArKey and OneTime Pass

Management Analytics
PO Box 1480
Hudson, OH 44236
Email: fc@all.net
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.

ArKey:
This is the original Argued Key system that mutu



This page was created Wed Aug 11 13:00:10 EDT 1999


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