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This chapter is from the book

Considerations for Network Sniffing

In order to do ethical and productive sniffing, you should follow the following guidelines.

Always Get Permission

Network sniffing, like many other security functions, has the potential for abuse. By capturing every transmission on the wire, you are very likely to see passwords for various systems, contents of e-mails, and other sensitive data, both internal and external, since most systems don't encrypt their traffic on a local LAN. This data, in the wrong hands, could obviously lead to serious security breaches. In addition, it could be a violation of your employees' privacy, depending on your company policies. For example, you might observe employees logging into their employee benefits or 401(k) accounts. Always get written permission from a supervisor, and preferably upper management, before you start this kind of activity. And you should consider what to do with the data after getting it. Besides passwords, it may contain other sensitive data. Generally, network-sniffing logs should be purged from your system unless they are needed for a criminal or civil prosecution. There are documented cases of well-intentioned system administrators being fired for capturing data in this manner without permission.

Understand Your Network Topology

Make sure you fully understand the physical and logical layout of your network before setting up your sniffer. Sniffing from the wrong place on the network will cause you either to not see what you are looking for or to get erroneous results. Make sure there is not a router between your sniffing workstation and what you are trying to observe. Routers will only direct traffic onto a network segment if it is addressed to a node located there. Also, if you are on a switched network, you will need to configure the port you are plugged into to be a "monitor" or "mirror" port. Various manufacturers use different terminology, but basically you need the port to act like a hub rather than a switch, so it should see all the traffic on that switch, not just what is addressed to your workstation. Without this setting, all your monitor port will see is the traffic addressed to the specific port you are plugged into and the network's broadcast traffic.

Use Tight Search Criteria

Depending on what you are looking for, using an open filter (that is, seeing everything) will make the output data voluminous and hard to analyze. Use specific search criteria to narrow down the output that your sniffer shows. Even if you are not exactly sure what you are looking for, you can still write a filter to limit your search results. If you are looking for an internal machine, set your criteria to see only source addresses within your network. If you are trying to track down a specific type of traffic, say FTP traffic, then limit the results to only those on the port that application uses. Doing this will make your sniffer results much more usable.

Establish a Baseline for Your Network

If you use your sniffer to analyze your network during normal operation and record the summary results, you will then have a baseline to compare it to when you are trying to isolate a problem. The Ethereal sniffer discussed in this chapter creates several nice reports for this. You will also have some data to track your network utilization over time. You can use this data to decide when your network is becoming saturated and what the primary causes are. It might be a busy server, more users, or a change in the type of traffic. If you know what you started with, you can more easily tell what and where your culprit is.

Tcpdump: An Ethernet Traffic Analyzer


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University of California, Lawrence Berkeley Laboratories

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There are many sniffers available, both free and commercial, but Tcpdump is the most widely available and inexpensive. It comes with most UNIX distributions, including Linux and BSD. In fact, if you have a fairly current Linux distribution, chances are you already have Tcpdump installed and ready to go.

Installing Tcpdump

Tcpdump does exactly what its name implies: it dumps the contents of the TCP/IP packets passing through an interface to an output device, usually the screen or to a file.

  1. In order for Tcpdump to work, it must be able to put your network card into what is called promiscuous mode. This means that the network card will intercept all traffic on the Ethernet wire, not just that addressed to it. Each operating system processes traffic from the Ethernet card in a different fashion. To provide a common reference for programmers, a library called pcap was created. On UNIX this is known as libpcap and on Windows as WinPcap. These low-level drivers can modify the way the card would normally handle traffic. They must be installed before you can install Tcpdump.

    If Tcpdump is already on your system, then you already have this driver installed. If not, they are provided on the CD-ROM that accompanies this book in the misc directory, or you can get them from the Tcpdump Web site. Make sure you install them before you install Tcpdump.

    Note: Libpcap also requires the Flex and Bison scripting languages, or Lex and Yacc as a substitute. If you don't have these, get them from your OS distribution disks or online and install them so libpcap will install successfully.

  2. Install libpcap by unpacking it and issuing the standard compilation commands:

    make install

    If you get a warning something like "Cannot determine packet capture interface" during the compilation process, then your network card doesn't support promiscuous mode operation and you will have to get another card to use Tcpdump. Most cards these days should support this mode of operation.

  3. Once libpcap is installed, unpack the Tcpdump package and change to that directory.

  4. Run the same compilation commands:

    make install

    Now you are ready to use Tcpdump.

Running Tcpdump

There are a number of filter operations you can perform on the output to look for a specific type of traffic or lessen the overall amount of output. Indeed, on a busy network, unfiltered Tcpdump output will cause your screen to scroll faster than you can read it! However, for a quick demo of the power of Tcpdump, invoke it from the command line by simply typing:


You will see all the TCP traffic passing your machine's Ethernet card, unfiltered. It might look something like the example in Listing 6.1.

Example 6.1. Tcpdump Example

12:25:38.504619 > . ack
  1418369642 win 31856 <nop,nop,timestamp 72821542 25475802> (DF)

12:25:38.504758 > . ack
  1 win 40544 <nop,nop,timestamp 25486047 72811295> (DF)

12:25:38.507753 >
  11414+ PTR? (44) (DF)

12:25:38.561481 >
  11414 NXDomain*- 0/1/0 (113)

12:25:38.562754 >
  11415+ PTR? (42) (DF)

12:25:38.609588 >
  11415 NXDomain 0/1/0 (119)

12:25:38.610428 >
 1416+ PTR? (44) (DF)

12:25:38.649808 >
  11416 1/0/0 (69)

12:25:43.497909 arp who-has tell

12:25:43.498153 arp reply is-at 0:6:25:9f:34:ac

12:25:43.498943 >
  11417+ PTR? (42) (DF)

12:25:43.533126 >
  11417 NXDomain 0/1/0 (119)

12:25:44.578546 >
  Trap(35) E:3955.2.2.1 enterpriseSpecific[specific-   trap(1)!=0] 43525500
ccc.gif [|snmp]

This might look a little confusing at first, but if you break it down it starts to make more sense. The first number is a timestamp, broken down into fractions of a second, because on a busy network there will be many packets per second on the wire. The next number is the source IP address of the packet followed by > (a greater than sign), and then the destination address. Finally, there may be some comments and other data. You can see several different kinds of traffic in this example, including DNS traffic (domain), ARP, and SNMP.

By default, Tcpdump runs until stopped by you pressing Control+C or another interrupt signal. When Tcpdump stops, it prints a summary of all the traffic it saw. The summary statistics include:

  • Packets received by filter. This is the count of packets processed by the Tcpdump filter. It is not a count of all the TCP packets on the wire unless you ran Tcpdump without any filter criteria.

  • Packets dropped by kernel. The number of packets that were dropped due to a lack of resources on your system. This feature may not be supported on all systems. Even when it is, it may not be accurate if there is a lot of saturation on the network or your sniffer machine is very slow.

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