The Dark Side of Packet Capture

by O'Reilly Press

Pretty much anything you can discover through packet capture can be discovered by anyone else using packet capture in a similar manner. Moreover, some technologies that were once thought to be immune to packet capture, such as switches, are not as safe as once believed. Make sure the Dark Side stays with you by reading this excerpt from the O'Reilly book, Network Troubleshooting Tools.

Network Troubleshooting Tools
by Joseph D. Sloan

Network Troubleshooting Tools - click to go to publisher's site

1.7. Dark Side of Packet Capture

What you can do, others can do. Pretty much anything you can discover through packet capture can be discovered by anyone else using packet capture in a similar manner. Moreover, some technologies that were once thought to be immune to packet capture, such as switches, are not as safe as once believed.

1.7.1. Switch Security

Switches are often cited as a way to protect traffic from sniffing. And they really do provide some degree of protection from casual sniffing. Unfortunately, there are several ways to defeat the protection that switches provide.

First, many switches will operate as hubs, forwarding traffic out on every port, whenever their address tables are full. When first initialized, this is the default behavior until the address table is built. Unfortunately, tools like macof, part of the dsniff suite of tools, will flood switches with MAC addresses overflowing a switch's address table. If your switch is susceptible, all you need to do to circumvent security is run the program.

Second, if two machines have the same MAC address, some switches will forward traffic to both machines. So if you want copies of traffic sent to a particular machine on your switch, you can change the MAC address on your interface to match the target devices' MAC address. This is easily done on many Unix computers with the ifconfig command.

A third approach, sometimes called ARP poisoning, is to send a forged ARP packet to the source device. This can be done with a tool like arpredirect, also part of dsniff. The idea is to substitute the packet capture device's MAC address for the destination's MAC address. Traffic will be sent to a packet capture device, which can then forward the traffic to its destination. Of course, the forged ARP packets can be sent to any number of devices on the switch.

The result, with any of these three techniques, is that traffic will be copied to a device that can capture it. Not all switches are susceptible to all of these attacks. Some switches provide various types of port security including static ARP assignments. You can also use tools like arpwatch to watch for suspicious activities on your network. (arpwatch is described in .) If sniffing is a concern, you may want to investigate what options you have with your switches.

While these techniques could be used to routinely capture traffic as part of normal management, the techniques previously suggested are preferable. Flooding the address table can significantly degrade network performance. Duplicating a MAC address will allow you to watch traffic only to a single host. ARP poisoning is a lot of work when monitoring more than one host and can introduce traffic delays. Consequently, these aren't really techniques that you'll want to use if you have a choice.

1.7.2. Protecting Yourself

Because of the potential for abuse, you should be very circumspect about who has access to packet capture tools. If you are operating in a Unix-only environment, you may have some success in restricting access to capture programs. packet capture programs should always be configured as privileged commands. If you want to allow access to a group of users, the recommended approach is to create an administrative group, restrict execution of packet capture programs to that group, and give group membership only to a small number of trusted individuals. This amounts to setting the SUID bit for the program, but limiting execution to the owner and any group members.

With some versions of Unix, you might even consider recompiling the kernel so the packet capture software can't be run on machines where it isn't needed. For example, with FreeBSD, it is very straightforward to disable the Berkeley packet filter in the kernel. (With older versions of FreeBSD, you needed to explicitly enable it.) Another possibility is to use interfaces that don't support promiscuous mode. Unfortunately, these can be hard to find.

There is also software that can be used to check to see if your interface is in promiscuous mode. You can do this manually with the ifconfig command. Look for PROMISC in the flags for the interface. For example, here is the output for one interface in promiscuous mode:

bsd2# ifconfig ep0
        inet netmask 0xffffff00 broadcast
        inet6 fe80::260:97ff:fe06:2222%ep0 prefixlen 64 scopeid 0x2
        ether 00:60:97:06:22:22
        media: 10baseT/UTP
        supported media: 10baseT/UTP

Of course, you'll want to check every interface.

Alternately, you could use a program like cpm, check promiscuous mode from CERT/CC. lsof, described in , can be used to look for large open files that might be packet sniffer output. But if you have Microsoft Windows computers on your network or allow user-controlled computers on your network, this approach isn't enough.

While it may appear that packet capture is a purely passive activity that is undetectable, this is often not the case. There are several techniques and tools that can be used to indicate packet capture or to test remote interfaces to see if they are in promiscuous mode. One of the simplest techniques is to turn your packet capture software on, ping an unused IP address, and watch for DNS queries trying to resolve that IP address. An unused address should be ignored. If someone is trying to resolve the address, it is likely they have captured a packet.

Another possibility is the tool antisniff from L0pht Heavy Industries. This is a commercial tool, but a version is available for noncommercial uses. There are subtle changes in the behavior of an interface when placed in promiscuous mode. This tool is designed to look for those changes. It can probe the systems on a network, examine their responses, and usually determine which devices have an interface in promiscuous mode.

Another approach is to restructure your network for greater security. To the extent you can limit access to traffic, you can reduce the packet capture. Use of virtual LANs can help, but no approach is really foolproof. Ultimately, strong encryption is your best bet. This won't stop sniffing, but it will protect your data. Finally, it is always helpful to have clearly defined policies. Make sure your users know that unauthorized packet capture is not acceptable.

1.8. Microsoft Windows

In general, it is inadvisable to leave packet capture programs installed on Windows systems unless you are quite comfortable with the physical security you provide for those machines. Certainly, packet capture programs should never be installed on publicly accessible computers using consumer versions of Windows.

The programs WinDump95 and WinDump are ports of tcpdump to Windows 95/98 and Windows NT, respectively. Each requires the installation of the appropriate drivers. They are run in DOS windows and have the same basic syntax as tcpdump. As tcpdump has already been described, there is little to add here.

ethereal is also available for Windows and, on the whole, works quite well. The one area in which the port doesn't seem to work is in sending output directly to a printer. However, printing to files works nicely so you can save any output you want and then print it.

One of the more notable capture programs available for Windows platforms is netmon (Network Monitor), a basic version of which is included with Windows NT Server. The netmon program was originally included with Windows NT 3.5 as a means of collecting data to send to Microsoft's technical support. As such, it was not widely advertised. Figure 1-5 shows the packet display window.

Figure 1-5 Figure 1-5. netmon for Windows
Click image to view full-sized in a new window
(Click image for larger view in a new window)

The basic version supplied with Windows NT Server is quite limited in scope. It restricts capture to traffic to or from the server and severely limits the services it provides. The full version is included as part of the Systems Management Server (SMS), part of the BackOffice suite, and is an extremely powerful program. Of concern with any capture and analysis program is what protocols can be effectively decoded. As might be expected, netmon is extremely capable when dealing with Microsoft protocols but offers only basic decoding of Novell protocols. (For Novell protocols, consider Novell's LANalyzer.)

One particularly nice feature of netmon is the ability to set up collection agents on any Windows NT workstation and have them collect data remotely. The collected data resides with the agent until needed, thus minimizing traffic over the network.

The program is, by default, not installed. The program can be added as a service under network configuration in the setup window. It is included under Administrative Tools (Common). The program, once started, is very intuitive and has a strong help system.

Network Troubleshooting Tools - click to go to publisher's site

That was our final segment from the O'Reilly book, Network Troubleshooting Tools. For further information on that title, click the cover image at right.

This article was originally published on Tuesday Dec 4th 2001