Sniffing and spoofing are security threats that target the lower layers of the networking infrastructure supporting applications that use the Internet. Users do not interact directly with these lower layers and are typically completely unaware that they exist. Without a deliber- ate consideration of these threats, it is impossible to build effective security into the higher levels.
Sniffing is a passive security attack in which a machine separate from the intended destination reads data on a
network. The term “sniffing” comes from the notion of “sniffing the ether” in an Ethernet network and is a
bad pun on the two meanings of the word “ether.” Passive security attacks are those that do not alter the
normal flow of data on a communication link or inject data into the link.
Spoofing is an active security attack in which one machine on the network masquerades as a different machine. As an active attack, it disrupts the normal flow of data and may involve injecting data into the communications link between other machines. This masquerade aims to fool other machines on the network into accepting the impostor as an original, either to lure the other machines into sending it data or to allow it to alter data. The meaning of “spoof” here is not “a lighthearted parody,” but rather “a deception intended to trick one into accept- ing as genuine something that is actually false.” Such deception can have grave consequences because notions of trust are central to many networking systems. Sniffing may seem innocuous
(depending on just how sensitive and confidential you consider the information on your network), some network security attacks use sniffing as a prelude to spoofing. Sniffing gathers sufficient information to make the deception believable.
Sniffing
Sniffing is the use of a network interface to receive data not intended for the machine in which the interface resides. A variety of types of machines need to have this capability. A token-ring bridge, for example, typically has two network interfaces that normally receive all packets traveling on the media on one interface and retransmit some, but not all, of these packets on the other interface. Another example of a device that incorporates sniffing is one typically marketed as a “network analyzer.” A network analyzer helps network administrators diagnose a variety of obscure problems that may not be visible on any one particular host. These problems can involve unusual interactions between more than just one or two machines and sometimes
involve a variety of protocols interacting in strange ways.
Devices that incorporate sniffing are useful and necessary. However, their very existence implies that a malicious person could use such a device or modify an existing machine to snoop on network traffic. Sniffing programs could be used to gather passwords, read inter-machine e-mail, and examine client-server database records in transit. Besides these high-level data, low- level information might be used to mount an active attack on data in another computer system.
Sniffing: How It Is Done
In a shared media network, such as Ethernet, all network interfaces on a network segment have access to all of the data that travels on the media. Each network interface has a hardware-layer address that should differ from all hardware-layer addresses of all other network interfaces on the network. Each network also has at least one broadcast address that corresponds not to an individual network interface, but to the set of all network interfaces. Normally, a network interface will only respond to a data frame carrying either its own hardware-layer address in the frame’s destination field or the “broadcast address” in the destination field. It responds to these frames by generating a hardware interrupt to the CPU. This interrupt gets the attention
of the operating system, and passes the data in the frame to the operating system for further processing.
Note The term “broadcast address” is somewhat misleading. When the sender wants to get the attention of the operating systems of all hosts on the network, he or she uses the “broadcast address.” Most network interfaces are capable of being put into a “promiscuous mode.” In promiscuous mode, network interfaces generate a hard- ware interrupt to the CPU for every frame they encounter, not just the ones with
their own address or the “broadcast address.” The term “shared media” indicates to the reader that such networks broadcast all frames—the frames travel on all the physical media that make up the network.
At times, you may hear network administrators talk about their networking trouble spots— when they observe failures in a localized area. They will say a particular area of the Ethernet is busier than other areas of the Ethernet where there are no problems. All of the packets travel through all parts of the Ethernet segment. Interconnection devices that do not pass all the frames from one side of the device to the other form the boundaries of a segment. Bridges, switches, and routers divide segments from each other, but low-level devices that operate on one bit at a time, such as repeaters and hubs, do not divide segments from each other. If only low-level devices separate two parts of the network, both are part of a single segment. All frames traveling in one part of the segment also travel in the other part.
The broadcast nature of shared media networks affects network performance and reliability so greatly that networking professionals use a network analyzer, or sniffer, to troubleshoot problems. A sniffer puts a network interface in promiscuous mode so that the sniffer can monitor each data packet on the network segment. In the hands of an experienced system administrator, a sniffer is an invaluable aid in determining why a network is behaving (or misbehaving) the way it is. With an analyzer, you can determine how much of the traffic is due to which network protocols, which hosts are the source of most of the traffic, and which hosts
are the destination of most of the traffic. You can also examine data traveling between a particular pair of hosts and categorize it by protocol and store it for later analysis offline. With a sufficiently powerful CPU, you can also do the analysis in real time.
Most commercial network sniffers are rather expensive, costing thousands of dollars. When you examine these closely, you notice that they are nothing more than a portable computer with an Ethernet card and some special software. The only item that differentiates a sniffer from an ordinary computer is software. It is also easy to download shareware and freeware sniffing software from the Internet or various bulletin board systems. The ease of access to sniffing software is great for network administrators because this type of
software helps them become better network troubleshooters. However, the availability of this software also means that malicious computer users with access to a network can capture all the data flowing through the network. The sniffer can capture all the data for a short period of time or selected portions of the data for a fairly long period of time. Eventually, the malicious user will run out of space to store the data—the network I use often has 1000 packets per second flowing on it. Just capturing the first 64 bytes of data from each packet fills up my system’s local disk space within the hour.
Note Esniff.c is a simple 300-line C language program that works on SunOS 4.x. When run by the root user on a Sun workstation, Esniff captures the first 300 bytes of each TCP/IP connection on the local network. It is quite effective at capturing all usernames and passwords entered by users for telnet, rlogin, and FTP. TCPDump 3.0.2 is a common, more sophisticated, and more portable Unix sniffing program written by Van Jacobson, a famous developer of high-quality TCP/IP software. It uses the libpcap library for portably interfacing with promiscuous mode network interfaces. The most recent version is available via anonymous FTP to ftp.ee.lbl.gov.
NetMan contains a more sophisticated, portable Unix sniffer in several programs in its network management suite. The latest version of NetMan is available via anonymous FTP to ftp.cs.curtin.edu.au in the directory /pub/netman. EthDump is a sniffer that runs under DOS and can be obtained via anonymous FTP from ftp.eu.germany.net in the directory /pub/networking/inet/ethernet/. On some Unix systems, TCPDump comes bundled with the vendor OS. When run by an ordinary, unprivileged user, it does not put the network interface into promiscuous mode. With this command available, a user can only see data being sent to the Unix host, but is not limited to seeing data sent to processes owned by the user. Systems administrators concerned about sniffing should remove user execution privileges from this program.
