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Chapter: Security in Computing : Security in Networks

Denial of Service

Communications fail for many reasons. For instance, a line is cut. Or network noise makes a packet unrecognizable or undeliverable. A machine along the transmission path fails for hardware or software reasons.

Denial of Service


So far, we have discussed attacks that lead to failures of confidentiality or integrityproblems we have also seen in the contexts of operating systems, databases, and applications. Availability attacks, sometimes called denial-of-service or DOS attacks, are much more significant in networks than in other contexts. There are many accidental and malicious threats to availability or continued service.


Transmission Failure


Communications fail for many reasons. For instance, a line is cut. Or network noise makes a packet unrecognizable or undeliverable. A machine along the transmission path fails for hardware or software reasons. A device is removed from service for repair or testing. A device is saturated and rejects incoming data until it can clear its overload. Many of these problems are temporary or automatically fixed (circumvented) in major networks, including the Internet.


However, some failures cannot be easily repaired. A break in the single communications line to your computer (for example, from the network to your network interface card or the telephone line to your modem) can be fixed only by establishment of an alternative link or repair of the damaged one. The network administrator will say "service to the rest of the network was unaffected," but that is of little consolation to you.



From a malicious standpoint, you can see that anyone who can sever, interrupt, or overload capacity to you can deny you service. The physical threats are pretty obvious. We consider instead several electronic attacks that can cause a denial of service.


Connection Flooding


The most primitive denial-of-service attack is flooding a connection. If an attacker sends you as much data as your communications system can handle, you are prevented from receiving any other data. Even if an occasional packet reaches you from someone else, communication to you will be seriously degraded.


More sophisticated attacks use elements of Internet protocols. In addition to TCP and UDP, there is a third class of protocols, called ICMP or Internet Control Message Protocols. Normally used for system diagnostics, these protocols do not have associated user applications. ICMP protocols include

ping, which requests a destination to return a reply, intended to show that the destination system is reachable and functioning


echo, which requests a destination to return the data sent to it, intended to show that the connection link is reliable (ping is actually a version of echo)


destination unreachable, which indicates that a destination address cannot be accessed


source quench, which means that the destination is becoming saturated and the source should suspend sending packets for a while


These protocols have important uses for network management. But they can also be used to attack a system. The protocols are handled within the network stack, so the attacks may be difficult to detect or block on the receiving host. We examine how these protocols can be used to attack a victim.




This attack works between two hosts. Chargen is a protocol that generates a stream of packets; it is used to test the network's capacity. The attacker sets up a chargen process on host A that generates its packets as echo packets with a destination of host B. Then, host A produces a stream of packets to which host B replies by echoing them back to host A. This series puts the network infrastructures of A and B into an endless loop. If the attacker makes B both the source and destination address of the first packet, B hangs in a loop, constantly creating and replying to its own messages.


Ping of Death


A ping of death is a simple attack. Since ping requires the recipient to respond to the ping request, all the attacker needs to do is send a flood of pings to the intended victim. The attack is limited by the smallest bandwidth on the attack route. If the attacker is on a 10-megabyte (MB) connection and the path to the victim is 100 MB or more, the attacker cannot mathematically flood the victim alone. But the attack succeeds if the numbers are reversed: The attacker on a 100-MB connection can easily flood a 10-MB victim. The ping packets will saturate the victim's bandwidth.




The smurf attack is a variation of a ping attack. It uses the same vehicle, a ping packet, with two extra twists. First, the attacker chooses a network of unwitting victims. The attacker spoofs the source address in the ping packet so that it appears to come from the victim. Then, the attacker sends this request to the network in broadcast mode by setting the last byte of the address to all 1s; broadcast mode packets are distributed to all hosts on the network. The attack is shown in Figure 7-16.

Syn Flood


Another popular denial-of-service attack is the syn flood. This attack uses the TCP protocol suite, making the session-oriented nature of these protocols work against the victim.


For a protocol such as Telnet, the protocol peers establish a virtual connection, called a session, to synchronize the back-and -forth, command-response nature of the Telnet terminal emulation. A session is established with a three-way TCP handshake. Each TCP packet has flag bits, two of which are denoted SYN and ACK. To initiate a TCP connection, the originator sends a packet with the SYN bit on. If the recipient is ready to establish a connection, it replies with a packet with both the SYN and ACK bits on. The first party then completes the exchange to demonstrate a clear and complete communication channel by sending a packet with the ACK bit on, as shown in Figure 7-17.

Occasionally packets get lost or damaged in transmission. The destination maintains a queue called the SYN_RECV connections, tracking those items for which a SYNACK has been sent but no corresponding ACK has yet been received. Normally, these connections are completed in a short time. If the SYNACK (2) or the ACK (3) packet is lost, eventually the destination host will time out the incomplete connection and discard it from its waiting queue.


The attacker can deny service to the target by sending many SYN requests and never responding with ACKs, thereby filling the victim's SYN_RECV queue. Typically, the SYN_RECV queue is quite small, such as 10 or 20 entries. Because of potential routing delays in the Internet, typical holding times for the SYN_RECV queue can be minutes. So the attacker need only send a new SYN request every few seconds and it will fill the queue.


Attackers using this approach usually do one more thing: They spoof the nonexistent return address in the initial SYN packet. Why? For two reasons. First, the attacker does not want to disclose the real source address in case someone should inspect the packets in the SYN_RECV queue to try to identify the attacker. Second, the attacker wants to make the SYN packets indistinguishable from legitimate SYN packets to establish real connections. Choosing a different (spoofed) source address for each one makes them unique. A SYNACK packet to a nonexistent address results in an ICMP Destination Unreachable response, but this is not the ACK for which the TCP connection is waiting. (Remember that TCP and ICMP are different protocol suites, so an ICMP reply does not necessarily get back to the sender's TCP handler.)




The teardrop attack misuses a feature designed to improve network communication. A network IP datagram is a variable-length object. To support different applications and conditions, the datagram protocol permits a single data unit to be fragmented, that is, broken into pieces and transmitted separately. Each fragment indicates its length and relative position within the data unit. The receiving end is responsible for reassembling the fragments into a single data unit.


In the teardrop attack, the attacker sends a series of datagrams that cannot fit together properly. One datagram might say it is position 0 for length 60 bytes, another position 30 for 90 bytes, and another position 41 for 173 bytes. These three pieces overlap, so they cannot be reassembled properly. In an extreme case, the operating system locks up with these partial data units it cannot reassemble, thus leading to denial of service.


For more on these and other denial of service threats, see [CER99 and MAR05].


Traffic Redirection


As we saw earlier, at the network layer, a router is a device that forwards traffic on its way through intermediate networks between a source host's network and a destination's network. So if an attacker can corrupt the routing, traffic can disappear.


Routers use complex algorithms to decide how to route traffic. No matter the algorithm, they essentially seek the best path (where "best" is measured in some combination of distance, time, cost, quality, and the like). Routers are aware only of the routers with which they share a direct network connection, and they use gateway protocols to share information about their capabilities. Each router advises its neighbors about how well it can reach other network addresses. This characteristic allows an attacker to disrupt the network.


To see how, keep in mind that, in spite of its sophistication, a router is simply a computer with two or more network interfaces. Suppose a router advertises to its neighbors that it has the best path to every other address in the whole network. Soon all routers will direct all traffic to that one router. The one router may become flooded, or it may simply drop much of its traffic. In either case, a lot of traffic never makes it to the intended destination.


DNS Attacks


Our final denial-of-service attack is actually a class of attacks based on the concept of domain name server. A domain name server (DNS) is a table that converts domain names like ATT.COM into network addresses like; this process is called resolving the domain name. A domain name server queries other name servers to resolve domain names it does not know. For efficiency, it caches the answers it receives so it can resolve that name more rapidly in the future. A pointer to a DNS server can be retained for weeks or months.


In the most common implementations of Unix, name servers run software called Berkeley Internet Name Domain or BIND or named (a shorthand for "name daemon"). There have been numerous flaws in BIND, including the now-familiar buffer overflow.


By overtaking a name server or causing it to cache spurious entries (called DNS cache poisoning), an attacker can redirect the routing of any traffic, with an obvious implication for denial of service.


In October 2002, a massive flood of traffic inundated the top -level domain DNS servers, the servers that form the foundation of the Internet addressing structure. Roughly half the traffic came from just 200 addresses. Although some people think the problem was a set of misconfigured firewalls, nobody knows for sure what caused the attack.


An attack in March 2005 used a flaw in a Symantec firewall to allow a change in the DNS records used on Windows machines. The objective of this attack was not denial of service, however. In this attack, the poisoned DNS cache redirected users to advertising sites that received money from clients each time a user visited the site. Nevertheless, the attack also prevented users from accessing the legitimate sites.

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