DoS/DDoS Countermeasures

There are several ways to detect, halt, or prevent DoS attacks. The following are common security features:

• Network-Ingress Filtering All network access providers should implement network-ingress filtering to stop any downstream networks from injecting packets with faked or spoofed addresses into the Internet. Although this doesn't stop an attack from occurring, it does make it much easier to track down the source of the attack and terminate the attack quickly. Most IDS, firewalls, and routers provide network-ingress filtering capabilities.
• Rate-Limiting Network Traffic A number of routers on the market today have features that let you limit the amount of bandwidth some types of traffic can consume. This is sometimes referred to as traffic shaping.
• Intrusion Detection Systems Use an intrusion detection system (IDS) to detect attackers who are communicating with slave, master, or agent machines. Doing so lets you know whether a machine in your network is being used to launch a known attack but probably won't detect new variations of these attacks or the tools that implement them. Most IDS vendors have signatures to detect Trinoo, TFN, or Stacheldraht network traffic.
• Automated Network-Tracing Tools Tracing streams of packets with spoofed addresses through the network is a time-consuming task that requires the cooperation of all networks carrying the traffic and that must be completed while the attack is in progress.
• Host-Auditing and Network-Auditing Tools File-scanning tools are available that attempt to detect the existence of known DDoS tool client and server binaries in a system. Network-scanning tools attempt to detect the presence of DDoS agents running on hosts on your network.

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DoS Scanning Tools

Find_ddos is a tool that scans a local system that likely contains a DDoS program. It can detect several known DoS attack tools.

SARA gathers information about remote hosts and networks by examining network services. This includes information about the network information services as well as potential security flaws, such as incorrectly set up or configured network services, well-known bugs in the system or network utilities system software vulnerabilities listed in the Common Vulnerabilities and Exposures (CVE) database, and weak policy decisions.

RID is a free scanning tool that detects the presence of Trinoo, TFN, or Stacheldraht clients.

Zombie Zapper instructs zombie routines to go to sleep, thus stopping their attack. You can use the same commands an attacker would use to stop the attack.

How DDoS Attacks Work

DDoS is an advanced version of the DoS attack. Like DoS, DDoS tries to deny access to services running on a system by sending packets to the destination system in a way that the destination system can't handle. The key of a DDoS attack is that it relays attacks from many different hosts (which must first be compromised), rather than from a single host like DoS. DDoS is a large-scale, coordinated attack on a victim system.

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Hacking Tools

Trinoo is a tool that sends User Datagram Protocol (UDP) traffic to create a DDoS attack. The Trinoo master is a system used to launch a DoS attack against one or more target systems. The master instructs agent processes (called daemons) on previously compromised systems (secondary victims) to attack one or more IP addresses. This attack occurs for a specified period of time. The Trinoo agent or daemon is installed on a system that suffers from a buffer overflow vulnerability. WinTrinoo is a Windows version of Trinoo and has the same functionality as Trinoo.

Shaft is a derivative of the Trinoo tool that uses UDP communication between masters and agents. Shaft provides statistics on the flood attack that attackers can use to know when the victim system is shut down; Shaft provides UDP, ICMP, and TCP flooding attack options.

Tribal Flood Network (TFN) allows an attacker to use both bandwidth-depletion and resource-depletion attacks. TFN does UDP and ICMP flooding as well as TCP SYN and smurf attacks. TFN2K is based on TFN, with features designed specifically to make TFN2K traffic difficult to recognize and filter. It remotely executes commands, hides the source of the attack using IP address spoofing, and uses multiple transport protocols (including UDP, TCP, and ICMP).

Stacheldraht is similar to TFN and includes ICMP flood, UDP flood, and TCP SYN attack options. It also provides a secure telnet connection (using symmetric key encryption) between the attacker and the agent systems (secondary victims). This prevents system administrators from intercepting and identifying this traffic.

Mstream uses spoofed TCP packets with the ACK flag set to attack a target. It consists of a handler and an agent portion, but access to the handler is password protected.

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The services under attack are those of the primary victim; the compromised systems used to launch the attack are secondary victims. These compromised systems, which send the DDoS to the primary victim, are sometimes called zombies or BOTs. They're usually compromised through another attack and then used to launch an attack on the primary victim at a certain time or under certain conditions. It can be difficult to track the source of the attacks because they originate from several IP addresses.

Normally, DDoS consists of three parts:

• Master/handler
• Slave/secondary victim/zombie/agent/BOT/BOTNET
• Victim/primary victim

The master is the attack launcher. A slave is a host that is compromised by and controlled by the master. The victim is the target system. The master directs the slaves to launch the attack on the victim system. See Figure 1.

 
Figure 1: Master and Slaves in a DDoS Attack

DDoS is done in two phases. In the intrusion phase, the hacker compromises weak systems in different networks around the world and installs DDoS tools on those compromised slave systems. In the DDoS attack phase, the slave systems are triggered to cause them to attack the primary victim. See Figure 2.

 
Figure 2: Bots or Zombie systems

Denial of Service

A DoS attack is an attempt by a hacker to flood a user's or an organization's system. As a CEH, you need to be familiar with the types of DoS attacks and should understand how DoS and DDoS attacks work. You should also be familiar with robots (BOTs) and robot networks (BOTNETs), as well as smurf attacks and SYN flooding. Finally, as a CEH, you need to be familiar with various DoS and DDoS countermeasures.

There are two main categories of DoS attacks:

• Attacks sent by a single system to a single target (simple DoS)
• Attacks sent by many systems to a single target (distributed denial of service, or DDoS)

The goal of DoS isn't to gain unauthorized access to machines or data, but to prevent legitimate users of a service from using it. A DoS attack may do the following:

• Flood a network with traffic, thereby preventing legitimate network traffic.
• Disrupt connections between two machines, thereby preventing access to a service.
• Prevent a particular individual from accessing a service.
• Disrupt service to a specific system or person.

Different tools use different types of traffic to flood a victim, but the result is the same: a service on the system or the entire system is unavailable to a user because it's kept busy trying to respond to an exorbitant number of requests.

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Real World Scenario: A Denial of Service Attack

On the evening of May 28, 2008, the company I was working for (alfasystems.com) suddenly dropped off the Internet. Their web servers were no longer accessible from the Internet.

Within a minute of the start of the attack, it was clear to the Alpha Systems engineers that they were experiencing a "packet flooding" attack of some sort. After looking at the log files of their Cisco router, it showed that both of their two T1 trunk interfaces to the Internet were receiving some sort of traffic at their maximum 1.54 megabit rate, while their outbound traffic had fallen to nearly zero. They were drowning in a flood of malicious traffic and valid traffic was unable to get out. Alpha Systems was the victim of a denial-of-service attack, more commonly referred to as a DoS. The engineers knew they had to do something quickly to stop the attack and get the web servers back up and accessible for their customers. But no one really knew what to do as this had never happened to the systems before. Then someone thought of the packet filtering capabilities of the router.

Luckily, because this DoS attack was prone to filtering, Alpha Systems was able to weed out the bad packets and return their service to almost normal operation. In two minutes Alpha Systems engineers applied "brute force" filters to their routers, shutting down all UDP and ICMP traffic, and alfasystems.com instantly popped back onto the Internet.

It was finally determined that their server had been attacked by 474 security-compromised Windows PCs containing remote-control attack "zombies," in a classic DoS attack generated by the coordinated efforts of these hundreds of individual PCs.

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A DoS attack is usually an attack of last resort. It's considered an unsophisticated attack because it doesn't gain the hacker access to any information but rather annoys the target and interrupts their service. DoS attacks can be destructive and have a substantial impact when sent from multiple systems at the same time (DDoS attacks).

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Hacking Tools

Ping of Death is an attack that can cause a system to lock up by sending multiple IP packets, which will be too large for the receiving system when reassembled. Ping of Death can cause a DoS to clients trying to access the server that has been a victim of the attack.

SSPing is a program that sends several large fragmented, Internet Control Message Protocol (ICMP) data packets to a target system. This will cause the computer receiving the data packets to freeze when it tries to reassemble the fragments.

A LAND attack sends a packet to a system where the source IP is set to match the target system's IP address. As a result, the system attempts to reply to itself, causing the system to create a loop—which will tie up system resources and eventually may crash the OS.

CPUHog is a DoS attack tool that uses up the CPU resources on a target system, making it unavailable to the user.

WinNuke is a program that looks for a target system with port 139 open, and sends junk IP traffic to the system on that port. This attack is also known as an out-of-bounds (OOB) attack and causes the IP stack to become overloaded—eventually the system crashes.

Jolt2 is a DoS tool that sends a large number of fragmented IP packets to a Windows target. This ties up system resources and eventually locks up the system. Jolt2 isn't Windows specific; many Cisco routers and other gateways may be vulnerable to the Jolt2 attack.

Bubonic is a DoS tool that works by sending TCP packets with random settings, in order to increase the load of the target machine so that it eventually crashes.

Targa is a program that can be used to run eight different DoS attacks. The attacker has the option to either launch individual attacks or try all of the attacks until one is successful.

RPC Locator is a service that, if unpatched, has a vulnerability to overflows. Details on patching a system to prevent RPC vulnerabilities will be covered later in the chapter. The RPC Locator service in Windows allows distributed applications to run on the network. It is susceptible to DoS attacks, and many of the tools that perform DoS attacks exploit this vulnerability.

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Note

Because DoS attacks are so powerful and can cripple a production system or network, this chapter does not include any DoS tool exercises. If you want to test the tools listed here, ensure that you are not using them on a production network or system. The DoS tools could render the target systems unusable.

DDoS attacks can be perpetrated by BOTs and BOTNETs, which are compromised systems that an attacker uses to launch the attack against the end victim. The system or network that has been compromised is a secondary victim, whereas the DoS and DDoS attacks flood the primary victim or target.

NetBIOS DoS Attacks

A NetBIOS denial-of-service (DoS) attack sends a NetBIOS Name Release message to the NetBIOS Name Service on a target Windows systems and forces the system to place its name in conflict so that the name can no longer be used. This essentially blocks the client from participating in the NetBIOS network and creates a network DoS for that system.

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Hacking Tool

NBName can disable entire LANs and prevent machines from rejoining them. Nodes on a NetBIOS network infected by the tool think that their names are already in use by other machines.

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Another way to create a more secure and memorable password is to follow a repeatable pattern, which will enable to password to be re-created when needed.

1. Start with a memorable phrase, such as
   • Maryhadalittlelamb
2. Change every other character to uppercase, resulting in
   • MaRyHaDaLiTtLeLaMb
3. Change a to @ and i to 1 to yield
   • M@RyH@D@L1TtLeL@Mb
4. Drop every other pair to result in a secure repeatable password or
   • M@H@L1LeMb

Now you have a password that meets all the requirements, yet can be "remade" if necessary.