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.

Hacking Tools

EtherFlood is used to flood an Ethernet switch with traffic to make it revert to a hub. By doing this, a hacker is able to capture all traffic on the network rather than just traffic going to and from their system, as would be the case with a switch.

Dsniff is a collection of Unix-executable tools designed to perform network auditing as well as network penetration. The following tools are contained in dsniff: filesnarf, mailsnarf, msgsnarf, urlsnarf, and webspy. These tools passively monitor a vulnerable shared network (such as a LAN where the sniffer sits behind any exterior firewall) for interesting data (passwords, email, files, and so on).

Sshmitm and webmitm implement active man-in-the-middle attacks against redirected Secure Shell (SSH) and HTTPS sessions.

Arpspoof, dnsspoof, and macof work on the interception of switched network traffic that is usually unavailable to a sniffer program because of switching. To get around the Layer 2 packet-switching issue, dsniff spoofs the network into thinking that it's a gateway that data must pass through to get outside the network.

IP Restrictions Scanner (IRS) is used to find the IP restrictions that have been set for a particular service on a host. It combines ARP poisoning with a TCP stealth or half-scan technique and exhaustively tests all possible spoofed TCP connections to the selected port of the target. IRS can find servers and network devices like routers and switches and identify access-control features like access control lists (ACLs), IP filters, and firewall rules.

sTerm is a telnet client with a unique feature: it can establish a bidirectional telnet session to a target host, without ever sending the real IP and MAC addresses in any packet. Using ARP poisoning, MAC spoofing, and IP spoofing techniques, sTerm can effectively bypass ACLs, firewall rules, and IP restrictions on servers and network devices.

Cain & Abel is a multipurpose hacking tool for Windows. It allows easy recovery of various kinds of passwords by sniffing the network; cracking encrypted passwords using dictionary or brute-force attacks; recording Voice over IP, or VoIP, conversations; decoding scrambled passwords; revealing password boxes; uncovering cached passwords; and analyzing routing protocols. The latest version contains a lot of new features like ARP Poison Routing (APR), which enables sniffing on switched LANs and man-in-the-middle attacks. The sniffer in this version can also analyze encrypted protocols such as SSH-1 and HTTPS, and it contains filters to capture credentials from a wide range of authentication mechanisms.

Packet Crafter is a tool used to create custom TCP/IP/UDP packets. The tool can change the source address of a packet to do IP spoofing and can control IP flags (such as checksums) and TCP flags (such as the state flags, sequence numbers, and ack numbers).

SMAC is a tool used to change the MAC address of a system. It lets a hacker spoof a MAC address when performing an attack.

MAC Changer is a tool used to spoof a MAC address on Unix. It can be used to set the network interface to a specific MAC address, set the MAC randomly, set a MAC of another vendor, set another MAC of the same vendor, set a MAC of the same kind, or display a vendor MAC list to choose from.

WinDNSSpoof is a simple DNS ID spoofing tool for Windows. To use it on a switched network, you must be able to sniff traffic of the computer being attacked. Therefore, it may need to be used in conjunction with an ARP spoofing or flooding tool.

Distributed DNS Flooder sends a large number of queries to create a DoS attack, disabling DNS. If DNS daemon software logs incorrect queries, the impact of this attack is amplified.

Understanding MAC Flooding and DNS Spoofing

A packet sniffer on a switched network can't capture all traffic as it can on a hub network; instead, it captures traffic either coming from or going to the system. It's necessary to use an additional tool to capture all traffic on a switched network. There are essentially two ways to perform active sniffing and make the switch send traffic to the system running the sniffer:

• ARP Spoofing This method involves using the MAC address of the network gateway and consequently receiving all traffic intended for the gateway on the sniffer system. A hacker can also flood a switch with so much traffic that it stops operating as a switch and instead reverts to acting as a hub, sending all traffic to all ports. This active sniffing attack allows the system with the sniffer to capture all traffic on the network.

Note

Many switches have been patched or redesigned to not be susceptible to the flooding vulnerability.

DNS Spoofing (or DNS Poisoning) This is a technique that tricks a DNS server into believing it has received authentic information when in reality it hasn't. Once the DNS server has been poisoned, the information is generally cached for a while, spreading the effect of the attack to the users of the server. When a user requests a certain website URL, the address is looked up on a DNS server to find the corresponding IP address. If the DNS server has been compromised, the user is redirected to a website other than the one that was requested, such as a fake website.

To perform a DNS attack, the attacker exploits a flaw in the DNS server software that can make it accept incorrect information. If the server doesn't correctly validate DNS responses to ensure that they come from an authoritative source, the server ends up caching the incorrect entries locally and serving them to users that make subsequent requests.

This technique can be used to replace arbitrary content for a set of victims with content of an attacker's choosing. For example, an attacker poisons the IP address's DNS entries for a target website on a given DNS server, replacing them with the IP address of a server the hacker controls. The hacker then creates fake entries for files on this server with names matching those on the target server. These files may contain malicious content, such as a worm or a virus. A user whose computer has referenced the poisoned DNS server is tricked into thinking the content comes from the target server and unknowingly downloads malicious content.

The types of DNS spoofing techniques are as follows:

• Intranet Spoofing Acting as a device on the same internal network
• Internet Spoofing Acting as a device on the Internet
• Proxy Server DNS Poisoning Modifying the DNS entries on a proxy server so the user is redirected to a different host system
• DNS Cache Poisoning Modifying the DNS entries on any system so the user is redirected to a different host

Wireshark Filters

Wireshark is a freeware sniffer that can capture packets from a wired or wireless LAN connection. It is a very powerful tool which can provide network and upper layer protocol data captured on a network. Like a lot of other network programs, Wireshark uses the pcap network library to capture packets.

Wireshark was called Ethereal until 2006 when the main developer decided to change its name because of copyright reasons with the Ethereal name, which was registered by the company he decided to leave in 2006.

In Exercise 1 you installed and began capturing packets using Wireshark. To narrow down the amount of information gathered by Wireshark, you can use filters. These filters limit the amount of information captured or displayed.

Here are some examples of Wireshark filters:

• ip.dst eq www.eccouncil.org This sets the filter to capture only packets destined for the web server www.eccouncil.org.
• ip.src == 192.168.1.1 This sets the filter to capture only packets coming from the host 192.168.1.1.
• eth.dst eq ff:ff:ff:ff:ff:ff This sets the filter to capture only Layer 2 broadcast packets.
• host 172.18.5.4 This sets the filter to capture only traffic to or from IP address 172.18.5.4.
• net 192.168.0.0/24 This sets the filter to capture traffic to or from a range of IP addresses.
• port 80 This sets the filter to capture traffic to destination port 80 (HTTP).
• port 80 and tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420 This sets the filter to capture HTTP GET requests. The filter looks for the bytes "G", "E", "T", and " " (hex values 47, 45, 54, and 20) just after the TCP header. "tcp[12:1] & 0xf0) >> 2" figures out the TCP header length.

Exercise 1 shows you how to write filters in Wireshark.

Exercise 1: Create a Wireshark filter to capture only traffic to or from an IP address

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1. Open Wireshark.
   
   
   
2. Click the active Network Interface to capture traffic.
3. Click Capture, then select filters.
   
   
   
4. Click the new button to create a new filter.
5. Name the new filter in the filter name field.
6. Type host IPaddress in the filter string field.
7. Click OK.
8. Select the capture menu and click start to begin the capture.

Repeat the above steps to create filters using the following strings:

• net 192.168.0.0/24 To capture traffic to or from a range of IP addresses.
• src net 192.168.0.0/24 To capture traffic from a range of IP addresses.
• dst net 192.168.0.0/24 To capture traffic to a range of IP addresses.
• port 53 To capture only DNS (port 53) traffic.
• host www.example.com and not (port 80 or port 25) To capture non-HTTP and non-SMTP traffic on your server.
• port not 53 and not arp To capture all except ARP and DNS traffic.
• tcp portrange 1501-1549 To capture traffic within a range of ports.
• not broadcast and not multicast Capture only unicast traffic. Useful to get rid of noise on the network if you only want to see traffic to and from your machine.

Practice writing filters in Wireshark that capture only one type of protocol traffic or traffic from a specific source IP or MAC address. Use your PC's IP or MAC address to test that the filter is working.

It's important to understand how to create these filters before you attempt the CEH exam.

Bypassing the Limitations of Switches

Because of the way Ethernet switches operate, it is more difficult to gather useful information when sniffing on a switched network. Since most modern networks have been upgraded from hub to switches, it takes a little more effort to sniff on a switched network. One of the ways to do that is to trick the switch into sending the data to the hackers' computer using ARP poisoning.

How ARP Works

ARP allows the network to translate IP addresses into MAC addresses. When one host using TCP/IP on a LAN tries to contact another, it needs the MAC address or hardware address of the host it's trying to reach. It first looks in its ARP cache to see if it already has the MAC address; if it doesn't, it broadcasts an ARP request asking, "Who has the IP address I'm looking for?" If the host that has that IP address hears the ARP query, it responds with its own MAC address, and a conversation can begin using TCP/IP.

ARP poisoning is a technique that's used to attack an Ethernet network and that may let an attacker sniff data frames on a switched LAN or stop the traffic altogether. ARP poisoning utilizes ARP spoofing, where the purpose is to send fake, or spoofed, ARP messages to an Ethernet LAN. These frames contain false MAC addresses that confuse network devices such as network switches. As a result, frames intended for one machine can be mistakenly sent to another (allowing the packets to be sniffed) or to an unreachable host (a denial-of-service, or DoS, attack). ARP spoofing can also be used in a man-in-the-middle attack, in which all traffic is forwarded through a host by means of ARP spoofing and analyzed for passwords and other information.

ARP Spoofing and Poisoning Countermeasures

To prevent ARP spoofing, permanently add the MAC address of the gateway to the ARP cache on a system. You can do this on a Windows system by using the ARP -scommand at the command line and appending the gateway's IP and MAC addresses. Doing so prevents a hacker from overwriting the ARP cache to perform ARP spoofing on the system but can be difficult to manage in a large environment because of the number of systems. In an enterprise environment, port-based security can be enabled on a switch to allow only one MAC address per switch port.

In Exercise 1 you will use Wireshark to sniff traffic.

Exercise 1: Use Wireshark to Sniff Traffic

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1. Download and install the latest stable version of Wireshark from www.wireshark.org.
2. Click on the Capture menu and then select interfaces.
   
   
   
3. Click the Start button next to the interface that shows packets being sent and received. If you have multiple interfaces with packet activity, choose one of them—preferably the interface with the most activity.
4. Click on a packet to analyze that single packet. The detailed headers will be displayed beneath the packet capture screen.
5. Expand each header (IP, TCP) of a packet and identify the address information.

This exercise will provide much more network traffic if performed on a hub rather than a switch. A wireless network can be used, as a wireless LAN is a shared network segment similar to how a hub operates.

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Hacking Tools

Wireshark is a freeware sniffer that can capture packets from a wired or wireless LAN connection. The software was previously called Ethereal. Wireshark is a common and popular program because it is free, but it has some drawbacks. An untrained user may find it difficult to write filters in Wireshark to capture only certain types of traffic.

Snort is an intrusion detection system (IDS) that also has sniffer capabilities. It can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, Common Gateway Interface (CGI) attacks, Server Message Block (SMB) probes, and OS fingerprinting attempts.

WinDump is the Windows version of tcpdump, the command-line network analyzer for Unix. WinDump is fully compatible with tcpdump and can be used to watch, diagnose, and save to disk network traffic according to various rules.

EtherPeek is a great sniffer for wired networks with extensive filtering and TCP/IP conversation tracking capabilities. The latest version of EtherPeek has been renamed OmniPeek.

WinSniffer is an efficient password sniffer. It monitors incoming and outgoing network traffic and decodes FTP, POP3, HTTP, ICQ, Simple Mail Transfer Protocol (SMTP), telnet, Internet Message Access Protocol (IMAP), and Network News Transfer Protocol (NNTP) usernames and passwords.

Iris is an advanced data- and network-traffic analyzer that collects, stores, organizes, and reports all data traffic on a network. Unlike other network sniffers, Iris is able to reconstruct network traffic, such as graphics, documents, and emails including attachments.

How a Sniffer Works

Sniffer software works by capturing packets not destined for the sniffer system's MAC address but rather for a target's destination MAC address. This is known as promiscuous mode. Normally, a system on the network reads and responds only to traffic sent directly to its MAC address. However, many hacking tools change the system's NIC to promiscuous mode. In promiscuous mode, a NIC reads all traffic and sends it to the sniffer for processing. Promiscuous mode is enabled on a network card with the installation of special driver software. Many of the hacking tools for sniffing include a promiscuous-mode driver to facilitate this process. Not all Windows drivers support promiscuous mode, so when using hacking tools ensure that the driver will support the necessary mode.

Any protocols that don't encrypt data are susceptible to sniffing. Protocols such as HTTP, POP3, Simple Network Management Protocol (SNMP), and FTP are most commonly captured using a sniffer and viewed by a hacker to gather valuable information such as usernames and passwords.

There are two different types of sniffing: passive and active. Passive sniffing involves listening and capturing traffic, and is useful in a network connected by hubs; active sniffinginvolves launching an Address Resolution Protocol (ARP) spoofing or traffic-flooding attack against a switch in order to capture traffic. As the names indicate, active sniffing is detectable but passive sniffing is not detectable.

In networks that use hubs or wireless media to connect systems, all hosts on the network can see all traffic; therefore, a passive packet sniffer can capture traffic going to and from all hosts connected via the hub. A switched network operates differently. The switch looks at the data sent to it and tries to forward packets to their intended recipients based on MAC address. The switch maintains a MAC table of all the systems and the port numbers to which they're connected. This enables the switch to segment the network traffic and send traffic only to the correct destination MAC addresses. A switch network has greatly improved throughput and is more secure than a shared network connected via hubs.

Another way to sniff data through a switch is to use a span port or port mirroring to enable all data sent to a physical switch port to be duplicated to another port. In many cases, span ports are used by network administrators to monitor traffic for legitimate purposes.