Distributed Denial of Service Attack (DDoS) Part 1

DoS stands for “Distributed Denial of Service.” A DDoS attack is a malicious attempt to make a server or a network resource unavailable to users, usually by temporarily interrupting or suspending the services of a host connected to the Internet.

Unlike a Denial of Service (DoS) attack, in which one computer and one internet connection is used to flood targeted resource with packets, a DDoS attack uses many computers and many Internet connections,

DDoS attacks can be broadly divided in three types:

Volume Based Attacks – includes UDP floods, ICMP floods, and other spoofed-packet floods. The attack’s goal is to saturate the bandwidth of the attacked site, and magnitude is measured in bits per second (Bps).
Protocol Attacks – includes SYN floods, fragmented packet attacks, Ping of Death, Smurf DDoS and more. This type of attack consumes actual server resources, or those of intermediate communication equipment, such as firewalls and load balancers, and is measured in Packets per second.
Application Layer Attacks – includes Slowloris, Zero-day DDoS attacks, DDoS attacks that target Apache, Windows or OpenBSD vulnerabilities and more. Comprised of seemingly legitimate and innocent requests, the goal of these attacks is to crash the web server, and the magnitude is measured in Requests per second.

Specific DDoS Attacks Types

Some specific and particularly popular and dangerous types of DDoS attacks include:

UDP Flood – this DDoS attack leverages the User Datagram Protocol (UDP), a sessionless networking protocol. This type of attack floods random ports on a remote host with numerous UDP packets, causing the host to repeatedly check for the application listening at that port, and (when no application is found) reply with an ICMP Destination Unreachable packet. This process saps host resources, and can ultimately lead to inaccessibility.
ICMP (Ping) Flood – similar in principle to the UDP flood attack, an ICMP flood overwhelms the target resource with ICMP Echo Request (ping) packets, generally sending packets as fast as possible without waiting for replies. This type of attack can consume both outgoing and incoming bandwidth, since the victim’s servers will often attempt to respond with ICMP Echo Reply packets, resulting a significant overall system slowdown.
SYN Flood – A SYN flood DDoS attack exploits an known weakness in the TCP connection sequence (the “three-way handshake”), wherein a SYN request to initiate a TCP connection with a host must be answered by a SYN-ACK response from that host, and then confirmed by an ACK response from the requester. In a SYN flood scenario, the requester sends multiple SYN requests, but either does not respond to the host’s SYN-ACK response, or sends the SYN requests from a spoofed IP address. Either way, the host system continues to wait for acknowledgement for each of the requests, binding resources until no new connections can be made, and ultimately resulting in denial of service.
Ping of Death – a ping of death ("POD") attack involves the attacker sending multiple malformed or malicious pings to a computer. The maximum packet length of an IP packet (including header) is 65,535 bytes. However, the Data Link Layer usually poses limits to the maximum frame size - for example 1500 bytes over an Ethernet network. In this case, a large IP packet is split across multiple IP packets (known as fragments), and the recipient host reassembles the IP fragments into the complete packet. In a Ping of Death scenario, following malicious manipulation of fragment content, the recipient ends up with an IP packet which is larger than 65,535 bytes when reassembled. This can overflow memory buffers allocated for the packet, causing denial of service for legitimate packets.
Slowloris – especially dangerous to hosts running Apache, dhttpd, Tomcat and GoAhead WebServer, Slowloris is a highly-targeted attack, enabling one web server to take down another server, without affecting other services or ports on the target network. Slowloris does this by holding as many connections to the target web server open for as long as possible. It accomplishes this by creating connections to the target server, but sending only a partial request. Slowloris constantly sends more HTTP headers, but never completes a request. The targeted server keeps each of these false connections open. This eventually overflows the maximum concurrent connection pool, and leads to denial of additional connections from legitimate clients.
Zero-day DDoS – “Zero-day” are simply unknown or new attacks, exploiting vulnerabilities for which no patch has yet been released. The term is well-known hacker community, and trading Zero-day vulnerabilities that can be used in attacks has become a popular activity.