A Denial of Service (DoS) 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.

DoS and DDoS Attack

It is important to differentiate between Denial of Service (DoS) and Distributed Denial of Service (DDoS) attacks.

In a DoS attack, one computer and one internet connection is used to flood a server with packets, with the aim of overloading the targeted server’s bandwidth and resources.

A DDoS attack, uses many devices and multiple Internet connections, often distributed globally into what is referred to as a botnet. A DDoS attack is, therefore, much harder to deflect, simply because there is no single attacker to defend from, as the targeted resource will be flooded with requests from many hundreds and thousands of multiple sources.

Types of DoS Attacks

The most common type of Denial of Service attack involves flooding the target resource with external communication requests. This overload prevents the resource from responding to legitimate traffic, or slows its response so significantly that it is rendered effectively unavailable.

Resources targeted in a DoS attack can be a specific computer, a port or service on the targeted system, an entire network, a component of a given network any system component. DoS attacks may also target human-system communications (e.g. disabling an alarm or printer), or human-response systems (e.g. disabling an important technician's phone or laptop).

DoS attacks can also target tangible system resources, such as computational resources (bandwidth, disk space, processor time); configuration information (routing information, etc.); state information (for example, unsolicited TCP session resetting). Moreover, a DoS attack can be designed to: execute malware that maxes out the processor, preventing usage; trigger errors in machine microcode or sequencing of instructions, forcing the computer into an unstable state; exploit operating system vulnerabilities to sap system resources; crash the operating system altogether.
The overriding similarity in these examples is that, as a result of the successful Denial of Service attack, the system in question does not respond as before, and service is either denied or severly limited.

Types of DDoS Attacks

DDoS attacks can divided in three types:

• Volume Based Attacks - This type of attack includes UDP floods, ICMP floods, and other spoofed packet floods. The goal of this DDoS attack is to saturate the bandwidth of the attacked site. The magnitude of a volume-based attack is usually measured in Bits per second.
• Protocol Attacks - This type of DDoS attack consumes the resources of either the servers themselves, or of intermediate communication equipment, such as routers, load balancers and even some firewalls. Some examples of protocol attacks include SYN floods, fragmented packet attacks, Ping of Death, Smurf DDoS and more. Protocol attacks are usually measured in Packets per second.
• Application Layer Attacks - Perhaps the most dangerous type of DDoS attack, application layer attacks are comprised of seemingly legitimate and innocent requests. The intent of these attacks is to crash the web server. SDome examples of application layer attacks include Slowloris, Zero-day DDoS attacks, DDoS attacks that target Apache, Windows or OpenBSD vulnerabilities and more. The magnitude of this type of attack is measured in Requests per second.

Symptoms and Manifestations

The United States Computer Emergency Readiness Team (US-CERT) defines symptoms of denial-of-service attacks to include:

• Unusually slow network performance (opening files or accessing web sites)
• Unavailability of a particular web site
• Inability to access any web site
• Dramatic increase in the number of spam emails received—(this type of DoS attack is considered an e-mail bomb)[2]
• Disconnection of a wireless or wired internet connection
• The term "hit offline" being used on you, then you (the target) may disconnect from the internet

Denial-of-service attacks can also lead to problems in the network 'branches' around the actual computer being attacked. For example, the bandwidth of a router between the Internet and a LAN may be consumed by an attack, compromising not only the intended computer, but also the entire network.

If the attack is conducted on a sufficiently large scale, entire geographical regions of Internet connectivity can be compromised without the attacker's knowledge or intent by incorrectly configured or flimsy network infrastructure equipment.

Methods of attack

A "Denial-of-Service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. There are two general forms of DoS attacks: those that crash services and those that flood services.

A DoS attack can be perpetrated in a number of ways. The five basic types of attack are:

1. Consumption of computational resources, such as bandwidth, disk space, or processor time.
2. Disruption of configuration information, such as routing information.
3. Disruption of state information, such as unsolicited resetting of TCP sessions.
4. Disruption of physical network components.
5. Obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

A DoS attack may include execution of malware intended to:[citation needed]

1. Max out the processor's usage, preventing any work from occurring.
2. Trigger errors in the microcode of the machine.
3. Trigger errors in the sequencing of instructions, so as to force the computer into an unstable state or lock-up.
4. Exploit errors in the operating system, causing resource starvation and/or thrashing, i.e. to use up all available facilities so no real work can be accomplished or it can crash the system itself
5. Crash the operating system itself.

Preventing DoS and DDoS Vulnerabilities

Defending against Denial of Service attacks typically involves the use of a combination of attack detection, traffic classification and response tools, aiming to block traffic that they identify as illegitimate and allow traffic that they identify as legitimate. A list of prevention and response tools is provided below:

Firewalls
Firewalls can be setup to have simple rules such to allow or deny protocols, ports or IP addresses. In the case of a simple attack coming from a small number of unusual IP addresses for instance, one could put up a simple rule to drop all incoming traffic from those attackers.

More complex attacks will however be hard to block with simple rules: for example, if there is an ongoing attack on port 80 (web service), it is not possible to drop all incoming traffic on this port because doing so will prevent the server from serving legitimate traffic. Additionally, firewalls may be too deep in the network hierarchy. Routers may be affected before the traffic gets to the firewall. Nonetheless, firewalls can effectively prevent users from launching simple flooding type attacks from machines behind the firewall.

Some stateful firewalls, like OpenBSD's pf packet filter, can act as a proxy for connections: the handshake is validated (with the client) instead of simply forwarding the packet to the destination. It is available for other BSDs as well. In that context, it is called "synproxy".

Switches
Most switches have some rate-limiting and ACL capability. Some switches provide automatic and/or system-wide rate limiting, traffic shaping, delayed binding (TCP splicing), deep packet inspection and Bogon filtering (bogus IP filtering) to detect and remediate denial of service attacks through automatic rate filtering and WAN Link failover and balancing.

These schemes will work as long as the DoS attacks are something that can be prevented by using them. For example SYN flood can be prevented using delayed binding or TCP splicing. Similarly content based DoS may be prevented using deep packet inspection. Attacks originating from dark addresses or going to dark addresses can be prevented using Bogon filtering. Automatic rate filtering can work as long as you have set rate-thresholds correctly and granularly. Wan-link failover will work as long as both links have DoS/DDoS prevention mechanism.

Routers
Similar to switches, routers have some rate-limiting and ACL capability. They, too, are manually set. Most routers can be easily overwhelmed under DoS attack. Cisco IOS has features that prevent flooding, i.e. example settings.

Application Front-end Hardware
Application front end hardware is intelligent hardware placed on the network before traffic reaches the servers. It can be used on networks in conjunction with routers and switches. Application front end hardware analyzes data packets as they enter the system, and then identifies them as priority, regular, or dangerous. There are more than 25 bandwidth management vendors.

IPS Based Prevention
Intrusion-prevention systems (IPS) are effective if the attacks have signatures associated with them. However, the trend among the attacks is to have legitimate content but bad intent. Intrusion-prevention systems which work on content recognition cannot block behavior-based DoS attacks.

An ASIC based IPS may detect and block denial of service attacks because they have the processing power and the granularity to analyze the attacks and act like a circuit breaker in an automated way.

A rate-based IPS (RBIPS) must analyze traffic granularly and continuously monitor the traffic pattern and determine if there is traffic anomaly. It must let the legitimate traffic flow while blocking the DoS attack traffic.

DDS Based Defense
More focused on the problem than IPS, a DoS Defense System (DDS) is able to block connection-based DoS attacks and those with legitimate content but bad intent. A DDS can also address both protocol attacks (such as Teardrop and Ping of death) and rate-based attacks (such as ICMP floods and SYN floods).

Like IPS, a purpose-built system, such as the well-known Top Layer IPS products, can detect and block denial of service attacks at much nearer line speed than a software based system.

Blackholing and Sinkholing
With blackholing, all the traffic to the attacked DNS or IP address is sent to a "black hole" (null interface, non-existent server, ...). To be more efficient and avoid affecting network connectivity, it can be managed by the ISP.

Sinkholing routes to a valid IP address which analyzes traffic and rejects bad ones. Sinkholing is not efficient for most severe attacks.

Clean Pipes
All traffic is passed through a "cleaning center" or a "scrubbing center" via various methods such as proxies, tunnels or even direct circuits, which separates "bad" traffic (DDoS and also other common internet attacks) and only sends good traffic beyond to the server. The provider needs central connectivity to the Internet to manage this kind of service unless they happen to be located within the same facility as the "cleaning center" or "scrubbing center".