What is a DDoS/DoS Attack? | DDoS Meaning

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What is a DDoS/DoS (Distributed Denial-of-Service) Attack?

A distributed denial-of-service (DDoS/DoS) attack is a malicious attempt to disrupt the normal traffic of a targeted server, service or network by overwhelming the target or its surrounding infrastructure with a flood of Internet traffic.

DDoS attacks achieve effectiveness by utilizing multiple compromised computer systems as sources of attack traffic. Exploited machines can include computers and other networked resources such as IoT devices.

From a high level, a DDoS attack is like an unexpected traffic jam clogging up the highway, preventing regular traffic from arriving at its destination.

How does a DDoS/DoS attack work?

DDoS attacks are carried out with networks of Internet-connected machines.

These networks consist of computers and other devices (such as IoT devices)which have been infected with malware, allowing them to be controlled remotely by an attacker. These individual devices are referred to as bots (or zombies), and a group of bots is called a botnet.

Once a botnet has been established, the attacker is able to direct an attack by sending remote instructions to each bot.

When a victim’s server or network is targeted by the botnet, each bot sends requests to the target’s IP address, potentially causing the server or network to become overwhelmed, resulting in a denial-of-service to normal traffic.

Because each bot is a legitimate Internet device, separating the attack traffic from normal traffic can be difficult.

DoS attacks typically fall in 2 categories:

Buffer overflow attacks

An attack type in which a memory buffer overflow can cause a machine to consume all available hard disk space, memory, or CPU time. This form of exploit often results in sluggish behavior, system crashes, or other deleterious server behaviors, resulting in denial-of-service.

Flood attacks

By saturating a targeted server with an overwhelming amount of packets, a malicious actor is able to oversaturate server capacity, resulting in denial-of-service. In order for most DoS flood attacks to be successful, the malicious actor must have more available bandwidth than the target.

How can you tell if a computer is experiencing a DoS attack?

While it can be difficult to separate an attack from other network connectivity errors or heavy bandwidth consumption, some characteristics may indicate an attack is underway.

Indicators of a DoS attack include:


  • Atypically slow network performance such as long load times for files or websites
  • The inability to load a particular website such as your web property
  • A sudden loss of connectivity across devices on the same network

What is the difference between a DDoS attack and a DOS attack?

The distinguishing difference between DDoS and DoS is the number of connections utilized in the attack. Some DoS attacks, such as “low and slow” attacks like Slowloris, derive their power in the simplicity and minimal requirements needed to them be effective.

dos vs ddos

DoS utilizes a single connection, while a DDoS attack utilizes many sources of attack traffic, often in the form of a botnet. Generally speaking, many of the attacks are fundamentally similar and can be attempted using one more many sources of malicious traffic.

What are some historically significant DoS attacks?

Historically, DoS attacks typically exploited security vulnerabilities present in network, software and hardware design. These attacks have become less prevalent as DDoS attacks have a greater disruptive capability and are relatively easy to create given the available tools. In reality, most DoS attacks can also be turned into DDoS attacks.


A few common historic DoS attacks include:

Smurf Attack

a previously exploited DoS attack in which a malicious actor utilizes the broadcast address of vulnerable network by sending spoofed packets, resulting in the flooding of a targeted IP address.

Ping Flood

this simple denial-of-service attack is based on overwhelming a target with ICMP (ping) packets. By inundating a target with more pings than it is able to respond to efficiently, denial-of-service can occur. This attack can also be used as a DDoS attack.

Ping of Death

often conflated with a ping flood attack, a ping of death attack involves sending a malformed packet to a targeted machine, resulting in deleterious behavior such as system crashes.

How to identify a DDoS attack?

The most obvious symptom of a DDoS attack is a site or service suddenly becoming slow or unavailable. But since a number of causes — such a legitimate spike in traffic — can create similar performance issues, further investigation is usually required.

Traffic analytics tools can help you spot some of these telltale signs of a DDoS attack:


  • Suspicious amounts of traffic originating from a single IP address or IP range
  • A flood of traffic from users who share a single behavioral profile, such as device type, geolocation, or web browser version
  • An unexplained surge in requests to a single page or endpoint
  • Odd traffic patterns such as spikes at odd hours of the day or patterns that appear to be unnatural (e.g. a spike every 10 minutes)


There are other, more specific signs of DDoS attack that can vary depending on the type of attack.

What are some common types of DDoS attacks?

Different types of DDoS attacks target varying components of a network connection. In order to understand how different DDoS attacks work, it is necessary to know how a network connection is made.


A network connection on the Internet is composed of many different components or “layers”. Like building a house from the ground up, each layer in the model has a different purpose.


The OSI model, shown below, is a conceptual framework used to describe network connectivity in 7 distinct layers.

osi layer for ddos attack knowledge

While nearly all DDoS attacks involve overwhelming a target device or network with traffic, attacks can be divided into three categories. An attacker may use one or more different attack vectors, or cycle attack vectors in response to counter measures taken by the target.

Application layer attacks

The goal of the attack

Sometimes referred to as a layer 7 DDoS attack (in reference to the 7th layer of the OSI model), the goal of these attacks is to exhaust the target’s resources to create a denial-of-service.

The attacks target the layer where web pages are generated on the server and delivered in response to HTTP requests. A single HTTP request is computationally cheap to execute on the client side, but it can be expensive for the target server to respond to, as the server often loads multiple files and runs database queries in order to create a web page.

Layer 7 attacks are difficult to defend against, since it can be hard to differentiate malicious traffic from legitimate traffic.

Application layer attack example

application ddos attack example

HTTP flood

This attack is similar to pressing refresh in a web browser over and over on many different computers at once – large numbers of HTTP requests flood the server, resulting in denial-of-service.


This type of attack ranges from simple to complex.


Simpler implementations may access one URL with the same range of attacking IP addresses, referrers and user agents. Complex versions may use a large number of attacking IP addresses, and target random urls using random referrers and user agents.

Protocol attacks

The goal of the attack

Protocol attacks, also known as a state-exhaustion attacks, cause a service disruption by over-consuming server resources and/or the resources of network equipment like firewalls and load balancers.

Protocol attacks utilize weaknesses in layer 3 and layer 4 of the protocol stack to render the target inaccessible.

Protocol attack example

ddos protocol attack

SYN flood

A SYN Flood is analogous to a worker in a supply room receiving requests from the front of the store.

The worker receives a request, goes and gets the package, and waits for confirmation before bringing the package out front. The worker then gets many more package requests without confirmation until they can’t carry any more packages, become overwhelmed, and requests start going unanswered.

This attack exploits the TCP handshake — the sequence of communications by which two computers initiate a network connection — by sending a target a large number of TCP “Initial Connection Request” SYN packets with spoofed source IP addresses.

The target machine responds to each connection request and then waits for the final step in the handshake, which never occurs, exhausting the target’s resources in the process.

Volumetric attacks

The goal of the attack

This category of attacks attempts to create congestion by consuming all available bandwidth between the target and the larger Internet. Large amounts of data are sent to a target by using a form of amplification or another means of creating massive traffic, such as requests from a botnet.

Amplification example:

ddos amplication attack

DNS Amplification

A DNS amplification is like if someone were to call a restaurant and say “I’ll have one of everything, please call me back and repeat my whole order,” where the callback number actually belongs to the victim.


With very little effort, a long response is generated and sent to the victim.
By making a request to an open DNS server with a spoofed IP address (the IP address of the victim), the target IP address then receives a response from the server.

What is a DDoS botnet?

Botnet attacks are responsible for the largest DDoS attacks on record. Learn how devices become infected with botnet malware, how bots are remotely controlled, and how to protect a network from a botnet infestation.

What is a Botnet?

A botnet refers to a group of computers which have been infected by malware and have come under the control of a malicious actor. The term botnet is a portmanteau from the words robot and network and each infected device is called a bot. Botnets can be designed to accomplish illegal or malicious tasks including sending spam, stealing data, ransomware, fraudulently clicking on ads or distributed denial-of-service (DDoS) attacks.

While some malware, such as ransomware, will have a direct impact on the owner of the device, DDoS botnet malware can have different levels of visibility; some malware is designed to take total control of a device, while other malware runs silently as a background process while waiting silently for instructions from the attacker or “bot herder.”

Self-propagating botnets recruit additional bots through a variety of different channels. Pathways for infection include the exploitation of website vulnerabilities, Trojan horse malware, and cracking weak authentication to gain remote access. Once access has been obtained, all of these methods for infection result in the installation of malware on the target device, allowing remote control by the operator of the botnet. Once a device is infected, it may attempt to self-propagate the botnet malware by recruiting other hardware devices in the surrounding network.

While it’s infeasible to pinpoint the exact numbers of bots in a particular botnet, estimations for total number of bots in a sophisticated botnet have ranged in size from a few thousand to greater than a million.

Why are botnets created?

Reasons for using a botnet ranges from activism to state-sponsored disruption, with many attacks being carried out simply for profit. Hiring botnet services online is relatively inexpensive, especially in relationship to the amount of damage they can cause. The barrier to creating a botnet is also low enough to make it a lucrative business for some software developers, especially in geographic locations where regulation and law enforcement are limited. This combination has led to a proliferation of online services offering attack-for-hire.

How is a botnet controlled?

A core characteristic of a botnet is the ability to receive updated instructions from the bot herder. The ability to communicate with each bot in the network allows the attacker to alternate attack vectors, change the targeted IP address, terminate an attack, and other customized actions. Botnet designs vary, but the control structures can be broken down into two general categories:

The client/server botnet model

The client/server model mimics the traditional remote workstation workflow where each individual machine connects to a centralized server (or a small number of centralized servers) in order to access information. In this model each bot will connect to a command-and-control center (CnC) resource like a web domain or an IRC channel in order to receive instructions. By using these centralized repositories to serve up new commands for the botnet, an attacker simply needs to modify the source material that each botnet consumes from a command center in order to update instructions to the infected machines. The centralized server in control of the botnet may be a device owned and operated by the attacker, or it may be an infected device.


A number of popular centralized botnet topologies have been observed, including:

Star Network Topology

Multi Server Network Topology

Hierarchical Network Topology

In any of these client/server models, each bot will connect to a command center resource like a web domain or an IRC channel in order to receive instructions. By using these centralized repositories to serve up new commands for the botnet, an attacker simply needs to modify the source material that each botnet consumes from a command center in order to update instructions to the infected machines.


Hand-in-hand with the simplicity of updating instructions to the botnet from a limited number of centralized sources is the vulnerability of those machines; in order to remove a botnet with a centralized server, only the server needs to be disrupted. As a result of this vulnerability, the creators of botnet malware have evolved and moved towards a new model that is less susceptible to disruption via a single or a few points of failure.

The peer-to-peer botnet model

To circumvent the vulnerabilities of the client/server model, botnets have more recently been designed using components of decentralized peer-to-peer filesharing. Embedding the control structure inside the botnet eliminates the single point-of-failure present in a botnet with a centralized server, making mitigation efforts more difficult. P2P bots can be both clients and command centers, working hand-in-hand with their neighboring nodes to propagate data.


Peer to peer botnets maintain a list of trusted computers with which they can give and receive communications and update their malware. By limiting the number of other machines the bot connects to, each bot is only exposed to adjacent devices, making it harder to track and more difficult to mitigate. Lacking a centralized command server makes a peer-to-peer botnet more vulnerable to control by someone other than the botnet’s creator. To protect against loss of control, decentralized botnets are typically encrypted so that access is limited.

How do IoT devices become a botnet?

No one does their Internet banking through the wireless CCTV camera they put in the backyard to watch the bird feeder, but that doesn’t mean the device is incapable of making the necessary network requests. The power of IoT devices coupled with weak or poorly configured security creates an opening for botnet malware to recruit new bots into the collective. An uptick in IoT devices has resulted in a new landscape for DDoS attacks, as many devices are poorly configured and vulnerable.


If an IoT device’s vulnerability is hardcoded into firmware, updates are more difficult. To mitigate risk, IoT devices with outdated firmware should be updated as default credentials commonly remain unchanged from the initial installation of the device. Many discount manufacturers of hardware are not incentivized to make their devices more secure, making the vulnerability posed from botnet malware to IoT devices remain an unsolved security risk.

How is an existing botnet disabled?

Disable a botnet’s control centers

Botnets designed using a command-and-control schema can be more easily disabled once the control centers can be identified. Cutting off the head at the points of failure can take the whole botnet offline. As a result, system administrators and law enforcement officials focus on closing down the control centers of these botnets. This process is more difficult if the command center operates in a country where law enforcement is less capable or willing to intervene.

Eliminate infection on individual devices

For individual computers, strategies to regain control over the machine include running antivirus software, reinstalling software from a safe backup, or starting over from a clean machine after reformatting the system. For IoT devices, strategies may include flashing the firmware, running a factory reset or otherwise formatting the device. If these option are infeasible, other strategies may be available from the device’s manufacturer or a system administrator.

How can you protect devices from becoming part of a botnet?

Create secure passwords

For many vulnerable devices, reducing exposure to botnet vulnerability can be as simple as changing the administrative credentials to something other than the default username and password. Creating a secure password makes brute force cracking difficult, creating a very secure password makes brute force cracking virtually impossible. For example, a device infected with the Mirai malware will scan IP addresses looking for responding devices. Once a device responds to a ping request, the bot will attempt to login to that found device with a preset list of default credentials. If the default password has been changed and a secure password has been implemented, the bot will give up and move on, looking for more vulnerable devices.

Allow only trusted execution of third-party code

If you adopt the mobile phone model of software execution, only allowed applications may run, granting more control to terminate software deemed as malicious, botnets included. Only an exploitation of the supervisor software (i.e. kernel) may result in exploitation of the device. This hinges on having a secure kernel in the first place, which most IoT devices do not have, and is more applicable to machines that are running third party software.

Periodic system wipe/restores

Restoring to a known good state after a set time will remove any gunk a system has collected, botnet software included. This strategy, when used as a preventative measure, ensures even silently running malware gets thrown out with trash.

Implement good ingress and egress filtering practices

Other more advanced strategies include filtering practices at network routers and firewalls. A principle of secure network design is layering: you have the least restriction around publicly accessible resources, while continually beefing up security for things you deem sensitive. Additionally, anything that crosses these boundaries has to be scrutinized: network traffic, usb drives, etc. Quality filtering practices increase the likelihood that DDoS malware and their methods of propagation and communication will be caught before entering or leaving the network.

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