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What Is a Denial of Service Attack (DoS)?

  • By Gcore
  • June 8, 2023
  • 9 min read
What Is a Denial of Service Attack (DoS)?

The world of cybersecurity is ever-evolving, with new threats appearing almost every day. Denial of service (DoS) attacks are not new, but they can have a serious impact on a wide variety of services. Estimates suggest there are thousands of DoS attacks happening every single day and the numbers keep increasing. Their goal is to interrupt a device’s or service’s normal functioning.

When these attacks are successful, legitimate users won’t be able to access the service. Stopping such an attack once it happens is possible, but not always easy, which is why they can cost organizations a lot of time and money. Here’s everything you need to know about DoS attacks.

What Is a Denial of Service (DoS) Attack?

A denial of service (DoS) attack is a cyber attack that aims to make a device, service, network, or other information system unavailable to legitimate users. The hacker uses a single machine and typically floods the target with an extremely high number of requests. Eventually, the target machine can no longer process normal traffic.

When thinking of a cyber attack, for many people what comes to mind is someone trying to access data illegally. A DoS attack is not necessarily about accessing or stealing someone else’s data. The goal, in most cases, is to block users from accessing a service. Revenge, competition, extortion, and even activism are some reasons people resort to DoS attacks.

Sometimes, denial of service attacks serve as a precursor to other, more severe attacks. Once a device or service becomes inactive, the attackers can infiltrate other areas of the system and attempt to install malware with the aim of conducting a bigger attack in the future.

How Do DoS Attacks Differ from Distributed Denial of Service (DDoS) Attacks?

A distributed denial of service (DDoS) attack is a type of DoS attack that uses several distributed machines to launch the attack instead of a single machine.

DoS attacks come from one IP and are relatively easy to counter. DDoS attacks come from multiple IPs, which makes them more difficult to stop.

When the attack comes from distributed sources, it can be much harder to differentiate malicious traffic from normal traffic. As a result, DDoS attacks are harder to detect before they cause real damage. With a DoS attack, only one machine needs to be detected and stopped.

Types of DoS Attacks

Denial of service attacks are usually divided into two categories:

  • Buffer overflows, which crash web-based services. This is the most common type of DoS attack. The hacker drives high traffic and data to a network point. To handle the traffic, the system will need to use all its resources and memory, eventually causing it to crash.
  • Flood attacks, which flood services and devices. These attacks are carried out by sending high volumes of data packets, connection requests, and other types of traffic to a network or system.

Both types come with several other subtypes.

Types of Buffer Overflow Attack

The most notable buffer overflow attacks include:

  • Stack overflow. Here, the attacker sends more information to a device, program, or network than it can handle. The program will respond by using an area of the memory called “the stack.” When the stack is full, information can overflow to other parts of the program until it crashes.
  • Unicode overflow. This type of attack is designed specifically for programs that process text. The program usually expects to receive ASCII. Instead, the attacker sends Unicode characters beyond the program’s capacity. ASCII comprises 128 characters, the numbers, and letters you use every day. Unicode contains a much larger set of characters and symbols, up to 221 (roughly 2,000,000,) which won’t be readable to a program that expects ASCII. The extra text can overflow in other parts of the memory, causing the program to crash.

Types of Flood Attack

Flood attacks also come in different forms, such as:

  • ICMP flood. In this attack, the hacker hits the targeted network with a flood of Internet Control Message Protocol (ICMP) packets. ICMP is a protocol normally used to send error messages over the internet. When a network is flooded with such packets, it can’t handle any more legitimate traffic and may crash.
  • SYN flood. Here, the attacker tries to overwhelm a network by flooding it with connection requests; namely SYN packets. Devices communicate over the internet through the TCP protocol. To start the communication, one device will send an SYN (synchronize) packet to the others. To respond, the other device sends an SYN-ACK (synchronize-acknowledge) packet. During an SYN Flood attack, the attacker sends many SYN packets but never responds to SYN-ACK packets. The network begins slowing down, is unable to handle legitimate traffic, and could eventually crash.

Denial of Service Attack Examples

Denial of service attacks have a long history. The first documented attack of this kind is the Robert Morris worm attack that took place in 1988. The worm installed itself on systems connected to the internet, triggering DoS attacks and buffer overflows. At the time, there weren’t many computers connected to the internet and most were used in academia and research. However, estimates suggest up to 10% of the computers in the US were affected. Since then, these attacks have evolved, but their goal has stayed the same: limiting users’ access to certain services or devices.

Among recent examples, the most significant one is an attack from 2019—the DoS ransomware attack that affected Baltimore. During the attack, many of the city’s critical systems were taken down, including email systems, bill payment services, and even the 911 emergency dispatch. Over 1,000 home sales had to be delayed as a result of the attack.

From DoS to DDoS Attacks

Today, DoS attacks are not that popular anymore. Because they come from a single IP, they are fairly easy to counter with modern technologies. They can still happen, but most companies are ready to stop them within minutes or even seconds.

DDoS attacks, on the other hand, are more prevalent and still pose a huge danger. As an example, we had a 650 Gbps attack on our infrastructure in January 2023. Cybercriminals tried to take down a service belonging to one of our clients. The incident lasted for fifteen minutes, after which the attack stopped because we prevented any losses for our client, meaning the attack was ineffective.

An older, infamous example is the attack against GitHub on February 28, 2018. The attack originated from tens of thousands of points and managed to take GitHub down for 10 minutes. Another major DDoS attack happened in June 2022 and targeted a Chinese telecommunications company. The attack lasted a total of four hours, during which time 25.3 billion requests were sent. The hackers used a botnet with 170,000 IPs and managed to compromise servers located in 180 countries.

Preventing DoS Attacks

As the saying goes, prevention is better than cure. It’s true for your health and certainly true when it comes to DoS attacks. Preventing an attack is almost always easier than trying to stop one that’s already in progress. Here are a few easy steps to reduce the likelihood of experiencing a denial of service attack.

1. Use a Firewall

A firewall may sound like a simple solution, but can be very effective. In fact, the simplicity of it means many people overlook this step. If you manage a large network with a large number of people connecting to it—such as a workplace—make sure that everyone understands the importance of using a firewall.

This tool may not be completely bulletproof, but it can help block out much of the unwanted traffic. It may also spot suspicious activity and unauthorized traffic sources, making a DoS attack less likely to be successful.

2. Limit Connections

Many DoS attacks come from perfectly harmless-looking sources. One way to counter this is by limiting the number of connections that can come from a single IP address in a given time period. This can help prevent flooding attacks from one IP address, though DDoS attacks could still be successful.

3. Consider Network Segmentation

Network segmentation—splitting large networks into smaller ones—helps reduce the impact of a denial of service attack. To achieve this, you can create several VLANs and firewalls that will help limit the spread of the attack.

DoS attacks can still happen, but they will only affect one segment. The other parts of the network will keep working, reducing the damage to your services and the users. Network segmentation could even allow you to end the attack sooner by limiting the attack’s potential spread.

4. Use Load Balancing

Load balancing is a similar concept to network segmentation. In this case, though, you’re distributing traffic across multiple servers. If one server is overloaded or becomes unresponsive, the load balancer will send the traffic to another server.

5. Use Intrusion Detection and Prevention Systems

Intrusion detection and prevention systems to analyze incoming traffic and look for suspicious patterns. They can also limit the number of connections within a time frame from a certain IP if they detect unusual traffic coming from that source. They can be useful in preventing various types of malicious attacks, including DoS and DDoS attacks.

6. The Power of Education

When users are aware of potential DoS attacks, how they look, and when they can happen, prevention becomes easier. You can’t prevent something if you don’t know what it looks like or that it even exists!

Not every person has the knowledge to understand all the intricacies of DoS attacks, but by being aware of them and their potential risks, they can take simple, small steps to prevent them. For example, they can become more diligent with keeping their software updated, using strong passwords and changing them periodically, and using a firewall. Education is key to ensuring broad implementation of basic steps to prevent DoS attacks.

7. Conduct Periodic Penetration Tests

Penetration testing (pen testing) can help you spot vulnerabilities in your systems. These may not be visible “to the naked eye,” but you can be sure hackers will find them sooner or later, and use them against you.

Conducting periodic pen tests will help you stay one step ahead of attackers and address vulnerabilities before attackers can target them.

Mitigating DoS Attacks—What to Do if You’ve Already Been Hit?

Prevention is king, but sometimes hackers can exploit the smallest vulnerabilities, and attacks happen. It’s very difficult to stop a denial of service attack entirely. However, there are a few ways to mitigate and reduce their impact on your systems.

1. Traffic Filtering

Filtering can be effective both as prevention and mitigation. When you know you’re being flooded with unwanted traffic, filtering can be an effective solution to stop it in its tracks.

The filter will check all incoming traffic and make sure that only legitimate sources are allowed in. Sometimes it won’t stop the flood entirely, but it can reduce its load on your system, preventing it from becoming unavailable or crashing.

2. Scrubbing Services

Scrubbing services are like the cleaning crew that comes after the party. Their role is to check all existing traffic and remove malicious sources, while allowing legitimate traffic to continue.

Scrubbing centers are effective against all denial-of-service attacks—both DoS and DDoS. For example, scrubbing techniques stopped one of the most famous DDoS attacks of the past few years—the 2018 attack against GitHub. The traffic directed at GitHub was redirected to other data centers, where it was then “scrubbed.” As a result, only the legitimate traffic remained, and GitHub services were once again available.

3. Blackhole Routing

Blackhole routing means redirecting traffic to a null route, or a “black hole.” It is effective whenever a network is flooded with traffic from one or several IP addresses.

This technique has one major downside: it doesn’t always differentiate between malicious and legitimate traffic. In other words, you could be stopping access to your services for real users. It can still be a quick and effective way to mitigate a DoS attack in progress, though most agree it is not a great tool for prevention.

4. Encrypt Data and Use Backups

Encryption and backups will not stop a cyber attack in its tracks, but they are life savers if you do experience a DoS attack. They will reduce the risk of damage or theft, which can sometimes occur during or immediately after a DoS attack. Plus, if some of your systems do become unresponsive or corrupt, a backup will minimize your losses and will help you restore your workflow quickly.

How to Know if You’re Experiencing a DoS Attack?

Acting quickly is essential when you’re experiencing a DoS attack, but to do that, you must know what one looks like. The answer will vary slightly depending on your systems, any preventative measures you have in place, and the type of attack. But there are a few telltale signs that can help you recognize a denial of service attack.

1. Slow or Unresponsive Network

Before systems crash, they start slowing down for no apparent reason. You may notice that it’s taking longer to load websites, download files, or even send emails. If there’s no other reason for the slowdown, you may be experiencing a DoS attack.

2. Websites or Services Become Unavailable

Sometimes the network doesn’t crash all at once. You may have certain preventative measures like segmentation or load balancing that help reroute floods of traffic. Instead, you may notice that certain services become unavailable.

For example, your email could suddenly become unavailable. If you’re not having connectivity or other server issues, it is possible that your email server is being targeted by a DoS attack.

3. Unusual Network Traffic

Unusual and unexpected network traffic is never a good sign. Systems will soon become slow or unresponsive, and it won’t be long until no one can access any services.

A firewall can usually spot unusual network traffic and even stop it. If it doesn’t, you may need to start considering DoS attack mitigation tools like traffic filtering.

4. High CPU or Memory Usage

Certain DoS attacks—like buffer overflows—affect the memory of the systems they target. The first signs will usually include a slower-than-normal system and unresponsive programs. These issues can have other causes, though. High CPU usage could simply be an incompatible program, for instance, while unresponsive services could be caused by a connectivity issue.

You will need to keep an eye on your systems at all times and double-check every time you’re experiencing what could be symptoms of a DoS attack. Connectivity, software, or hardware issues will usually be easy to find and will cease once the culprit is identified and remedied, whereas a DoS attack will get worse until it is stopped.

Conclusion

A denial of service attack prevents legitimate users from accessing a device, service, or network. The disruption can have serious consequences for users and businesses alike and include loss of revenue, reputation, and sensitive data.

DoS attacks come in many forms, including buffer overflows and flooding, with the attack having a single source. You may also encounter distributed denial of service attacks. These are similar to DoS attacks, but they come from multiple IPs, which makes them harder to detect and stop.

The good news is there are ways to prevent DoS attacks. Some are more simple, like using a firewall and educating users about what DoS attacks look like. Others are more complex and involve using load balancing techniques, intrusion detection and prevention systems, encryption, and pen testing.

Keep your systems secure against DoS and DDoS attacks with Gcore’s DDoS protection solution. It can keep your services, apps, and websites safe, and has over 1 Tbps total filtering capacity. Connect with one of our experts to learn more.

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You’ll be asked to update your DNS records so they point to the Gcore CDN, allowing Gcore to issue the certificates later.Figure 4: Create CDN resourceNext, open the resource settings by selecting your CDN resource from the list in the center (Figure 5).Figure 5: Select the CDN resourceEnable HTTPS in the resource settings, as shown in Figure 6:Select SSL in the left navigationClick the Enable HTTPS checkboxClick Get SSL certificateFigure 6: Get an SSL certificateYour certificate will usually be issued within 30 minutes.Our Commitment to Online SecurityAt Gcore, we’re committed to making the internet secure for everyone. As part of this mission, we offer free CDN and free TLS certificates. Take advantage and protect your resources efficiently for free!Get TLS encryption on Gcore CDN free

How to Detect and Stop Bad Bots

A bot, short for “robot,” is a software program that can perform tasks automatically, quickly, and efficiently. Both good bots and bad bots exist; Googlebot facilitates web page indexing, but LizardStresser orchestrates DDoS attacks. Because good and bad bots share certain traits, distinguishing between them can be tricky unless the correct bot detection techniques are used. In this article, we examine the evolution of bot detection techniques in response to the ever-changing threat landscape and discuss how bots can be detected and, when desirable, stopped.What Is Bot Detection?Bot detection is the process of identifying and distinguishing between legitimate human users, good bots, and bad bots. Because bots can mimic certain legitimate user behaviors, such as mouse movements and keystrokes, cybersecurity professionals and business leaders should implement bot detection as an integral component of their security strategy. Otherwise, you could end up with misleading analytics, compromised user experiences, and potential security breaches that can harm your organization’s reputation and bottom line.Bot detection helps to mitigate malicious bot activities such as unethical web scraping, spamming, account takeover, click fraud, and DDoS attacks, without interfering with good bots such as website uptime monitors. Effective bot detection enhances cybersecurity and improves the web user’s overall experience.Botnet Detection TechniquesOver the decades, different botnet mitigation techniques have been developed to deal with the challenges of stopping bad bots while allowing good bots to continue their activities. These techniques typically involve identifying the command-and-control infrastructure coordinating the botnet activities. However, since botnets keep evolving to bypass mitigation measures, new and better botnet detection and mitigation strategies are continuously being developed.Let’s examine botnet detection techniques. We’ll start with the oldest and then look at contemporary techniques. However, new techniques build on the old, and all these techniques still play a part in botnet detection today.Intrusion Detection SystemsFigure 1: How a basic intrusion detection system worksIntrusion detection systems (IDS) emerged in the late 1980s to monitor and analyze network traffic for security incidents like unauthorized access and policy violations. IDS can detect threats, such as botnets, and alert security teams. Intrusion prevention systems (IPS) can proactively mitigate detected threats. Modern IDPS (intrusion detection and prevention systems) combine IDS and IPS functions.IDS is trained on data from sources like network traffic, system logs, and application activity. Botnet-focused IDS can be anomaly-based (monitoring abnormal behaviors) or signature-based (matching patterns with known botnets).When a potential botnet is detected, the IDS generates alerts or notifications based on severity. Depending on cybersecurity policies, the IDS may block traffic, isolate systems, or alert security teams. IDS also generates incident logs and reports, detailing the time of incidents, detected threats, countermeasures, and recommendations for improvement.Intrusion detection systems can be grouped into six types:Network-Based Intrusion Detection Systems (NIDS): These monitor real-time network traffic and analyze packets on network segments or devices to detect attacks like DoS, port scanning, and reconnaissance.Protocol-Based Intrusion Detection Systems (PIDS): A type of NIDS that targets specific network communication protocols (e.g., P2P, HTTP, IRC) to protect against intrusion and policy violations. PIDS is limited in scope.Machine Learning-Based Intrusion Detection Systems (ML-IDS): Subset of NIDS using machine learning algorithms to detect network intrusions and malicious activities by learning from historical data. ML-IDS is more efficient than traditional rule-based systems but requires fine-tuning to minimize false positives.Host-Based Intrusion Detection Systems (HIDS): Monitor the computer infrastructure they are installed on (e.g., computers, servers) to safeguard against attacks. They gather data, analyze traffic, and log suspicious behavior, providing insights into system health and security. HIDS is an approach that’s most suitable for small teams with lean overheads.Hybrid Intrusion Detection Systems: Combine different detection techniques (e.g., NIDS, HIDS, anomaly-based, signature-based) in a single framework to effectively detect botnet activity and provide insightful data. Problematically, they create a single point of failure and are complex to troubleshoot.Multi-Layered Intrusion Detection Systems: These systems combine different detection techniques (e.g., NIDS, HIDS, anomaly-based, signature-based) in a layered approach, with each IDS as a separate component. They eliminate a single point of failure and simplify troubleshooting but complicate setup, management, and reporting.To summarize, intrusion detection systems (IDS) enhance network security by monitoring and analyzing traffic to detect potential threats, providing valuable insights and real-time response capabilities. However, they can produce false positives, require ongoing maintenance and fine-tuning, and may be complex to manage and integrate into existing security frameworks.HoneynetFirst used around the year 2000, a honeynet is a network of traps or decoy networks (honeypots) set up with built-in vulnerabilities to attract cyberattacks. A typical honeynet comprises two or more honeypots. Honeynets aid in botnet detection by deliberately exposing vulnerabilities that attract malicious attacks. This deception technique allows botnet attacks to be studied in a controlled environment or managed and stopped, as needed.Figure 2: Honeynet setupAs such, there are two main types of honeynets: research honeynets and production honeynets. Research honeynets are primarily set up to study attack vector tactics, techniques, and procedures, while production honeynets are deployed within production environments.Despite their effectiveness, honeynets have limitations, such as setup complexity, limited network coverage, and high maintenance overhead, especially for high-capacity setups. Additionally, honeynets can sometimes be detected, bypassed, armed, and deployed against the production network itself.DNS-Based Botnet DetectionFigure 3: DNS-based botnet detectionAround 2005, the DNS-based botnet detection technique started to gain popularity. DNS-based botnet detection works by monitoring the way computers use the Domain Name System (DNS) to find websites. When you enter a website address into your browser, your computer uses DNS to find the numerical IP address that corresponds to that website. Botnets, which are networks of infected computers controlled by cybercriminals, often need to communicate with the attackers’ servers to receive instructions. They use DNS to find these servers.A botnet detection system monitors all DNS requests made by network computers. They analyze which domain names are being requested and how often. Since botnets often use unusual domain names that people don’t typically visit, the systems look for patterns that indicate suspicious activity, such as frequent requests to these strange or newly created domains. They can then block the requests to these malicious domains, preventing the infected computers from communicating with the cybercriminals.Although they provide real-time detection, network-wide coverage, low false-positive rates, and threat intelligence gathering, they are prone to evasion techniques and are limited by their reliance on external threat intelligence sources for domain reputation data.Comparison of Botnet Detection TechniquesHere’s how these three botnet detection techniques compare.FeatureIntrusion Detection Systems (IDS)HoneynetDNS-Based Botnet DetectionDefinitionNetwork security tools monitor and analyze network traffic for potential threatsNetwork of traps or decoy networks designed to attract cyberattacksTechnique monitoring and analyzing DNS traffic for botnet activityDetection focusNetwork traffic, system logs, and application activityCyberattackers’ behavior and tacticsDNS traffic patterns, requests, and responsesDetection methodsSignature-based, anomaly-based, machine learningDeception through vulnerabilitiesDomain reputation checks, anomaly detectionData collectedNetwork traffic, system logs, application activityAttack interactions with honeypotsDNS traffic, requests, responsesAlerting and responseGenerates alerts, blocks traffic, isolates systemsStudies attacks, handles malicious interactionsBlocks connections, redirects to sinkholes, alertsUse casesPrevents unauthorized access, breaches, policy violationsStudies attack tactics, gathers threat intelligenceReal-time botnet detection, low false positivesComplexityVaries based on IDS type (NIDS, HIDS, hybrid, multi-layered)Moderate to high due to setup and maintenanceModerate, relies on DNS traffic analysisEffectivenessEffective for detecting network-based threatsEffective for studying attacks, gathering threat intelEffective for real-time botnet detectionLimitationsCan be bypassed by sophisticated attacksSetup complexity, limited network coverageProne to evasion techniques, reliance on external dataDeploymentNetwork-wide, host-based, hybrid, multi-layeredControlled environment, production networksDNS infrastructure monitoringPopularityWidely used in cybersecurityLess common due to complexityIncreasing popularityFuture evolutionEvolving to integrate AI, threat intelligenceEvolving to address evasion techniquesEvolving to handle DNS tunnelingManagement overheadVaried based on IDS type and deploymentHigh for setup, maintenance, and monitoringModerate for DNS traffic analysisHow to Stop BotnetsNow we know how undesirable botnets are detected, let’s turn to how they can be stopped. Three main options exist: CAPTCHA, rate limiting, and bot protection.A. JS Challenges/CAPTCHAOne way to stop bad bot activity is by implementing JS Challenges and CAPTCHA on your websites or web applications. Both are effective security mechanisms used to protect against malicious bots, automated scripts, and other unauthorized automated activities, such as web scraping.Figure 4: CAPTCHAGcore provides JS Challenge and JS CAPTCHA solutions as part of Gcore WAAP. First, a JS challenge runs a small piece of JavaScript code in the user’s browser, which a bot typically cannot execute. This code checks for typical human behavior and browser characteristics to ensure the request comes from a legitimate user. Next, a CAPTCHA presents a task that is easy for humans but difficult for bots, such as identifying objects in images or solving simple puzzles. By completing these tasks, users prove they are human, thereby preventing automated systems from accessing or abusing web services.But there’s a downside: CAPTCHAs do not distinguish between beneficial bots (such as search engine crawlers or monitoring tools) and malicious bots. They can impede good bots from performing their intended functions. To allow good bots while still protecting against malicious ones, website administrators need to create exceptions or use alternative verification methods that can recognize and permit trusted bots. Gcore manages this process with our WAAP customers to ensure good bots continue to function effectively.B. Rate LimitingFigure 5: Rate LimitingA key characteristic of bots is their ability to automate and rapidly scale tasks. For example, bots can fill and submit forms much faster than humans, sending a large number of requests to the server and receiving an equally large number of responses. This can drain server resources and degrade site performance.Rate limiting controls the number of requests an IP address or IP range can make to a resource within a certain timeframe. This method mitigates bad bot activity on websites or web applications. Good bots don’t engage in this kind of behavior, so there’s not much risk of stopping their activity with a rate limiter.Gcore Rate Limiter protects your websites and web applications from excessive requests that signal bad bot activity. You can specify a set of rules dictating how many requests are allowed per IP address per second. Once this limit is exceeded, the requester will receive an HTTP 429 (Too Many Requests) error message.Stop Bad Bots with Gcore WAAPWhile bot detection techniques such as honeynets, DNS-based bot detectors, and intrusion detection systems (IDSs) are effective in their own right, a hybrid or multi-layered bot detection approach is the most accurate way to detect bot activity. Gcore WAAP (Web Application Firewall + API Protection) is the ultimate all-in-one bot detection and protection solution for your websites and web applications. Gcore WAAP incorporates bot protection with a web application firewall, API security, and advanced DDoS protection to offer enhanced enterprise-grade security.We protect against threats including and beyond the OWASP Top 10, addressing unpatched vulnerabilities and zero-day attacks by leveraging machine learning technologies. With Gcore WAAP, you enjoy API-specific protection and security against credential stuffing, account takeover, brute force attacks, and L7 DDoS attacks.Gcore WAAP is scalable to meet your needs, regardless of industry. It is also easy to deploy—no additional hardware, software, or changes in the code are required on your part. Once you send a request, Gcore will start protecting your web resources immediately. Request Gcore WAAP today and enjoy bot-free websites and web applications.ConclusionDetecting and stopping bad bots involves a combination of advanced techniques tailored to identify and mitigate malicious activities while allowing beneficial bots to operate. Implementing a multi-layered bot detection strategy, such as Gcore WAAP, ensures comprehensive protection against various threats while maintaining website performance and user experience.Gcore WAAP is integrated into Gcore’s global infrastructure, operating on 180+ global points of presence in Tier III and IV data centers, ensuring optimal performance, low latency worldwide, and outstanding security at the network’s edge. Secure your web applications and APIs against the most sophisticated cyber threats to safeguard your business’ reputation.Discover Gcore WAAP

What Is TCP DDoS Protection? | How Does It Work?

TCP DDoS (distributed denial-of-service) attacks pose a significant threat to network infrastructure. They exploit vulnerabilities in the TCP protocol to overwhelm servers and disrupt legitimate traffic. This article explains the mechanics of TCP DDoS attacks, their potential impact, and effective strategies for prevention and mitigation. We’ll explore different types of TCP DDoS attacks, such as SYN floods and ACK floods, and discuss how to safeguard your systems against these threats using techniques like SYN cookies, traffic filtering, and load balancing.What Is a TCP DDoS Attack?A TCP DDoS attack (Transmission Control Protocol distributed denial-of-service) is a type of cyberattack that targets the TCP protocol, a fundamental communication protocol used in networked systems. DDoS attacks aim to disrupt normal traffic on a network by overwhelming it with a flood of malicious traffic.In a TCP DDoS attack, the attacker uses multiple compromised devices, often referred to as a botnet, to send a huge number of packets (small units of data) to a target network or server. The goal is to overload the network components, making them unable to handle legitimate requests from real users.The principle of a DDoS attackTCP is widely used for internet communications, including web browsing and email, but it wasn’t originally designed with robust security features. This lack of built-in security makes it vulnerable to exploitation. Hackers can send different types of TCP packets to execute their attacks. To understand this, let’s examine how TCP normally functions and what happens during a DDoS attack in the three most common TCP attack methods.How Does a TCP DDoS Attack Work?When a device, like your computer, wants to connect to a server, such as a website, it follows a three-step process known as the TCP handshake.How TCP normally worksEach type of TCP DDoS attack (SYN flood, SYN-ACK flood, and ACK flood) targets a different stage of this handshake. By overwhelming the server at each stage, the attacker disrupts the handshake process, causing downtime and service disruption for legitimate users.Let’s look at how TCP works and how different types of attacks exploit each stage.Initiating the Connection: SYN FloodNormal process: Your device sends a SYN packet to the server to request a connection.SYN flood attack: The attacker sends a massive number of SYN packets to the server, pretending to request connections. The server tries to respond to each request by sending SYN-ACK packets. However, since the attacker never completes the handshake, the server is left waiting with numerous half-open connections, consuming its resources and making it unable to handle legitimate requests.In an SYN flood attack, the attacker can use their own resources to send the requests or use TCP/IP hijacking to spoof legitimate IP addresses, making the malicious requests appear genuine. The spoofed systems do not respond to the SYN-ACK packets because they are unaware of the SYN packets sent on their behalf. Even though the server eventually drops these half-open connections, the sheer volume can exceed the server’s capacity, rendering it unresponsive to real clients.How a SYN flood attack worksAcknowledging the Request: SYN-ACK FloodNormal process: The server receives the SYN packet and responds with a SYN-ACK packet to acknowledge the connection request.SYN-ACK flood attack: The attacker sends a large number of SYN-ACK packets to the server without any corresponding SYN requests. The server has to process these unexpected packets, which uses up its resources and disrupts its ability to function properly.A SYN-ACK flood leverages an unusual approach by sending SYN-ACK packets to the server. Even though they are not part of a legitimate handshake, these packets must be processed, and a significant number of such packets can cause server-side disruptions.Completing the Handshake: ACK PacketsNormal process: Your device responds to the server’s SYN-ACK packet with an ACK packet, completing the handshake and establishing the connection.ACK flood attack: The attacker sends a flood of ACK packets to the server. These packets look like legitimate responses, so the server spends its resources processing them. This overwhelms the server, making it difficult for it to handle actual connections from real users.An ACK flood exploits the TCP function for data delivery acknowledgment. When a client receives the requested data, it sends an ACK message to inform the server that the data was successfully received. Attackers take advantage of this by sending a large number of forged ACK packets, which look identical to legitimate ones. This overwhelms the server, as it cannot distinguish between real and fake ACK messages, leading to resource exhaustion and service disruption.What Damage Can a TCP DDoS Attack Cause?A successful TCP DDoS attack overwhelms the target server with fake requests, rendering the server unavailable to real users and the services relying on it. The duration of the downtime and the criticality of the affected services can result in significant financial losses and reputational damage for the business. Unprotected businesses lose an average of $6,000 per minute during a DDoS attack. The unavailability of an online banking service or a medical institution can cause real distress and disruption with serious consequences for customers.DDoS attacks can also act as a diversion for the target company’s IT and security teams, acting as a smokescreen for other malicious activities. While the incident response team is focused on restoring the attacked server’s performance, hackers might exploit the distraction to carry out phishing attempts or other attacks on sensitive information stored on the server.How to Prevent TCP DDoS AttacksWhen an attack succeeds, it’s already too late to prevent it. Therefore, the best tactics against all kinds of DDoS attacks are defensive measures.One option is to use a comprehensive third-party DDoS mitigation service, like Gcore DDoS Protection, and leave it to the specialists. We recommend this approach, as using a robust, proven protection service is usually the most convenient and effective strategy.Alternatively, you can attempt to apply prevention mechanisms yourself. For TCP DDoS attacks, these include the following:SYN cookies: Protect against SYN flood attacks by adding a special value to SYN-ACK packets. The server will only restore the connection if this value is returned in the client’s ACK message, preventing half-open connections from consuming resources.Filtering and blocking: Identify and block traffic from irrelevant sources or patterns that seem suspicious.Blocking unused ports: Reduce the potential attack surface by closing ports that aren’t in use.Rate limiting: To prevent overload, set thresholds for the amount of traffic the server can handle, including the number of SYN requests.Load balancing and redundancy: Distribute traffic using load balancers and have backup servers ready to handle increased traffic volumes.Caching and CDNs: Use caching and content delivery networks to minimize the resources needed to handle repetitive requests.However, following best security practices doesn’t eliminate the risk of an attack entirely, so it’s also important to monitor traffic for spikes and analyze such spikes for anomalies in order to react promptly in case of an attack. If you’re undertaking DDoS mitigation yourself, this will involve some manual processes. Third-party providers will analyze traffic on your behalf, and some—like Gcore—offer real-time visibility into traffic so you can watch DDoS attacks being stopped as they happen.Thwart Attackers with Gcore DDoS ProtectionOur global network of scrubbing centers is engineered to keep your business operational during a DDoS attack. Your customers will experience uninterrupted functionality even during a DDoS attack, no matter the type. Gcore scrubbing centers are equipped with backup copies of critical systems and network equipment, underscoring our dedication to providing continuous service and robust security.Gcore DDoS Protection offers businesses the following benefits:Robust infrastructure: With over 148 Tbps of network capacity and a global network that’s constantly learning from millions of internet properties, Gcore protects you against the largest and most sophisticated attacks.Proprietary DDoS protection solution: Tailored specifically to ward off a broad spectrum of DDoS threats.Detection of low-frequency attacks from the first query: Even the most subtle attacks are detected.Exceptionally low false-positive rate (less than 0.01%): Maintains normal operations by accurately distinguishing between legitimate traffic and attack vectors.Real-time statistics in the control panel: Offers immediate insights into traffic patterns and potential threats, allowing for swift action.Server protection in your data center: Extends Gcore’s protective measures directly to your infrastructure through a Generic Routing Encapsulation (GRE) tunneling protocol, regardless of location.24/7, highly qualified technical support: Ensures that expert help is always on hand, day or night, to address any concerns or attacks.Exceptional uptime rate with 99.99% SLA: A seamless and uninterrupted user experience backed up by Tier III and IV data centers.ConclusionDDoS attacks, including various kinds of TCP attacks, are still a real threat to online services and can cause real damage to businesses. Preparing for the attacks in advance, however, can help to mitigate their consequences or avoid losses completely.Experience the peace of mind that comes with advanced protection with Gcore DDoS Protection for comprehensive security against DDoS attacks. With over 1 Tbps of total filtering capacity and a 99.99% SLA, your digital assets remain protected from even the most complex, sophisticated, and sustained attacks. Gcore helps to maintain the continuity of your online services, regardless of potential attackers’ motivations.Explore Gcore DDoS Protection

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