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SYN Flood Attacks

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SYN Flood Attacks: The Silent Killers of the Internet​

Imagine that you are hosting a party at your home. You have invited a few friends and prepared some snacks and drinks. You are expecting a fun and relaxing evening. However, as soon as you open the door, you are greeted by hundreds of strangers who claim to be your guests. They push their way into your house, fill up every room, and demand your attention. They don't leave any space for your real friends, who are stuck outside or unable to reach you. You are overwhelmed, frustrated, and helpless.

This is what a SYN flood attack feels like for a server. A SYN flood attack is a type of cyberattack that exploits a flaw in the way computers communicate over the Internet. The attacker sends a barrage of fake requests to initiate a connection with the server, but never completes the process. The server has to keep track of all these pending requests, which consume its memory and processing power. Eventually, the server becomes so overloaded that it can't handle any legitimate requests from real users. The server is effectively paralyzed by the attacker.

A SYN flood attack is one of the most common and dangerous forms of denial-of-service (DoS) or distributed denial-of-service (DDoS) attacks. It can target any system that provides services over the Internet using the TCP protocol, such as web servers, email servers, or file transfer servers. A SYN flood attack can cause severe damage to the availability and performance of these systems, affecting millions of users and businesses around the world. A SYN flood attack can also result in financial losses, reputational harm and legal issues for the victims.

What are SYN Flood Attacks?​

A SYN flood attack is a form of denial-of-service (DoS) or distributed denial-of-service (DDoS) attack that exploits the TCP handshake process to overwhelm a server with open connections. The attacker sends massive numbers of SYN packets, which are part of the initial connection request, without responding to the corresponding acknowledgements. The server has to spend resources waiting for half-opened connections, which can consume enough resources to make the system unresponsive to legitimate traffic. A SYN flood is also known as a half-open attack or a protocol attack.

A SYN flood attack is dangerous because it can take down even high-capacity devices capable of maintaining millions of connections. It can also bypass some firewalls and intrusion prevention systems that are not designed to handle this type of attack. A SYN flood attack can disrupt the availability and performance of any system connected to the Internet and providing TCP services, such as web servers, email servers, or file transfer servers. A SYN flood attack can also cause financial losses, reputational damage, and legal consequences for the victims.

How a SYN Flood Attack can be detected?​

A SYN flood attack can be hard to detect and distinguish from legitimate traffic spikes, especially if the attacker uses IP spoofing or randomizes the source ports. There are some signs and methods that can help you identify a SYN flood attack on your server, for example:

  • Monitor network traffic for suspicious activity. You can use network packet capture and analysis tools to inspect the traffic coming to and from your server and look for anomalies, such as unusually high levels of traffic, traffic coming from unusual locations or sources, or a large number of SYN packets without corresponding ACK packets.
  • Check the status of your server resources. You can use commands such as netstat, ss, or iptraf to check the status of your TCP connections and see if there are a lot of half-open connections (SYN_RECV) or connection requests (SYN_SENT) that are not completed. You can also check the CPU usage, memory consumption, and network bandwidth of your server and see if they are abnormally high or maxed out.
  • Use SYN cookies or other SYN flood protection mechanisms. SYN cookies are a technique that allows the server to handle SYN packets without allocating resources until the final ACK packet is received. This way, the server can avoid keeping half-open connections and wasting resources. Other SYN flood protection mechanisms include firewalls, load balancers, proxies, or DDoS mitigation services that can filter out malicious traffic and block SYN packets from spoofed IP addresses.

Detecting SYN Flood with netstat​

One way to detect a SYN flood attack is to use the netstat command, which displays network connections, routing tables, interface statistics, and more. You can use netstat with various options to filter and analyze the output. For example, you can use netstat -n to show numerical addresses instead of resolving hostnames, netstat -p to show the process ID and name of each connection, netstat -t to show only TCP connections, and netstat -a to show all connections and listening ports.

To detect a SYN flood attack, you can look for indicators like a high number of SYN packets or SYN-ACK packets compared to other types of packets, a high number of half-open connections or SYN_RECV state connections, a high number of connections with unknown or spoofed source IP addresses, a high rate of connection requests or timeouts, and a low rate of connection completions or established connections.

For example, you can use netstat -ant | grep SYN_RECV to show all TCP connections in the SYN_RECV state, which means the server has sent a SYN-ACK packet but has not received an ACK packet yet. If you see a large number of connections in this state, especially from different source IP addresses, it may indicate a SYN flood attack.

You can also use netstat -s to show statistics for each protocol. You can look for metrics like the number of active TCP openings (SYN packets sent), passive TCP openings (SYN packets received), failed connection attempts (SYN-ACK packets sent but no ACK received), and reset connections (RST packets sent or received). If you see a high ratio of failed connection attempts to passive TCP openings, or a high ratio of reset connections to active TCP openings, it may indicate a SYN flood attack.

How a SYN Flood Attack can be mitigated?​

A SYN flood attack can cause serious damage to your server and website, affecting your availability, performance, and reputation. Therefore, you need to take immediate action to mitigate the attack and restore your service as soon as possible. Here are some of the techniques that can help you do that:

  • Rate limiting. This technique involves limiting the number of SYN requests that can be sent to your server at any one time. This can help you prevent your server from being overwhelmed by the attack and preserve some resources for legitimate traffic.
  • New Generation Web Application Firewall (NGWAF). A NGWAF is a device or service that can protect your web applications from various types of cyberattacks, including SYN Flood attacks. It can analyze and filter incoming traffic at the application layer and block malicious requests before they reach your server. One example of a NGWAF is PowerWAF, which is a cloud-based solution that can provide real-time protection, scalability, and performance for your web applications.
  • Network segmentation. This technique involves dividing your network into smaller, more secure subnetworks to limit the potential damage from a SYN flood attack. You can isolate your critical servers or applications from the rest of the network and protect them with additional security measures.
  • Load balancing. This technique involves distributing incoming traffic across multiple servers, reducing the risk of a single server being overloaded by a SYN flood attack. You can use load balancing devices or services that can balance the load among your servers and redirect traffic away from the affected ones.
  • SYN cookies. This technique involves using a cryptographic hashing method to handle SYN packets without allocating resources until the final ACK packet is received. This way, your server can avoid keeping half-open connections and wasting resources. SYN cookies are a feature that can be enabled on some operating systems or servers.

Differences between SYN Flood and other DDoS attacks​

A DDoS attack is a cyberattack that aims to disrupt the normal functioning of a server, service, or network by overwhelming it with a large amount of traffic from multiple sources. There are different types of DDoS attacks, depending on the layer of the network protocol stack they target, the technique they use, or the goal they have. A SYN flood attack is a specific type of DDoS attack that targets the TCP layer (layer 4) of the network protocol stack and exploits the TCP handshake process to create half-open connections on the server. Here are some of the main differences between a SYN flood attack and other types of DDoS attacks:

  • Volumetric attacks. These are the most common types of DDoS attacks. They aim to consume the network bandwidth or resources of the target by sending a large volume of traffic, such as UDP packets, ICMP packets, or spoofed packets. A SYN flood attack can be considered a type of volumetric attack, but it is more focused on exhausting the server's connection pool rather than its bandwidth.
  • Protocol attacks. These are types of DDoS attacks that exploit weaknesses or vulnerabilities in the network protocols or devices, such as routers, firewalls, or load balancers. They aim to consume the processing power or memory of the target by sending malformed or invalid packets, such as TCP flags, ping of death, or fragmented packets. A SYN flood attack can be considered a type of protocol attack, but it is more focused on exploiting the TCP handshake process rather than other protocol features.
  • Application layer attacks. These are types of DDoS attacks that target the application layer (layer 7) of the network protocol stack and mimic legitimate requests from users or clients. They aim to consume the application resources or logic of the target by sending requests that trigger intensive tasks, such as HTTP GET or POST requests, SQL queries, or login attempts. A SYN flood attack is different from an application layer attack because it does not target a specific application or service, but rather any system that uses TCP connections.

These types are not mutually exclusive and can be combined or used together to create more complex and powerful attacks. You should be prepared for any type of DDoS attack and use multiple layers of defense to protect websites.

Preventing SYN Flood Attacks​

A SYN flood attack is a serious threat to your website and server, and it can cause significant damage to your availability, performance, and reputation. Therefore, you should take proactive measures to prevent or reduce the likelihood or impact of a SYN flood attack in the future. Here are some of the best practices and tools for preventing SYN flood attacks:

  • Keep your system updated and patched. You should always keep your operating system, server software, applications, and security tools updated and patched with the latest versions and fixes. This can help you avoid known vulnerabilities or bugs that can be exploited by a SYN flood attack or other types of cyberattacks.
  • Use strong authentication and encryption. You should use strong authentication and encryption methods to protect your server and website from unauthorized access or tampering. For example, you should use HTTPS instead of HTTP, SSL/TLS certificates instead of self-signed certificates, and two-factor authentication instead of passwords.
  • Implement network security policies and rules. You should implement network security policies and rules that can help you control and monitor the traffic coming to and from your server and website. For example, you should use firewalls, routers, or switches to filter out unwanted or malicious traffic, such as SYN packets from spoofed IP addresses or unusual sources. You should also use IDS or IPS devices or services to detect and block potential SYN flood attacks or other types of cyberattacks.
  • Use load balancing and redundancy. You should use load balancing and redundancy techniques to distribute incoming traffic across multiple servers or locations, reducing the risk of a single server being overloaded by a SYN flood attack or other types of cyberattacks. You should also have backup servers or services that can take over in case of a failure or outage.
  • Use New Generation Web Application Firewall (NGWAF). A NGWAF is a device or service that can protect your web applications from various types of cyberattacks, including SYN flood attacks. It can analyze and filter incoming traffic at the application layer and block malicious requests before they reach your server. One example of a NGWAF is PowerWAF, which is a cloud-based solution that can provide real-time protection, scalability, and performance for your web applications.

These are some of the best practices and tools for preventing SYN flood attacks. You should always be prepared for any possible scenario and have a contingency plan in case of an emergency.

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