5.1 15 Analyze A Syn Flood Attack

A SYN flood attack is a type of denial-of-service (DoS) attack that exploits the TCP handshake process. It overwhelms a server with a flood of SYN packets, the first step in establishing a TCP connection, preventing legitimate users from connecting. Let's delve into analyzing a SYN flood attack to understand its mechanics, detection, and mitigation strategies.

Understanding the TCP Handshake

Before diving into the attack itself, it's crucial to understand the normal TCP handshake:

1. SYN (Synchronize): The client sends a SYN packet to the server, requesting a connection.
2. SYN-ACK (Synchronize-Acknowledge): The server responds with a SYN-ACK packet, acknowledging the client's request and proposing its own parameters.
3. ACK (Acknowledge): The client sends an ACK packet back to the server, confirming the connection.

This three-way handshake establishes a reliable connection between the client and server. However, the vulnerability lies in the server's handling of the SYN-ACK response.

What is a SYN Flood Attack?

In a SYN flood attack, the attacker sends a massive number of SYN packets to the target server. However, unlike legitimate connection attempts, the attacker either:

• Doesn't complete the handshake: The attacker doesn't send the final ACK packet.
• Spoofs the source IP address: The SYN packets are sent with a forged source IP address, making it impossible for the server to send the SYN-ACK back to the originating host.

In both scenarios, the server allocates resources (memory and CPU) for each incoming SYN request and keeps a "half-open" connection waiting for the final ACK. Because the attacker floods the server with these SYN requests, the server's resources are quickly exhausted. This leads to:

• Denial of Service: Legitimate users trying to connect to the server are unable to do so, as the server is overwhelmed with half-open connections.
• Server Slowdown or Crash: The server may become extremely slow or even crash due to resource exhaustion.

Analyzing a SYN Flood Attack: A Step-by-Step Approach

Analyzing a SYN flood attack involves several steps:

1. Detection:

• Monitor Network Traffic: The first step is to identify an unusual surge in network traffic directed towards the target server, specifically SYN packets. Tools like Wireshark, tcpdump, and network monitoring systems (NMS) can be used for this purpose.
• Analyze Server Logs: Examine the server's logs for a high number of incomplete TCP connections (SYN_RECEIVED state). This indicates that the server is receiving SYN packets but not completing the handshake.
• Check Server Performance: Monitor CPU usage, memory utilization, and network latency. A sudden spike in CPU usage and memory consumption, coupled with increased network latency, can be a sign of a SYN flood attack.
• Intrusion Detection Systems (IDS): Utilize IDS to detect suspicious patterns of network activity, such as a large number of SYN packets originating from a single source or multiple sources with spoofed IP addresses.
• Firewall Logs: Review firewall logs for dropped connections and unusual traffic patterns that might indicate an ongoing SYN flood attack.

2. Capturing Network Traffic:

• Use Packet Sniffers: Capture network traffic using tools like Wireshark or tcpdump on the server itself or on a network device close to the server. This will provide detailed information about the packets being sent to the server.
• Filter for SYN Packets: Filter the captured traffic to isolate SYN packets (TCP flag SYN is set). This will help you focus on the packets that are contributing to the attack. The Wireshark filter would be tcp.flags.syn == 1 and tcp.flags.ack == 0. The tcpdump filter would be tcp[tcpflags] & tcp-syn != 0 and tcp[tcpflags] & tcp-ack == 0.

3. Examining Captured Packets:

• Source IP Addresses: Analyze the source IP addresses of the SYN packets.
  • Spoofed IP Addresses: If the source IP addresses are random or belong to non-routable networks, it's a strong indication that the attacker is spoofing the IP addresses. This makes it difficult to trace the attack back to its origin.
  • Distributed Attack: If the SYN packets are coming from a large number of unique IP addresses, it suggests a distributed SYN flood attack, potentially launched from a botnet.
• Source Ports: Examine the source ports of the SYN packets. Are they random or sequential? Random source ports are often used to bypass simple filtering rules.
• Destination Port: The destination port is usually the port on which the target server is listening for connections (e.g., port 80 for HTTP, port 443 for HTTPS). Verify that the destination port is the expected one.
• TCP Flags: Ensure that only the SYN flag is set. If other flags, such as FIN or RST, are also set, it might indicate a different type of attack or a misconfigured client.
• Packet Size: SYN packets are typically small. Check the size of the packets to ensure they are within the expected range. Unusually large SYN packets might indicate an attempt to exploit vulnerabilities.
• TTL (Time to Live): Analyze the TTL values of the SYN packets. Consistent TTL values from different source IP addresses could suggest that the attacker is using a specific network infrastructure.
• TCP Options: Check the TCP options field for any unusual or suspicious options. Attackers might use specific TCP options to evade detection or exploit vulnerabilities.

4. Identifying the Attack Source (If Possible):

• Geolocation: If the source IP addresses are not spoofed, use geolocation tools to determine the geographic location of the attackers. This information can be used to block traffic from specific regions.
• Reverse DNS Lookup: Perform reverse DNS lookups on the source IP addresses to identify the hostnames associated with them. This might provide clues about the attacker's infrastructure.
• Contacting ISPs: If you can identify the Internet Service Providers (ISPs) of the attacking IP addresses, contact them and report the attack. They might be able to take action to stop the attack.
• Blacklisting IP Addresses: Implement blacklisting rules on your firewall to block traffic from the identified attacking IP addresses. However, be aware that attackers can easily change their IP addresses, so blacklisting is not always an effective long-term solution.

5. Analyzing the Attack Pattern:

• Attack Duration: How long has the attack been going on? Is it a short burst or a sustained attack?
• Attack Intensity: How many SYN packets are being sent per second? Is the attack increasing in intensity over time?
• Attack Frequency: Is the attack a one-time event or are there recurring attacks?
• Targeted Services: Which services are being targeted by the attack? Are specific ports being targeted? Understanding the attack pattern helps in implementing appropriate mitigation strategies.

Mitigation Strategies for SYN Flood Attacks

Several mitigation techniques can be employed to protect against SYN flood attacks:

1. SYN Cookies:

• How it works: SYN cookies are a defense mechanism where the server doesn't store connection-specific data when it receives a SYN packet. Instead, it generates a cryptographic hash (the "cookie") based on the SYN packet's source IP address, source port, destination IP address, destination port, and a secret key. This cookie is sent back to the client in the SYN-ACK packet.
• Verification: When the client sends the ACK packet, the server recomputes the hash using the same information. If the recomputed hash matches the cookie in the ACK packet, the server knows that the connection is legitimate and allocates resources for it.
• Benefits: SYN cookies prevent the server from being overwhelmed by half-open connections, as it doesn't need to store any state until the handshake is complete.
• Drawbacks: SYN cookies can limit the TCP options that can be used and might impact performance slightly.

2. Increasing Backlog Queue Size:

• How it works: The backlog queue is the number of incomplete connection requests that the server can hold in memory. Increasing the size of the backlog queue allows the server to handle a larger number of SYN requests before becoming overwhelmed.
• Implementation: The backlog queue size can be configured in the server's operating system or application settings.
• Benefits: A larger backlog queue can absorb a higher volume of SYN packets, giving the server more time to process legitimate connection requests.
• Drawbacks: Increasing the backlog queue size only delays the inevitable if the attack is sustained and intense enough. It also consumes more memory.

3. Reducing SYN-ACK Timeout:

• How it works: The SYN-ACK timeout is the amount of time the server waits for the final ACK packet after sending the SYN-ACK. Reducing this timeout frees up resources more quickly if the ACK is not received.
• Implementation: The SYN-ACK timeout can be configured in the server's operating system settings.
• Benefits: A shorter timeout reduces the amount of time resources are tied up waiting for the ACK, allowing the server to handle more connection requests.
• Drawbacks: A very short timeout might cause legitimate connections to be dropped if the client is slow to respond.

4. Rate Limiting:

• How it works: Rate limiting involves limiting the number of SYN packets that the server accepts from a particular source IP address within a given time period.
• Implementation: Rate limiting can be implemented using firewalls, intrusion prevention systems (IPS), or specialized DDoS mitigation devices.
• Benefits: Rate limiting can prevent a single attacker from overwhelming the server with SYN packets.
• Drawbacks: Rate limiting might also affect legitimate users if they are behind a shared IP address or if the rate limit is set too low.

5. SYN Proxy:

• How it works: A SYN proxy acts as an intermediary between the client and the server. The proxy intercepts SYN packets from clients and completes the TCP handshake on behalf of the server. Only legitimate connections are then forwarded to the server.
• Benefits: SYN proxies protect the server from being directly exposed to SYN flood attacks.
• Drawbacks: SYN proxies add complexity to the network infrastructure and can introduce latency.

6. Firewalls and Intrusion Prevention Systems (IPS):

• How it works: Firewalls and IPS can be configured to detect and block SYN flood attacks based on various criteria, such as the number of SYN packets per second, the source IP address, and the TCP flags.
• Benefits: Firewalls and IPS provide a first line of defense against SYN flood attacks.
• Drawbacks: Firewalls and IPS need to be properly configured and maintained to be effective. They might also generate false positives, blocking legitimate traffic.

7. Web Application Firewalls (WAFs):

• How it works: WAFs are designed to protect web applications from various types of attacks, including SYN flood attacks. They can analyze HTTP traffic and block malicious requests.
• Benefits: WAFs provide an additional layer of security for web applications.
• Drawbacks: WAFs need to be properly configured and maintained to be effective.

8. Content Delivery Networks (CDNs):

• How it works: CDNs distribute content across multiple servers, making it more difficult for attackers to target a single server. They can also absorb a large volume of traffic, mitigating the impact of SYN flood attacks.
• Benefits: CDNs improve website performance and availability while also providing DDoS protection.
• Drawbacks: Using a CDN can add cost and complexity to the network infrastructure.

9. DDoS Mitigation Services:

• How it works: DDoS mitigation services specialize in protecting websites and networks from DDoS attacks, including SYN flood attacks. They use a variety of techniques, such as traffic scrubbing, rate limiting, and SYN cookies, to mitigate attacks.
• Benefits: DDoS mitigation services provide comprehensive protection against DDoS attacks.
• Drawbacks: DDoS mitigation services can be expensive.

Tools for Analyzing SYN Flood Attacks

• Wireshark: A powerful packet analyzer that can capture and analyze network traffic in real-time. It allows you to filter and inspect packets to identify SYN flood attacks.
• tcpdump: A command-line packet analyzer that is similar to Wireshark. It can be used to capture and filter network traffic.
• Nmap: A network scanning tool that can be used to identify open ports and services on a target server. It can also be used to detect SYN flood vulnerabilities.
• Hping3: A command-line packet crafting tool that can be used to send custom TCP packets, including SYN packets, to a target server. It can be used to simulate SYN flood attacks for testing purposes.
• Network Monitoring Systems (NMS): Tools like Nagios, Zabbix, and SolarWinds can monitor network traffic and server performance, alerting administrators to potential SYN flood attacks.

Prevention is Better Than Cure: Hardening Your Systems

While mitigation strategies are essential, proactive measures can significantly reduce the risk of SYN flood attacks:

• Keep Systems Updated: Regularly update your operating systems, applications, and security software to patch vulnerabilities that attackers could exploit.
• Implement Strong Security Policies: Enforce strong password policies, restrict access to sensitive resources, and implement multi-factor authentication.
• Regular Security Audits: Conduct regular security audits to identify vulnerabilities and weaknesses in your systems.
• Intrusion Detection and Prevention Systems (IDS/IPS): Deploy and properly configure IDS/IPS to detect and block malicious network traffic.
• Network Segmentation: Segment your network to isolate critical systems and limit the impact of a successful attack.
• Educate Users: Educate users about phishing attacks and other social engineering techniques that attackers might use to gain access to your systems.

Example Scenario: Analyzing a SYN Flood Attack with Wireshark

Let's imagine you suspect a SYN flood attack on your web server. You decide to use Wireshark to analyze the network traffic.

1. Capture Traffic: Start Wireshark and begin capturing traffic on the network interface connected to your web server.

2. Apply Filter: Apply the filter tcp.flags.syn == 1 and tcp.flags.ack == 0 to isolate SYN packets.

3. Analyze Source IP Addresses: Examine the list of source IP addresses. You notice a large number of SYN packets coming from a wide range of seemingly random IP addresses.

4. Analyze Packet Rate: Observe the number of SYN packets per second. You see a sustained rate of thousands of SYN packets per second, significantly higher than normal.

5. Identify Spoofed Addresses: You perform reverse DNS lookups on some of the source IP addresses and find that they either don't resolve or resolve to unrelated hostnames. This indicates that the IP addresses are likely being spoofed.

6. Conclusion: Based on your analysis, you conclude that your web server is indeed under a SYN flood attack with spoofed IP addresses.

7. Mitigation: You then implement SYN cookies on your server and configure your firewall to rate limit SYN packets from suspicious IP addresses. You also contact your ISP to report the attack and request assistance.

Conclusion

Analyzing a SYN flood attack requires a systematic approach, involving careful monitoring, packet capture, and detailed examination of network traffic. By understanding the attack's characteristics and implementing appropriate mitigation strategies, you can protect your servers and networks from the devastating effects of SYN flood attacks. Remember that a multi-layered approach, combining proactive security measures with reactive mitigation techniques, is the most effective way to defend against these types of attacks. Continuous monitoring and analysis are crucial for detecting and responding to evolving threats.