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How can you identify the source of a SYN flood attack?

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A SYN flood attack is a type of denial-of-service (DoS) attack that exploits a vulnerability in the TCP protocol. TCP is a connection-oriented protocol that requires a three-way handshake between the client and the server before exchanging data. A SYN flood attack sends a large number of SYN packets (requests to initiate a connection) to the target server, but never completes the handshake by sending ACK packets (acknowledgements). This causes the server to allocate resources for half-open connections that never materialize, eventually exhausting its memory and processing capacity. In this article, you will learn how to identify the source of a SYN flood attack using some common tools and techniques.

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John Cookson

1 Check the symptoms

Identifying the source of a SYN flood attack begins by examining the symptoms on the target server. Common signs of a SYN flood attack include high CPU and memory usage, slow or unresponsive network services, an increased number of half-open connections, and decreased number of established connections. Additionally, SYN packets with spoofed or random source IP addresses can indicate a SYN flood attack. To monitor the network traffic and connection status on the server, tools such as netstat, hping, or tcpdump can be used. For example, running the command netstat -s | grep SYN will display the number of SYN packets received and sent by the server.

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John Cookson
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Look for the IP in parallel to your response - typically, numerous IPs generate small traffic than one sending lots. Response includes configure load balancer and Firewall to discard high SYN requests, implement appropriate rate and SYN reply limits, consider using SYN cookies and honeypots, integrate failover servers, or build servers capable of handling significantly more load than anticipated can help and also future-proofs the system For a proactive approach, allocating budget for hosting critical servers with providers offering DOS/DDOS protection is advisable. Alternatively, subscribing to a DDOS service where the service provider manages the handshake process to critical IPs can be advantageous.

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Dhananjay Rathod
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These attacks mess up the TCP handshake by sending tons of SYN packets to the server but never finishing the connection. This makes the server run out of resources and slow down or crash. To find out who’s behind the attack, you need to look at the symptoms on the server and check the network traffic with tools like netstat, ping, or tcpdump. These tools can help you spot weird patterns of SYN packets, like too many, too few, or fake source IP addresses. By following these packets, you can track down the source of the attack and stop it.

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Aneek Bera

Resident Engineer @ Oracle | Kubernetes Expert | MBA Candidate @ SMU Cox School of Business
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One key symptom is an unusually high volume of SYN packets compared to the normal traffic pattern. This can cause a significant increase in the number of half-open connections, leading to resource exhaustion on the targeted server more like the DOS attacks. Apart from this, there are other kinds of attacks as well namely; Direct SYN Flood Attacks, and SYN Spoofed Attacks.

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2 Trace the packets

The next step to identify the source of a SYN flood attack is to trace the packets that are flooding the server. You can use tools such as Wireshark, Nmap, or traceroute to analyze the packet headers and the route they take to reach the server. For example, you can use the command wireshark -i eth0 -f "tcp[13] & 2 != 0" to capture only the SYN packets on the interface eth0.

However, tracing the packets may not be enough to find the actual source of the attack, as the attacker may use spoofing techniques to hide or change the source IP address of the packets. In that case, you may need to contact your Internet service provider (ISP) or network administrator to help you track down the origin of the attack.

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Aneek Bera

Resident Engineer @ Oracle | Kubernetes Expert | MBA Candidate @ SMU Cox School of Business
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Tools like Wireshark can filter and analyze network traffic to pinpoint the origin of the attack. Additionally, leveraging network flow data from routers and switches can provide insights into the flow of traffic and help trace the attack back to its source.

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3 Mitigate the attack

The final step to identify the source of a SYN flood attack is to mitigate the attack and prevent further damage to the server. You can use tools such as iptables, fail2ban, or syncookies to filter or limit the incoming SYN packets and reduce the impact of the attack. For example, you can use the command iptables -A INPUT -p tcp --syn -m limit --limit 1/s -j ACCEPT to allow only one SYN packet per second from each source IP address.

Additionally, you can use tools such as firewalls, load balancers, or proxy servers to protect your server from future attacks. These tools can help you distribute the load, block malicious traffic, or hide your server's IP address from potential attackers.

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Aneek Bera

Resident Engineer @ Oracle | Kubernetes Expert | MBA Candidate @ SMU Cox School of Business
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Increasing backlog queues buffers against SYN flood attacks by accommodating more connections. Filtering blocks malicious requests based on patterns or IPs. Reducing SYN-RECEIVED timers deallocates resources faster. SYN cache minimizes resource use by handling valid requests only. SYN cookies delay resource allocation until connection legitimacy is confirmed. Load balancers distribute traffic across servers, mitigating attacks. Firewalls and proxies filter traffic before reaching servers. Honeypots divert and study attacks, while rate limiting sets connection thresholds. Combining these techniques creates a layered defense. Cloud-based DDoS protection offloads and filters traffic, offering proactive defense against sophisticated attacks.

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4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can you identify the source of a SYN flood attack?

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4

1 Check the symptoms

2 Trace the packets

3 Mitigate the attack

4 Here’s what else to consider

Network Security

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How can you identify the source of a SYN flood attack?

1

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4

1 Check the symptoms

2 Trace the packets

3 Mitigate the attack

4 Here’s what else to consider

Network Security

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