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About TCP SYN packets & How to Protect Your Server from SYN Flood Attacks

SYN flood attacks are a type of denial-of-service (DoS) attack that aim to exhaust the resources of a server by sending a large number of TCP SYN packets with fake or spoofed source IP addresses or ports. The server responds to each SYN packet with a SYN-ACK packet, but never receives the final ACK packet from the client, leaving many half-open connections that consume memory and CPU. As a result, the server becomes unable to accept new legitimate connections and provide normal services.

In this blog post, we will explain how SYN-ACK packets work, how they are used in SYN flood attacks, and how you can defend your server from such attacks.

What are SYN-ACK packets?

SYN-ACK packets are part of the TCP three-way handshake process, which is used to establish a reliable connection between a client and a server. The TCP three-way handshake consists of the following steps:

  • The client sends a SYN packet to the server, indicating its intention to start a communication and providing its initial sequence number.
  • The server replies with a SYN-ACK packet, acknowledging the client’s SYN packet and providing its own initial sequence number.
  • The client sends an ACK packet to the server, confirming the receipt of the server’s SYN-ACK packet and completing the handshake.

The SYN-ACK packet contains two flags: SYN and ACK. The SYN flag indicates that the packet is used to synchronize the sequence numbers between the client and the server. The ACK flag indicates that the packet is used to acknowledge the previous packet received from the other party. The SYN-ACK packet also contains the sequence number of the server and the acknowledgment number of the client.

How are SYN-ACK packets used in SYN flood attacks?

In a SYN flood attack, the attacker sends a large number of SYN packets to the server, either with fake or spoofed source IP addresses or ports, or with real source IP addresses but using tools that do not respond to SYN-ACK packets. The server responds to each SYN packet with a SYN-ACK packet, expecting to receive an ACK packet from the client. However, since the ACK packet is never sent or received, the server keeps waiting for it until the connection times out. This creates a large number of half-open connections that occupy the server’s resources and prevent it from accepting new legitimate connections.

How to defend against SYN flood attacks?

There are several ways that you can protect your network and server from SYN flood attacks, such as:

  • Firewall configuration: You can configure your firewall to limit the incoming traffic and block the incoming connections from suspicious or known malicious IP addresses or ports. You can also use rate-limiting or SYN proxy techniques to filter out excessive or invalid SYN packets.
  • SYN cookies: SYN cookies are a special type of cookie that is used to track SYN requests. When a server receives a SYN packet, it will send back a SYN-ACK packet with a SYN cookie to the client. The SYN cookie is a cryptographic hash that encodes the server’s sequence number and some other information. The server does not need to store any state information for the connection until it receives the ACK packet from the client. The ACK packet must contain the same SYN cookie that the server sent. The server can then verify the SYN cookie and reconstruct the state information for the connection. This way, the server can avoid creating half-open connections and save resources.
  • Network equipment upgrade: You can upgrade your network equipment, such as routers, switches, and load balancers, to have more memory and processing power to handle the increased traffic and connections. You can also use clustering or load balancing techniques to distribute the traffic and connections among multiple servers.
  • Monitoring tools: You can use commercial or open-source monitoring tools to detect and analyze the traffic and connections on your network and server. You can also set up alerts and notifications to inform you of any abnormal or suspicious activity or behavior. You can then take appropriate actions to mitigate or stop the attack.

SYN flood attacks are one of the most common and dangerous types of DoS attacks that can severely affect the performance and availability of your server. By understanding how SYN-ACK packets work and how they are used in SYN flood attacks, you can better prepare and protect your server from such attacks. You can also use the methods and tools mentioned above to enhance your security and resilience against SYN flood attacks.


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