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What are SYN cookies?

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What are SYN cookies?

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SYN cookies are an implementation of TCP that can respond to the TCP SYN request with a cookie. Following the descriptions above, in normal TCP implementation, when the server received a SYN packet, it responds with a SYN-ACK to acknowledge, and enter the TCP_SYN_RECV state (half-open connection) to wait the last ACK. The server uses a data structure describing all pending connections, and the data structure is of finite size. Therefore, the attacker may fill up the structure. In the SYN cookies implementation of TCP, when the server received a SYN packet, it responds a SYN-ACK packet with the ACK sequence number calculated from source address, source port, source sequence, destination address, destination port and a secret seed. Then the server releases state. If an ACK comes from the client, the server can recalculate it to determine if it is a response to the former SYN-ACK. If it is, the server can directly enter the TCP_ESTABLISHED state and open the connection. In this way, the s

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