The server accepts the SYN, adds an entry to its connection table and sends back a SYN-ACK. However, the sending client(s) will not respond to the SYN-ACK which leaves the connection half-open. The entry in the server’s connection table remains until it times out (normally taking up to a minute) and thus sending a large number of SYNs in a short time period can exhaust a server’s memory.
Delayed binding can help protect real servers from this issue, where proxies are concerned, by ensuring a client-side connection is correctly established before establishing one server-side.
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