How SYN cookies mitigate SYN flooding attacks?
SYN cookies is a technical attack mitigation technique whereby the server replies to TCP SYN requests with crafted SYN-ACKs, without inserting a new record to its SYN Queue. Only when the client replies this crafted response a new record is added.
Do SYN cookies manipulate TCP protocol?
The use of SYN cookies does not break any protocol specifications, and therefore should be compatible with all TCP implementations.
How are SYN cookies used in the Internet?
SYN cookies. SYN cookie is a technique used to resist IP Spoofing attacks. The technique’s primary inventor Daniel J. Bernstein defines SYN cookies as “particular choices of initial TCP sequence numbers by TCP servers.”. In particular, the use of SYN cookies allows a server to avoid dropping connections when the SYN queue fills up.
What happens when SYN cookie queue fills up?
A server that uses SYN cookies doesn’t have to drop connections when its SYN queue fills up. Instead it sends back a SYN+ACK, exactly as if the SYN queue had been larger. (Exceptions: the server must reject TCP options such as large windows, and it must use one of the eight MSS values that it can encode.)
How are SYN cookies compliant with the TCP protocol?
Reality: SYN cookies are fully compliant with the TCP protocol. Every packet sent by a SYN-cookie server is something that could also have been sent by a non-SYN-cookie server. SYN cookies “do not allow to use TCP extensions” such as large windows. Reality: SYN cookies don’t hurt TCP extensions.
Why do we use sequence number in SYN cookies?
This choice of sequence number complies with the basic TCP requirement that sequence numbers increase slowly; the server’s initial sequence number increases slightly faster than the client’s initial sequence number. A server that uses SYN cookies doesn’t have to drop connections when its SYN queue fills up.