The SYN flood attack sends TCP connections requests faster than a machine can process them.
Attacker creates a random source address for each packet
SYN flag set in each packet is a request to open a new connection to the server from the
spoofed IP address
Victim responds to spoofed IP address,then waits for confirmation that never arrives (timeout
wait is about 3 minutes)
Victim’s connection table fills up waiting for replies and ignores new connections
Legitimate users are ignored and will not be able to access the server
How do you protect your network against SYN Flood attacks?

A.
SYN cookies. Instead of allocating a record,send a SYN-ACK with a carefully constructed
sequence number generated as a hash of the clients IP address,port number,and other
information. When the client responds with a normal ACK,that special sequence number will
beincluded,which the server then verifies. Thus,the server first allocates memory on the third
packet of the handshake,not the first.

B.
RST cookies – The server sends a wrong SYN/ACK back to the client. The client should then
generate a RST packet telling the server that something is wrong. At this point,the server knows
the client is valid and will now accept incoming connections from that client normally

C.
Check the incoming packet’s IP address with the SPAM database on the Internet and enable
the filter using ACLs at the Firewall

D.
Stack Tweaking. TCP stacks can be tweaked in order to reduce the effect of SYN floods.
Reduce the timeout before a stack frees up the memory allocated for a connection

E.
Micro Blocks. Instead of allocating a complete connection,simply allocate a micro record of 16-bytes for the incoming SYN object