For example, the disabling of packet retransmissions in some environments with low bandwidth can cause a legitimate request to fail. So, after the modification we have to make sure that our server can properly communicate with other hosts. In some cases the values can be too strict. ![]() We should remember that our modification of variables will change the behavior of the TCP/IP stack. However, we can still increase the likelihood of creating a full connection with legitimate clients by performing the above operations. Note that an attacker can simply send more packets with the SYN flag set and then the above tasks will not solve the problem. A Packet with the ACK flag finalizes the process of the three-way handshake. The process of packet retransmissions is performed by a server when it doesn't receive an ACK packet from a client. This method is accomplished by decreasing the time of the first packet retransmission and by either decreasing the number of packet retransmissions or by turning off packet retransmissions entirely. Decreasing the time period of keeping a pending connection in the SYN RECEIVED state in the queue.Increasing the queue of half-open connections (in the SYN RECEIVED state).Changing the default values of stack variables can be another layer of protection and help better secure your hosts. Additionally, in some cases it is worth tuning parameters of the TCP/IP stack. What can an administrator do when his servers are under a classic, non-bandwidth flooding SYN attack? One of most important steps is to enable the operating system's built-in protection mechanisms like SYN cookies or SynAttackProtect. This kind of flood is outside the scope of scope of this article, as is the filtering of packets which has been discussed elsewhere. It should be noted that some SYN attacks do not always attempt to upset servers, but instead try to consume all of the bandwidth of your Internet connection. ![]() While SYN attacks may not be entirely preventable, tuning the TCP/IP stack will help reduce the impact of SYN attacks while still allowing legitimate client traffic through. This method, the tuning of the TCP/IP stack in various operating systems, will be described in depth in this article. In addition to creating packet filters, the modification of the TCP/IP stack of a given operating system can be performed by an administrator. In almost every case proper filtering of packets is a viable solution. Several methods, more or less effective, are usually used. By Mariusz Burdach Most people know how problematic protection against SYN denial of service attacks can be.
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