syn-cookie (IDS MS-DPC)
Description
Enable SYN-cookie defenses against SYN attacks when using the MS-DPC. By default, SYN-cookie techniques are not applied.
When SYN cookie is enabled on Junos OS and becomes the TCP-negotiating proxy for the destination server, it replies to each incoming SYN segment with a SYN/ACK containing an encrypted cookie as its initial sequence number (ISN). The cookie is an MD5 hash of the original source address and port number, destination address and port number, and ISN from the original SYN packet. After sending the cookie, Junos OS drops the original SYN packet and deletes the calculated cookie from memory. If there is no response to the packet containing the cookie, the attack is noted as an active SYN attack and is effectively stopped.
If the initiating host responds with a TCP packet containing the cookie +1 in the TCP ACK field, Junos OS extracts the cookie, subtracts 1 from the value, and recomputes the cookie to validate that it is a legitimate ACK. If it is legitimate, Junos OS starts the TCP proxy process by setting up a session and sending a SYN to the server containing the source information from the original SYN. When Junos OS receives a SYN/ACK from the server, it sends ACKs to the server and to the initiation host. At this point the connection is established and the host and server are able to communicate directly.
Options
The remaining statements are described separately.
Required Privilege Level
interface—To view this statement in the configuration.
interface-control—To add this statement to the configuration.
Release Information
Statement introduced before Junos OS Release 7.4.