Junos services support a limited set of IDSs to help detect attacks such as port scanning and anomalies in traffic patterns. It also supports some attack prevention by limiting the number of flows, sessions, and rates. In addition, it protects against SYN attacks by implementing a SYN cookie mechanism. Since the intrusion detection and prevention (IDP) service does not support higher-layer application signatures, we must examine another solution.
The IDP solution is really more of a monitoring tool than an actual prevention tool. So, how does Juniper make the IDP claim? One response is that protection against a SYN attack can be configured. To prevent a SYN attack, the router will operate as a type of SYN “proxy” and will utilize cookie values. Essentially, when this feature is turned on, the router will respond to the initial SYN packet with a SYN-ACK packet that contains a unique cookie value in the sequence number field. If the initiator responds with the same cookie in the sequence field, the TCP flow is accepted; if the responder does not respond or if it responds with the wrong cookie, the flow is dropped. To kick off this defense, we must configure a SYN cookie threshold.
To enable the SYN cookie defense, an IDS rule action must contain a threshold that indicates when the feature should be enabled and an MSS value to avoid having the router manage segmented fragments when acting as a SYN proxy:
 lab@PBR# set services
ids rule simple-ids term 1then syn-cookie ? Possible ...