Cisco Firewalls and Routers: Validating Source with SYN-ACK Packets

SYN-ACK Packet Validation for Cisco Firewalls and Routers

Prev Question Next Question


Cisco firewalls and routers can respond to a TCP SYN packet that is destined for a protected resource, by using a SYN-ACK packet to validate the source of the SYN packet.

What is this feature called?



Click on the arrows to vote for the correct answer

A. B. C. D.


The feature described in the question is TCP intercept.

TCP intercept is a security feature found in Cisco firewalls and routers that helps protect networks from TCP-based attacks by intercepting and validating incoming TCP connections. When a TCP SYN packet is received by the firewall or router, the TCP intercept feature sends a SYN-ACK packet back to the source IP address of the incoming packet, which is typically a client attempting to establish a connection with a server on the protected network. The client is required to respond with an ACK packet before the connection is established.

This process helps to validate the source of the SYN packet and ensure that it is a legitimate connection attempt rather than a potential attack or a spoofed packet. TCP intercept can also be configured to apply different actions based on the results of the validation, such as allowing the connection to proceed or dropping the packet.

In contrast, IP reverse path verification and TCP reverse path verification are both related to source address validation, but they work differently from TCP intercept. IP reverse path verification checks the source IP address of incoming packets against the routing table to determine whether the packet is arriving on the correct interface, while TCP reverse path verification checks the source IP address against the routing table and the TCP session state to ensure that the packet is part of an existing TCP connection. TCP sequence number randomization is another security feature that helps protect against TCP-based attacks by randomizing the sequence numbers used in TCP packets to make it more difficult for attackers to predict the next sequence number and hijack the connection.