NetComm NB712 manual Denial of Service Attack, Circuit Gateway, Application Gateway

2.1.2 Circuit Gateway

Also called a “Circuit Level Gateway,” this is a firewall approach that validates connections before allowing data to be exchanged. What this means is that the firewall doesn’t simply allow or disallow packets but also determines whether the connection between both ends is valid according to configurable rules, then opens a session and permits traffic only from the allowed source and possibly only for a limited period of time.

Level 5: Application

Level 4: TCP

Level 3: IP

Level 2: Data Link

Destination IP address and/ or source IP address and/or time of day

protocol user password

Level 1: Physical

2.1.3 Application Gateway

The Application Level Gateway acts as a proxy for applications, performing all data exchanges with the remote system on their behalf. This can render a computer behind the firewall all but invisible to the remote system. It can allow or disallow traffic according to very specific rules; permitting some commands to a server but not others, limiting file access to certain types, varying rules according to authenticated users and so forth. This type of firewall may also perform very detailed logging of traffic and monitoring of events on the host system, and can often be instructed to sound alarms or notify an operator under defined conditions. Application-level gateways are generally regarded as the most secure type of firewall.

Level 5: Application
Level 4: TCP	Telnet
	FTP
Level 3: IP	HTT:
	SMTP
Level 2: Data Link
Level 1: Physical

2.2 Denial of Service Attack

Denial of service (DoS) attacks typically come in two varieties: resource starvation and resource overload. DoS attacks can occur when there is a legitimate demand for a resource that is greater than the supply (i.e. too many web requests to an already overloaded web server). Software vulnerability or system misconfigurations can also cause DoS situations. The difference between a malicious denial of service and simple system overload is the requirement of an individual with malicious intent (attacker) using or attempting to use resources specifically to deny those resources to other users.

10	NB712 / NB714 User Guide
	YML829 Rev1