D-Link DFL-2500 - Manual (Page 16)

1.2. NetDefendOS Architecture
1.2.1. State-based Architecture
The NetDefendOS architecture is centered around the concept of state-based connections.
Traditional IP routers or switches commonly inspect all packets and then perform forwarding
decisions based on information found in the packet headers. With this approach, packets are
forwarded without any sense of context which eliminates any possibility to detect and analyze
complex protocols and enforce corresponding security policies.
Stateful Inspection
NetDefendOS employs a technique called stateful inspection which means that it inspects and
forwards traffic on a per-connection basis. NetDefendOS detects when a new connection is being
established, and keeps a small piece of information or state in it's state table for the lifetime of that
connection. By doing this, NetDefendOS is able to understand the context of the network traffic,
which enables it to perform in-depth traffic scanning, apply bandwidth management and much
more.
The stateful inspection approach additionally provides high throughput performance with the added
advantage of a design that is highly scalable. The NetDefendOS subsystem that implements stateful
inspection will sometimes be referred to in documentation as the NetDefendOS state-engine.
1.2.2. NetDefendOS Building Blocks
The basic building blocks in NetDefendOS are interfaces, logical objects and various types of rules
(or rule sets).
Interfaces
Interfaces are the doorways for network traffic passing through, to or from the system. Without
interfaces, a NetDefendOS system has no means for receiving or sending traffic. Several types of
interfaces are supported; Physical Interfaces, Physical Sub-Interfaces and Tunnel Interfaces.
Physical interfaces corresponds to actual physical Ethernet ports; physical sub-interfaces include
VLAN and PPPoE interfaces while tunnel interfaces are used for receiving and sending traffic in
VPN tunnels.
Interface Symmetry
The NetDefendOS interface design is symmetric, meaning that the interfaces of the device are not
fixed as being on the "insecure outside" or "secure inside" of a network topology. The notion of
what is inside and outside is totally for the administrator to define.
Logical Objects
Logical objects can be seen as pre-defined building blocks for use by the rule sets. The address
book, for instance, contains named objects representing host and network addresses. Another
example of logical objects are services , representing specific protocol and port combinations. Also
important are the Application Layer Gateway (ALG) objects which are used to define additional
parameters on specific protocols such as HTTP, FTP, SMTP and H.323.
NetDefendOS Rule Sets
Finally, rules which are defined by the administrator in the various rule sets are used for actually
implementing NetDefendOS security policies. The most fundamental set of rules are the IP Rules,
which are used to define the layer 3 IP filtering policy as well as carrying out address translation and
server load balancing. The Traffic Shaping Rules define the policy for bandwidth management, the
IDP Rules control the behavior of the intrusion prevention engine and so on.
1.2. NetDefendOS Architecture Chapter 1. Product Overview
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