D-Link DFL-1600 - Security Appliance User Manual Download

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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 its 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 a variety of
other functions.

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 through which network traffic enters or leaves the NetDefend Firewall.
Without interfaces, a NetDefendOS system has no means for receiving or sending traffic.

The following types of interface are supported in NetDefendOS:

•

Physical interfaces - These correspond to the actual physical Ethernet interfaces.

•

Sub-interfaces - These include VLAN and PPPoE interfaces.

•

Tunnel interfaces - Used for receiving and sending traffic through 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 predefined 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 which represent 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.

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