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rule.

3.5.3. IP Rule Actions

A rule consists of two parts: the filtering parameters and the action to take if there is a match with
those parameters. As described above, the parameters of any NetDefendOS rule, including IP rules
are:

•

Source Interface

•

Source Network

•

Destination Interface

•

Destination Network

•

Service

When an IP rule is triggered by a match then one of the following Actions can occur:

Allow

The packet is allowed to pass. As the rule is applied to only the opening of a
connection, an entry in the "state table" is made to record that a connection is open.
The remaining packets related to this connection will pass through the NetDefendOS
"stateful engine".

FwdFast

Let the packet pass through the NetDefend Firewall without setting up a state for it in
the state table. This means that the stateful inspection process is bypassed and is
therefore less secure than Allow or NAT rules. Packet processing time is also slower
than Allow rules since every packet is checked against the entire rule set.

NAT

This functions like an Allow rule, but with dynamic address translation (NAT) enabled
(see Section 7.2, “NAT” in Chapter 7, Address Translation for a detailed description).

SAT

This tells NetDefendOS to perform static address translation. A SAT rule always
requires a matching Allow, NAT or FwdFast IP rule further down the rule set (see
Section 7.4, “SAT” in Chapter 7, Address Translation for a detailed description).

Drop

This tells NetDefendOS to immediately discard the packet. This is an "impolite"
version of Reject in that no reply is sent back to the sender. It is often preferable since
it gives a potential attacker no clues about what happened to their packets.

Reject

This acts like Drop but will return a TCP RST or ICMP Unreachable message,
informing the sending computer that the packet was dropped. This is a "polite" version
of the Drop IP rule action.

Reject is useful where applications that send traffic wait for a timeout to occur before
realizing that the traffic was dropped. If an explicit reply is sent indicating that the
traffic was dropped, the application need not wait for the timeout.

Bi-directional Connections

A common mistake when setting up IP Rules is to define two rules, one rule for traffic in one
direction and another rule for traffic coming back in the other direction. In fact nearly all IP Rules
types allow bi-directional traffic flow once the initial connection is set up. The Source Network
and Source Interface in the rule means the source of the initial connection request. If a connection
is permitted and then becomes established, traffic can flow in either direction over it.

The exception to this bi-directional flow is FwdFast rules. If the FwdFast action is used, the rule
will not allow traffic to flow from the destination back to the source. If bi-directional flow is
required then two FwdFast rules are needed, one for either direction. This is also the case if a
FwdFast rule is used with a SAT rule.

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