D-Link NetDefend DFL-1660, User Manual

Page: 142 / 595

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.
Note: Some actions alter TCP sequence numbers
In some situations with certain types of network equipment, the TCP sequence number
needs to remain the same as data traffic traverses the firewall.
It is therefore important to know that only the FwdFast action guarantees that the
TCP sequence number is unaltered. Other IP rule actions, such as Allow and NAT
change the TCP sequence number as traffic flows through NetDefendOS.
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.
Using Reject
In certain situations the Reject action is recommended instead of the Drop action because a "polite"
reply is required from NetDefendOS. An example of such a situation is when responding to the
IDENT user identification protocol. Some applications will pause for a timeout if Drop is used and
Reject can avoid such processing delays.
3.6.4. Editing IP rule set Entries
After adding various rules to the rule set editing any rule can be achieved in the Web Interface by
right clicking on that line.
A context menu will appear with the following options:
Edit This allows the contents of the rule to be changed.
Delete
This will remove the rule permanently from the rule set.
Disable/Enable This allows the rule to be disabled but left in the rule set. While disabled the
rule set line will not affect traffic flow and will appear grayed out in the user
interface. It can be re-enabled at any time.
Move options The last section of the context menu allows the rule to be moved to a
different position in the rule set and therefore have a different precedence
3.6.4. Editing IP rule set Entries Chapter 3. Fundamentals
142