Policy-Based Routing With Multiple Isps - D-Link NetDefendOS User Manual

5. At this point, using the routing table selected, the actual route lookup is done to find the
packet's destination interface. Note that the routing table's Ordering property is used to
determine how the actual lookup is done and the options for this are described in the next
section. To implement virtual systems, the property should be set to the value Only.
6. The connection is then subject to the normal IP rule set. If a SAT rule is encountered, address
translation will be performed. The decision of which routing table to use is made before
carrying out address translation but the actual route lookup is performed on the altered
address. Note that the original route lookup to find the destination interface used for all rule
look-ups was done with the original, untranslated address.
7. If allowed by the IP rule set, the new connection is opened in the NetDefendOS state table
and the packet forwarded through this connection.
The Ordering parameter
Once the routing table for a new connection is chosen and that table is an alternate routing
table, the Ordering parameter associated with the table is used to decide how the alternate table
is combined with the main table to lookup the appropriate route. The three available options
are:
1. Default
The default behavior is to first look up the route in the main table. If no matching route is
found, or a default route is found (a route with the destination all-nets), a lookup for a
matching route in the alternate table is performed. If no match is found in the alternate
table then the default route in the main table will be used.
2. First
This behavior is to first look up the connection's route in the alternate table. If no matching
route is found there then the main table is used for the lookup. The default all-nets route
will be counted as a match in the alternate table if it is found there.
3. Only
This option ignores the existence of any other table except the alternate table so that is the
only one used for the lookup.
One application of this option is to give the administrator a way to dedicate a single routing
table to one set of interfaces. The Only option should be used when creating virtual systems
since it can dedicate a routing table to a set of interfaces.
The first two options can be regarded as combining the alternate table with the main table and
assigning one route if there is a match in both tables.
Important: Ensure an all-nets route appears in the main table
A common mistake when setting up policy-based routing is the absence of a default
all-nets route in the main routing table. If there is no matching route in main, the traffic
will be dropped, unless the receiving interface is a member of a specific routing table.
This is explained further under the heading "The Routing Table Selection Process" above.
Example 4.8. Policy-based Routing with Multiple ISPs
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Chapter 4: Routing