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12.3
DNS
BAT54-Rail/F..
Release
7.54
06/08
U How does the DNS server react to the request?
When processing requests for specific names, the DNS server takes advan-
tage of all of the information available to it:
D First, the DNS server checks whether access to the name is not prohibited
by the filter list. If that is the case, an error message is returned to the re-
questing computer stating that access to the address has been denied.
D Next, it searches in its own static DNS table for suitable entries.
D If the address cannot be found in the DNS table, it searches the dynamic
DHCP table. The use of DHCP information can be disabled if required.
D If no information on the name can be located in the previous tables, the
DNS server then searches the lists of the NetBIOS module. The use of
the NetBIOS information can also be disabled if necessary.
D Finally, the DNS server checks whether the request to another DNS serv-
er is to be forwarded to another DNS server via a WAN interface (special
DNS forwarding via the DNS destination table).
If the requested name cannot be found in any of the information sources
available to it, the DNS server sends the request to another server—that of
the Internet provider, for example—using the general DNS forwarding mech-
anism, or returns an error message to the requesting computer.
12.3.2 DNS forwarding
If it cannot serve the request from its own DNS tables, the DNS server for-
wards the request to other DNS servers. This process is called DNS forward-
ing.
Here a distinction is made between
D special DNS forwarding
Requests for certain name areas are forwarded to certain DNS servers.
D general DNS forwarding
All other names not specified in detail are forwarded to the “higher-
level” DNS server.
U Special DNS forwarding
With “special DNS forwarding” name areas can be defined for the resolution
of which specified DNS server are addressed.
A typical application for special DNS forwarding results for a home worksta-
tion: The user wants to be able to connect to the company intranet and direct-
ly to the Internet at the same time. The requests sent into the intranet must
be routed to the company DNS server, and all other requests to the DNS
server of the provider.
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