BinTec X3200, Руководство пользователя

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X3200 User’s Guide 231
Basic IP settings
6
DNS cache If a DNS request is passed by X3200 to a DNS and this DNS answers with a
DNS record, the resolved name is saved with the associated IP address as a
positive dynamic entry in the DNS cache of X3200 . This means that once a
name has been resolved and is required again, X3200 can answer the request
from the cache and a new request to an external name server is not necessary.
These requests can therefore be answered more quickly, bandwidth is reduced
on the WAN connections and the costs of unnecessary connections are saved.
If a DNS request cannot be answered by any of the DNS asked, this is saved in
the cache as a negative dynamic entry. As failed DNS requests (requests that
cannot be answered) are not usually saved by applications or IP stacks, these
negative dynamic entries saved in the cache prevent frequent unsuccessful
connection setups to external DNS.
The validity of the positive dynamic entries in the cache is given by the TTL
(Time To Live), which is contained in the DNS record. Negative entries are as-
signed the value Maximum TTL for Neg Cache Entries. A dynamic entry is de-
leted from the cache when the TTL expires.
Static name entries You use positive static entries to enter names with the associated IP addresses
in X3200 . If you save frequently needed IP addresses in this way, X3200 can
answer relevant DNS requests itself and the connection to an external name
server is not necessary. This speeds up access to these addresses. For a small
network, such a name server can be configured in X3200 . The installation of a
separate DNS and the tedious updating of HOSTS files on the PCs in the LAN
is not necessary.
With negative static entries, a name is not assigned an IP address, a corre-
sponding DNS request is answered negatively and not passed to any other
name server either.
Filter function By using negative static entries, you can limit name resolution in X3200 using a
filter function. This makes access to certain domains much more difficult for us-
ers in the LAN, as it prevents the corresponding names being resolved. You can
use wildcards (*) when entering the name.
You can easily change a dynamic entry to a static entry "at the press of a but-
ton" in
IP
➧
DNS
➧
D
YNAMIC
C
ACHE
(see table 6-43, page 240 ).