BinTec X4000, Руководство пользователя

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248 X4000 User’s Guide
Advanced Configuration of the Basic Unit with the Setup Tool
8
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 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
assigned the value Maximum TTL for Neg Cache Entries. A dynamic entry is
deleted from the cache when the TTL expires.
Static
name entries
You use positive static entries to enter names with the associated IP addresses
in X4000 . If you save frequently needed IP addresses in this way, X4000 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 X4000 . 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
corresponding 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 X4000 using a
filter function. This makes access to certain domains much more difficult for
users in the LAN, as it prevents the corresponding names being resolved. You
can use wildcards (*) when entering the name.
When you enter a static entry, you define how long this assignment of name and
IP address is valid by setting the TTL. This TTL is entered in each DNS record
with which X4000 answers a relevant DNS request.
You can easily change a dynamic entry to a static entry "at the press of a
button" in
IP
➧
DNS
➧
D
YNAMIC
C
ACHE
(see Table 8-43, page 258 ).