User`s guide

Configuration through the web interface
99
DNS Proxy Settings
 Enable DNS Proxy Service: Enables the DNS Proxy feature on this Digi device. DNS
Proxy permits DNS client hosts to communicate with this Digi device as if it were a
DNS Server. It forwards the DNS client's request to one of the DNS servers configured
in its network settings. The response from the actual DNS server will be relayed to the
requesting client when it is received by the DNS Proxy. The DNS Proxy does not cache
the actual detailed client requests nor the responses received from the DNS servers.
Rather, it acts as a request/response relay agent between the DNS clients and servers.
The DNS Proxy will cycle through the DNS servers that are configured in the Digi
device. DNS client requests are identified by the client's IP address and the unique
Query ID in the DNS request message. For each new DNS client request (new Query
ID), the DNS Proxy uses the first DNS server in its list of DNS servers. If the client
retries the same request (same Query ID), the DNS Proxy will recognize that retry
message and will either send the retry request to the same DNS server as the previous
request for this client, or it will move to the next DNS server in its list of DNS servers.
The DNS Proxy feature determines when to retry the same DNS server, or move to the
next DNS server, according to the DNS Proxy: Request Retries Per DNS Server
configuration setting (see below). The DNS Proxy itself does not perform unsolicited
retries of DNS client requests.
Note The DHCP Server feature on the Digi device may be configured to use the DNS
Proxy feature. For more information, see "DHCP server settings" on page 64. The
DNS server list may be dynamic in its content. For example, when DNS server IP
addresses are received from a mobile service provider's network, they are added to
the DNS server list of this Digi device. Those DNS server IP addresses may or may
not be configured when the DHCP Server offers a lease to a DHCP client. As a
result, the DHCP client may have no DNS servers provided to it in the lease, and
domain name resolution may fail for that client. A significant benefit of the DNS
Proxy feature is that the DHCP Server can offer its own IP address as a DNS server
in the client lease, and the DNS Proxy will forward DNS requests and responses as
stated above. Since the DHCP protocol does not allow a DHCP Server to force an
unsolicited DNS server list update to its clients, the DNS Proxy feature provides an
indirect method by which such updates may be made effective for the client.
 Request Cache Size Maximum: Specifies the maximum number of DNS client request
records that the DNS Proxy will maintain concurrently in its cache. A large cache
consumes more system resources than does a small cache. However, if the maximum
cache size is too small, new DNS client requests may be quietly discarded until the
cache has room to add new client request records, or existing cache entries may be
replaced by the new requests. If a large number of concurrent DNS client lookups is
anticipated, configuring a larger maximum cache size is recommended. See also the
setting For new client requests received when the request cache is full below.