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. 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.
• Request Idle Time-To-Live: Specifies the period of time, in seconds, that a DNS client request will remain in the DNS Proxy cache, before it is deleted. This is a period of idle time, during which neither a DNS client request retry is received by the DNS Proxy, nor a DNS server response is received by the DNS Proxy, for a specific DNS client request. A shorter Idle TTL results in the DNS Proxy using resources more efficiently, since the client request cache is reduced in size and the request buffers are released more quickly for future use for other DNS client requests.

• Request Retries Per DNS Server: Specifies the number of retries using the same DNS server, for a specific DNS client request that is retried (retransmitted) by the DNS client. There is always one “try” but the number of retries is configurable.

  For new client requests received when the request cache is full:
  
  Specifies how to handle new client requests when the maximum number of client request entries is already being serviced (the request cache is full). There are two choices for this option:
  
  Replace the Least Recently Used (LRU) client request with the new request: Remove the least recently used entry from the cache, and add an entry for the new client request.

  Discard (ignore) new requests until some existing requests have expired:
  Silently discard the new client request, and do this for all future new requests until one or more entries have expired and been removed from the request cache.