RL-ARM User's Guide

DNS Client

The DNS Client allows you to resolve IP addresses of internet hosts that are identified by a host name. The address is resolved using a protocol in which a piece of information is sent by a client process (executing on the local computer) to a server process (executing on a remote computer). The address resolution procedure is completed when the client receives a response from the server containing the required address.

• DNS Client switch enables or disables the DNS Client service in your application. It is enabled when this value is set to 1. It should be set to 0 when the DNS Client is not used.

  #define DNS_ENABLE     1
  

• Cache Table size specifies the size of the DNS Cache by defining the number of entries for the DNS Cache table. When the IP address is resolved, it is also stored to the local cache.

  When a request for resolving an IP address is received, the DNS Client first checks the local cache memory. If a valid entry is found there, the IP address is taken from the cache, and the request is not sent on to the remote DNS Server.

  #define DNS_TABSIZE    20
  

Note

• DNS Cache entries expire after a Time to Live (TTL) timeout. This is defined by the DNS Server. The TTL value for resolved IP addresses is received in an answer packet from the DNS Server. The DNS Client manages the timeouts. When a timeout counter expires, the DNS Cache entry is deleted from the Cache.
• When the DNS client is not enabled, the ARM linker does not link the DNS support modules to your application and thus reduces the code size and memory usage.