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RL-ARM User's Guide (MDK v4)

RL-RTX RL-FlashFS RL-TCPnet TCP Socket Opening TCP Connection TCP Active Open TCP Passive Open Sending TCP Data Example for Sending Data Multiple TCP Connections UDP Socket Opening UDP Connection Sending UDP Data When DHCP Enabled When ARP Cache Empty Example for Sending Data IP Multicasting Multiple UDP Connections Configuring RL-TCPnet Static Configuration System Definitions Ethernet Network Interface PPP Network Interface SLIP Network Interface UDP Socket TCP Socket BSD Socket HTTP Server Telnet Server TFTP Server TFTP Client FTP Server FTP Client DNS Client SMTP Client SNMP Agent SNTP Client Error Function Runtime Configuration Library Files Using RL-TCPnet Stand Alone With RTX Kernel Event Driven Operation IP Address Assignment Ethernet Interface PPP Interface SLIP Interface Localhost Applications HTTP Server Script Language CGI Functions Ajax Support Using XML XML Example How it works SOAP Support SOAP Interface Large POST Messages Web Pages Default Page Error Pages Web on SD Card Web Update File System Interface Http Caching How it works Internal Web External Web Multi-user Authentication Using RAM File System FCARM File Converter PRINT Directive NOPRINT Directive PAGEWIDTH Directive PAGELENGTH Directive ROOT Directive Telnet Server Command Line Interface Multi-user Authentication Sending Reply Message Short Reply Long Reply Continuous Screen Update TFTP Server File System Interface TFTP Client File System Interface FTP Server File System Interface Multi-user Authentication Supported Commands FTP Client File System Interface SMTP Client SNMP Agent MIB Database MIB Interface MIB Entry MIB Table DNS Resolver Starting DNS Device Drivers Ethernet Driver Interrupt Mode Modem Driver Serial Driver Using Serial Link Cable Connection Modem Connection Windows Dial-up Add Direct Serial Link New Dial-up Connection Configure PPP Dial-up Configure SLIP Dial-up Debugging Enabling Debug Debug Level Redirecting Output Function Overview BSD Routines CGI Routines Ethernet Routines FTP Routines HTTP Routines IGMP Routines Miscellaneous Routines Modem Routines PPP Routines Serial Routines SLIP Routines SMTP Routines SNMP Routines System Functions TCP Routines Telnet Routines TFTP Routines UDP Routines RL-CAN RL-USB Example Programs Library Reference Appendix

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


  • 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.
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