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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 Resolver

Domain Name System (DNS) servers store and manage information about domains and respond to resolution requests for clients (in some cases millions of times each day). The DNS database is a distributed name database stored on many DNS servers. DNS uses a hierarchical tree structure for its name space and a hierarchical tree for name authorities and registration.

Since information in DNS is stored in a distributed form, there is no single server that has information about every domain in the system. The process of resolution instead relies on the hierarchy of name servers as described above.

At the top of the DNS hierarchy is the root domain and the root name servers. These are the most important servers because they maintain information about the top-level domains within the root. They also know the servers that can be used to resolve domains one level below them. Those servers can reference servers that are responsible for second-level domains. Thus, a DNS resolution requests might be sent to more than one server.

An Embedded DNS Resolver is capable of resolving the IP address of a host from the host's name. It does this by sending DNS requests to a DNS Server. The IP address of a DNS Server is specified in the configuration or can be obtained from the DHCP Server for the Local Area Network.

The Embedded DNS Resolver caches the resolved IP addresses. The length of time the resolved host IP address is kept in the local cache depends on the Time to Live (TTL) timeout. This is returned in an answering packet from the DNS Server. The next time a DNS is requested, the cache table is checked first. If a valid host is found, the IP address is resolved from the cache and no actual DNS request is sent to the DNS Server.

You must use the DNS Resolver when a remote host uses a Dynamic IP, which changes each time the remote host logs on to the internet.

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