DNS

The lwIP stack provides a basic DNS client (introduced in 1.3.0) to allow other applications to resolve host names to addresses using the DNS (Dynamic Name Service) protocol. LWIP_DNS must be #defined in lwipopts.h to a nonzero value to enable DNS support in your lwIP platform.

Raw API applications can use the dns_gethostbyname() function to request a lookup, and specify a callback function to notify the application when the lookup is complete. For socket based apps, a gethostbyname() wrapper function is provided that blocks during the lookup if necessary.

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* err_t
* dns_gethostbyname(const char *hostname, struct ip_addr *addr, dns_found_callback found,
*                  void *callback_arg)

* Resolve a hostname (string) into an IP address.
* NON-BLOCKING callback version for use with raw API!!!
*
* Returns immediately with one of err_t return codes:
* - ERR_OK if hostname is a valid IP address string or the host
*   name is already in the local names table.
* - ERR_INPROGRESS enqueue a request to be sent to the DNS server
*   for resolution if no errors are present.
*
* @param hostname the hostname that is to be queried
* @param addr pointer to a struct ip_addr where to store the address if it is already
*             cached in the dns_table (only valid if ERR_OK is returned!)
* @param found a callback function to be called on success, failure or timeout (only if
*              ERR_INPROGRESS is returned!)
* @param callback_arg argument to pass to the callback function
*    callback function and argument defined as follows:
*     A function of this type must be implemented by the application using the DNS resolver.
*      @param name pointer to the name that was looked up.
*      @param ipaddr pointer to a struct ip_addr containing the IP address of the hostname,
*        or NULL if the name could not be found (or on any other error).
*      @param callback_arg a user-specified callback argument passed to dns_gethostbyname:

* typedef void (*dns_found_callback)(const char *name, struct ip_addr *ipaddr, void *callback_arg);

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Future enhancements planned:

• return multiple ip addresses
• return other RR types (MX, SRV...)
• implement a "dynamic dns update" message

[Category:lwIP Application Developers Guide]