LwIP Application Developers Manual5---高层协议之DNS

1.前言

lwIP提供一个基本的DNS客户端（1.3.0后引进），通过使用DNS(Domain Name System)协议来允许应用程序解决主机名到地址的转换。

在文件lwipopts.h里面定义LWIP_DNS的值为非零值可以使能DNS。

如果DHCP与lwIP DNS客户端一起工作，那么DNS将会自动被配置使用提供的DNS服务器（如果DHCP提供一个）。

2.Application DNS requests with Raw/Native API

Raw API应用可以使用dns_gethostbyname()函数来请求一次查找，并指定一个回调函数当查找结束时来通知应用程序。

你可以通过如下来阅读函数头部，该函数会马上返回。如果请求的地址已经知道，那么该函数会马上通过指针参数返回。

当一个DNS服务器的请求结束时，你的回调函数将会被调用

* 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);

A sample call:

struct ip_addr resolved;

  switch(dns_gethostbyname("www.lwIP.com", &resolved, smtp_serverFound, NULL)){
  case ERR_OK:
    // numeric or cached, returned in resolved
    smtp.serverIP.addr = resolved->addr;
    smtp.state = SMTP_NAME_RESOLVED;
    break;
  case ERR_INPROGRESS:
    // need to ask, will return data via callback
    smtp.state = SMTP_NAME_RESOLVING;
    break;
  default:
    // bad arguments in function call
    break;
  }

A sample DNS callback function:

void smtp_serverFound(const char *name, struct ip_addr *ipaddr, void *arg)
{
  if ((ipaddr) && (ipaddr->addr))
  {
    smtp.serverIP.addr = ipaddr->addr;
    smtp.state = SMTP_NAME_RESOLVED;
    if (smtp_connect() == ERR_OK)
      return;
    smtp.lastError = SMTP_CONNECT_FAILED;
  }
  else
    smtp.lastError = SMTP_UNKNOWN_HOST;
  smtp.state = SMTP_IDLE;
}

3.Application DNS requests with Netconn API

err_t netconn_gethostbyname(const char *name, ip_addr_t *addr)

See also netconn_gethostbyname().

4.Application DNS requests with Sockets API

For socket based apps, a gethostbyname() wrapper function is provided that blocks during the lookup if necessary. Use the following functions:

• gethostbyname() - standard implementation, blocks if necessary until lookup complete or fails
• gethostbyname_r() - thread-safe version of gethostbyname (separate buffer for each invokation)

5.External References

DNS-related RFCs

6.Revisions

• Local static host lookup table support (added in 1.3.1)
• Return multiple IP addresses (future)
• Return other RR (record) types e.g. MX, SRV... (future)
• implement a "Dynamic DNS update" message (future)