IOS 下三种 DNS 解析方式分析（LocalDns）

1：struct hostent	*gethostbyname(const char *);
2：struct hostent	*gethostbyname2(const char *, int);

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

   char   *ptr, **pptr;
   struct hostent *hptr;
   char   str[32];
   ptr = "www.meitu.com";
   NSMutableArray * ips = [NSMutableArray array];

    if((hptr = gethostbyname(ptr)) == NULL)
   {
       return;
   }

   for(pptr=hptr->h_addr_list; *pptr!=NULL; pptr++) {
        NSString * ipStr = [NSString stringWithCString:inet_ntop(hptr->h_addrtype, *pptr, str, sizeof(str)) encoding:NSUTF8StringEncoding];
        [ips addObject:ipStr?:@""];
   }

   CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
   NSLog(@"22222 === ip === %@ === time cost: %0.3fs", ips,end - start);

struct hostent {
	char	*h_name;	/* official name of host */
	char	**h_aliases;	/* alias list */
	int	h_addrtype;	/* host address type */
	int	h_length;	/* length of address */
	char	**h_addr_list;	/* list of addresses from name server */
#if !defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE)
#define	h_addr	h_addr_list[0]	/* address, for backward compatibility */
#endif /* (!_POSIX_C_SOURCE || _DARWIN_C_SOURCE) */
};

int		res_query __P((const char *, int, int, u_char *, int));
函数原型为：
int res_query(const char *dname, int class, int type, unsigned char *answer, int anslen)

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

    unsigned char auResult[512];
    int nBytesRead = 0;

    nBytesRead = res_query("www.meitu.com", ns_c_in, ns_t_a, auResult, sizeof(auResult));

    ns_msg handle;
    ns_initparse(auResult, nBytesRead, &handle);

    NSMutableArray *ipList = nil;
    int msg_count = ns_msg_count(handle, ns_s_an);
    if (msg_count > 0) {
        ipList = [[NSMutableArray alloc] initWithCapacity:msg_count];
        for(int rrnum = 0; rrnum < msg_count; rrnum++) {
            ns_rr rr;
            if(ns_parserr(&handle, ns_s_an, rrnum, &rr) == 0) {
                char ip1[16];
                strcpy(ip1, inet_ntoa(*(struct in_addr *)ns_rr_rdata(rr)));
                NSString *ipString = [[NSString alloc] initWithCString:ip1 encoding:NSASCIIStringEncoding];
                if (![ipString isEqualToString:@""]) {

                    //将提取到的IP地址放到数组中
                    [ipList addObject:ipString];
                }
            }
        }
        CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
        NSLog(@"11111 === ip === %@ === time cost: %0.3fs", ipList,end - start);
    }

Boolean CFHostStartInfoResolution (CFHostRef theHost, CFHostInfoType info, CFStreamError *error);

Boolean result,bResolved;
CFHostRef hostRef;
CFArrayRef addresses = NULL;
NSMutableArray * ipsArr = [[NSMutableArray alloc] init];

CFStringRef hostNameRef = CFStringCreateWithCString(kCFAllocatorDefault, "www.meitu.com", kCFStringEncodingASCII);

hostRef = CFHostCreateWithName(kCFAllocatorDefault, hostNameRef);
CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();
result = CFHostStartInfoResolution(hostRef, kCFHostAddresses, NULL);
if (result == TRUE) {
    addresses = CFHostGetAddressing(hostRef, &result);
}
bResolved = result == TRUE ? true : false;

if(bResolved)
{
    struct sockaddr_in* remoteAddr;
    for(int i = 0; i < CFArrayGetCount(addresses); i++)
    {
        CFDataRef saData = (CFDataRef)CFArrayGetValueAtIndex(addresses, i);
        remoteAddr = (struct sockaddr_in*)CFDataGetBytePtr(saData);

        if(remoteAddr != NULL)
        {
            //获取IP地址
            char ip[16];
            strcpy(ip, inet_ntoa(remoteAddr->sin_addr));
            NSString * ipStr = [NSString stringWithCString:ip encoding:NSUTF8StringEncoding];
            [ipsArr addObject:ipStr];
        }
    }
}
CFAbsoluteTime end = CFAbsoluteTimeGetCurrent();
NSLog(@"33333 === ip === %@ === time cost: %0.3fs", ipsArr,end - start);
CFRelease(hostNameRef);
CFRelease(hostRef);

/*
 *  CFHostStartInfoResolution()
 *  
 *  Discussion:
 *	Performs a lookup for the given host.  It will search for the
 *	requested information if there is no other active request.
 *	Previously cached information of the given type will be released.
 *  
 *  Mac OS X threading:
 *	Thread safe
 *  
 *  Parameters:
 *
 *	theHost:  //需要被解决的CFHostRef的对象
 *	  The CFHostRef which should be resolved. Must be non-NULL. If
 *	  this reference is not a valid CFHostRef, the behavior is
 *	  undefined.
 *
 *	info: 返回值的类型 数组/Data/string..
 *	  The enum representing the type of information to be retrieved.
 *	  If the value is not a valid type, the behavior is undefined.
 *
 *	error: 错误
 *	  A reference to a CFStreamError structure which will be filled
 *	  with any error information should an error occur.  May be set
 *	  to NULL if error information is not wanted.
 *  
 *  Result: 解析结果成功还是失败
 *	Returns TRUE on success and FALSE on failure.  In asynchronous
 *	mode, this function will return immediately.  In synchronous
 *	mode, it will block until the resolve has completed or until the
 *	resolve is cancelled.
 *  
 */
CFN_EXPORT __nullable CFArrayRef
CFHostGetAddressing(CFHostRef theHost, Boolean * __nullable hasBeenResolved) CF_AVAILABLE(10_3, 2_0);