iOS 获取本机DNS服务器，根据域名获取IP

/// 获取本机DNS服务器
- (NSString *)outPutDNSServers
{
    res_state res = malloc(sizeof(struct __res_state));

    int result = res_ninit(res);

    NSMutableArray *dnsArray = @[].mutableCopy;

    if ( result == 0 )
    {
        for ( int i = 0; i < res->nscount; i++ )
        {
            NSString *s = [NSString stringWithUTF8String :  inet_ntoa(res->nsaddr_list[i].sin_addr)];

            [dnsArray addObject:s];
        }
    }
    else{
        NSLog(@"%@",@" res_init result != 0");
    }

    res_nclose(res);

    return dnsArray.firstObject;
}

/// 根据域名获取IP地址
- (NSString*)getIPWithHostName:(const NSString*)hostName
{
    const char *hostN= [hostName UTF8String];

    // 记录主机的信息，包括主机名、别名、地址类型、地址长度和地址列表 结构体
    struct hostent *phot;

    @try {
        // 返回对应于给定主机名的包含主机名字和地址信息的hostent结构指针
        phot = gethostbyname(hostN);

        struct in_addr ip_addr;

        memcpy(&ip_addr, phot->h_addr_list[0], 4);

        char ip[20] = {0};

        inet_ntop(AF_INET, &ip_addr, ip, sizeof(ip));

        NSString* strIPAddress = [NSString stringWithUTF8String:ip];

        return strIPAddress;

    }
        @catch (NSException *exception) {
            return nil;
    }
}