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这个问题纠结了很久，子域名下ns为添加子域名授权dns服务器，以为域名本身@的ns记录是ns再授权，使用过程中并非如此，在其它www.snooda.com/read/278看到一段描述，暂且转过来。
很多顶级域的dns都会有@域的ns记录指向自己，之前理解有误，其实这些ns记录是对二级域生效的。这样设置是表示二级域和顶级域由相同的ns服务器负责解析。即使不设置这个记录，顶级域dns服务器也是可以解析子域的，但顶级域的ns记录缓存时间是不可控的（设置时没有ttl），有可能很快过期，这样的话解析二级域时就需要多次去com域dns上查询，而有了ns记录，可以自己控制ttl，在ttl没有过期的时间内，查询时可以直接命中ns记录去查询二级域，有效提高速度。

理论上顶级域dns只负责解析顶级域自身的a、mx、cname、ns记录等，其他二级域记录需要顶级域的ns记录指定的dns来解析，而一般大家使用dns托管的话，二级域名和顶级域往往是一起管理的，所以ns是指向自身的。

<?php
class DomainAge{
  private $WHOIS_SERVERS=array(
    "com"               =>  array("whois.verisign-grs.com","/Creation Date:(.*)/"),
    "net"               =>  array("whois.verisign-grs.com","/Creation Date:(.*)/"),
    "org"               =>  array("whois.pir.org","/Created On:(.*)/"),
    "info"              =>  array("whois.afilias.info","/Created On:(.*)/"),
    "biz"               =>  array("whois.neulevel.biz","/Domain Registration Date:(.*)/"),
    "us"                =>  array("whois.nic.us","/Domain Registration Date:(.*)/"),
    "uk"                =>  array("whois.nic.uk","/Registered on:(.*)/"),
    "ca"                =>  array("whois.cira.ca","/Creation date:(.*)/"),
    "tel"               =>  array("whois.nic.tel","/Domain Registration Date:(.*)/"),
    "ie"                =>  array("whois.iedr.ie","/registration:(.*)/"),
    "it"                =>  array("whois.nic.it","/Created:(.*)/"),
    "cc"                =>  array("whois.nic.cc","/Creation Date:(.*)/"),
    "ws"                =>  array("whois.nic.ws","/Domain Created:(.*)/"),
    "sc"                =>  array("whois2.afilias-grs.net","/Created On:(.*)/"),
    "mobi"              =>  array("whois.dotmobiregistry.net","/Created On:(.*)/"),
    "pro"               =>  array("whois.registrypro.pro","/Created On:(.*)/"),
    "edu"               =>  array("whois.educause.net","/Domain record activated:(.*)/"),
    "tv"                =>  array("whois.nic.tv","/Creation Date:(.*)/"),
    "travel"            =>  array("whois.nic.travel","/Domain Registration Date:(.*)/"),
    "in"                =>  array("whois.inregistry.net","/Created On:(.*)/"),
    "me"                =>  array("whois.nic.me","/Domain Create Date:(.*)/"),
    "cn"                =>  array("whois.cnnic.cn","/Registration Date:(.*)/"),
    "asia"              =>  array("whois.nic.asia","/Domain Create Date:(.*)/"),
    "ro"                =>  array("whois.rotld.ro","/Registered On:(.*)/"),
    "aero"              =>  array("whois.aero","/Created On:(.*)/"),
    "nu"                =>  array("whois.nic.nu","/created:(.*)/")
  );
  public function age($domain)
  {
    $domain = trim($domain); //remove space from start and end of domain
    if(substr(strtolower($domain), 0, 7) == "http://") $domain = substr($domain, 7); // remove http:// if included
    if(substr(strtolower($domain), 0, 4) == "www.") $domain = substr($domain, 4);//remove www from domain
    if(preg_match("/^([-a-z0-9]{2,100})\.([a-z\.]{2,8})$/i",$domain))
    {
      $domain_parts = explode(".", $domain);
      //取后缀
      $tld = strtolower(array_pop($domain_parts));
      if(!$server=$this->WHOIS_SERVERS[$tld][0]) {
        return false;
      }
      $res=$this->queryWhois($server,$domain);
      if(preg_match($this->WHOIS_SERVERS[$tld][1],$res,$match))
      {
        date_default_timezone_set('UTC');
        $time = time() - strtotime($match[1]);
        $years = floor($time / 31556926);
        $days = floor(($time % 31556926) / 86400);
        if($years == "1") {$y= "1 year";}
        else {$y = $years . " years";}
        if($days == "1") {$d = "1 day";}
        else {$d = $days . " days";}
        return "$y, $d";
      }
      else
        return false;
    }
    else
      return false;
  }
  private function queryWhois($server,$domain)
  {
    $fp = @fsockopen($server, 43, $errno, $errstr, 20) or die("Socket Error " . $errno . " - " . $errstr);
    if($server=="whois.verisign-grs.com")
      $domain="=".$domain;
    //echo $domain;
    fputs($fp, $domain . "\r\n");
    $out = "";
    while(!feof($fp)){
      $out .= fgets($fp);
    }
    fclose($fp);
    //echo $out;
    return $out;
  }
}
$w=new DomainAge();
$age = $w->age("baidu.com");
echo $age."\n";
?>

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