gethostbyname()

(PHP 4, PHP 5, PHP 7)

返回主机名对应的 IPv4地址。

说明

返回主机名$hostname对应的 IPv4 互联网地址。

参数

主机名

返回值

成功时返回 IPv4 地址，失败时原封不动返回$hostname字符串。

范例

简单的gethostbyname()例子

参见

• gethostbyaddr() 获取指定的IP地址对应的主机名
• gethostbynamel() 获取互联网主机名对应的 IPv4 地址列表
• inet_pton()Converts a human readable IP address to its packed in_addr representation
• inet_ntop()Converts a packed internet address to a human readable representation

If you do a gethostbyname() and there is no trailing dot after a domainname that does not resolve, this domainname will ultimately be appended to the server-FQDN by nslookup.
So if you do a lookup for nonexistentdomainname.be your server may return the ip for nonexistentdomainname.be.yourhostname.com, which is the server-ip.
To avoid this behaviour, just add a trailing dot to the domainname; i.e. gethostbyname('nonexistentdomainname.be.')

Options for the underlying resolver functions can be supplied by using the RES_OPTIONS environmental variable. (at least under Linux, see man resolv.conf)
Set timeout and retries to 1 to have a maximum execution time of 1 second for the DNS lookup:

You should also use fully qualified domain names ending in a dot. This prevents the resolver from walking though all search domains and retrying the domain with the search domain appended.

This function says "Returns the IPv4 address or a string containing the unmodified hostname on failure.
This isn't entirely true, any hostname with a null byte in it will only return the characters BEFORE the null byte.

Results:
string 'foobar' (length=7)
string 'foo' (length=3)

Important note: You should avoid its use in production.
DNS Resolution may take from 0.5 to 4 seconds, and during this time your script is NOT being executed.
Your customers may think that the server is slow, but actually it is just waiting for the DNS resolution response.
You can use it, but if you want performance, you should avoid it, or schedule it to some CRON script...

For doing basic RBL (Real Time Blacklist) lookups with this function do:

Tomas V.V.Cox

gethostbyname and gethostbynamel does not ask for AAAA records. I have written two functions to implement this. gethostbyname6 and gethostbynamel6. I don't believe this issue has been addressed yet.
They are made to replace gethostbyname[l], in a way that if $try_a is true, if it fails to get AAAA records it will fall back on trying to get A records.
Feel free to correct any errors, I realise that it is asking for *both* A and AAAA records, so this means two DNS calls.. probably would be more efficient if it checked $try_a before making the query, but this works for me so I'll leave that up to someone else to implement in their own work.. the tip is out there now anyway..
Here is the code:
  function gethostbyname6($host, $try_a = false) {
    // get AAAA record for $host
    // if $try_a is true, if AAAA fails, it tries for A
    // the first match found is returned
    // otherwise returns false
    $dns = gethostbynamel6($host, $try_a);
    if ($dns == false) { return false; }
    else { return $dns[0]; }
  }
  function gethostbynamel6($host, $try_a = false) {
    // get AAAA records for $host,
    // if $try_a is true, if AAAA fails, it tries for A
    // results are returned in an array of ips found matching type
    // otherwise returns false
    $dns6 = dns_get_record($host, DNS_AAAA);
    if ($try_a == true) {
      $dns4 = dns_get_record($host, DNS_A);
      $dns = array_merge($dns4, $dns6);
    }
    else { $dns = $dns6; }
    $ip6 = array();
    $ip4 = array();
    foreach ($dns as $record) {
      if ($record["type"] == "A") {
        $ip4[] = $record["ip"];
      }
      if ($record["type"] == "AAAA") {
        $ip6[] = $record["ipv6"];
      }
    }
    if (count($ip6) 
For the "dig" solution, here is a better one:

Using tail because dig, even in the +short form, may return CNAME entries first, and also may return many, many entries (see "dig google.com").
Usig the dot at the end of the host to not perform domain searches.
This works also when $host = IP address already
Will return empty string in case o resolve failure (invalid hostname)
Always add the root . at the end or your server can add it's own suffix.
example: abc.com could become abc.com.yourcompany.com if you don't use abc.com.
I also find this function a litle bit limited because it doesn't tell you if the resolve failed.
TO adress all those issues I use :
function host2ip($host)
{
  $host=trim($host.'.'); // clean and add root .
  $ip= gethostbyname($host);
  if($ip==$host) $ip='';// empty IP if there is no ip
  return $ip;
}
referring to ralphbolton at mail2sexy dot com comment:
(at least in 5.2.0 + djbdns-dnscache) gethostbyname does not really seem to cache entries. If somebody notices a speed-up after the second lookup of the same domain - that's most likely your dns-cache itself, not some php-internal dns-cache.
It does cache the entries in your /etc/resolv.conf (e.g. what dns to use) so I agree with him, that stopping and starting apache, will reload the resolv.conf.
If name resolution fails with apache2, mod_chroot and php5, add
LoadFile /lib/libnss_dns.so.2
to the mod_chroot config.
When using PHP and Apache in a chroot environment on RedHat Linux, I have found that I need to bind-mount /var/run/nscd to get this to work. Apparently, the socket in that directory is needed for all the DNS things.
One note about using gethostbyname() for checking email address domains:
If the name doesn't resolve, follow up with getdnsrr() and make sure they don't have an MX entry before returning an error.
It is possible for a domain name not to have an A record, but still have an MX entry.
In PHP4 you can use gethostbyname() but I have found this unreliable when doing lookups on entries that return A records on the private network. PHP5 has a much better routine -- dns_get_record(). If you are stuck with PHP4 or don't want to upgrade you can use dig: 
Function returns boolean: 
 

鹏仔微信 15129739599 鹏仔QQ344225443 鹏仔前端 pjxi.com 共享博客 sharedbk.com