17.10.1 Problem
17.10.2 Solution
Use gethostbyname( ) and gethostbyaddr( ):
$ip = gethostbyname('www.example.com'); // 192.0.34.72
$host = gethostbyaddr('192.0.34.72'); // www.example.com
17.10.3 Discussion
You can't trust the name returned by gethostbyaddr(
) . A DNS server with
authority for a particular IP address can return any hostname at all. Usually,
administrators set up DNS servers to reply with a correct hostname, but a
malicious user may configure her DNS server to reply with incorrect hostnames.
One way to combat this trickery is to call gethostbyname( ) on the hostname returned from gethostbyaddr( )
and make sure the name resolves to the original IP address.
If either function can't successfully look up the IP address or
the domain name, it doesn't return false, but instead returns the
argument passed to it. To check for failure, do this:
if ($host == ($ip = gethostbyname($host))) {
// failure
}
This assigns the return value of gethostbyname( ) to
$ip and also checks that $ip is not equal to the original
$host.
Sometimes a single hostname can map to multiple IP addresses.
To find all hosts, use gethostbynamel( ):
$hosts = gethostbynamel('www.yahoo.com');
print_r($hosts);
Array
(
[0] => 64.58.76.176
[1] => 64.58.76.224
[2] => 64.58.76.177
[3] => 64.58.76.227
[4] => 64.58.76.179
[5] => 64.58.76.225
[6] => 64.58.76.178
[7] => 64.58.76.229
[8] => 64.58.76.223
)
In contrast to gethostbyname( ) and gethostbyaddr(
), gethostbynamel( ) returns an array, not a string.
You can also do more complicated DNS-related tasks. For
instance, you can get the MX
records using getmxrr( ):
getmxrr('yahoo.com', $hosts, $weight);
for ($i = 0; $i < count($hosts); $i++) {
echo "$weight[$i] $hosts[$i]\n";
}
5 mx4.mail.yahoo.com
1 mx2.mail.yahoo.com
1 mx1.mail.yahoo.com
