Recipe 17.10 Performing DNS Lookups
17.10.1 Problem
You want to look up a domain name or
an IP address.
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
To perform zone transfers, dynamic DNS updates,
and more, see PEAR's Net_DNS
package.
17.10.4 See Also
Documentation on gethostbyname( ) at
http://www.php.net/gethostbyname,
gethostbyaddr( )
http://www.php.net/gethostbyaddr,
gethostbyaddrl( ) at
http://www.php.net/gethostbyaddrl, and
getmxrr( ) at
http://www.php.net/getmxrr;
PEAR's Net_DNS package at
http://pear.php.net/package-info.php?package=Net_DNS;
DNS and BIND by Paul Albitz and Cricket Liu
(O'Reilly) .
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