Question

I am facing an issue with $_SERVER[‘REMOTE_ADDR’] in PHP
It is giving a IPv6 like value even though the server is using IPv4.

Can anyone help me to resolve this issue.

Answer 1

The server is then accepting connections on an IPv6 socket. Some operating systems can do both IPv4 and IPv6 on an IPv6 socket. When that happens the IPv6 address will look like ::ffff:192.0.2.123 or ::ffff:c000:027b which is the same address but written in hexadecimal.

If you see IPv6 addresses like 2a00:8640:1::224:36ff:feef:1d89 then your webserver really is reachable over IPv6 🙂

Anyway, to convert everything back to a canonical form you can use something like:

// Known prefix
$v4mapped_prefix_hex = '00000000000000000000ffff';

$v4mapped_prefix_bin = hex2bin($v4mapped_prefix_hex); 

// Parse
$addr = $_SERVER['REMOTE_ADDR'];
$addr_bin = inet_pton($addr);
if( $addr_bin === FALSE ) {
  // Unparsable? How did they connect?!?
  die('Invalid IP address');
}

// Check prefix
if( substr($addr_bin, 0, strlen($v4mapped_prefix_bin)) == $v4mapped_prefix_bin) {
  // Strip prefix
  $addr_bin = substr($addr_bin, strlen($v4mapped_prefix_bin));
}

// Convert back to printable address in canonical form
$addr = inet_ntop($addr_bin);

Using this code, when you input one of the following:

::ffff:192.000.002.123
::ffff:192.0.2.123
0000:0000:0000:0000:0000:ffff:c000:027b
::ffff:c000:027b
::ffff:c000:27b
192.000.002.123
192.0.2.123

you always get the canonical IPv4 address 192.0.2.123 as output.

And of course IPv6 addresses get returned as canonical IPv6 addresses: 2a00:8640:0001:0000:0224:36ff:feef:1d89 becomes 2a00:8640:1::224:36ff:feef:1d89 etc.