I’m using my computer as a development platform and I usually let people in by editing my firewall active table
but for those with dynamic ip addresses I had to figure out a different solution
so I made a small script in PHP to check for my friends dynamic ip address by looking up his dyndns.org hostname
it checks IP then goes sleeping for 5 minutes
when it detects an ip change it updates firewall by flushing my firends chain and adding a rule to let him access my webserver
this script works in my firewall iptables setup, it could work on yours but you have to be smart and figure that out for yourself, I’m not giving any guaranteess for this, use it as GPL V3 code 
if you want me to help you set it up, just contact me
here’s the code:
#!/usr/bin/php
<?php
/*
use this code as GPL V3 licence says
Copyright (C) 2008, Miroslav Madzarevic, All Rights Reserved
*/
$old_ip = '';
while (1) {
sleep (300);
$host = 'friend.dyndns.org';
if (!preg_match('/^'.$host.' has address ((?:\d{1,3}\.){3}\d{1,3})$/',
exec ("/usr/bin/host $host"), $matches)) {
echo "IP problems\n";
continue;
}
$ip = $matches[1];
$ip_parts = preg_split ('/\./', $ip, -1, PREG_SPLIT_NO_EMPTY);
if (count($ip_parts) != 4) {
echo "wrong ip parts count\n";
exit;
}
$good = array();
foreach ($ip_parts as $ip_part) {
$ip_part = (int) $ip_part;
if ($ip_part < 1 or $ip_part > 254) {
echo "wrong ip part, $ip_part\n";
exit;
}
$good[] = $ip_part;
}
$new_ip = join('.', $good);
if ($old_ip != $new_ip) {
`/sbin/iptables -F friend`;
`/sbin/iptables -A friend -s $new_ip/32 -p tcp -m tcp --dport 80 -j ACCEPT`;
$old_ip = $new_ip;
}
}
?>
) and adds an entry to /etc/rc.local so that it starts next time you reboot your box automatically
