To get customer’s IP, behind a proxy or not, you can use this build in function in magento

 Mage::app()->getRequest()->getClientIp(true);

refers to lib/Zend/Controller/Request/Http.php

 /**
     * Get the client's IP addres
     *
     * @param  boolean $checkProxy
     * @return string
     */
    public function getClientIp($checkProxy = true)
    {
        if ($checkProxy && $this->getServer('HTTP_CLIENT_IP') != null) {
            $ip = $this->getServer('HTTP_CLIENT_IP');
        } else if ($checkProxy && $this->getServer('HTTP_X_FORWARDED_FOR') != null) {
            $ip = $this->getServer('HTTP_X_FORWARDED_FOR');
        } else {
            $ip = $this->getServer('REMOTE_ADDR');
        }

        return $ip;
    }