Please go to your server root and open config.php to setup custom domain

cd /var/www/html/

nano  config.php
 * The base MySQL settings of Osclass
define('MULTISITE', 0);
/** MySQL database name for Osclass */
define('DB_NAME', 'osclass');
/** MySQL database username */
define('DB_USER', 'user');
/** MySQL database password */
/** MySQL hostname */
define('DB_HOST', 'localhost');
/** Database Table prefix */
define('DB_TABLE_PREFIX', 'oc_');
define('REL_WEB_URL', '/');
define('WEB_PATH', 'http://yourIPaddress/');

change this with your domain like mentioned below

define('WEB_PATH', '');

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With a heavy heart Debian  mourns the passing of Ian Murdock, stalwart proponent of Free Open Source Software, Father, Son,