Log user IP to debug.log on login.

log-user-ip.php

<?php
/**
 * Log User IP.
 *
 * @package Fragen\Log_User_IP
 *
 * Plugin Name:       Log User IP
 * Plugin URI:        https://gist.github.com/afragen/dda81b07997c5cc1d58fd90396adeb0a
 * Description:       Log user IP to debug.log at login.
 * Version:           0.1.0
 * Author:            Andy Fragen
 * License:           MIT
 * Requires at least: 5.2
 * Requires PHP:      7.1
 * Gist Plugin URI:   https://gist.github.com/afragen/dda81b07997c5cc1d58fd90396adeb0a
 */

namespace Fragen;

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

add_action(
	'login_redirect',
	function ( $redirect_to, $requested_redirect_to, $user ) {
		if ( $user instanceof \WP_User && $user->exists() ) {
			if ( ! empty( $_SERVER['HTTP_CLIENT_IP'] ) ) {
				$ip = $_SERVER['HTTP_CLIENT_IP'];
			} elseif ( ! empty( $_SERVER['HTTP_X_FORWARDED_FOR'] ) ) {
				$ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
			} else {
				$ip = $_SERVER['REMOTE_ADDR'];
			}
			error_log( $user->data->user_login . ' is logged in from IP ' . $ip );
		}

		return $redirect_to;
	},
	10,
	3
);