In a WordPress Multisite setup, administrators may encounter login failures on sub-sites, especially when the main site and sub-sites are on different domains. This guide aims to address this issue to ensure seamless access and login to sub-sites.

Problem Description

In a typical WordPress Multisite network, the main site and sub-sites may share the same top-level domain. However, there might be instances where completely different domains are used. For instance, the network structure might be as follows:

• example.com (Main site)
• sub.example.com (Sub-site)
• example.net (Sub-site)

In this structure, an error stating “Cookies are blocked or not supported by your browser” may arise when attempting to access or login to example.net, even though this issue doesn’t occur when switching from example.com to sub.example.com.

Solution

1. Update the wp-config.php File:
   Add the following code in your wp-config.php file to ensure that cookies work correctly across different sub-domains:

   define('COOKIE_DOMAIN', $_SERVER['HTTP_HOST']);

This way, WordPress will set the appropriate cookies for your browser regardless of which site you are accessing.

2. Clear Browser Cookies and Cache:
   After making the changes, clear your browser’s cookies and cache, then try logging in again.
3. Use Plugins:
   There are plugins like WP Multisite SSO that might help with single sign-on across different domains. Although we haven’t tried it personally, it might be an option to resolve this issue.

Understanding COOKIE_DOMAIN

The COOKIE_DOMAIN constant tells WordPress how to set cookies. When you log in, WordPress sets a cookie in your browser to mark you as authenticated. The COOKIE_DOMAIN sets the scope of this cookie, i.e., the browser will only send the cookie back to sites that match its scope. For example, if you log into example.com, the scope of this cookie will be set to example.com, and your browser will send this cookie only when you access this domain or its sub-domains.

Additional Resources

• WordPress Multisite with Different Domains Setup
• Mozilla Documentation on Cookie Scope

Through the steps outlined above, you should be able to resolve the sub-site login failure issue in a WordPress Multisite network, particularly when different domains are utilized.