In AEM, the Externalizer is an OSGI service that allows you to programmatically transform a resource path (e.g. /path/to/my/page ) into an external and absolute URL (for example, https://www.mycompany.com/path/to/my/page ) by prefixing the path with a pre-configured DNS.
Because an instance can not know its externally visible URL if it is running behind a web layer, and because sometimes a link has to be created outside of the request scope, this service provides a central place to configure those external URLs and build them.
Configuring the Externalizer service
The Externalizer service allows you to centrally define multiple domains that can be used to programmatically prefix resource paths. Each domain is identified by a unique name that is used to programmatically reference the domain.
To define a domain mapping for the Externalizer service:
1. Navigate to the configuration manager via Tools , then Web Console , or enter https://:/system/console/configMgr.
2. Click Day CQ Link Externalizer to open the configuration dialog box.