External Link is a very small and simple app for Data Center & Server instances. Just install the app, and all external URLs and attachments in the attachment overviews will open in a new browser tab.
Clicking on a link to an external page will open the page in a new tab.
Clicking on links leading to pages inside Confluence opens them inside Confluence.
Optionally, you can define a CSS class that will be added to every external URL and attachment link element (in attachment overviews only) in Confluence. This allows you to style external links differently in the Confluence Stylesheet.
You can find the External Links configuration at
⚙ → Manage Apps → External Links for Confluence → Configure