TYPO3 Core

sorry forgot to set it as a feature!

This could be really handy, but has to be done by involving front-end rendering.
A "link" could also occour as pseudo code e.g. [link]123[/link] or could just be a number in a field of an extension treated as internal link.

Thus, such a link validator should search for links in HTML code and validate them. To build a relationship between a found invalid link and its originating record could be very trick.

refindex could be handy for this task

why not use an external tool like XENU?

This is in TYPO3 4.5 with a separate extension.