TYPO3 Core

Since this feature request is still open after almnost five years, it seems that there is not much interest in implementing this feature.

However, another common use case, where the described feature could be very useful, is for internal environments. Imagine an intranet in combination with a separated DMS application, a bugtracker or something else, where a lot of links from the TYPO3 intranet point to the other systems and which the linkvalidator alerts as false positive, because it has no access to it, maybe because of SSO.
Instead of getting a lot of false positives, the domains of these external systems could be excluded from the link checking.

@Christopher There is currently no simple way to do this with Linkvalidator out of the box.

However, Link Validator is very well customizable. You can create your own custom Link type, extending the external link type. Please see the Link Validator documentation" and look in the source code of ExternalLinktype.php

In fact, I would personally recommend to do that and not use the ExternalLinktype without customization anyway, for the following reasons:

1. problem of false positives: Links are detected as broken even if they are not. This seems to be a problem which is getting worse. There are a number of reasons for this but the most common seem to be (1) incomplete certificate chain of the server (which is resolved in the browser because the browser fetches intermediate certificates, but curl (which is by default used by Guzzle, which is used by TYPO3) does not and (2) Cloudflare protected sites. If you customize the class, you can define what is checked and what is not and if specific error types or domains should be excluded
2. if there are lots of external links, it might make sense to implement a crawl delay and make sure external sites are not checked too often

Alternatively, you can look at my extension Broken Link Fixer (brofix), which is based on linkvalidator but has implemented some changes, such as:

- link target cache and crawl delay when fetching to reduce access of external sites
- Button in link list for "ignoring" a target URL by domain or URL (exclude list), exclude list can be maintained in the BE module

You can look at the implementation in "Broken Link Fixer" (brofix) as a start to create your own or you can try out the extension.

Even with the "ignore" button, I have now deactivated the external link checking for our university TYPO3 installation except for a few known internal domains in the university because the "false positives" was too much of a problem.

@Sybille Peters wow, just read your brofix extension documentation and this is exactly what I have been having in mind (also having an ignore button in the list). Thank you very much for the hint and your work on that extension.

Can't this get into the core, replacing the Linkvalidator?

@Christopher I can't make that decision.

I am just a contributor, I am not in core team.

You can contact me on Slack if you have further questions. Anyway, the "ignore button" is just a workaround, it is not a silver bullet and the problem of the "false positives" is quite tricky to solve.

If you want to talk about linkvalidator you can also use the core channel, see contribution guide: https://docs.typo3.org/m/typo3/guide-contributionworkflow/main/en-us/Community/Index.html