Contribute D8 version of an existing D7 contrib project?
You can use Browse code repository to see your source code on your project page. Currently when you go to list of branches, the 8.x-1.x doesn't exist yet.

To maintain both D7 and D8, you need to have one branch for each, 7.x-1.x for Drupal 7, and 8.x-1.x for Drupal 8.

To create Drupal 8 out of Drupal 7 version, you should first clone your repository as per these Git instructions:

git clone --branch 7.x-1.x your_user@git.drupal.org:project/check_dns.git


then create a new branch:

git checkout -b 8.x-1.x


which allows you to freely modify your files for Drupal 8 version.

Once you did your conversion, you can push your branch back to Drupal.org, e.g.

git push origin 8.x-1.x


To switch between branches, use git checkout command, so you can contribute to both version.

Ideally, you should read more about managing source code using Git. Checkout this GitHub Cheat Sheet.