If you're getting a DNS error, it's not related to your robots.txt - it's related to your DNS settings. You can check for DNS errors with online tools like: DNS Health
If you need additional help with that, I'd suggest making a screenshot of your DNS table settings from your DNS service provider and asking another question with that screenshot pasted in it (i.e., example).
By the way, the accepted answer regarding your other robots.txt question is correct. I would stick with that for what you were asking about previously - robots.txt directives are not the cause of any DNS errors.