Quick answer
Host suggests preferred domain.
robots.txt Host Directive
Host suggests preferred domain. Only Google uses it; optional.
Common causes
- Wrong Host syntax.
- Expecting all engines to use.
How to fix
- Host: example.com (no scheme).
- Optional; Google only.
FAQ
- Host directive?
- Google only, optional.
- Syntax?
- Host: domain.com.
Fix it now
Try in validator (prefill this example)