>> https://www.securecoding.cert.org/confluence/display/cplusplus/PRE06-CPP.+Enclose+header+files+in+an+inclusion+guard
[...]
> the link is not available.

The example above worked for me again a moment ago.
Would you like to try a shorter link out?
https://www.securecoding.cert.org/confluence/pages/viewpage.action?pageId=637

Regards,
Markus