Since the DOS tag is ambiguous, I've just created the denial-of-service tag. I already started retagging a few questions.
The problem is that denial-of-service won't appear as a possible choice when somebody starts typing dos (or ddos) in the "Tags" field.
To fix this, I'd like these two tag synonyms to be created:
ddos makes sense to remap to denial-of-service but dos-attack does not since it no longer is called up by the system and by the time you've typed up dos you should see ddos which is now denial-of-service which brings you to the start again.
[dos-attack] never existed; I just wanted a shortcut to the new tag. But you're right: [ddos] already achieves that.