1. Mites or mange. A Vet would have to take some scraping to confirm. Dogs whose immune systems are compromised or undeveloped (e.g. puppies) can readily get mites.

2. Severe untreated allergy.

In cases 1,2 the smell might be secondary infection of bacteria, yeast, or fungi secondary to the underlying condition.

3. Thyroid disfunction. Older dogs frequently start to have thyroid problems, with the result of dry dandruff like skin, loss of hair, and again secondary infection setting in. A blood panel monitoring for T4 hormone is the only way to tell for sure.

A trip to the Vet needs to be made for diagnosis and treatment.