Is headwear restricted by NFHS?

[buckrog64]

Fellow officials:

I've been through the books a time or two and cannot find any NFHS rule restricting the wearing of any headwear under the football helmet. The only thing I thought I heard once was the idea of wearing a bandana and that if part of the bandana stuck out from underneath the helmet, that would be illegal. Would you please advise on the legality of wearing something under the helmet? If you know it is illegal, please let me know where to find it. I know of no state restrictions that apply to this rule in the state of Iowa, where I work. Thanks.