Is it rude to specify the attire your guests should wear?

I think that this action can definately turn people off and possibly offend them. Its fine to stress the importance of a formal or casual dress code, but actually telling the people in specifics to that degree is too far. Stating that formal attire is sufficient enough. Trust me, there will be people that will feel they may be underdressed as is when they get there and by stating what they have to wear will only make their fears or anxieties about the event worse. Just go with the formal requirement :)