I follow a variant of ketogenic diet, and here are the rules.

Only eat:

• eggs
• meat
• fish
• chicken
• cheeses
• cream
• butter
• olive oil
• coconut oil
• common mushrooms, check nutrients for other mushrooms
• all spices, seasonings and hot sauces are fine if they don't include sugar

Never eat the following:

• no sugar or honey in any shape or form
• no grains like wheat or rice so no bread or pasta
• no seafood except fish
• no vegetables
• no berries
• no fruits
• no nuts
• no milk

Drinks:

• anything containing absorbing carbohydrates (sugar, fructose) is a big no
• coffee/tea with a ton of cream or coconut oil, optional artificial sweetener
• sugar-free sodas are usually fine
• hard spirits are fine (whiskey, vodka, gin)
• most liqueurs (the sweet kind) are out of the question
• dry wines with less than 3 grams of sugar per liter
• beer is not really suitable, but one or two never hurts if you are strict

Supplements recommended:

• vitamin Bs
• vitamin D
• calcium
• magnesium at the evening

Sources

• Wikipedia - Ketogenic diet