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

Only eat:

• eggs
• meat
• fish
• chicken
• cheeses
• butter
• heavy cream
• olive oil
• coconut oil
• common mushrooms, check nutrients for other mushrooms
• all spices, seasonings and hot sauces are fine as the amounts are so small

Especially avoid eating:

• no sugar or honey in any shape or form
• no grains like wheat or rice, so no bread or pasta
• no vegetables
• no berries
• no fruits
• tiny amounts of non-fish seafood
• tiny amounts of nut
• tiny amounts of milk

Drink options:

• coffee and tea with a ton of cream or coconut oil, optional artificial sweetener
• anything containing absorbing carbohydrates (sugar, fructose) is a big no-no
• sugar-free sodas are totally fine for me, but I've heard that they don't work for everyone
• hard spirits are fine (whiskey, vodka, gin)
• most liqueurs (the sweet kind) are out of the question
• dry wines are mostly fine, those with less than 3 grams of sugar per liter
• beer is not really suitable, but a few won't hurt if you are otherwise strict

Supplements recommended:

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

Sources

• Wikipedia - Ketogenic diet