I have several white shirt and they're in yellowish color. I've tried to boil them with bleach + detergent but it's failed. And also there are rust stain too. How can I make my shirts perfecly white again?
There are essentially two types of bleach, chlorine & oxygen.
I assume you have tried the first, but not the second.
Chlorine bleach is the most common - the one that smells like a really strong swimming pool, the one most likely to be kept under the sink or by the toilet.
The other type is oxygen bleach - particularly effective on organic fibres.
Oxygen bleach, hydrogen peroxide, is pretty much odourless, can be made at home by adding sodium percarbonate to water. It has quite a short shelf-life once made up domestically by this method & any left over should be disposed of immediately. Continuing reaction in a sealed container can cause the container to leak, or burst because of the increased gas pressure. Any spills should be immediately flushed with copious water.
Any left over can safely be poured down the sink & flushed down with water. The byproducts are just hydrogen & oxygen.
The name I know best as a household washing whitener which contains peroxide is 'Vanish' - though whether that is UK market only, or truly international, I don't know. Supermarkets also have own-brands, very similar.
Products of that type will contain other detergents & optical brighteners [blue & fluorescent dyes] to enhance the appearance still further.
You can actually buy sodium percarbonate on eBay & similar sites & make the solution up yourself. It's considerably cheaper that way, but needs careful dry storage (eg an old Vanish container, suitably labelled ;-) & of course contains no additional detergent/brightener, so needs to be used in conjunction with a regular washing detergent for best results.
There are also liquid cleaning products tried on the domestic/household market - usually as surface cleaners rather than laundry - but they don't seem to have gained much traction & are harder to find than they once were. Look for 'Oxy' or 'Oxygen' somewhere in the product name or description.
Oxygen bleach is also pretty good on 'organic' stains, but not 'chemical' - wine will come out better than marker pen, for example.
Note that cotton & other natural fibres are far more likely to come out white with this method than artificial fibres, which can yellow over time & cannot be brought back to white by bleaching.