Which of the MS-DOS header fields are mandatory/op

Question

The above is the complete list of MS-DOS header fields, but I don't know which of them are mandatory and which are optional, does anyone know?

Answer 1

Well back in 2006 someone wanted to create the world most tiny PE. For this he wrote a small PE Fuzzer. With the smallest codebase posible.

return 42;

He managed to get the following sizes of PE's you are too busy to read the entire page, here is a summary of the results:

• Smallest possible PE file: 97 bytes
• Smallest possible PE file on Windows 2000: 133 bytes
• Smallest PE file that downloads a file over WebDAV and executes it: 133 bytes

You can check his work here: http://www.phreedom.org/research/tinype/

He also states the required header values. These are:

• e_magic
• e_lfanew
• Machine
• NumberOfSections
• SizeOfOptionalHeader
• Characteristics

OptionalHeader:

• Magic
• AddressOfEntryPoint
• ImageBase
• SectionAlignment
• FileAlignment
• MajorSubsystemVersion
• SizeOfImage
• SizeOfHeaders
• Subsystem
• SizeOfStackCommit
• SizeOfHeapReserve

Answer 2

If you're trying to create PE Image, e_magic(Magic number) and elfanew(File address of new exe header) are the only mandatory fields that you have to fill in. elfanew should point to the PE IMAGE_NT_HEADER structure.

Answer 3

For MS-DOS, all of the headers are mandatory.

For Win9x and above, e_lfanew must be the offset from the start of the image to the start of the IMAGE_NT_HEADERS, and e_magic must be IMAGE_DOS_SIGNATURE ('MZ').