What are valid characters for creating a multipart form boundary?

Question

In an HTML form post what are valid characters for creating a multipart boundary?

Answer 1

According to RFC 2046, section 5.1.1:

 boundary := 0*69<bchars> bcharsnospace

 bchars := bcharsnospace / " "

 bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" /
                  "+" / "_" / "," / "-" / "." /
                  "/" / ":" / "=" / "?"

So it can be between 1 and 70 characters long, consisting of alphanumeric, and the punctuation you see in the list. Spaces are allowed except at the end.

Answer 2

There are no rules as of the content of the boundary but as it must not occur in any of the parts of your message content is usually a randomly generated sequence of numbers, letters or combination of both in order to guarantee uniqueness and differentiate from any possible dictionary words. So as you start your message each data type section is separated by “–” followed by the boundary sequence and the content type + encoding. After the last section “–” followed by the boundary followed by “–” is used to delimit the end of the message. The way multipart content works is by specifying a boundary in the “Content-type:” header of your email. The boundary is used to separate the different content types and looks something like this:

Content-type: multipart/mixed; boundary="fU3W4Vzr4G3D54f3"