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In an HTML form post what are valid characters for creating a multipart boundary?
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The boundary is included to separate name/value pair in the multipart/form-data . The boundary parameter acts like a marker for each pair of name and value in the multipart/form-data. The boundary parameter is automatically added to the Content-Type in the http (Hyper Text Transfer Protocol) request header.
multipart/form-data contains boundary to separate name/value pairs. The boundary acts like a marker of each chunk of name/value pairs passed when a form gets submitted. The boundary is automatically added to a content-type of a request header.
A multipart/form-data request body contains a series of parts separated by a boundary delimiter, constructed using Carriage Return Line Feed (CRLF), "--", and also the value of the boundary parameters. The boundary delimiter must not appear inside any of the encapsulated parts.
The boundary is specified like this: Content-Type: multipart/form-data; boundary=AaB03x . So, without a proper content-type, you almost can't have true multipart/form-data.
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.
103
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"
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