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I'm writing a script that uploads a file to a cgi script that expects a multipart request, such as a form on a HTML page. The boundary is a unique token that annotates the file contents in the request body. Here's an example body:
--BOUNDARY Content-Disposition: form-data; name="paramname"; filename="foo.txt" Content-Type: text/plain ... file contents here ... --BOUNDARY-- The boundary cannot be present in the file contents, for obvious reasons.
What should I do in order to create an unique boundary? Should I generate a random string, check to see if it is in the file contents, and if it is, generate a new, rinse and repeat, until I have a unique string? Or would a "pretty random token" (say, combination of timestamp, process id, etc) be enough?
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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.
A boundary is just the 'key' to separate the multiple "parts" of a multipart payload. Normally something like '&' is enough to separate the variables but you need something more unique to separate the payloads within the payload.
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.
Each item in a multipart message is separated by a boundary marker. Webkit based browsers put "WebKitFormBoundary" in the name of that boundary. The Network tab of developer tools do not show file data in a multipart message report: They can be too big.
If you use something random enough like a GUID there shouldn't be any need to hunt through the payload to check for an alias of the boundary. Something like:-
----=NextPart_3676416B-9AD6-440C-B3C8-FC66DDC7DB45
Header:....
Payload
----=NextPart_3676416B-9AD6-440C-B3C8-FC66DDC7DB45--
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