What is the boundary in multipart/form-data?

Question

I want to ask a question about the multipart/form-data. In the HTTP header, I find that the Content-Type: multipart/form-data; boundary=???.

Is the ??? free to be defined by the user? Or is it generated from the HTML? Is it possible for me to define the ??? = abcdefg?

Answer 1

Is the ??? free to be defined by the user?

Yes.

or is it supplied by the HTML?

No. HTML has nothing to do with that. Read below.

Is it possible for me to define the ??? as abcdefg?

Yes.

If you want to send the following data to the web server:

name = John
age = 12

using application/x-www-form-urlencoded would be like this:

name=John&age=12

As you can see, the server knows that parameters are separated by an ampersand &. If & is required for a parameter value then it must be encoded.

So how does the server know where a parameter value starts and ends when it receives an HTTP request using multipart/form-data?

Using the boundary, similar to &.

For example:

--XXX
Content-Disposition: form-data; name="name"

John
--XXX
Content-Disposition: form-data; name="age"

12
--XXX--

In that case, the boundary value is XXX. You specify it in the Content-Type header so that the server knows how to split the data it receives.

So you need to:

• Use a value that won't appear in the HTTP data sent to the server.

• Be consistent and use the same value everywhere in the request message.