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Multipart formposts
A multipart formpost is what an HTTP client sends when an HTML form is submitted with enctype set to "multipart/form-data". It is an HTTP POST request sent with the request body specially formatted as a series of "parts", separated with MIME boundaries.
An example piece of HTML would look like this:
<form action="submit.cgi" method="post" enctype="multipart/form-data">
Name: <input type="text" name="person"><br>
File: <input type="file" name="secret"><br>
<input type="submit" value="Submit">
</form>
Which could look something like this in a web browser:
a multipart form
A user can fill in text in the 'Name' field and by pressing the 'Browse' button a local file can be selected that will be uploaded when 'Submit' is pressed.

Sending such a form with curl

With curl, you add each separate multipart with one -F (or --form) flag and you then continue and add one -F for every input field in the form that you want to send.
The above small example form has two parts, one named 'person' that is a plain text field and one named 'secret' that is a file.
Send your data to that form like this:
curl -F person=anonymous -F [email protected] http://example.com/submit.cgi

The HTTP this generates

The action specifies where the POST is sent. method says it is a POST and enctype tells us it is a multipart formpost.
With the fields filled in as shown above, curl generates and sends these HTTP request headers to the host example.com:
POST /submit.cgi HTTP/1.1
Host: example.com
User-Agent: curl/7.46.0
Accept: */*
Content-Length: 313
Expect: 100-continue
Content-Type: multipart/form-data; boundary=------------------------d74496d66958873e
Content-Length, of course, tells the server how much data to expect. This example's 313 bytes is really small.
The Expect header is explained in the Expect 100 continue chapter.
The Content-Type header is a bit special. It tells that this is a multipart formpost and then it sets the "boundary" string. The boundary string is a line of characters with a bunch of random digits somewhere in it, that serves as a separator between the different parts of the form that will be submitted. The particular boundary you see in this example has the random part d74496d66958873e but you will, of course, get something different when you run curl (or when you submit such a form with a browser).
So after that initial set of headers follows the request body
--------------------------d74496d66958873e
Content-Disposition: form-data; name="person"
anonymous
--------------------------d74496d66958873e
Content-Disposition: form-data; name="secret"; filename="file.txt"
Content-Type: text/plain
contents of the file
--------------------------d74496d66958873e--
Here you clearly see the two parts sent, separated with the boundary strings. Each part starts with one or more headers describing the individual part with its name and possibly some more details. Then after the part's headers come the actual data of the part, without any sort of encoding.
The last boundary string has two extra dashes -- appended to signal the end.

Content-Type

POSTing with curl's -F option will make it include a default Content-Type header in its request, as shown in the above example. This says multipart/form-data and then specifies the MIME boundary string. That content-type is the default for multipart formposts but you can, of course, still modify that for your own commands and if you do, curl is clever enough to still append the boundary magic to the replaced header. You cannot really alter the boundary string, since curl needs that for producing the POST stream.
To replace the header, use -H like this:
curl -F 'name=Dan' -H 'Content-Type: multipart/magic' https://example.com

Converting a web form

There are a few different ways to figure out how to write a curl command line to submit a multipart form as seen in HTML.
  1. 1.
    Save the HTML locally, run nc locally to listen on a chosen port number, change the action URL to submit the POST to your local nc instance. Submit the form and watch how nc shows it. Then translate into a curl command line.
  2. 2.
    Use the "development tools" in your favorite browser and inspect the POST request in the network tab after you have submitted it. Then convert that HTTP data to a curl command line. Unfortunately, the copy as curl feature in the browsers usually do not actually do multipart formposts particularly well.
  3. 3.
    Inspect the source HTML and convert into a curl command line directly from that.

From <form> to -F

In a <form> that uses enctype="multipart/form-data", the first step is to find the action= property as that tells the target for the POST. You need to convert that into a full URL for your curl command line.
An example action looks like this:
<form action="submit.cgi" method="post" enctype="multipart/form-data">
If the form is found in a web page hosted on a URL like for example https://example.com/user/login the action=submit.cgi is a relative path within the same "directory" as the form itself. The full URL to submit this form thus becomes https://example.com/user/submit.cgi. That is the URL to use in the curl command line.
Next, you must identify every <input> tag used within the form, including the ones that are marked as "hidden". Hidden just means that they are not shown in the web page, but they should still be sent in the POST.
For every <input> in the form there should be a corresponding -F in the command line.

text input

A regular tag using type "text" in the style like
<input type="text" name="person">
should then set the field name with content like this:
curl -F "person=Mr Smith" https://example.com/

file input

When the input type is set to a "file", like in:
<input type="file" name="image">
You provide a file for this part by specifying the file name and use @ and the path to the file to include:
curl -F [email protected] https://example.com/

hidden input

A common technique to pass on state from a form is to set a number of <input> tags as type="hidden". This is basically the same thing as an already filled in form field, so you convert this to a command line by using the name and value. For example:
<input type="hidden" name="username" value="bob123">
This is converted like for the normal text field, and here you know what the content should be:
curl -F "username=bob123" https://example.com/

All fields at once

If we toy with the idea that all the three different <input> tags showed in the examples above were used in the same <form>, then a complete curl command line to send, including the correct URL as extracted above, would look like:
curl -F "person=Mr Smith" -F [email protected] -F "username=bob123" \
https://example.com/user/submit.cgi
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Sending such a form with curl
The HTTP this generates
Content-Type
Converting a web form
From <form> to -F
text input
file input
hidden input
All fields at once