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Multipart form data: The ENCTYPE attribute of <form> tag specifies the method of encoding for the form data. It is one of the two ways of encoding the HTML form. It is specifically used when file uploading is required in HTML form. It sends the form data to server in multiple parts because of large size of file.
Multipart/Form-Data is a popular format for REST APIs, since it can represent each key-value pair as a “part” with its own content type and disposition. Each part is separated by a specific boundary string, and we don't explicitly need Percent Encoding for their values.
x-www-form-urlencoded vs multipart/form-data 3) Both content types are used while sending form data as a POST request. 4) The x-www-form-urlencoded is used more generally to send text data to the server while multipart/form-data is used to send binary data, most notably for uploading files to the server.
EDIT: I am maintaining a similar, but more in-depth answer at: https://stackoverflow.com/a/28380690/895245
To see exactly what is happening, use nc -l or an ECHO server and an user agent like a browser or cURL.
Save the form to an .html file:
<form action="http://localhost:8000" method="post" enctype="multipart/form-data">
<p><input type="text" name="text" value="text default">
<p><input type="file" name="file1">
<p><input type="file" name="file2">
<p><button type="submit">Submit</button>
</form>
Create files to upload:
echo 'Content of a.txt.' > a.txt
echo '<!DOCTYPE html><title>Content of a.html.</title>' > a.html
Run:
nc -l localhost 8000
Open the HTML on your browser, select the files and click on submit and check the terminal.
nc prints the request received. Firefox sent:
POST / HTTP/1.1
Host: localhost:8000
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:29.0) Gecko/20100101 Firefox/29.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Cookie: __atuvc=34%7C7; permanent=0; _gitlab_session=226ad8a0be43681acf38c2fab9497240; __profilin=p%3Dt; request_method=GET
Connection: keep-alive
Content-Type: multipart/form-data; boundary=---------------------------9051914041544843365972754266
Content-Length: 554
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="text"
text default
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="file1"; filename="a.txt"
Content-Type: text/plain
Content of a.txt.
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name="file2"; filename="a.html"
Content-Type: text/html
<!DOCTYPE html><title>Content of a.html.</title>
-----------------------------9051914041544843365972754266--
Aternativelly, cURL should send the same POST request as your a browser form:
nc -l localhost 8000
curl -F "text=default" -F "[email protected]" -F "[email protected]" localhost:8000
You can do multiple tests with:
while true; do printf '' | nc -l localhost 8000; done
Many thanks to @Ciro Santilli answer! I found that his choice for boundary is quite "unhappy" because all of thoose hyphens: in fact, as @Fake Name commented, when you are using your boundary inside request it comes with two more hyphens on front:
Example:
POST / HTTP/1.1
HOST: host.example.com
Cookie: some_cookies...
Connection: Keep-Alive
Content-Type: multipart/form-data; boundary=12345
--12345
Content-Disposition: form-data; name="sometext"
some text that you wrote in your html form ...
--12345
Content-Disposition: form-data; name="name_of_post_request" filename="filename.xyz"
content of filename.xyz that you upload in your form with input[type=file]
--12345
Content-Disposition: form-data; name="image" filename="picture_of_sunset.jpg"
content of picture_of_sunset.jpg ...
--12345--
I found on this w3.org page that is possible to incapsulate multipart/mixed header in a multipart/form-data, simply choosing another boundary string inside multipart/mixed and using that one to incapsulate data. At the end, you must "close" all boundary used in FILO order to close the POST request (like:
POST / HTTP/1.1
...
Content-Type: multipart/form-data; boundary=12345
--12345
Content-Disposition: form-data; name="sometext"
some text sent via post...
--12345
Content-Disposition: form-data; name="files"
Content-Type: multipart/mixed; boundary=abcde
--abcde
Content-Disposition: file; file="picture.jpg"
content of jpg...
--abcde
Content-Disposition: file; file="test.py"
content of test.py file ....
--abcde--
--12345--
Take a look at the link above.
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