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

User · Answer

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 amp age 12   As you can see  the server knows that parameters are separated by an ampersand  amp   If  amp  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  amp    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

User · Answer

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    The form with enctype  multipart form-data  attribute will have a request header Content-Type    multipart form-data  boundary  --- WebKit193844043-h  browser generated vaue    The payload passed looks something like this   Content-Type  multipart form-data  boundary ---WebKitFormBoundary7MA4YWxkTrZu0gW      -----WebKitFormBoundary7MA4YWxkTrZu0gW     Content-Disposition  form-data  name    file     filename    captcha        Content-Type       -----WebKitFormBoundary7MA4YWxkTrZu0gW     Content-Disposition  form-data  name    action         submit     -----WebKitFormBoundary7MA4YWxkTrZu0gW--   On the webservice side  it s consumed in  Consumes  multipart form-data   form   Beware  when testing your webservice using chrome postman  you need to check the form data option radio button  and File menu from the dropdown box to send attachment  Explicit provision of content-type as multipart form-data throws an error  Because boundary is missing as it overrides the curl request of post man to server with content-type by appending the boundary which works fine   See RFC1341 sec7 2 The Multipart Content-Type

User · Answer

The exact answer to the question is  yes  you can use an arbitrary value for the boundary parameter  given it does not exceed 70 bytes in length and consists only of 7-bit US-ASCII  printable  characters   If you are using one of multipart   content types  you are actually required to specify the boundary parameter in the Content-Type header  otherwise the server  in the case of an HTTP request  will not be able to parse the payload   You probably also want to set the charset parameter to UTF-8 in your Content-Type header  unless you can be absolutely sure that only US-ASCII charset will be used in the payload data   A few relevant excerpts from the RFC2046    4 1 2  Charset Parameter      Unlike some other parameter values  the values of the charset parameter are NOT case sensitive   The default character set  which must be assumed in the absence of a charset parameter  is US-ASCII   5 1   Multipart Media Type     As stated in the definition of the Content-Transfer-Encoding field  RFC 2045   no encoding other than  7bit    8bit   or  binary  is permitted for entities of type  multipart    The  multipart  boundary delimiters and header fields are always represented as 7bit US-ASCII in any case  though the header fields may encode non-US-ASCII header text as per RFC 2047  and data within the body parts can be encoded on a part-by-part basis  with Content-Transfer-Encoding fields for each appropriate body part       The Content-Type field for multipart entities requires one parameter   boundary   The boundary delimiter line is then defined as a line consisting entirely of two hyphen characters   -   decimal value 45  followed by the boundary parameter value from the Content-Type header field  optional linear whitespace  and a terminating CRLF       Boundary delimiters must not appear within the encapsulated material  and must be no longer than 70 characters  not counting the two leading hyphens       The boundary delimiter line following the last body part is a distinguished delimiter that indicates that no further body parts will follow   Such a delimiter line is identical to the previous delimiter lines  with the addition of two more hyphens after the boundary parameter value     Here is an example using an arbitrary boundary   Content-Type  multipart form-data  charset utf-8  boundary  another cool boundary   --another cool boundary Content-Disposition  form-data  name  foo   bar --another cool boundary Content-Disposition  form-data  name  baz   quux --another cool boundary--

[html] What is the boundary in multipart/form-data?

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