mod_http_upload_external

Introduction

This module implements XEP-0363, which lets clients upload files over HTTP to an external web server.

This module generates URLs that are signed using a HMAC. Any web service that can authenticate these URLs can be used. There is a PHP implementation available here. To implement your own service compatible with this module, check out the implementation notes below (and if you publish your implementation - let us know!).

Configuration

Add "http_upload_external" to modules_enabled in your global section, or under the host(s) you wish to use it on.

External URL

You need to provide the path to the external service. Ensure it ends with '/'.

For example, to use the PHP implementation linked above, you might set it to:

http_upload_external_base_url = "https://your.example.com/path/to/share.php/"

Secret

Set a long and unpredictable string as your secret. This is so the upload service can verify that the upload comes from mod_http_upload_external, and random strangers can't upload to your server.

http_upload_external_secret = "this is a secret string!"

You need to set exactly the same secret string in your external service.

Limits

A maximum file size can be set by:

http_upload_external_file_size_limit = 123 -- bytes

Default is 100MB (100*1024*1024).

Compatibility

Works with Prosody 0.9.x and later.

Implementation

To implement your own external service that is compatible with this module, you need to expose a simple API that allows the HTTP GET, HEAD and PUT methods on arbitrary URLs located on your service.

For example, if http_upload_external_base_url is set to https://example.com/upload/ then your service might receive the following requests:

Upload a new file:

PUT https://example.com/upload/foo/bar.jpg?v=49e9309ff543ace93d25be90635ba8e9965c4f23fc885b2d86c947a5d59e55b2

Recipient checks the file size and other headers:

HEAD https://example.com/upload/foo/bar.jpg

Recipient downloads the file:

GET https://example.com/upload/foo/bar.jpg

The only tricky logic is in validation of the PUT request. Firstly, don't overwrite existing files (return 409 Conflict).

Then you need to validate the auth token. This will be in the URL query parameter 'v'. If it is absent, fail with 403 Forbidden.

Calculate the expected auth token by reading the value of the Content-Length header of the PUT request. E.g. for a 1MB file will have a Content-Length of '1048576'. Append this to the uploaded file name, separated by a space (0x20) character.

For the above example, you would end up with the following string: "foo/bar.jpg 1048576"

The auth token is a SHA256 HMAC of this string, using the configured secret as the key. E.g.

calculated_auth_token = hmac_sha256("foo/bar.jpg 1048576", "secret string")

If this is not equal to the 'v' parameter provided in the upload URL, reject the upload with 403 Forbidden.

Note: your language/environment may provide a function for doing a constant-time comparison of these, to guard against timing attacks that may be used to discover the secret key.