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External Authentication

To enable external authentication, you must set the absolute path of your authentication program or an HTTP URL using the external_auth_hook key in your configuration file.

The external program can read the following environment variables to get info about the user trying to authenticate:

  • SFTPGO_AUTHD_USERNAME
  • SFTPGO_AUTHD_PASSWORD, not empty for password authentication
  • SFTPGO_AUTHD_PUBLIC_KEY, not empty for public key authentication
  • SFTPGO_AUTHD_KEYBOARD_INTERACTIVE, not empty for keyboard interactive authentication

Previous global environment variables aren't cleared when the script is called. The content of these variables is not quoted. They may contain special characters. They are under the control of a possibly malicious remote user. The program must write, on its standard output, a valid SFTPGo user serialized as JSON if the authentication succeed or a user with an empty username if the authentication fails.

If the hook is an HTTP URL then it will be invoked as HTTP POST. The request body will contain a JSON serialized struct with the following fields:

  • username
  • password, not empty for password authentication
  • public_key, not empty for public key authentication
  • keyboard_interactive, not empty for keyboard interactive authentication

If authentication succeed the HTTP response code must be 200 and the response body a valid SFTPGo user serialized as JSON. If the authentication fails the HTTP response code must be != 200 or the response body must be empty.

If the authentication succeeds, the user will be automatically added/updated inside the defined data provider. Actions defined for users added/updated will not be executed in this case. The external hook should check authentication only. If there are login restrictions such as user disabled, expired, or login allowed only from specific IP addresses, it is enough to populate the matching user fields, and these conditions will be checked in the same way as for built-in users. The program hook must finish within 30 seconds, the HTTP hook timeout will use the global configuration for HTTP clients.

This method is slower than built-in authentication, but it's very flexible as anyone can easily write his own authentication hooks. You can also restrict the authentication scope for the hook using the external_auth_scope configuration key:

  • 0 means all supported authetication scopes. The external hook will be used for password, public key and keyboard interactive authentication
  • 1 means passwords only
  • 2 means public keys only
  • 4 means keyboard interactive only

You can combine the scopes. For example, 3 means password and public key, 5 means password and keyboard interactive, and so on.

Let's see a very basic example. Our sample authentication program will only accept user test_user with any password or public key.

#!/bin/sh

if test "$SFTPGO_AUTHD_USERNAME" = "test_user"; then
  echo '{"status":1,"username":"test_user","expiration_date":0,"home_dir":"/tmp/test_user","uid":0,"gid":0,"max_sessions":0,"quota_size":0,"quota_files":100000,"permissions":{"/":["*"],"/somedir":["list","download"]},"upload_bandwidth":0,"download_bandwidth":0,"filters":{"allowed_ip":[],"denied_ip":[]},"public_keys":[]}'
else
  echo '{"username":""}'
fi

An example authentication program allowing to authenticate against an LDAP server can be found inside the source tree ldapauth directory.

An example server, to use as HTTP authentication hook, allowing to authenticate against an LDAP server can be found inside the source tree ldapauthserver directory.

If you have an external authentication hook that could be useful to others too, please let us know and/or please send a pull request.