Platform Team/Server Kit/sugar-server

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Summary

The core Sugar Server Kit component.

The singular program requires only Python, and obvious dependencies like coreutils, to allow all its services to function properly. It provides basic sugar related services, and uses one CLI tool to manage all its functionality.

Functionality

Registry

During the sugar-server work, all its components have access to a registry, an analog of a database to keep information about registered users or activated machines. The registry is file based, i.e., every item is kept in one file (in JSON notation) in the <root-option>/home/registry directory.

There are two subdirectories, i.e., "tables":

  • users, for registered users, and
  • machines, for machines that requested activation leases on bootstrap.

Registry items might contain the following keys:

  • uid, user's identity got after registration on a server;
  • machine_sn, serial number of hardware Sugar user is running on, e.g., serial number of a XO laptop;
  • machine_uuid, UUID value of hardware Sugar user is running on, e.g., UUID of a XO laptop;
  • pubkey, SSH public key, this is the exact line that needs to be placed as-is to ~/.ssh/authorized_keys file; note that OLPC XS operates with stripped version of public keys.

Services

The list of services that sugar-server provides. Each service has a described interface to understand how it interact with clients. The counterpart for these interfaces is the sugar-client with a Client API it provides for clients, e.g., Sugar Shell.

If a particular service supports a RESTfull interface, the HTTP server is started on host with the httpd-port port. The content of requests and replies is transferred using dictionaries in JSON notation.

The resulting dictionary on success, will contain predefined keys:

  • success: OK;

The resulting dictionary on failure, will contain:

  • success: ERR;
  • error, error message.

registry

Provides Sugar users registration on a school server. After being registered, Sugar on user's side will set up interaction with a school server. See sugar-client project for details from the client point of view.

Interface:

POST /client/register

Process registration on a server.

The input dictionary, contains (see identity models section for additional details):

  • nickname, user's Sugar nick name;
  • machine_sn, machine's serial number, makes special sense only for XO anti-thief support and hardware based identity model,
  • machine_uuid, machine's UUID value, makes special sense only for XO anti-thief support,
  • pubkey, SSH public key passed as-is.

The resulting dictionary, contains:

  • uid, unique identity that needs to be used for further interaction with the server.
GET /client/status?uid=UID

Get information about client's status on a server.

The resulting dictionary, contains:

  • registered, if False, client needs to process registration;
  • jabber-url, if registered is True, Jabber server url;
  • backup-url, if registered is True, Rsync url to backup Journal to, needs to be used as-is.
  • pending-restore, is there a pending restore, i.e., if True, during the registration on a server, there was an existing backup; any further, post registration, backup or restore will clean up the pending-restore flag.

Requested uid might be NOT registered in cases like:

  • registration was removed from the server,
  • request was sent to a server that was not the same as where the uid was created,
  • someone registered the same serial number.

Requirements:

Backwards compatibility:

dict register(str machine_sn, str nickname, str machine_uuid, str pubkey)

To support OLPC's XS clients, RPC function to serve registration requests. The XML-RPC service will be listening to the 8080 port on the --host host.

backup

Process backup and restore for students' Journals. The service accepts requests that clients send before starting the backup process. If server accepts requests, clients start Rsync'ing Journal data via SSH. The Rsync url needs to be obtained from a /client/status request and be used without any modification on the client side, i.e., the process is fully server driven.

Users' backups are stored in the <root-option>/home/backups/<UID's-last-two-chars>/<UID> directories.

Interface:

GET /client/backup?uid=UID

Check if client can start transferring Journal files.

The resulting dictionary, contains:

  • accepted, if True, clients can start backup process.

Requirements:

  • The root configuration option should point to the home directory of the system user that starts sugar-server process. This is needed because clients will SSH to this user to perform Rsync backups.

Backwards compatibility:

Note, this backup functionality is not compatible with client tools that work with OLPC XS. The problem is that these tools construct a backup url on the client side (using only the server host name from a backup url given after registration on the server) so relying that every client has its own system user on the server side. That doesn't work with this new backup functionality, where there is only one system user on the server side, and authorisation happens at the SSH level (using the command option in ~/.ssh/authorized_keys file).

activation

This service is intended to process anti-thief requests that XO laptops send to the server during the boot process. It is looking for pre-created leases using <root-option>/share/leases/<SN's-last-two-chars>/<SN> paths. If there are no existing leases and the delegation option is True, it will ask the keyring service to generate a new one using delegated leases.

Interface:

GET /client/lease?machine_sn=MACHINE_SN

Get OLPC anti-thief lease for specified XO's serial number.

The resulting dictionary, contains:

  • lease, lease content

Requirements:

  • Current XO's bootstrap code uses TCP interface, having the following numbers hard coded:
    • host needs to be 172.18.0.1,
    • activation-port needs to be 191,
    • XOs use 172.18.96.1 as a gateway while connecting to 172.18.0.1.

Backwards compatibility:

Service is also listening to --activation-port port on the --host host to process TCP requests used by the XO's bootstrapping code. If sent data starts with a serial number, the service will reply with:

  • STOLEN text, if serial number is stated as stolen;
  • lease content found for serial number;
  • UNKNOWN for errors.

keyring

Keyring service is needed to work with the activation one. It signs delegated leases for activation requests if pre-existing leases weren't found.

Interface:

There is no public interface, the service is used internally by sugar-server.

User identity models

How sugar-server will identify users upon registration.

Hardware based model

The use case assumes:

  • the hardware is intended to run only Sugar,
  • it serves only one user,
  • there is an existing practice within XO deployments with workflows that need to be preserved.

To process registration, users need to provide:

  • valid SSH public key,
  • valid machine serial number, i.e., a serial number that the school server is aware of, e.g., there is an activation lease.

After re-flashing an XO (with loosing private key), it is possible to re-register the user by uploading a new public key. See below for related vulnerability issues.

The following TODO needs to be implemented:

  • Properly handle backups after hardware switching for the particular user.
  • More reliable identity scheme that will avoid compromising users.

SSH key based model

This is a low level model, since the major feature is uploading an SSH public key to the server. This model might be useful in cases like:

  • the hardware is intended to run not only Sugar (thus, might not be affected by "re-flashing" issue),
  • it might serve multiple users.

To process registration, users need to provide a

  • valid SSH public key.

To always have access to already-uploaded-to-the-server data, people need to take care about securing their SSH key. This key can be found in the ~/.sugar/<profile>/owner.key file.

Vulnerabilities

How current implementations are bad.

Compromised users

Current implementation of hardware based identity model has a security issue that might be described with the following use case:

  • an XO registered on a school server;
  • processed several Journal backups there;
  • got re-flashed and failed in keeping client side identities, like the Sugar profile private key;
  • the XO wants to re-register to restore Journal content from a previous backup. But from the school server point of view, this request is a regular registration request from a new XO (which provides the same serial number that the existing registration uses).

Sugar-server will process these registrations assuming that the request was sent from an XO that has exactly the same serial number that was provided with the request.

Compromised servers

Both identity models might process registration on fake servers. The only thing clients need to start a registration, is a properly resolved schoolserver host name.

Possible solutions:

Configuration

By default, configuration occurs based on several sources (sorted by order of application):

  • /etc/sugar-server.conf system-wide configuration file,
  • ~/.config/sugar-server/config user-wide configuration file,
  • sugar-server's command-line arguments.

Configuration files contain option names equal to command-line arguments. To get the current configuration, call:

sugar-server config

See sugar-server-templates sources for an example.

Getting involved

  • Report on bugs.
  • Read the HACKING file to know how to contribute with code.

Resources