Protection against "trusted" users
Protection against untrusted users

AUTHOR AND COPYRIGHT
SEE ALSO

NAME

RPC::PlServer - Perl extension for writing PlRPC servers

SYNOPSIS

  # Create a subclass of RPC::PlServer
  use RPC::PlServer;

  package MyServer;
  $MyServer::VERSION = '0.01';
  @MyServer::ISA = qw(RPC::PlServer);

  # Overwrite the Run() method to handle a single connection
  sub Run {
      my $self = shift;
      my $socket = $self->{'socket'};
  }

  # Create an instance of the MyServer class
  package main;
  my $server = MyServer->new({'localport' => '1234'}, \@ARGV);

  # Bind the server to its port to make it actually running
  $server->Bind();

DESCRIPTION

PlRPC (Perl RPC) is a package for implementing servers and clients that are written in Perl entirely. The name is borrowed from Sun's RPC (Remote Procedure Call), but it could as well be RMI like Java's "Remote Method Interface), because PlRPC gives you the complete power of Perl's OO framework in a very simple manner.

RPC::PlServer is the package used on the server side, and you guess what RPC::PlClient is for. Both share the package RPC::PlServer::Comm for communication purposes. See PlRPC::Client(3) and RPC::PlServer::Comm for these parts.

PlRPC works by defining a set of methods that may be executed by the client. For example, the server might offer a method "multiply" to the client. Now the clients method call

    @result = $client->multiply($a, $b);

will be immediately mapped to a method call

    @result = $server->multiply($a, $b);

on the server. The arguments and results will be transferred to or from the server automagically. (This magic has a name in Perl: It's the Storable module, my thanks to Raphael Manfredi for this excellent package.) Simple, eh? :-)

The RPC::PlServer and RPC::PlClient are abstract servers and clients: You have to derive your own classes from it.

Additional options

The RPC::PlServer inherits all of Net::Daemon's options and attributes and adds the following:

cipher: The attribute value is an instance of Crypt::DES, Crypt::IDEA or any other class with the same API for block encryption. If you supply such an attribute, the traffic between client and server will be encrypted using this option.
maxmessage (--maxmessage=size): The size of messages exchanged between client and server is restricted, in order to omit denial of service attacks. By default the limit is 65536 bytes.
users: This is an attribute of the client object used for Permit/Deny rules in the config file. It's value is an array ref of user names that are allowed to connect from the given client. See the example config file below. CONFIGURATION FILE.

Error Handling

Error handling is simple with the RPC package, because it is based on Perl exceptions completely. Thus your typical code looks like this:

  eval {
      # Do something here. Don't care for errors.
      ...
  };
  if ($@) {
      # An error occurred.
      ...
  }

Server Constructors

  my $server = RPC::PlServer(\%options, \@args);

(Class method) This constructor is immediately inherited from the Net::Daemon package. See Net::Daemon(3) for details.

Access Control

  $ok = $self->AcceptApplication($app);
  $ok = $self->AcceptVersion($version);
  $ok = $self->AcceptUser($user, $password);

The RPC::PlServer package has a very detailed access control scheme: First of all it inherits Net::Daemon's host based access control. It adds version control and user authorization. To achieve that, the method Accept from Net::Daemon is split into three methods, AcceptApplication, AcceptVersion and AcceptUser, each of them returning TRUE or FALSE. The client receives the arguments as the attributes application, version, user and password. A client is accepted only if all of the above methods are returning TRUE.

The default implementations are as follows: The AcceptApplication method returns TRUE, if $self is a subclass of $app. The AcceptVersion method returns TRUE, if the requested version is less or equal to ${$class}::VERSION, $self being an instance of $class. Whether a user is permitted to connect depends on the client configuration. See CONFIGURATION FILE below for examples.

Method based access control

Giving a client the ability to invoke arbitrary methods can be a terrible security hole. Thus the server has a methods attribute. This is a hash ref of class names as keys, the values being hash refs again with method names as the keys. That is, if your hash looks as follows:

    $self->{'methods'} = {
        'CalcServer' => {
            'NewHandle' => 1,
            'CallMethod' => 1 },
        'Calculator' => {
            'new' => 1,
            'multiply' => 1,
            'add' => 1,
            'divide' => 1,
            'subtract' => 1 }
        };

then the client may use the CalcServer's NewHandle method to create objects, but only via the permitted constructor Calculator->new. Once a Calculator object is created, the server may invoke the methods multiply, add, divide and subtract.

CONFIGURATION FILE

The server config file is inherited from Net::Daemon. It adds the users and cipher attribute to the client list. Thus a typical config file might look as follows:

    # Load external modules; this is not required unless you use
    # the chroot() option.
    #require DBD::mysql;
    #require DBD::CSV;

    # Create keys
    my $myhost_key = Crypt::IDEA->new('83fbd23390ade239');
    my $bob_key    = Crypt::IDEA->new('be39893df23f98a2');

    {
        # 'chroot' => '/var/dbiproxy',
        'facility' => 'daemon',
        'pidfile' => '/var/dbiproxy/dbiproxy.pid',
        'user' => 'nobody',
        'group' => 'nobody',
        'localport' => '1003',
        'mode' => 'fork',

        # Access control
        'clients' => [
            # Accept the local LAN (192.168.1.*)
            {
                'mask' => '^192\.168\.1\.\d+$',
                'accept' => 1,
                'users' => [ 'bob', 'jim' ],
                'cipher' => $myhost_key
            },
            # Accept myhost.company.com
            {
                'mask' => '^myhost\.company\.com$',
                'accept' => 1,
                'users' => [ {
                    'name' => 'bob',
                    'cipher' => $bob_key
                    } ]
            },
            # Deny everything else
            {
                'mask' => '.*',
                'accept' => 0
            }
        ]
    }

Things you should note: The user list of 192.168.1.* contains scalar values, but the user list of myhost.company.com contains hash refs: This is required, because the user configuration is more specific for user based encryption.

EXAMPLE

Enough wasted time, spread the example, not the word. :-) Let's write a simple server, say a server for MD5 digests. The server uses the external package MD5, but the client doesn't need to install the package. MD5(3). We present the server source here, the client is part of the RPC::PlClient man page. See RPC::PlClient(3).

    #!/usr/bin/perl -wT
    # Note the -T switch! This is always recommended for Perl servers.

    use strict;               # Always a good choice.

    require RPC::PlServer;
    require MD5;

    package MD5_Server;  # Clients need to request application
                         # "MD5_Server"

    $MD5_Server::VERSION = '1.0'; # Clients will be refused, if they
                                  # request version 1.1
    @MD5_Server::ISA = qw(RPC::PlServer);

    eval {
        # Server options below can be overwritten in the config file or
        # on the command line.
        my $server = MD5_Server->new({
	    'pidfile'    => '/var/run/md5serv.pid',
	    'configfile' => '/etc/md5serv.conf',
	    'facility'   => 'daemon', # Default
	    'user'       => 'nobody',
	    'group'      => 'nobody',
	    'localport'  => 2000,
	    'logfile'    => 0,        # Use syslog
            'mode'       => 'fork',   # Recommended for Unix
            'methods'    => {
	        'MD5_Server' => {
		    'ClientObject' => 1,
		    'CallMethod' => 1,
		    'NewHandle' => 1
		    },
	        'MD5' => {
		    'new' => 1,
		    'add' => 1,
		    'hexdigest' => 1
		    },
	        }
        });
        $server->Bind();
    };

SECURITY

It has to be said: PlRPC based servers are a potential security problem! I did my best to avoid security problems, but it is more than likely, that I missed something. Security was a design goal, but not *the* design goal. (A well known problem ...)

I highly recommend the following design principles:

Protection against "trusted" users

perlsec: Read the perl security FAQ (perldoc perlsec) and use the -T switch.
taintperl: Use the -T switch. I mean it!
Verify data: Never untaint strings withouth verification, better verify twice. For example the CallMethod function first checks, whether an object handle is valid before coercing a method on it.
Be restrictive: Think twice, before you give a client access to a method.
perlsec: And just in case I forgot it: Read the perlsec man page. :-)

Protection against untrusted users

Host based authorization: PlRPC has a builtin host based authorization scheme; use it! See /CONFIGURATION FILE.
User based authorization: PlRPC has a builtin user based authorization scheme; use it! See /CONFIGURATION FILE.
Encryption: Using encryption with PlRPC is extremely easy. There is absolutely no reason for communicating unencrypted with the clients. Even more: I recommend two phase encryption: The first phase is the login phase, where to use a host based key. As soon as the user has authorized, you should switch to a user based key. See the DBI::ProxyServer for an example.

AUTHOR AND COPYRIGHT

The PlRPC-modules are

  Copyright (C) 1998, Jochen Wiedmann
                      Am Eisteich 9
                      72555 Metzingen
                      Germany

                      Phone: +49 7123 14887
                      Email: joe@ispsoft.de

  All rights reserved.

You may distribute this package under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.