[ SEA-GHOST MINI SHELL]
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 ...)
Due to implementation of PlRPC, it's hard to use internal authentication
mechanisms properly to achieve secured remote calls. Therefore users are
advised to use an external authentication mechanism like TLS or IPsec.
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.
Use of Storable
Storable module used for serialization and deserialization
underneath is inherently insecure. Deserialized data can contain
objects which lead to loading foreign modules and executing possible
attached destructors. Do not accept host-based unauthorized
connections. The Storable module is exercised before checking user
password.
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.
Please note PlRPC encryption does not protect from reply attacks.
You should have implement it on the application or the cipher level.
AUTHOR AND COPYRIGHT
The PlRPC-modules are
Copyright (C) 1998, Jochen Wiedmann
Email: jochen.wiedmann at freenet.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.
SEE ALSO
RPC::PlClient(3), RPC::PlServer::Comm(3), Net::Daemon(3),
Net::Daemon::Log(3), Storable(3), Sys::Syslog(3), Win32::EventLog(3)
See DBI::ProxyServer(3) for an example application.
SEA-GHOST - SHELL CODING BY SEA-GHOST