=head1 NAME

Arc::Command - Abstract base class for ARCv2 commands

=head1 DESCRIPTION

ARC allows non-privileged users to run privileged commands on the server.
The server decides if the user is allowed to run this command through ACL.

This file is a part of the Perl ARCv2 module suite. ARCv2 is a 
rewrite of ARC by R.Toebbicke, CERN, Switzerland in Perl. 

=head1 ABSTRACT

From ARC by R. Toebbicke, modified by me:
User requests are shipped from a client machine to a server using a
SASL-authenticated socket connection. The purpose is to convey
requests such as privileged commands (e.g. AFS, Crontab) to be executed on the
server under appropriate privileges. Given that all privileges are
confined to the server and the server can be programmed as to filter and
check the command to be executed, the client machine can be less trusted
than the server.

Because ARC-v1-Commands are written in perl anyway, implementing the client/server
in perl makes sense. Platform-independence and "easy-to-read" source code are welcome
too. This package provides two perl command line scripts (arcx, arcxd). They can
be used for working with the ARC server from the command line, resp. to start the
server.

=head1 SYNOPSIS

This module is part of the module suite ARCv2.

This is the command module from ARCv2. If we would use C++, we would say
this is an abstract class of an ARC Command. All commands used by ARCv2 should
derive from this class.

=head1 HOW DOES IT WORKS

This abstract class is the base class for all existing ARCv2 commands and 
should be the base class for all new ARCv2 commands.
 
When starting the ARCv2 Server you have to pass a important hash:

 $arc = new Arc::Server ( 
    [..] Arc::Server vars [..],
   connection_vars => { 
      commands => { 
        'pv' => 'Arc::Command::Pv'
      } 
   }
 )  
 
 resp.

 $arc->{connection_vars}->{commands}

This hash describes the assignment of B<Command Name> and B<Command Class>. 
When a client has authenticated and wants to run a command, it will send 
the B<Command Name> and suitable, optional parameters. The server will look into 
the commands hash and creates an object of the B<Command Class> 
associated with B<Command Name>.

 my $perlcmd = $this->{commands}->{$cmd};
 [..]
 eval "require $perlcmd;"
 [..]
 (fork)
 my $ret = eval
 {
  my $object = new $perlcmd(
    _username => $this->{_username},
    _peeraddr => $this->{peeraddr},
    _peerport => $this->{peerport},
    _peername => $this->{peername},
    _cmd => $cmd,
    logfileprefix => "command",
  );
  $object->Execute(@a);
  $cmderr = $object->IsError();

  return -1;
 };
 
When everything went alright, the command will be executed. The command runs
in a separate process. Therefore STDIN, STDOUT and STDERR are duped to two
pipes, one for the in, one for the out direction. In the parent process data
from the encrypted network command connection is read from and written to these pipes.
Same situation on the client side, STDIN and STDOUT are used to put and get the 
data through the network.
 
                   encrypted
          /--->>---| net- |--->>-----\   
        / /---<<---| work |---<<-----\ \
      / /                              \ \
     | |     in                         | |    p2
  |--------|->>--\                  |--------|->>--\ 
  | Client | out   \                | Server | p1    \
  |--------|-<<-\    \              |--------|-<<-\    \ 
                /|\  \|/                          /|\  \|/
               |--------|                       |-----------|
               |  User  |                       |  Command  |
               |--------|                       |-----------|
 
This design makes it easy for ARCv2 Commands to get input and produce output.

B<Example>:
 sub Execute 
 {
  while ($_ = <STDIN>) { # ends on EOF
     s/a/b/g; print;
  }
 }
 
If you want to implement a new Command for ARCv2 you have to derive from 
Arc::Command and override the sub C<Execute>. See existing Arc::Command::* 
classes for examples. To get your Command recognised you have to assign a 
B<Command Name> to your command class. ARCv2 ignores the return code of
B<Execute>. If your command runs into an error use the _SetError function 
and return immediately. This is what ARCv2 will evaluate and send to the
client.

B<Example>:
 sub Execute
 {
  my $this = shift;
  my $pw = <>;
  if ($pw ne "klaus") {
	  return $this->_SetError("Wrong password.");
  }
 }
 
In ARCv2 some standard commands are already implemented: C<Arc::Command::Get>,
C<Arc::Command::Put>, C<Arc::Command::Uptime>, C<Arc::Command::Whoami>,
C<Arc::Command::Test>.

By default, these classes are mapped to B<Command Names> as follows (in the 
default arcxd.conf for arcxd):
  uptime => Arc::Command::Uptime,
  whoami => Arc::Command::Whoami,
  copy   => Arc::Command::Get,
  cp     => Arc::Command::Get,
  get    => Arc::Command::Get,
  put    => Arc::Command::Put,
  test   => Arc::Command::Test,
  help   => Arc::Command::Help,
 
B<Caution>: Especially take care of the C<Arc::Command::Get> and 
C<Arc::Command::Put> in production environment. As ARCv2 will probably
run as root and by default the Get and Put command do NOT have an access
control, everyone can get or put any files from/to your ARCv2 server.

There are some member variables, which contain information about the 
client. See 'Class VARIABLES' for a complete list of them. These values 
are filled by Arc::Connection::Server, when the client wants to run a command.


=head1 Class VARIABLES

=head3 PUBLIC MEMBERS

=over 2

=item logfileprefix I<reimplemented from Arc>

B<Default value>: "command"

=back 

=over 2

=item logdestination I<inherited from Arc>

B<Description>: Where should all the log output go to ('stderr','syslog')

B<Default value>: 'syslog'

=item loglevel I<inherited from Arc>

B<Description>: loglevel is combination of bits (1=AUTH,2=USER,4=ERR,8=CMDDEBUG,16=VERBSIDE,32=DEBUG) see _Log method

B<Default value>: 7

=back 

=over 2

=back 

=head3 PROTECTED MEMBERS

=over 2

=item _cmd 

B<Description>: user runs this command

B<Default value>: undef

=item _commands 

B<Description>: the "available commands"-hash from the server, 

B<Default value>: {}

=item _mech 

B<Description>: user uses this authentication mechanism (e.g. GSSAPI)

B<Default value>: undef

=item _peeraddr 

B<Description>: users ip address

B<Default value>: undef

=item _peername 

B<Description>: users host address in sockaddr_in format

B<Default value>: undef

=item _peerport 

B<Description>: users port

B<Default value>: undef

=item _realm 

B<Description>: the name of the realm, to which the user belongs (SASL)

B<Default value>: ""

=item _username 

B<Description>: user, who has authenticated against ARCv2 Server by using SASL

B<Default value>: ""

=back 

=over 2

=item _error I<inherited from Arc>

B<Description>: contains the error message

B<Default value>: undef

=item _syslog I<inherited from Arc>

B<Description>: log to syslog or to STDERR

B<Default value>: 1

=back 

=over 2

=back 

=head3 PRIVATE MEMBERS

=head1 Class METHODS

=head3 PUBLIC METHODS

=over 2

=item Execute ( ... (parameter from users request) ) 

B<Description>: execute this command.
This function is called by the ARCv2 Server when the user wants 
to execute this command. 


B<Returns:> true if the command has succeeded, false (and please set _SetError) if not.


=back 

=over 2

=item DESTROY (  ) I<inherited from Arc>

B<Description>: Destructor


=item IsError (  ) I<inherited from Arc>

B<Description>: User function to get the error msg.


B<Returns:> the error message if any otherwise undef


B<Example:>

unless (my $err = $arc->IsError()) { .. } else { print STDERR $err; }


=item Log ( $facility, ... (message) ) I<inherited from Arc>

B<Description>: Log function.
Logs messages to 'logdestination' if 'loglevel' is is set appropriatly.
loglevel behaviour has changed in the 1.0 release of ARCv2, the "Arc"-class can export
LOG_AUTH (authentication information), LOG_USER (connection information), LOG_ERR (errors), 
LOG_CMD (ARCv2 addition internal command information), LOG_SIDE (verbose client/server-specific
information), LOG_DEBUG (verbose debug information). It possible to combine the 
levels with or (resp. +) to allow a message to appear when not all loglevels are 
requested by the user.
Commonly used for logging errors from application level.


B<Returns:> always false


B<Example:>

return $arc->Log(LOG_ERR,"Message");


=item new ( %hash, key => val, ... ) I<inherited from Arc>

B<Description>: Constructor. 
Initializes the object and returns it blessed.
For all sub classes, please override C<_Init> to check the 
parameter which are passed to the C<new> function. This
is necessary because you are not able to call the the new method of a
parent class, when having a class name (new $class::SUPER::new, does not work.).


B<Returns:> blessed object of the class


B<Example:>

my $this = new Arc::Class ( key => value, key2 => value2 );


=back 

=over 2

=back 

=head3 PROTECTED METHODS

=over 2

=item _Debug ( ... (message) ) I<inherited from Arc>

B<Description>: Debug function.
Logs messages with "DEBUG" 


B<Returns:> always false


B<Example:>

$this->_Debug("hello","world"); # message will be "hello world"


=item _Init ( %hash, key => val, ... ) I<inherited from Arc>

B<Description>: Init function (initializes class context)
Module dependent initialization, every subclass shall override it
and call the _Init of its SUPER class. This method is called by the new method of C<Arc>.


B<Returns:> true, if all passed values are in their definition scope, otherwise false


B<Example:>

see source code of any non-abstract sub class of Arc


=item _SetError ( ... (message)  ) I<inherited from Arc>

B<Description>: SetError function.
This function prepends the error message (@_) to an existing error message (if any) and
logs the message with LOG_ERR facility.
Use this function for setting an error from class level. Users should use IsError 
to get the message if a function failed.


B<Returns:> always false


B<Example:>

return $this->_SetError("User is not allowed to do this."); # breaks when an error occured


=back 

=over 2

=back 

=head3 PRIVATE METHODS

=head1 SEE ALSO

L<Arc>, L<Arc::Command>, L<Arc::Connection>, 
L<Arc::Connection::Server>, L<Arc::Connection::Client>,
L<arcx>, L<arcxd>, L<Authen::SASL>, L<Authen::SASL::Cyrus>
L<Net::Server::PreFork>

=head1 AUTHOR

 Patrick Boettcher <patrick.boettcher@desy.de>

=head1 COPYRIGHT AND LICENSE

Copyright (c) 2003-5 Patrick Boettcher <patrick.boettcher@desy.de> and others.
All rights reserved.
Zeuthen, Germany, (old) Europe

 This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

 Special thanks go to:
DESY Zeuthen, in particular:

- Wolfgang Friebel for bleeding edge testing and heavy bug reporting (and the idea of reimplementing ARC).

- Waltraut Niepraschk and Andreas Haupt for their help and support during the development. 

=cut