Python interface to remctl

OVERVIEW

The Python interface to remctl provides Python bindings to the libremctl client library plus a high-level interface that translates the libremctl API into something closer to idiomatic Python.

This module provides three interfaces: a low-level interface that provides a minimal translation from the libremctl API; a simple interface in Python that performs a single call to a remctl server and returns the result as an object; and a full interface in Python which provides more control over the connection, returns individual output tokens, and allows multiple commands to be sent via the same connection.

REQUIREMENTS

The module has been tested with Python 2.5 and may not work with older versions of Python, although the only known incompatibility are parameters in setup.py not supported prior to Python 2.3.

SIMPLIFIED INTERFACE

remctl.remctl(host, port, principal, command)

Runs a command on the remote system and returns an object containing the results.

Arguments:
* host (required): string, the host to connect to
* port (optional): unsigned short, the port to connect to
* principal (optional): string, authentication identity of host
* command (required): sequence or iterator yielding the command

If port is not given, the library default is used (try 4373 first and fall back to attempting the legacy 4444 port). If principal is not given, the library default (host/<host> with the realm determined by the domain-realm mapping) is used. To use the defaults, only the host and command may be specified using named arguments, or None can be passed as the port and principal arguments.

command can be any sequence or iterator that returns a series of strings or things that can be converted to strings making up the command.

Returns an object of type RemctlSimpleResult with the following
attributes:
* stdout: string, the standard output from the command
* stderr: string, the standard error from the command
* status: integer, the exit status of the command

Exceptions:
* ValueError, TypeError: an invalid argument was supplied
* RemctlProtocolError: an error occurred in the remctl communication

The string value of the RemctlProtocolError exception contains the string returned by the remctl library. This may be an internal error (such as a network error) or an error returned by the remote server (such as an unknown command error).

Here is an example using the simplified interface:

    import remctl, sys
    command = ('test', 'test')
    try:
        result = remctl.remctl(host = 'foo.example.com', command = command)
    except remctl.RemctlProtocolError, error:
        print "Error:", str(error)
        sys.exit(1)
    if result.stdout:
        print "stdout:", result.stdout
    if result.stderr:
        print "stderr:", result.stderr
    print "exit status:", result.status

FULL INTERFACE

The full remctl interface requires the user to do more bookkeeping, but provides more flexibility and visibility into what is happening at a protocol level. It allows issuing multiple commands on the same persistant connection (provided that the remote server supports protocol version two; if it doesn't, the library will transparently fall back to opening a connection for each command).

To use the full interface, first create a connection object with remctl.Remctl() (either passing it the connection arguments or then calling its open() method), and then call the command() method to issue a command. Read output tokens with output() until a status token has been received. Then the command is complete and another command can be issued.

remctl.Remctl(host, port, principal)

The constructor. Create a new Remctl object. The arguments are optional; if given, the constructor immediately calls open() and passes those arguments to it. See below for their meaning and possible exceptions.

All further methods below must be called on a Remctl object as returned by the constructor.

Remctl.set_ccache(ccache)

Sets the credential cache for outgoing connections to ccache.

Arguments:
* ccache (required): string, the credential cache to use

This is normally the full path to a Kerberos ticket cache, but may have other valid forms depending on the underlying Kerberos implementation in use by GSS-API. This method will affect all subsequent open() calls on the same object, but will have no effect on connections that are already open.

If the remctl client library was built against a Kerberos library and the GSS-API library supported gss_krb5_import_cred, this call affects only this Remctl object. Otherwise, this will affect not only all subsequent open() calls for the same object, but all subsequent remctl connections of any kind from the same process, and even other GSS-API connections from the same process unrelated to remctl.

Not all GSS-API implementations support setting the credential cache. If this operation is not supported, a RemctlError exception will be thrown.

Exceptions:
* RemctlError: setting the credential cache isn't supported

Remctl.set_source_ip(source)

Sets the source IP for future outgoing connections. This will affect all subsequent open() calls on the same object, but will have no effect on connections that are already open.

Arguments:
* source (required): string, the source IP address

The source may be either an IPv4 or an IPv6 address (if IPv6 is supported). It must be an IP address, not a host name.

Exceptions:
* RemctlError: a memory allocation failure occurred

Remctl.set_timeout(timeout)

Sets the timeout for connections and commands. All subsequent operations on this object will be subject to this timeout, including open() if called prior to calling open(). If the timeout is 0, this clears any timeout that was previously set.

The timeout is a timeout on network activity from the server, not on a complete operation. So, for example, a timeout of ten seconds just requires that the server send some data every ten seconds. If the server sends only tiny amounts of data at a time, the complete operation could take much longer than ten seconds without triggering the timeout.

Arguments:
* timeout (required): long, the timeout in seconds or 0

Exceptions:
* RemctlError: the timeout was negative

Remctl.open(host, port, principal)

Open a connection to a remote server and authenticate. There is no return value; an exception is thrown on any error.

Arguments:
* host (required): string, the host to connect to
* port (optional): unsigned short, the port to connect to
* principal (optional): string, authentication identity of host

If port is not given, the library default is used (try 4373 first and fall back to attempting the legacy 4444 port). If principal is not given, the library default (host/<host> with the realm determined by the domain-realm mapping) is used.

Exceptions:
* ValueError, TypeError: an invalid argument was supplied
* RemctlError: a network or authentication error occurred

The string value of the RemctlError exception contains the string returned by the remctl library, similar to what would be returned by the error() method.

Remctl.command(command)

Send a command to the remote host. There is no return value; an exception is thrown on any error.

The Remctl object must already be connected. The command may, under the remctl protocol, contain any character, but be aware that most remctl servers will reject commands or arguments containing ASCII 0 (NUL). This currently therefore cannot be used for upload of arbitrary unencoded binary data.

Arguments:
* command (required): sequence or iterator yielding the command

command can be any sequence or iterator that returns a series of strings or things that can be converted to strings making up the command.

Exceptions:
* ValueError, TypeError: an invalid argument was supplied
* RemctlError: a network or authentication error occurred
* RemctlNotOpenedError: no connection currently open

The string value of the RemctlError exception contains the string returned by the remctl library, the same as would be returned by the error() method.

Remctl.output()

Reads an output token from the server and returns it as a tuple. A command will result in either one error token or zero or more output tokens followed by a status token. The output is complete as soon as any token other than an output token has been received, but the module will keep returning done tokens to the caller for as long as output() is called without another call to command().

The members of the returned tuple are: * type: string, "output", "status", "error", or "done" * output: string, the returned output or error * stream: integer, 1 for stdout or 2 for stderr for an output token * status: integer, exit status of command for a status token * error: integer, remctl protocol error for an error token

type will always be present. The other members of the tuple may be None depending on the type of token. Output tokens will have output and stream information, error tokens will have output and error information, and status tokens will have status information. done tokens will return None for all other elements.

For error tokens, error holds the numeric error code (see the remctl protocol specification), which is the recommended value for programs to check when looking for specific errors. output will contain an English text translation of the error code and the exact text may change.

If the server does not return an output token, output() will throw an exception.

Exceptions:
* RemctlError: a network or authentication error occurred
* RemctlNotOpenedError: no connection currently open

Remctl.noop()

Send a NOOP message to the server and read the reply. This is primarily used to keep a connection to a remctl server alive, such as through a firewall with a session timeout, while waiting to issue further commands. Returns true on success, false on failure.

The NOOP message requires protocol version 3 support in the server, so the caller should be prepared for this function to fail, indicating that the connection could not be kept alive and possibly that it was closed by the server. In this case, the client will need to explicitly reopen the connection with open().

Exceptions:
* RemctlError: a network or authentication error occurred
* RemctlNotOpenedError: no connection currently open

Remctl.close()

Close a connection. After calling this method, open() must be called for this object before sending any further commands. The connection will also be automatically closed when the object is destroyed, so calling this method is often not necessary.

Remctl.error()

Returns the error from the last failed Remctl method. This will be the same string as was returned as the string value of a RemctlError or RemctlProtocolError exception.

LOW-LEVEL INTERFACE

This module also provides a _remctl module, which exports a low-level interface with essentially the same API as the C API. It is very similar to the simplified and full interface above except that the simplified call returns its results as a tuple, status codes are returned from functions instead of throwing exceptions (exceptions are only thrown for things like out-of-memory errors), argument checking won't be as nice and in some cases relies on the underlying libremctl C library to do the error checking, and command arguments have to be lists and not arbitrary sequences or iterators.

The _remctl interface is not currently documented or intended for direct use. Use at your own risk.

HISTORY

The original implementation was written by Thomas L. Kula <kula@tproa.net> and was known as pyremctl. As of remctl 2.13 it is part of the stock remctl distribution and ongoing maintenance is done by Russ Allbery and Thomas L. Kula.

THANKS

Andrew Mortensen for general code formatting comments and a reminder to free malloc'd memory.

LICENSE

Copyright 2008 Thomas L. Kula <kula@tproa.net>
Copyright 2008, 2009, 2011, 2012
    The Board of Trustees of the Leland Stanford Junior University
Copyright 2014 Russ Allbery <eagle@eyrie.org>

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.

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