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secsh 0.1
"bulbasaur" release, 18 sep 2003
(c) 2003 Robey Pointer <robey@lag.net>
http://www.lag.net/~robey/secsh/
*** WHAT
secsh is a module for python 2.3 that implements the SSH2 protocol for secure
(encrypted and authenticated) connections to remote machines. unlike SSL (aka
TLS), SSH2 protocol does not require heirarchical certificates signed by a
powerful central authority. you may know SSH2 as the protocol that replaced
telnet and rsh for secure access to remote shells, but the protocol also
includes the ability to open arbitrary channels to remote services across the
encrypted tunnel (this is how sftp works, for example).
the module works by taking a socket-like object that you pass in, negotiating
with the remote server, authenticating (using a password or a given private
key), and opening flow-controled "channels" to the server, which are returned
as socket-like objects. you are responsible for verifying that the server's
host key is the one you expected to see, and you have control over which kinds
of encryption or hashing you prefer (if you care), but all of the heavy lifting
is done by the secsh module.
it is written entirely in python (no C or platform-dependent code) and is
released under the GNU LGPL (lesser GPL).
*** REQUIREMENTS
python 2.3 <http://www.python.org/>
pyCrypto <http://www.amk.ca/python/code/crypto.html>
*** PORTABILITY
i code and test this library on Linux and MacOS X. for that reason, i'm
pretty sure that it works for all posix platforms, including MacOS. i also
think it will work on Windows, though i've never tested it there. if you
run into Windows problems, send me a patch: portability is important to me.
the Channel object supports a "fileno()" call so that it can be passed into
select or poll, for polling on posix. once you call "fileno()" on a Channel,
it changes behavior in some fundamental ways, and these ways require posix.
so don't call "fileno()" on a Channel on Windows. (the problem is that pipes
are used to simulate an open socket, so that the ssh "socket" has an OS-level
file descriptor. i haven't figured out how to make pipes on Windows go into
non-blocking mode yet. [if you don't understand this last sentence, don't
be afraid. the point is to make the API simple enough that you don't HAVE to
know these screwy steps. i just don't understand windows enough.])
*** DEMO
the demo app (demo.py) is a raw implementation of the normal 'ssh' CLI tool.
while the secsh library should work on all platforms, the demo app will only
run on posix, because it uses select.
you can run demo.py with no arguments, or you can give a hostname (or
username@hostname) on the command line. if you don't, it'll prompt you for
a hostname and username. if you have an ".ssh/" folder, it will try to read
the host keys from there, though it's easily confused. you can choose to
authenticate with a password, or with an RSA or DSS key, but it can only
read your private key file(s) if they're not password-protected.
the demo app leaves a logfile called "demo.log" so you can see what secsh
logs as it works. but the most interesting part is probably the code itself,
which hopefully demonstrates how you can use the secsh library.
*** USE
(this section could probably be improved a lot.)
first, create a Transport by passing in an existing socket (connected to the
desired server). call "start_client(event)", passing in an event which will
be triggered when the negotiation is finished (either successfully or not).
the event is required because each new Transport creates a new worker thread
to handle incoming data asynchronously.
after the event triggers, use "is_active()" to determine if the Transport was
successfully connected. if so, you should check the server's host key to make
sure it's what you expected. don't worry, i don't mean "check" in any crypto
sense: i mean compare the key, byte for byte, with what you saw last time, to
make sure it's the same key. Transport will handle verifying that the server's
key works.
next, authenticate, using either "auth_key" or "auth_password". in the future,
this API may change to accomodate servers that require both forms of auth.
pass another event in so you can determine when the authentication dance is
over. if it was successful, "is_authenticated()" will return true.
once authentication is successful, the Transport is ready to use. call
"open_channel" or "open_session" to create new channels over the Transport
(SSH2 supports many different channels over the same connection). these calls
block until they succeed or fail, and return a Channel object on success, or
None on failure. Channel objects can be treated as "socket-like objects": they
implement:
recv(nbytes)
send(data)
settimeout(timeout_in_seconds)
close()
fileno() [* see note below]
because SSH2 has a windowing kind of flow control, if you stop reading data
from a Channel and its buffer fills up, the server will be unable to send you
any more data until you read some of it. (this won't affect other channels on
the Transport, though.)
* NOTE that if you use "fileno()", the behavior of the Channel will change
slightly, underneath. this shouldn't be noticable outside the library, but
this alternate implementation will not work on non-posix systems. so don't
try calling "fileno()" on Windows! this has the side effect that you can't
pass a Channel to "select" or "poll" on Windows (which should be fine, since
those calls don't exist on Windows). calling "fileno()" creates an OS-level
pipe and generates a real file descriptor which can be used for polling, BUT
should not be used for reading data from the channel: use "recv" instead.
because each Transport has a worker thread running in the background, you
must call "close()" on the Transport to kill this thread. on many platforms,
the python interpreter will refuse to exit cleanly if any of these threads
are still running (and you'll have to kill -9 from another shell window).
*** CHANGELOG
2003-08-24:
* implemented the other hashes: all 4 from the draft are working now
* added 'aes128-cbc' and '3des-cbc' cipher support
* fixed channel eof/close semantics
2003-09-12: version "aerodactyl"
* implemented group-exchange kex ("kex-gex")
* implemented RSA/DSA private key auth
2003-09-13:
* fixed inflate_long and deflate_long to handle negatives, even though
they're never used in the current ssh protocol
2003-09-14:
* fixed session_id handling: re-keying works now
* added the ability for a Channel to have a fileno() for select/poll
purposes, although this will cause worse window performance if the
client app isn't careful
2003-09-16: version "bulbasaur"
* fixed pipe (fileno) method to be nonblocking and it seems to work now
* fixed silly bug that caused large blocks to be truncated
2003-10-08:
* patch to fix Channel.invoke_subsystem and add Channel.exec_command
[vaclav dvorak]
* patch to add Channel.sendall [vaclav dvorak]
* patch to add Channel.shutdown [vaclav dvorak]
* patch to add Channel.makefile and a ChannelFile class which emulates
a python file object [vaclav dvorak]
2003-10-26:
* thread creation no longer happens during construction -- use the new
method "start_client(event)" to get things rolling
* re-keying now takes place after 1GB of data or 1 billion packets
(these limits can be easily changed per-session if needed)
*** MISSING LINKS
* ctr forms of ciphers are missing (blowfish-ctr, aes128-ctr, aes256-ctr)
* can't handle password-protected private key files
* multi-part auth not supported (ie, need username AND pk)
* should have a simple synchronous method that handles all auth & events,
by pre-seeding the password or key info, and the expected key
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