Dtls 1.2.3
Python Datagram Transport Layer Security
License: Apache-2.0

Datagram Transport Layer Security for Python
============================================

PyDTLS brings Datagram Transport Layer Security (DTLS - RFC 6347:
http://tools.ietf.org/html/rfc6347) to the Python environment. In a
nutshell, DTLS brings security (encryption, server authentication, user
authentication, and message authentication) to UDP datagram payloads in
a manner equivalent to what SSL/TLS does for TCP stream content.

DTLS is now very easy to use in Python. If you're familiar with the ssl
module in Python's standard library, you already know how. All it takes
is passing a datagram/UDP socket to the *wrap\_socket* function instead
of a stream/TCP socket. Here's how one sets up the client side of a
connection:

::

    import ssl
    from socket import socket, AF_INET, SOCK_DGRAM
    from dtls import do_patch
    do_patch()
    sock = ssl.wrap_socket(socket(AF_INET, SOCK_DGRAM))
    sock.connect(('foo.bar.com', 1234))
    sock.send('Hi there')

As of version 1.2.0, PyDTLS supports DTLS version 1.2 in addition to
version 1.0. This version also introduces forward secrecy using elliptic
curve cryptography and more fine-grained configuration options.

Installation
------------

To install from PyPI, on any supported platform enter:

::

    pip install Dtls

Design Goals
------------

The primary design goal of PyDTLS is broad availability. It has
therefore been built to be widely compatible with the following:

-  Operating systems: apart from the Python standard library, PyDTLS
   relies on the OpenSSL library only. OpenSSL is widely ported, and in
   fact the Python standard library's *ssl* module also uses it.
-  Python runtime environments: PyDTLS is a package consisting of pure
   Python modules only. It should therefore be portable to many
   interpreters and runtime environments. It interfaces with OpenSSL
   strictly through the standard library's *ctypes* foreign function
   library.
-  The Python standard library: the standard library's *ssl* module is
   Python's de facto interface to SSL/TLS. PyDTLS aims to be compatible
   with the full public interface presented by this module. The ssl
   module ought to behave identically with respect to all of its
   functions and options when used in conjunction with PyDTLS and
   datagram sockets as when used without PyDTLS and stream sockets.
-  Connection-based protocols: as outlined below, layering security on
   top of datagram sockets requires introducing certain connection
   constructs normally absent from datagram sockets. These constructs
   have been added in such a way as to be compatible with code that
   expects to interoperate with connection-oriented stream sockets. For
   example, code that expects to go through server-side
   bind/listen/accept connection establishment should be reusable with
   PyDTLS sockets.

Distributions
-------------

PyDTLS requires version 1.0.0 or higher of the OpenSSL library. Earlier
versions are reported not to offer stable DTLS support. Since packaged
distributions of this version of OpenSSL are available for many popular
operating systems, OpenSSL-1.0.0 is an installation requirement before
PyDTLS functionality can be called. On Ubuntu 12.04 LTS, for example,
the Python interpreter links with libcrypto.so.1.0.0 and
libssl.so.1.0.0, and so use of PyDTLS requires no further installation
steps.

In comparison, installation of OpenSSL on Microsoft Windows operating
systems is inconvenient. For this reason, source distributions of PyDTLS
are available that include OpenSSL dll's for 32-bit and 64-bit Windows.
All dll's have been linked with the Visual Studio 2008 version of the
Microsoft C runtime library, msvcr90.dll, the version used by CPython
2.7. Installation of Microsoft redistributable runtime packages should
therefore not be required on machines with CPython 2.7. The version of
OpenSSL distributed with PyDTLS 0.1.0 is 1.0.1c. The version distributed
with PyDTLS 1.2.0 is commit 248cf959672041f38f4d80a4a09ee01d8ab04fe8
(branch OpenSSL\_1\_0\_2-stable, 1.0.2l-dev, containing a desirable fix
to DTLSv1\_listen not present in 1.0.2k, the stable version at the time
of PyDTLS 1.2.0 release).

The OpenSSL version used by PyDTLS can be determined from the values of
*sslconnection's* DTLS\_OPENSSL\_VERSION\_NUMBER,
DTLS\_OPENSSL\_VERSION, and DTLS\_OPENSSL\_VERSION\_INFO. These
variables are available through the *ssl* module also if *do\_patch* has
been called (see below). Note that the OpenSSL version used by PyDTLS
may differ from the one used by the *ssl* module.

Interfaces
----------

PyDTLS' top-level package, *dtls*, provides DTLS support through the
**SSLConnection** class of its *sslconnection* module. **SSLConnection**
is in-line documented, and dtls/test/echo\_seq.py demonstrates how to
take a simple echo server through a listen/accept/echo/shutdown sequence
using this class. The corresponding client side can look like the
snippet at the top of this document, followed by a call to the *unwrap*
method for shutdown (or a call to the **SSLConnection** *shutdown*
method, if an instance of this class is used for the client side also).
Note that the *dtls* package does not depend on the standard library's
*ssl* module, and **SSLConnection** can therefore be used in
environments where *ssl* is unavailable or incompatible.

It is expected that with the *ssl* module being an established, familiar
interface to TLS, it will be the preferred module through which to
access DTLS. To do so, one must call the *dtls* package's *do\_patch*
function before passing sockets of type SOCK\_DGRAM to either *ssl's*
*wrap\_socket* function, or *ssl's* **SSLSocket** constructor.

It should be noted that once *do\_patch* is called, *dtls* will raise
exceptions of type **ssl.SSLError** instead of its default
**dtls.err.SSLError**. This allows users' error handling code paths to
remain identical when interfacing with *ssl* across stream and datagram
sockets.

Connection Handling
-------------------

The DTLS protocol implies a connection as an association between two
network peers where the overall association state is characterized by
the handshake status of each peer endpoint (see RFC 6347). The OpenSSL
library records this handshake status in "SSL" type instances (a.k.a.
struct ssl\_st). Datagrams can be securely sent and received by
referring to a unique "SSL" instance after handshaking has been
completed with this instance and its network peer. A connection is
implied in that traffic may be directed to or received from only that
network peer with whose "SSL" instance the handshake has been completed.
The fact that the underlying network protocol, UDP in most cases, is
itself connectionless is immaterial.

Further, in order to prevent denial-of-service attacks on UDP DTLS
servers, clients must undergo a cookie exchange phase early in the
handshaking protocol, and before server-side resources are committed to
a particular client (see section 4.2.1 of RFC 6347). The cookie exchange
proves to the server that a client can indeed receive IP traffic at the
source IP address with which its handshake-initiating ClientHello
datagram is marked.

PyDTLS implements this connection establishment through the *connect*
method on the client side, and the *accept* method on the server side.
The latter returns a new **dtls.SSLConnection** or **ssl.SSLSocket**
object (depending on which interface is used, see above), which is
"connected" to its peer. In addition to the *read* and *write* methods
(at both interface levels), **SSLSocket's** *send* and *recv* methods
can be used; use of *sendto* and *recvfrom* on connected sockets is
prohibited by *ssl*. *accept* returns peer address information, as
expected. Note that when using the *ssl* interface to *dtls*, *listen*
must be called before calling *accept*.

Demultiplexing
--------------

At the network io layer, only datagrams from its connected peer must be
passed to a **SSLConnection** or **SSLSocket** object (unless the object
is unconnected on the server-side, in which case it can be in listening
mode, the initial server-side socket whose role it is to listen for
incoming client connection requests).

The initial server-side listening socket is not useful for performing
this datagram routing function. This is because it must remain
unconnected and ready to receive additional connection requests from
new, unknown clients.

The function of passing incoming datagrams to the proper connection is
performed by the *dtls.demux* package. **SSLConnection** requests a new
connection from the demux when a handshake has cleared the cookie
exchange phase. An efficient implementation of this request is provided
by the *osnet* module of the demux package: it creates a new socket that
is bound to the same network interface and port as the listening socket,
but connected to the peer. UDP stacks such as the one included with
Linux route incoming datagrams to such a connected socket in preference
to an unconnected socket bound to the same port.

Unfortunately such is not the behavior on Microsoft Windows. Windows UDP
routes datagrams to whichever currently existing socket bound to the
particular port the earliest (and whether or not that socket is
unconnected, or connected to the datagram's peer, or a different peer).
Other sockets bound to the same port will not receive traffic, if and
until they become the earliest bound socket because another socket is
closed.

The demux package therefore provides and automatically selects the
module *router* on Windows platforms. This module also creates a new
socket when receiving a new connection request; but instead of binding
this socket to the same port as the listening socket, it binds to a new
ephemeral port. *router* then forwards datagrams originating from the
peer for which a connection was requested to the corresponding socket.

For efficiency's sake, no forwarding is performed on outgoing traffic.
Instead, **SSLConnection** directs outgoing traffic from the original
listening socket, using *sendto*. At the OpenSSL level this requires
separate read and write datagram BIO's for an "SSL" instance, one in
"connected" state and one in "peer set" state, respectively, and
associated with two separate network sockets.

>From the perspective of a PyDTLS user, this selection of and difference
between demux implementations should be transparent, with the possible
exception of performance deviation. This transparency does however have
some limits: for example, when *router* is in use, the *accept* methods
can return *None*. This happens when **SSLConnection** detects that the
demux has forwarded a datagram to a known connection instead of
initiating a connection to a new peer through *accept*. Returning *None*
in this case is important whenever non-blocking sockets or sockets with
timeouts are used, since another socket might now be readable as a
result of the forwarded datagram. *accept* must return so that the
application can iterate on its asynchronous *select* loop.

Shutdown and Unwrapping
-----------------------

PyDTLS implements the SSL/TLS shutdown protocol as it has been adapted
for DTLS. **SSLConnection's** *shutdown* and **SSLSocket's** *unwrap*
invoke this protocol. As is the case with DTLS handshaking in general,
applications must be prepared to use the *get\_timeout* and
*handle\_timeout* methods in addition to re-invoking *shutdown* or
*unwrap* when sockets become readable and an exception carried
SSL\_ERROR\_WANT\_READ. (See more on asynchronous IO in the Testing
section.)

**SSLConnection's** *shutdown* and **SSLSocket's** *unwrap* return a
(possibly new) socket that can be used for unsecured communication with
the peer, as set forth by the *ssl* module. The demux infrastructure
remains in use for this communication until the returned socket is
cleaned up. Note that when the *router* demux is in use, the object
returned will be one derived from *socket.socket*. This is because the
send and recv paths must still be directed to two different OS sockets.
In addition, the right thing happens if secured communication is resumed
over such a socket by passing it to *ssl.wrap\_socket* or the
**SSLConnection** constructor. If *osnet* is used, an actual
*socket.socket* instance is returned.

Framework Compatibility
-----------------------

PyDTLS sockets have been tested under the following usage modes:

-  Using blocking sockets and sockets with timeouts in multi-threaded
   UDP servers
-  Using non-blocking sockets, and in conjunction with the asynchronous
   socket handler, asyncore
-  Using blocking sockets, and in conjunction with the network server
   framework SocketServer - ThreadingTCPServer (this works because of
   PyDTLS's emulation of connection-related calls)

Multi-thread Support
--------------------

Using multiple threads with OpenSSL requires implementing a locking
callback. PyDTLS does implement this, and therefore multi-threaded
programming with PyDTLS is safe in any environment. However, being a
pure Python library, these callbacks do carry some overhead. The *ssl*
module implements an equivalent locking callback in its C extension
module. Not requiring interpreter re-entry, this approach can be
expected to perform better. PyDTLS therefore queries OpenSSL as to
whether a locking callback is already in place, and does not overwrite
it if there is. Loading *ssl* can therefore improve performance, even
when only the *sslconnection* interface is used.

Note that loading order does not matter: to obtain the performance
benefit, *ssl* can be loaded before or after the dtls package. This is
because *ssl* does not do an equivalent existing locking callback check,
and will simply overwrite the PyDTLS callback if it has already been
installed. But *ssl* should not be loaded while *dtls* operation is
already in progress, when some locks may be in their acquired state.

Also note that this performance enhancement is available only on
platforms where PyDTLS loads the same OpenSSL shared object as *ssl*. On
Ubuntu 12.04, for example, this is the case, but on Microsoft Windows it
is not.

Testing
-------

A simple echo server is available to be executed from the project root
directory with ``python -m dtls.test.echo_seq``. The echo server is
reachable using the code snippet at the top of this document, using port
28000 at "localhost".

Unit test suites can be executed from the project root directory with
``python -m dtls.test.unit [-v]`` and
``python -m dtls.test.unit_wrapper`` (for the client and server
wrappers)

Almost all of the Python standard library's *ssl* unit tests from the
module *test\_ssl.py* have been ported to *dtls.test.unit.py*. All tests
have been adjusted to operate with datagram sockets. On Linux, each test
is executed four times, varying the address family among IPv4 and IPv6
and the demux among *osnet* and *router*. On Windows, where *osnet* is
unavailable, each test is run twice, once with IPv4 and once with IPv6.

The unit test suite includes tests for each of the above-mentioned
compatible frameworks. The class **AsyncoreEchoServer** serves as an
example of how to use non-blocking datagram sockets and implement the
resulting timeout detection requirements. DTLS in general and OpenSSL in
particular require being called back when used with non-blocking sockets
(or sockets with timeout option) after DTLS timeouts expire to handle
packet loss using re-transmission during a handshake. Handshaking may
occur during any read or write operation, even after an initial
handshake completes successfully, in case renegotiation is requested by
a peer.

Running with the -v switch executes all unit tests in verbose mode.

dtls/test/test\_perf.py implements an interactive performance test suite
that compares the raw throughput of TCP, UDP, SSL, and DTLS. It can be
executed locally through the loopback interface, or between remote
clients and servers. In the latter case, test jobs are sent to remote
connected clients whenever a suite run is initiated through the
interactive interface. Run test\_perf.py -h for more information.

It should be noted that comparing the performance of protocols that
don't offer congestion control (UDP and DTLS) with those that do (TCP
and SSL) is a difficult undertaking. Raw throughput even across gigabit
network links can be expected to suffer without congestion control and
peers that generate data as fast as possible without throttling (as this
test does): the link's throughput will drop significantly as it enters
congestion collapse. Similarly, loopback is an imperfect test interface
since it rarely drops packets, and never duplicates or reorders them
(thus negating the relative performance benefits of DTLS over SSL).
Nevertheless, some useful insights can be gained by observing the
operation of test\_perf.py, including software stack behavior in the
presence of some amount of packet loss.

Logging
-------

The *dtls* package and its sub-packages log various occurrences,
primarily events that can aid debugging. Especially *router* emits many
messages when the logging level is set to at least *logging.DEBUG*.
dtls/test/echo\_seq.py activates this logging level during its
operation.

Currently Supported Platforms
-----------------------------

At the time of initial release, PyDTLS 0.1.0 has been tested on Ubuntu
12.04.1 LTS 32-bit and 64-bit, as well as Microsoft Windows 7 32-bit and
64-bit, using CPython 2.7.3. Patches with additional platform ports are
welcome.

As of release 1.2.0, PyDTLS is tested on Ubuntu 16.04 LTS as well as
Microsoft Windows 10, using CPython 2.7.13.