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msg = client_sock.recv(8192)
if not msg:
break
client_sock.sendall(msg)
def echo_server(address):
s = socket(AF_INETSOCK_STREAM)
s.bind(address)
s.listen(5)
while True:
c,a = s.accept()
echo_handler(c)
echo_server((''18000))
Within a client, you would do this:
from socket import socketAF_INETSOCK_STREAM
secret_key = b'peekaboo'
s = socket(AF_INETSOCK_STREAM)
s.connect(('localhost'18000))
client_authenticate(ssecret_key)
s.send(b'Hello World')
resp = s.recv(1024)
...
Discussion
A common use of hmac authentication is in internal messaging systems and interprocess
communication. For example, if you are writing a system that involves multiple pro‐
cesses communicating across a cluster of machines, you can use this approach to make
sure that only allowed processes are allowed to connect to one another. In fact, HMAC-
based authentication is used internally by the multiprocessing library when it sets up
communication with subprocesses.
It’s important to stress that authenticating a connection is not the same as encryption.
Subsequent communication on an authenticated connection is sent in the clear, and
would be visible to anyone inclined to sniff the traffic (although the secret key known
to both sides is never transmitted).
The authentication algorithm used by hmac is based on cryptographic hashing functions,
such as MD5 and SHA-1, and is described in detail in IETF RFC 2104
11.9. Authenticating Clients Simply  |  463
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11.10. Adding SSL to Network Services
Problem
You want to implement a network service involving sockets where servers and clients
authenticate themselves and encrypt the transmitted data using SSL.
Solution
The ssl module provides support for adding SSL to low-level socket connections. In
particular, the ssl.wrap_socket() function takes an existing socket and wraps an SSL
layer around it. For example, here’s an example of a simple echo server that presents a
server certificate to connecting clients:
from socket import socketAF_INETSOCK_STREAM
import ssl
KEYFILE = 'server_key.pem'   # Private key of the server
CERTFILE = 'server_cert.pem' # Server certificate (given to client)
def echo_client(s):
while True:
data = s.recv(8192)
if data == b'':
break
s.send(data)
s.close()
print('Connection closed')
def echo_server(address):
s = socket(AF_INETSOCK_STREAM)
s.bind(address)
s.listen(1)
# Wrap with an SSL layer requiring client certs
s_ssl = ssl.wrap_socket(s,
keyfile=KEYFILE,
certfile=CERTFILE,
server_side=True
)
# Wait for connections
while True:
try:
c,a = s_ssl.accept()
print('Got connection'ca)
echo_client(c)
except Exception as e:
print('{}: {}'.format(e.__class__.__name__e))
echo_server((''20000))
464  |  Chapter 11: Network and Web Programming
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Here’s an interactive session that shows how to connect to the server as a client. The
client requires the server to present its certificate and verifies it:
>>> from socket import socketAF_INETSOCK_STREAM
>>> import ssl
>>> s = socket(AF_INETSOCK_STREAM)
>>> s_ssl = ssl.wrap_socket(s,
...                         cert_reqs=ssl.CERT_REQUIRED,
...                         ca_certs = 'server_cert.pem')
>>> s_ssl.connect(('localhost'20000))
>>> s_ssl.send(b'Hello World?')
12
>>> s_ssl.recv(8192)
b'Hello World?'
>>>
The problem with all of this low-level socket hacking is that it doesn’t play well with
existing network services already implemented in the standard library. For example,
most server code (HTTP, XML-RPC, etc.) is actually based on the socketserver library.
Client code is also implemented at a higher level. It is possible to add SSL to existing
services, but a slightly different approach is needed.
First, for servers, SSL can be added through the use of a mixin class like this:
import ssl
class SSLMixin:
'''
Mixin class that adds support for SSL to existing servers based
on the socketserver module.
'''
def __init__(self*args,
keyfile=Nonecertfile=Noneca_certs=None,
cert_reqs=ssl.NONE,
**kwargs):
self._keyfile = keyfile
self._certfile = certfile
self._ca_certs = ca_certs
self._cert_reqs = cert_reqs
super().__init__(*args**kwargs)
def get_request(self):
clientaddr = super().get_request()
client_ssl = ssl.wrap_socket(client,
keyfile = self._keyfile,
certfile = self._certfile,
ca_certs = self._ca_certs,
cert_reqs = self._cert_reqs,
server_side = True)
return client_ssladdr
11.10. Adding SSL to Network Services  |  465
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To use this mixin class, you can mix it with other server classes. For example, here’s an
example of defining an XML-RPC server that operates over SSL:
# XML-RPC server with SSL
from xmlrpc.server import SimpleXMLRPCServer
class SSLSimpleXMLRPCServer(SSLMixinSimpleXMLRPCServer):
pass
Here’s the XML-RPC server from Recipe 11.6 modified only slightly to use SSL:
import ssl
from xmlrpc.server import SimpleXMLRPCServer
from sslmixin import SSLMixin
class SSLSimpleXMLRPCServer(SSLMixinSimpleXMLRPCServer):
pass
class KeyValueServer:
_rpc_methods_ = ['get', 'set', 'delete''exists', 'keys']
def __init__(self*args**kwargs):
self._data = {}
self._serv = SSLSimpleXMLRPCServer(*argsallow_none=True, **kwargs)
for name in self._rpc_methods_:
self._serv.register_function(getattr(selfname))
def get(selfname):
return self._data[name]
def set(selfnamevalue):
self._data[name= value
def delete(selfname):
del self._data[name]
def exists(selfname):
return name in self._data
def keys(self):
return list(self._data)
def serve_forever(self):
self._serv.serve_forever()
if __name__ == '__main__':
KEYFILE='server_key.pem'    # Private key of the server
CERTFILE='server_cert.pem'  # Server certificate
kvserv = KeyValueServer(('', 15000),
keyfile=KEYFILE,
certfile=CERTFILE),
kvserv.serve_forever()
466  |  Chapter 11: Network and Web Programming
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To use this server, you can connect using the normal xmlrpc.client module. Just spec‐
ify a https: in the URL. For example:
>>> from xmlrpc.client import ServerProxy
>>> s = ServerProxy('https://localhost:15000'allow_none=True)
>>> s.set('foo','bar')
>>> s.set('spam', [123])
>>> s.keys()
['spam', 'foo']
>>> s.get('foo')
'bar'
>>> s.get('spam')
[1, 2, 3]
>>> s.delete('spam')
>>> s.exists('spam')
False
>>>
One complicated issue with SSL clients is performing extra steps to verify the server
certificate or to present a server with client credentials (such as a client certificate).
Unfortunately, there seems to be no standardized way to accomplish this, so research is
often required. However, here is an example of how to set up a secure XML-RPC con‐
nection that verifies the server’s certificate:
from xmlrpc.client import SafeTransportServerProxy
import ssl
class VerifyCertSafeTransport(SafeTransport):
def __init__(selfcafile, certfile=Nonekeyfile=None):
SafeTransport.__init__(self)
self._ssl_context = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
self._ssl_context.load_verify_locations(cafile)
if cert:
self._ssl_context.load_cert_chain(certfilekeyfile)
self._ssl_context.verify_mode = ssl.CERT_REQUIRED
def make_connection(selfhost):
# Items in the passed dictionary are passed as keyword
# arguments to the http.client.HTTPSConnection() constructor.
# The context argument allows an ssl.SSLContext instance to
# be passed with information about the SSL configuration
s = super().make_connection((host, {'context'self._ssl_context}))
return s
# Create the client proxy
s = ServerProxy('https://localhost:15000',
transport=VerifyCertSafeTransport('server_cert.pem'),
allow_none=True)
11.10. Adding SSL to Network Services  |  467
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As shown, the server presents a certificate to the client and the client verifies it. This
verification can go both directions. If the server wants to verify the client, change the
server startup to the following:
if __name__ == '__main__':
KEYFILE='server_key.pem'   # Private key of the server
CERTFILE='server_cert.pem' # Server certificate
CA_CERTS='client_cert.pem' # Certificates of accepted clients
kvserv = KeyValueServer(('', 15000),
keyfile=KEYFILE,
certfile=CERTFILE,
ca_certs=CA_CERTS,
cert_reqs=ssl.CERT_REQUIRED,
)
kvserv.serve_forever()
To make the XML-RPC client present its certificates, change the ServerProxy initiali‐
zation to this:
# Create the client proxy
s = ServerProxy('https://localhost:15000',
transport=VerifyCertSafeTransport('server_cert.pem',
'client_cert.pem',
'client_key.pem'),
allow_none=True)
Discussion
Getting this recipe to work will test your system configuration skills and understanding
of SSL. Perhaps the biggest challenge is simply getting the initial configuration of keys,
certificates, and other matters in order.
To clarify what’s required, each endpoint of an SSL connection typically has a private
key and a signed certificate file. The certificate file contains the public key and is pre‐
sented to the remote peer on each connection. For public servers, certificates are nor‐
mally signed by a certificate authority such as Verisign, Equifax, or similar organization
(something that costs money). To verify server certificates, clients maintain a file con‐
taining the certificates of trusted certificate authorities. For example, web browsers
maintain certificates corresponding to the major certificate authorities and use them to
verify the integrity of certificates presented by web servers during HTTPS connections.
For the purposes of this recipe, you can create what’s known as a self-signed certificate.
Here’s how you do it:
bash % openssl req -new -x509 -days 365 -nodes -out server_cert.pem \
-keyout server_key.pem
Generating a 1024 bit RSA private key
..........................................++++++
...++++++
468  |  Chapter 11: Network and Web Programming
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-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:Illinois
Locality Name (eg, city) []:Chicago
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Dabeaz, LLC
Organizational Unit Name (eg, section) []:
Common Name (eg, YOUR name) []:localhost
Email Address []:
bash %
When creating the certificate, the values for the various fields are often arbitrary. How‐
ever, the “Common Name” field often contains the DNS hostname of servers. If you’re
just testing things out on your own machine, use “localhost.” Otherwise, use the domain
name of the machine that’s going to run the server.
As a result of this configuration, you will have a server_key.pem file that contains the
private key. It looks like this:
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
The server certificate in server_cert.pem looks similar:
-----BEGIN CERTIFICATE-----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.10. Adding SSL to Network Services  |  469
O5OnkTRwGVFJwi+dPmL48i8re68i0o0rioQnCbG2YD8CAwEAAaOBwTCBvjAdBgNV
HQ4EFgQUrtoLHHgXiDZTr26NMmgKJLJLFtIwgY4GA1UdIwSBhjCBg4AUrtoLHHgX
iDZTr26NMmgKJLJLFtKhYKReMFwxCzAJBgNVBAYTAlVTMREwDwYDVQQIEwhJbGxp
bm9pczEQMA4GA1UEBxMHQ2hpY2FnbzEUMBIGA1UEChMLRGFiZWF6LCBMTEMxEjAQ
BgNVBAMTCWxvY2FsaG9zdIIJAPMd+vi45js3MAwGA1UdEwQFMAMBAf8wDQYJKoZI
hvcNAQEFBQADgYEAFci+dqvMG4xF8UTnbGVvZJPIzJDRee6Nbt6AHQo9pOdAIMAu
WsGCplSOaDNdKKzl+b2UT2Zp3AIW4Qd51bouSNnR4M/gnr9ZD1ZctFd3jS+C5XRp
D3vvcW5lAnCCC80P6rXy7d7hTeFu5EYKtRGXNvVNd/06NALGDflrrOwxF3Y=
-----END CERTIFICATE-----
In server-related code, both the private key and certificate file will be presented to the
various SSL-related wrapping functions. The certificate is what gets presented to clients.
The private key should be protected and remains on the server.
In client-related code, a special file of valid certificate authorities needs to be maintained
to verify the server’s certificate. If you have no such file, then at the very least, you can
put a copy of the server’s certificate on the client machine and use that as a means for
verification. During connection, the server will present its certificate, and then you’ll
use the stored certificate you already have to verify that it’s correct.
Servers can also elect to verify the identity of clients. To do that, clients need to have
their own private key and certificate key. The server would also need to maintain a file
of trusted certificate authorities for verifying the client certificates.
If you intend to add SSL support to a network service for real, this recipe really only
gives a small taste of how to set it up. You will definitely want to consult the documen‐
tation for more of the finer points. Be prepared to spend a significant amount of time
experimenting with it to get things to work.
11.11. Passing a Socket File Descriptor Between Processes
Problem
You have multiple Python interpreter processes running and you want to pass an open
file descriptor from one interpreter to the other. For instance, perhaps there is a server
process that is responsible for receiving connections, but the actual servicing of clients
is to be handled by a different interpreter.
Solution
To pass a file descriptor between processes, you first need to connect the processes
together. On Unix machines, you might use a Unix domain socket, whereas on Win‐
dows, you could use a named pipe. However, rather than deal with such low-level
mechanics, it is often easier to use the multiprocessing module to set up such a
connection.
470  |  Chapter 11: Network and Web Programming
Once a connection is established, you can use the send_handle() and recv_handle()
functions in multiprocessing.reduction to send file descriptors between processes.
The following example illustrates the basics:
import multiprocessing
from multiprocessing.reduction import recv_handlesend_handle
import socket
def worker(in_pout_p):
out_p.close()
while True:
fd = recv_handle(in_p)
print('CHILD: GOT FD'fd)
with socket.socket(socket.AF_INETsocket.SOCK_STREAM, fileno=fdas s:
while True:
msg = s.recv(1024)
if not msg:
break
print('CHILD: RECV {!r}'.format(msg))
s.send(msg)
def server(addressin_pout_pworker_pid):
in_p.close()
s = socket.socket(socket.AF_INETsocket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKETsocket.SO_REUSEADDRTrue)
s.bind(address)
s.listen(1)
while True:
clientaddr = s.accept()
print('SERVER: Got connection from'addr)
send_handle(out_pclient.fileno(), worker_pid)
client.close()
if __name__ == '__main__':
c1c2 = multiprocessing.Pipe()
worker_p = multiprocessing.Process(target=workerargs=(c1,c2))
worker_p.start()
server_p = multiprocessing.Process(target=server,
args=((''15000), c1, c2worker_p.pid))
server_p.start()
c1.close()
c2.close()
In this example, two processes are spawned and connected by a multiprocessing Pipe
object. The server process opens a socket and waits for client connections. The worker
process merely waits to receive a file descriptor on the pipe using recv_handle(). When
the server receives a connection, it sends the resulting socket file descriptor to the worker
11.11. Passing a Socket File Descriptor Between Processes  |  471
using send_handle(). The worker takes over the socket and echoes data back to the
client until the connection is closed.
If you connect to the running server using Telnet or a similar tool, here is an example
of what you might see:
bash % python3 passfd.py
SERVER: Got connection from ('127.0.0.1', 55543)
CHILD: GOT FD 7
CHILD: RECV b'Hello\r\n'
CHILD: RECV b'World\r\n'
The most important part of this example is the fact that the client socket accepted in the
server is actually serviced by a completely different process. The server merely hands it
off, closes it, and waits for the next connection.
Discussion
Passing file descriptors between processes is something that many programmers don’t
even realize is possible. However, it can sometimes be a useful tool in building scalable
systems. For example, on a multicore machine, you could have multiple instances of the
Python interpreter and use file descriptor passing to more evenly balance the number
of clients being handled by each interpreter.
The send_handle() and recv_handle() functions shown in the solution really only
work with multiprocessing connections. Instead of using a pipe, you can connect in‐
terpreters as shown in Recipe 11.7, and it will work as long as you use UNIX domain
sockets or Windows pipes. For example, you could implement the server and worker
as completely separate programs to be started separately. Here is the implementation of
the server:
# servermp.py
from multiprocessing.connection import Listener
from multiprocessing.reduction import send_handle
import socket
def server(work_addressport):
# Wait for the worker to connect
work_serv = Listener(work_address, authkey=b'peekaboo')
worker = work_serv.accept()
worker_pid = worker.recv()
# Now run a TCP/IP server and send clients to worker
s = socket.socket(socket.AF_INETsocket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKETsocket.SO_REUSEADDRTrue)
s.bind((''port))
s.listen(1)
while True:
clientaddr = s.accept()
print('SERVER: Got connection from'addr)
472  |  Chapter 11: Network and Web Programming
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