HTTP - basic authentication example.

Michael Foord fuzzyman at gmail.com
Fri Sep 17 14:27:57 CEST 2004


"""This example is now updated to show an example at the end that uses
HTTPBasicAuthHandler *and* HTTPPasswordMgrWithDefaultRealm.
This is probably the 'right' way to do it... but is more of a pain
IMHO... (using a password manager either means *already* knowing the
realm... or *never* knowing the realm..)
"""

#!/usr/bin/python -u
#  16-09-04
# v1.0.0

# auth_example.py
# A simple CGI script manually demonstrating basic authentication.

# Copyright Michael Foord
# Free to use, modify and relicense.
# No warranty express or implied for the accuracy, fitness to purpose
or otherwise for this code....
# Use at your own risk !!!

# E-mail or michael AT foord DOT me DOT uk
# Maintained at www.voidspace.org.uk/atlantibots/pythonutils.html

"""
There is a system for requiring a username/password before a client
can visit a webpage. This is called authentication and is implemented
by the server - it actually allows for a whole set of pages (called a
realm) to be protected by authentication. This scheme (or schemes) are
actually defined by the HTTP spec, and so whilst python supports
authentication it doesn't document it very well. The HTTP
documentation is in the form of RFCs
(http://www.faqs.org/rfcs/rfc2617.html for basic and digest
authentication) which are technical documents and so not the most
readable !!

When I searched the web for details on authentication with python I
found lots of people asking questions, but a lack of clear answers.
This document and example code shows how to manually do basic
authentication with python. It is an example for performing a simple
operation rather than a technical document.

I am doing it manually rather than using an auth handler because my
script is a CGI which runs once for each page access. I have to store
the username/passwords between each access. A 'manual' explanation
also shows more clearly what is happening.

I've seen references to three authentication schemes, BASIC, NTLM and
DIGEST. It's possible there are more - but BASIC authentication is
overwhelmingly the most common. This tutorial/example only covers
BASIC authentication although some of the details may be applicable to
the other schemes.

--

In all these examples we will be using the python standard library
urllib2 to fetch web pages.

A client is any program that makes requests over the internet. It
could be a browser - or it could be a python program. When a client
requests a web page it sends a request to the server. That request
consists of headers with certain information about the request. Here
we are calling these headers 'http request headers'. If the request
fails to reach a server (the server name doesn't exist or there is no
internet connection for example) then the request will just fail. If
the request is made by python then an exception will be raised. This
exception will have a 'reason' attribute that is a tuple describing
the error.

The next example shows us creating a urllib2 request object, adding a
fake 'User-Agent' request header and making a request. The resulting
error shows what happens if you try to fetch a webpage without an
internet connection ! The User-Agent request header tells the server
what program is asking for the web page - some sites (e.g. google)
won't allow requests from anything other than a browser... so we might
have to pretend to be a browser. (Which is generally considered bad
client behaviour ? so I might get my knuckles rapped for including it
here).

>>> import urllib2
>>> req = urllib2.Request('http://www.google.co.uk')
>>> req.add_header('User-Agent', 'Mozilla/4.0 (compatible; MSIE 5.5;
Windows NT)')
>>> try:
	handle = urllib2.urlopen(req)
except IOError, e:
	if hasattr(e, 'reason'):
		print 'Reason : '
		print e.reason
else:
	print handle.read()                  # if we had a connection this
would print the page

		
Reason : 
(7, 'getaddrinfo failed')
>>> 

The actual exception is an URLError which is a subclass of IOError -
the one tested for above in the try-except block. If you did a dir(e)
in the above example then you would see all the attributes of the
exception object.

If however the request reaches a server then the server will send a
response back. Whether or not the request succeeds the response will
still contain headers from the server (or CGI script!!). These we call
here 'http response headers'. If there is a problem then this response
will include an error code that describes the problem. You will
already be familiar with some of these codes - 404 : Page not found,
500 : Internal Server Error etc. If this happens an exception will
still be raised by urllib2, but instead of a 'reason' attribute it
will have a 'code' attribute. The code attribute is an integer that
corresponds to the http error code. (see
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html for a *full*
list of codes).

If a page requires authentication then the error code is 401. Included
in the response headers is a 'WWW-authenticate' header that tells you
what authentication scheme the server is using for this page *and*
also something called a realm. It is rarely just a single page that is
protected by authentication but a section - a 'realm' of a website.
The name of the realm is included in this header line. If the client
*already knows* the username/password for this realm then it can
encode them into the request headers and try again. If the
username/password combination are correct, then the request will
succeed as normal. If the client doesn't know the username/password it
should ask the user. This means that if you enter a  protected 'realm'
the client effectively has to request each page twice. The first time
it will get an error code and be told what realm it is attempting to
access - the client can then get the right username/password for that
realm (on that server) and repeat the request.

HTTP is a 'stateless' protocol. This means that a server using basic
authentication won't 'remember' you are logged in and will need to be
sent the right header for every protected page you attempt to access.

Suppose we attempt to fetch a webpage protected by basic
authentication.

>>> theurl = 'http://www.someserver.com/somepath/someprotectedpage.html'
>>> try:
	handle = urllib2.urlopen(theurl)
except IOError, e:
	if hasattr(e, 'code'):
		if e.code != 401:
			print 'We got another error'
			print e.code
		else:
			print e.headers
			print e.headers['www-authenticate']
		# print e.headers.get('www-authenticate', '') might be a safer way
of doing this

Note the following things. We accessed the page directly from the url
instead of creating a request object. If the exception has a 'code'
attribute it also has an attribute called 'headers'. This is a
dictionary like object with all the headers in - but you can also
print it to display all the headers.  See the last line that displays
the 'www-authenticate' header line which ought to be present whenever
you get a 401 error.

Output from above example :

WWW-Authenticate: Basic realm="cPanel"
Connection: close
Set-Cookie: cprelogin=no; path=/
Server: cpsrvd/9.4.2

Content-type: text/html

Basic realm="cPanel"

You can see the  authentication scheme and the 'realm' part of the
'www-authenticate' header. Assuming you know the username and password
you can then navigate around that website - whenever you get a 401
error with *the same realm* you can just encode the username/password
into your request headers and your request should succeed.

Lets assume you need to access pages which are all in the same realm.
Assuming you have got the username and password from the user, you
save the name of the realm from the first access. Then whenever you
get a 401 error in the same realm (from the same server !) you know
the username/password to use. So the only detail left, is knowing how
to encode the username/password into the request header. This is done
by encoding it as a base 64 string. It doesn't actually look like
clear text - but it is only the most vaguest of 'encryption'. This
means basic authentication is just that - basic. Anyone sniffing your
traffic who sees an authentication request header will be able to
extract your username and password from it. Many websites like yahoo
or ebay, use javascript hashing/encryption and other tricks to
authenticate a login. This is much harder to detect and mimic from
python ! You may need to use a proxy client server and see what
information your browser is actually sending to the website (See
http://groups.google.co.uk/groups?hl=en&lr=&ie=UTF-8&threadm=6f402501.0409100632.47e403f3%40posting.google.com&rnum=2
for suggestions of several proxy servers that can do this).

There is a very simple recipe on the Activestate  Python Cookbook
(It's actually in the comments of this page
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/267197 )
showing how to encode a username/password into a request header. It
looks like this :

import base64
base64string = base64.encodestring('%s:%s' % (data['name'],
data['pass']))[:-1]
req.add_header("Authorization", "Basic %s" % base64string)

Where req is our request object like in the first example.

Let's wrap all this up with an example that shows accessing a page,
doing the authentication and saving the realm. I use a regular
expression to pull the scheme and realm out of the authentication
response header. I use urlparse to get the server part of the url. If
we store the username/password (or the whole request header line) then
we can re-use that information automatically if we come across another
page in that realm. When the code has run the contents of the page
we've fetched is saved as a string in the variable 'thepage'. If you
are writing an http client of any sort that has to deal with basic
authentication then this example should have everything you need to
know - but see the comment below about cookies.

Some websites may also use cookies with authentication. Luckily there
is a library that will allow you to have automatic cookie management
without thinking about it. This is ClientCookie
(http://wwwsearch.sourceforge.net/ClientCookie/ ). In python 2.4 it
becomes part of the python standard library as clientcookie. See my
cookbook example of how to use it
(http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/302930 ).
I've done a bigger example that displays a lot of http information
including cookies, headers, environment variables (the CGI
environment) and all this authentication stuff. You can find it at
http://www.voidspace.org.uk/atlantibots/recipebook.html#http - it's a
CGI and there's an online version to try. It shows lot's of the
information about http state, the CGI environment etc..

--

In actual fact the 'proper' way to do BASIC authentication with Python
is to install an authentication handler as an 'opener' (along with a
password manager) in urllib2. It doesn't show as clearly what is
happening and is less suitable for my needs (within a CGI), where a
pickled dictionary is more usefl. See at the bottom of this example
for an alternative example using an auth handler - this was sent to me
by Jaime Wyant (and amended by me)....
"""

import urllib2, sys, re, base64
from urlparse import urlparse
theurl = 'http://www.someserver.com/somepath/somepage.html'           
# if you want to run this example you'll need to supply a protected
page with your username and password
username = 'johnny'
password = 'XXXXXX'            # a very bad password

req = urllib2.Request(theurl)
try:
    handle = urllib2.urlopen(req)
except IOError, e:                  # here we are assuming we fail
    pass
else:                               # If we don't fail then the page
isn't protected
    print "This page isn't protected by authentication."
    sys.exit(1)
    
if not hasattr(e, 'code') or e.code != 401:                 # we got
an error - but not a 401 error
    print "This page isn't protected by authentication."
    print 'But we failed for another reason.'
    sys.exit(1)

authline = e.headers.get('www-authenticate', '')                # this
gets the www-authenticat line from the headers - which has the
authentication scheme and realm in it
if not authline:
    print 'A 401 error without an authentication response header -
very weird.'
    sys.exit(1)
    
authobj = re.compile(r'''(?:\s*www-authenticate\s*:)?\s*(\w*)\s+realm=['"](\w+)['"]''',
re.IGNORECASE)          # this regular expression is used to extract
scheme and realm
matchobj = authobj.match(authline)
if not matchobj:                                        # if the
authline isn't matched by the regular expression then something is
wrong
    print 'The authentication line is badly formed.'
    sys.exit(1)
scheme = matchobj.group(1) 
realm = matchobj.group(2)
if scheme.lower() != 'basic':
    print 'This example only works with BASIC authentication.'
    sys.exit(1)

base64string = base64.encodestring('%s:%s' % (username,
password))[:-1]
authheader =  "Basic %s" % base64string
req.add_header("Authorization", authheader)
try:
    handle = urllib2.urlopen(req)
except IOError, e:                  # here we shouldn't fail if the
username/password is right
    print "It looks like the username or password is wrong."
    sys.exit(1)
thepage = handle.read()
    
server = urlparse(theurl)[1].lower()            # server names are
case insensitive, so we will convert to lower case
test = server.find(':')
if test != -1: server = server[:test]           # remove the :port
information if present, we're working on the principle that realm
names per server are likely to be unique...

passdict = {(server, realm) : authheader }      # now if we get
another 401 we can test for an entry in passdict before having to ask
the user for a username/password

print 'Done successfully - information now stored in passdict.'
print 'The webpage is stored in thepage.'

"""
The proper way of actually doing this is to use an
HTTPBasicAuthHandler along with an HTTPPasswordMgr.
The python documentation on this is actually pretty minimal - so below
is an example showing how to do this.
The main problem with HTTPPasswordMgr is that you must *already* know
the realm - so we're going to use the HTTPPasswordMgrWithDefaultRealm
instead !!

theurl = 'http://www.someserver.com/highestlevelprotectedpath/somepage.htm'
username = 'johnny'
password = 'XXXXXX'         # a great password

passman = urllib2.HTTPPasswordMgrWithDefaultRealm()      # this
creates a password manager
passman.add_password(None, theurl, username, password)      # because
we have put None at the start it will always use this
username/password combination

authhandler = urllib2.HTTPBasicAuthHandler(passman)                 #
create the AuthHandler

opener = urllib2.build_opener(authhandler)                            
     # build an 'opener' using the handler we've created
# you can use the opener directly to open URLs
# *or* you can install it as the default opener so that all calls to
urllib2.urlopen use this opener
urllib2.install_opener(opener)

# All calls to urllib2.urlopen will now use our handler


ISSUES


CHANGELOG
16-09-04        Version 1.0.0
I think it's ok.
"""



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