#!/usr/bin/env python
# Startdate: 2021-06-17
# goals:
#   accept kerberos or ldap "authorization: basic gowinablz;nuiowekj==" auth, to create a cookie for a session that lasts for 15 minutes. use the cookie to get to protected URLs
# References:
#    https://code.tutsplus.com/tutorials/flask-authentication-with-ldap--cms-23101
#    https://www.techlifediary.com/python-web-development-tutorial-using-flask-session-cookies/
#    delete cookie https://stackoverflow.com/a/14386413/3569534
#    timeout sessions https://stackoverflow.com/a/11785722/3569534
#    future: https://code.tutsplus.com/tutorials/flask-authentication-with-ldap--cms-23101
#    better timeout session: https://stackoverflow.com/a/49891626/3569534
#    store "formdata" in session for changing the basic auth to form data for the ldap login https://stackoverflow.com/a/56904875/3569534
#    requires_session similar to requires_auth (kerberos) return to the passed function, from https://github.com/ArtemAngelchev/flask-basicauth-ldap/blob/master/flask_basicauth_ldap.py
#    modify url from urlparse https://stackoverflow.com/a/21629125/3569534
#    _unauthorized_kerberos meta redirect from https://billstclair.com/html-redirect2.html
#    preserve POST with code 307 https://stackoverflow.com/a/15480983/3569534
# Improve:
# Run:
#    FLASK_APP=session_app.py FLASK_DEBUG=1 flask run --host 0.0.0.0
# Dependencies:
#    apt-get install python3-flask python3-kerberos
#    pip3 install Flask-kerberos

from flask import Flask, Response, redirect, url_for, render_template, request, _request_ctx_stack as stack, make_response, session
from flask_kerberos import init_kerberos, requires_authentication, _unauthorized, _forbidden, _gssapi_authenticate
from functools import wraps
from urllib.parse import urlparse
import kerberos, binascii, datetime, os
import session_ldap

app = Flask(__name__)
if 'SESSION_CONFIG' in os.environ:
   conf_file=os.environ['SESSION_CONFIG']
else:
   conf_file="dev.cfg"
app.config.from_pyfile(conf_file, silent=False)
#app.config.from_object(__name__)
if 'DEBUG' in app.config and app.config['DEBUG']:
   app.debug=True
secret_key_value = os.urandom(24)
secret_key_value_hex_encoded = binascii.hexlify(secret_key_value)
app.config['SECRET_KEY'] = secret_key_value_hex_encoded
app.permanent_session_lifetime=datetime.timedelta(minutes=app.config['SESSION_DURATION_MINUTES'])

def requires_session(function):
   '''
   Requires a valid session, provided by cookie!
   '''
   @wraps(function)
   def decorated(*args, **kwargs):
      if not session:
         return Response("requires session",401)
      else:
         if 'user' not in session:
            return Response("User is not in this session.",401)
         s_user = session['user']
         s_groups = session['groups']
         c_user = request.cookies.get('user')
         print(f"session user: {s_user}")
         print(f"cookie user: {c_user}")
         if session['user'] != c_user:
            return Response("Wrong user for this session!.",401)
         # otherwise, everything is good!
         return function(s_user, s_groups, *args,**kwargs)
      # catch-all
      return Response("requires session",401)
   return decorated

# imported from flask_kerberos and modified, because I want custom 401 message
def requires_authn_kerberos(function):
   '''
   Require that the wrapped view function only be called by users
   authenticated with Kerberos. The view function will have the authenticated
   users principal passed to it as its first argument.

   :param function: flask view function
   :type function: function
   :returns: decorated function
   :rtype: function
   '''
   @wraps(function)
   def decorated(*args, **kwargs):
      header = request.headers.get("Authorization")
      if header:
         ctx = stack.top
         token = ''.join(header.split()[1:])
         rc = _gssapi_authenticate(token)
         if rc == kerberos.AUTH_GSS_COMPLETE:
            # if ldap config options are set, then do kerberos -> short username resolution
            user = ctx.kerberos_user
            if 'LDAP_USER_KERBEROS_PRINCIPAL_ATTRIB' in app.config:
               conn = session_ldap.get_ldap_connection(
                  server_uri=get_next_ldap_server(app),
                  bind_dn=app.config['LDAP_BIND_DN'],
                  bind_pw=app.config['LDAP_BIND_PASSWORD'],
               )
               this_user = session_ldap.get_ldap_attrib_from_krbPrincipalName(
                  connection = conn,
                  search_base=app.config['LDAP_USER_BASE'],
                  user_attrib="dn",
                  user_krbPrincipalName=user,
                  krbPrincipalName_attrib=app.config['LDAP_USER_KERBEROS_PRINCIPAL_ATTRIB']
               )
               #print(f"DEBUG: krb user {user} is ldap dn {this_user}")
               shortuser = session_ldap.get_ldap_username_attrib_from_dn(
                  authenticated_user=conn,
                  user_dn=this_user,
                  user_match_attrib=app.config['LDAP_USER_DISPLAY_ATTRIB']
               )
               #print(f"DEBUG: shortuser {shortuser}")
               groups = session_ldap.get_ldap_user_groups(
                  connection=conn,
                  user_dn=this_user,
                  user_attrib_memberof=app.config['LDAP_USER_ATTRIB_MEMBEROF'],
                  group_name_attrib=app.config['LDAP_GROUP_DISPLAY_ATTRIB'],
                  group_base=app.config['LDAP_GROUP_BASE']
               )
               #print(f"DEBUG: groups {groups}")
            response = function(shortuser, groups, *args, **kwargs)
            response = make_response(response)
            if ctx.kerberos_token is not None:
               response.headers['WWW-Authenticate'] = ' '.join(['negotiate', ctx.kerberos_token])
            return response
         elif rc != kerberos.AUTH_GSS_CONTINUE:
            return _forbidden()
      return _unauthorized_kerberos()
   return decorated

def requires_authn_ldap(function):
   '''
   Require that the wrapped view function only be called by users
   authenticated with ldap. The view function will have the authenticated
   users principal passed to it as its first argument.
   :param function: flask view function
   :type function: function
   :returns: decorated function
   :rtype: function
   '''
   @wraps(function)
   def decorated(*args, **kwargs):
      # formdata is in session if we are coming from login_basic()
      form = session.get('formdata', None)
      if form:
         #print(f"DEBUG: requires_authn_ldap form={form}")
         session.pop('formdata')
         if 'username' in form:
            username = form['username']
         if 'password' in form:
            pw = form['password']
      else:
         # then we are coming from the form with POST data
         if 'username' not in request.form or 'password' not in request.form:
            return _unauthorized_ldap()
         username = request.form['username']
         pw = request.form['password']
      #print(f"DEBUG: requires_authn_ldap with username={username} and pw={pw}")
      # learn dn of user
      this_uri = get_next_ldap_server(app)
      this_user = session_ldap.list_matching_users(
         server_uri=this_uri,
         bind_dn=app.config['LDAP_BIND_DN'],
         bind_pw=app.config['LDAP_BIND_PASSWORD'],
         user_base=app.config['LDAP_USER_BASE'],
         username=username,
         user_match_attrib=app.config['LDAP_USER_MATCH_ATTRIB']
      )
      # list_matching_users always returns list, so if it contains <> 1 we are in trouble
      if len(this_user) != 1:
         print(f"WARNING: cannot determine unique user for {app.config['LDAP_USER_MATCH_ATTRIB']}={username} which returned {this_user}")
         return _unauthorized_ldap()
      this_user = this_user[0]
      print(f"DEBUG: requires_authn_ldap: found in ldap the username {this_user}")
      ll = ldap_login(this_user,pw)
      if ll:
         shortuser = session_ldap.get_ldap_username_attrib_from_dn(
            authenticated_user=ll,
            user_match_attrib=app.config['LDAP_USER_DISPLAY_ATTRIB']
         )
         groups = session_ldap.get_ldap_user_groups(
            connection=ll,
            user_dn=this_user,
            user_attrib_memberof=app.config['LDAP_USER_ATTRIB_MEMBEROF'],
            group_name_attrib=app.config['LDAP_GROUP_DISPLAY_ATTRIB'],
            group_base=app.config['LDAP_GROUP_BASE']
         )
         print(f"DEBUG: user {shortuser} has groups {groups}")
         return function(shortuser, groups ,*args, **kwargs)
      else:
         return _unauthorized_ldap()
   return decorated

def _unauthorized_kerberos():
   '''
   Indicate that authentication is required
   '''
   return Response(f'<meta http-equiv="Refresh" content="4; url={url_for("login_ldap")}">Unauthorized! No kerberos auth provided. Trying <a href="{url_for("login_ldap")}">ldap</a> automatically in a moment.', 401, {'WWW-Authenticate': 'Negotiate'})

def _unauthorized_ldap():
   return Response(f'<meta http-equiv="Refresh" content="4; url={url_for("login")}">Unauthorized! Invalid ldap credentials... returning to login form', 401)

@app.route("/")
def index():
   return render_template('index.html')

@app.route("/protected/")
@requires_session
def protected_page(user=None,groups=None):
   return protected_page_real()

def protected_page_real():
   s_user = session['user']
   c_user = request.cookies.get('user')
   groups = session['groups']
   cookie=request.cookies
   print(cookie)
   return render_template('view.html', c_user = c_user, s_user=s_user, cookie=cookie, groups=groups)

@app.route("/login/new")
@app.route("/login/new/")
def login_new():
   return redirect(url_for("login", new=""))

@app.route("/login/", methods=['POST','GET'])
def login(user="None"):
   if request.method == "GET":
      if 'user' in session and request.cookies.get('user') == session['user'] and (not 'new' in request.args):
         return redirect(url_for("protected_page"))
      auth_header = request.headers.get("Authorization")
      if auth_header:
         if "negotiate" in auth_header:
            # assume we are already trying to log in with kerberos
            return redirect(url_for("login_kerberos"))
      # default, show login form
      return redirect(url_for("login_form"))
   elif request.method == "POST":
      if request.authorization:
         return redirect(url_for("login_basic"),code=307)
      return handle_login_ldap_from_non_ldap(request)

def get_next_ldap_server(app):
   # on first ldap_login attempt, cache this lookup result:
   if 'LDAP_HOSTS' not in app.config:
      this_domain = urlparse(app.config['LDAP_URI']).hostname
      app.config['LDAP_HOSTS'] = session_ldap.list_ldap_servers_for_domain(this_domain)
   else:
      # rotate them! So every ldap_login attempt will use the next ldap server in the list.
      this_list = app.config['LDAP_HOSTS']
      a = this_list[0]
      this_list.append(a)
      this_list.pop(0)
      app.config['LDAP_HOSTS'] = this_list
   # construct a new, full uri.
   this_netloc = app.config['LDAP_HOSTS'][0]
   up = urlparse(app.config['LDAP_URI'])
   if up.port:
      this_netloc += f":{up.port}"
   this_uri = up._replace(netloc=this_netloc).geturl()
   return this_uri

def ldap_login(username,password):
   #print(f"DEBUG: Trying user {username} with pw '{password}'")
   this_uri = get_next_ldap_server(app)
   # Perform the ldap interactions
   user = session_ldap.authenticated_user(
         server_uri=this_uri,
         user_dn=username,
         password=password
      )
   if user:
      return user
   else:
      return False
   return False

@app.route("/login/kerberos")
@app.route("/login/kerberos/")
@requires_authn_kerberos
def login_kerberos(user,groups=[]):
   resp = Response(f'<meta http-equiv="Refresh" content="1; url={url_for("protected_page")}">success with kerberos')
   #resp.headers['login'] = "from-kerberos"
   resp.set_cookie('type',"kerberos")
   resp = login_generic(session,resp,user,groups)
   return resp

@app.route("/login/ldap", methods=['POST','GET'])
@app.route("/login/ldap/", methods=['POST','GET'])
@requires_authn_ldap
def login_ldap(user,groups=[]):
   resp = Response(f'<meta http-equiv="Refresh" content="1; url={url_for("protected_page")}">success with ldap')
   resp.set_cookie('type',"ldap")
   resp = login_generic(session,resp,user,groups)
   return resp

def login_generic(session,resp,user,groups=[]):
   resp.set_cookie('user',user)
   end_time = datetime.datetime.now(datetime.timezone.utc) + app.permanent_session_lifetime
   end_time_str = datetime.datetime.strftime(end_time,"%FT%TZ")
   resp.set_cookie('timestamp',end_time_str)
   session.permanent = True
   session['user']=user
   session['end_time'] = end_time_str
   session['groups'] = groups
   return resp

@app.route("/login/form", methods=['POST','GET'])
@app.route("/login/form/", methods=['POST','GET'])
def login_form():
   if request.method == "GET":
      options = {
            "ldap": "ldap",
            "other": "other"
         }
      return render_template("login_form.html",
         login_url = url_for("login"),
         options=options,
         kerberos_url = url_for("login_kerberos")
      )
   else:
      # assume it is a POST
      return redirect(url_for("login_ldap"), code=307)

def handle_login_ldap_from_non_ldap(request):
   # set default logintype for user
   logintype = "ldap"
   # redirect to whichever option was chosen in the drop-down
   if 'logintype' in request.form:
      logintype = request.form['logintype']
   if "ldap" == logintype:
      return redirect(url_for("login_ldap"), code=307)
   else:
      return f"Authentication method {logintype} not supported yet.",400

@app.route("/logout")
@app.route("/logout/")
def logout():
   resp = Response(f'<meta http-equiv="Refresh" content="1; url={url_for("index")}">logged out')
   # not documented but is found on the Internet in a few random places:
   session.clear()
   resp.set_cookie('user','',expires=0)
   resp.set_cookie('type','',expires=0)
   resp.set_cookie('session','',expires=0)
   resp.set_cookie('timestamp','',expires=0)
   return resp

@app.route("/login/basic",methods=['POST','GET'])
@app.route("/login/basic/",methods=['POST','GET'])
def login_basic():
   if not request.authorization:
      return Response(f"Please provide username and password.",401,{'WWW-Authenticate': 'Basic'})
   if 'username' not in request.authorization:
      return Response(f"No username provided.",401)
   if 'password' not in request.authorization:
      return Response(f"No password provided.",401)
   username = request.authorization.username
   pw = request.authorization.password
   form={'username':username,'password':pw}
   session['formdata'] = form
   return redirect(url_for("login_ldap"),code=307)

@app.route("/protected/settings/", methods=['GET','POST'])
@requires_session
def protected_settings(user,groups):
   print(f"DEBUG: visit settings page as user {user}")
   print(f"DEBUG: with groups {groups}")
   if "admins" not in groups:
      #return Response(f'<h1>Not Found</h1><p>What you were looking for is just not there.<p><a href="{ url_for("index") }">Start over</a>', 404)
      #return Response(f'<h1>Not Found</h1><p>The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.</p>', 404)
      return Response(f'<h1>Not Authorized</h1><p>You are not authorized to access this page.</p>', 403)
   else:
      if request.method == "GET":
         return render_template(
            'settings.html',
            ldap_uri=app.config['LDAP_URI']
         )
      elif request.method == "POST":
         form = request.form
         print(f"Form: {form}")
         message = ""
         if 'ldap_uri' not in form:
            return Response("Invalid input.", 400)
         else:
            new_ldap_uri = form['ldap_uri']
            if new_ldap_uri != app.config['LDAP_URI']:
               app.config['LDAP_URI'] = new_ldap_uri
               # removing LDAP_HOSTS causes get_new_ldap_server to reidentify the ldap servers for this uri.
               if 'LDAP_HOSTS' in app.config:
                  app.config.pop('LDAP_HOSTS')
               message += "<li>LDAP_URI</li>"
         if "" != message:
            message = "Settings updated:<ul>" + message + "</ul>"
         message += f"<form action='{url_for('protected_settings')}' method='get'><input type='submit' value='Return to settings'/></form>"
         return Response(message, 200)

## This bumps the session lifetime to two minutes farther out from each web request with this session.
#@app.before_request
#def make_session_permanent():
#   session.permanent = True
#   session['end_time'] = datetime.datetime.now()+app.permanent_session_lifetime

os.environ['KRB5_KTNAME'] = app.config['KRB5_KTNAME']
if 'KRB5_TRACE' in app.config:
   os.environ['KRB5_TRACE'] = app.config['KRB5_TRACE']

init_kerberos(app, hostname=app.config['HOSTNAME'], service=app.config['KRB5_SERVICE'])
if __name__ == '__main__':
   #init_kerberos(app, hostname=app.config['HOSTNAME'], service=app.config['KRB5_SERVICE'])
   print("should listen to ",app.config['LISTEN_HOST'])
   app.run(
      host=app.config['LISTEN_HOST'],
      port=app.config['LISTEN_PORT'],
      debug=app.config['DEBUG']
   )