Implement checking the username against LDAP (#64)

This commit adds code to allow users that prefer to do user management
via LDAP to do so. Since Etebase does not store the password (proof) in
a LDAP compatible fashion, we compromise and instead two checks: One while
registering an account to see if the username is the LDAP directory and
one whenever the API is accessed. To prevent too many LDAP requests, the
result of the request is cached for an configurable amount of hours.

Make sure you have python-ldap installed and can successfully import
ldap. Then, if you use the easy config, add the following to your
config:

```
; [...]
; Regular etesync configuration

[ldap]
bind_dn = <Your LDAP "user" to bind as. See Note 1>
bind_pw = <The password to authenticate as your bind user>
; Or if you have the password in a file:
; bind_pw_file = /path/to/the/file.txt
server = <The URL to your LDAP server>
search_base = <Your search base>
filter = <Your LDAP filter query. See Note 2>
; In case a cache TTL of 1 hour is too short for you, set `cache_ttl` to
the preferred
; amount of hours a cache entry should be viewed as valid:
; cache_ttl = 5
```

With this config, I am able to make the EteSync server check with my
LDAP server if a user should be able to login or register.
Note that if a user is allowed to login or register, the password of the
LDAP user will be ignored. This LDAP patch is nothing more than an
additional check before the actual authentication.

A successful LDAP check will be cached, if not configured (correctly),
for one hour, after which the LDAP query will be performed again.

Note 1: This commit only works with a bind user
Note 2: The query must be specified. If an LDAP query returns more than
one or no result, then the authentication fails. If your query needs to
include the username that currently tries to perform a login or
registration, you can use %%s, which will be subsituted for the used
username.

etebase_server/myauth/ldap.py

@@ -0,0 +1,109 @@
+import logging
+
+from django.utils import timezone
+from django.conf import settings
+from django.core.exceptions import PermissionDenied as DjangoPermissionDenied
+from etebase_server.django.utils import CallbackContext
+from etebase_server.myauth.models import get_typed_user_model, UserType
+from etebase_server.fastapi.dependencies import get_authenticated_user
+from etebase_server.fastapi.exceptions import PermissionDenied as FastAPIPermissionDenied
+from fastapi import Depends
+
+import ldap
+
+User = get_typed_user_model()
+
+
+def ldap_setting(name, default):
+    """Wrapper around django.conf.settings"""
+    return getattr(settings, f"LDAP_{name}", default)
+
+
+class LDAPConnection:
+    __instance__ = None
+    __user_cache = {}  # Username -> Valid until
+
+    @staticmethod
+    def get_instance():
+        """To get a Singleton"""
+        if not LDAPConnection.__instance__:
+            return LDAPConnection()
+        else:
+            return LDAPConnection.__instance__
+
+    def __init__(self):
+        # Cache some settings
+        self.__LDAP_FILTER = ldap_setting("FILTER", "")
+        self.__LDAP_SEARCH_BASE = ldap_setting("SEARCH_BASE", "")
+
+        # The time a cache entry is valid (in hours)
+        try:
+            self.__LDAP_CACHE_TTL = int(ldap_setting("CACHE_TTL", ""))
+        except ValueError:
+            logging.error("Invalid value for cache_ttl. Defaulting to 1 hour")
+            self.__LDAP_CACHE_TTL = 1
+
+        password = ldap_setting("BIND_PW", "")
+        if not password:
+            pw_file = ldap_setting("BIND_PW_FILE", "")
+            if pw_file:
+                with open(pw_file, "r") as f:
+                    password = f.read().replace("\n", "")
+
+        self.__ldap_connection = ldap.initialize(ldap_setting("SERVER", ""))
+        try:
+            self.__ldap_connection.simple_bind_s(ldap_setting("BIND_DN", ""), password)
+        except ldap.LDAPError as err:
+            logging.error(f"LDAP Error occuring during bind: {err.desc}")
+
+    def __is_cache_valid(self, username):
+        """Returns True if the cache entry is still valid. Returns False otherwise."""
+        if username in self.__user_cache:
+            if timezone.now() <= self.__user_cache[username]:
+                # Cache entry is still valid
+                return True
+        return False
+
+    def __remove_cache(self, username):
+        del self.__user_cache[username]
+
+    def has_user(self, username):
+        """
+        Since we don't care about the password and so authentication
+        another way, all we care about is whether the user exists.
+        """
+        if self.__is_cache_valid(username):
+            return True
+        if username in self.__user_cache:
+            self.__remove_cache(username)
+
+        filterstr = self.__LDAP_FILTER.replace("%s", username)
+        try:
+            result = self.__ldap_connection.search_s(self.__LDAP_SEARCH_BASE, ldap.SCOPE_SUBTREE, filterstr=filterstr)
+        except ldap.NO_RESULTS_RETURNED:
+            # We handle the specific error first and the the generic error, as
+            # we may expect ldap.NO_RESULTS_RETURNED, but not any other error
+            return False
+        except ldap.LDAPError as err:
+            logging.error(f"Error occured while performing an LDAP query: {err.desc}")
+            return False
+
+        if len(result) == 1:
+            self.__user_cache[username] = timezone.now() + timezone.timedelta(hours=self.__LDAP_CACHE_TTL)
+            return True
+        return False
+
+
+def is_user_in_ldap(user: UserType = Depends(get_authenticated_user)):
+    if not LDAPConnection.get_instance().has_user(user.username):
+        raise FastAPIPermissionDenied(detail="User not in LDAP directory.")
+
+
+def create_user(context: CallbackContext, *args, **kwargs):
+    """
+    A create_user function which first checks if the user already exists in the
+    configured LDAP directory.
+    """
+    if not LDAPConnection.get_instance().has_user(kwargs["username"]):
+        raise DjangoPermissionDenied("User not in the LDAP directory.")
+    return User.objects.create_user(*args, **kwargs)

etebase_server/settings.py

@@ -140,6 +140,8 @@ config_locations = [
     "/etc/etebase-server/etebase-server.ini",
 ]
 
+ETEBASE_CREATE_USER_FUNC = "etebase_server.django.utils.create_user_blocked"
+
 # Use config file if present
 if any(os.path.isfile(x) for x in config_locations):
     config = configparser.ConfigParser()
@@ -168,7 +170,24 @@ if any(os.path.isfile(x) for x in config_locations):
     if "database-options" in config:
         DATABASES["default"]["OPTIONS"] = config["database-options"]
 
-ETEBASE_CREATE_USER_FUNC = "etebase_server.django.utils.create_user_blocked"
+    if "ldap" in config:
+        ldap = config["ldap"]
+        LDAP_SERVER = ldap.get("server", "")
+        LDAP_SEARCH_BASE = ldap.get("search_base", "")
+        LDAP_FILTER = ldap.get("filter", "")
+        LDAP_BIND_DN = ldap.get("bind_dn", "")
+        LDAP_BIND_PW = ldap.get("bind_pw", "")
+        LDAP_BIND_PW_FILE = ldap.get("bind_pw_file", "")
+        LDAP_CACHE_TTL = ldap.get("cache_ttl", "")
+
+        if not LDAP_BIND_DN:
+            raise Exception("LDAP enabled but bind_dn is not set!")
+        if not LDAP_BIND_PW and not LDAP_BIND_PW_FILE:
+            raise Exception("LDAP enabled but both bind_pw and bind_pw_file are not set!")
+
+        # Configure EteBase to use LDAP
+        ETEBASE_CREATE_USER_FUNC = "etebase_server.myauth.ldap.create_user"
+        ETEBASE_API_PERMISSIONS_READ = ["etebase_server.myauth.ldap.is_user_in_ldap"]
 
 # Efficient file streaming (for large files)
 SENDFILE_BACKEND = "etebase_server.fastapi.sendfile.backends.simple"