[mod] limiter -> botdetection: modularization and documentation

In order to be able to meet the outstanding requirements, the implementation is
modularized and supplemented with documentation.

This patch does not contain functional change, except it fixes issue #2455

----

Aktivate limiter in the settings.yml and simulate a bot request by::

    curl -H 'Accept-Language: de-DE,en-US;q=0.7,en;q=0.3' \
         -H 'Accept: text/html'
         -H 'User-Agent: xyz' \
         -H 'Accept-Encoding: gzip' \
         'http://127.0.0.1:8888/search?q=foo'

In the LOG:

    DEBUG   searx.botdetection.link_token : missing ping for this request: .....

Since ``BURST_MAX_SUSPICIOUS = 2`` you can repeat the query above two time
before you get a "Too Many Requests" response.

Closes: https://github.com/searxng/searxng/issues/2455
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

9 files changed, 471 insertions, 0 deletions

diff --git a/searx/botdetection/__init__.py b/searx/botdetection/__init__.py
new file mode 100644
index 00000000..78a7d30f
--- /dev/null
+++ b/searx/botdetection/__init__.py
@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+""".. _botdetection src:
+
+Bot detection methods
+---------------------
+
+The methods implemented in this python package are use by the :ref:`limiter src`.
+
+"""
+
+import flask
+
+
+def dump_request(request: flask.Request):
+    return (
+        "%s: '%s'" % (request.headers.get('X-Forwarded-For'), request.path)
+        + " || form: %s" % request.form
+        + " || Accept: %s" % request.headers.get('Accept')
+        + " || Accept-Language: %s" % request.headers.get('Accept-Language')
+        + " || Accept-Encoding: %s" % request.headers.get('Accept-Encoding')
+        + " || Content-Type: %s" % request.headers.get('Content-Type')
+        + " || Content-Length: %s" % request.headers.get('Content-Length')
+        + " || Connection: %s" % request.headers.get('Connection')
+        + " || User-Agent: %s" % request.headers.get('User-Agent')
+    )
diff --git a/searx/botdetection/http_accept.py b/searx/botdetection/http_accept.py
new file mode 100644
index 00000000..1ab7cb4c
--- /dev/null
+++ b/searx/botdetection/http_accept.py
@@ -0,0 +1,24 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``http_accept``
+----------------------
+
+The ``http_accept`` method evaluates a request as the request of a bot if the
+Accept_ header ..
+
+- did not contain ``text/html``
+
+.. _Accept:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept
+
+"""
+
+from typing import Optional, Tuple
+import flask
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    if 'text/html' not in request.accept_mimetypes:
+        return 429, "bot detected, HTTP header Accept did not contain text/html"
+    return None
diff --git a/searx/botdetection/http_accept_encoding.py b/searx/botdetection/http_accept_encoding.py
new file mode 100644
index 00000000..ae630fd6
--- /dev/null
+++ b/searx/botdetection/http_accept_encoding.py
@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``http_accept_encoding``
+-------------------------------
+
+The ``http_accept_encoding`` method evaluates a request as the request of a
+bot if the Accept-Encoding_ header ..
+
+- did not contain ``gzip`` AND ``deflate`` (if both values are missed)
+- did not contain ``text/html``
+
+.. _Accept-Encoding:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding
+
+"""
+
+from typing import Optional, Tuple
+import flask
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    accept_list = [l.strip() for l in request.headers.get('Accept-Encoding', '').split(',')]
+    if not ('gzip' in accept_list or 'deflate' in accept_list):
+        return 429, "bot detected, HTTP header Accept-Encoding did not contain gzip nor deflate"
+    return None
diff --git a/searx/botdetection/http_accept_language.py b/searx/botdetection/http_accept_language.py
new file mode 100644
index 00000000..06743802
--- /dev/null
+++ b/searx/botdetection/http_accept_language.py
@@ -0,0 +1,23 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``http_accept_language``
+-------------------------------
+
+The ``http_accept_language`` method evaluates a request as the request of a bot
+if the Accept-Language_ header is unset.
+
+.. _Accept-Language:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent
+
+"""
+
+
+from typing import Optional, Tuple
+import flask
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    if request.headers.get('Accept-Language', '').strip() == '':
+        return 429, "bot detected, missing HTTP header Accept-Language"
+    return None
diff --git a/searx/botdetection/http_connection.py b/searx/botdetection/http_connection.py
new file mode 100644
index 00000000..f61f5e48
--- /dev/null
+++ b/searx/botdetection/http_connection.py
@@ -0,0 +1,23 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``http_connection``
+--------------------------
+
+The ``http_connection`` method evaluates a request as the request of a bot if
+the Connection_ header is set to ``close``.
+
+.. _Connection:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Connection
+
+"""
+
+
+from typing import Optional, Tuple
+import flask
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    if request.headers.get('Connection', '').strip() == 'close':
+        return 429, "bot detected, HTTP header 'Connection=close'"
+    return None
diff --git a/searx/botdetection/http_user_agent.py b/searx/botdetection/http_user_agent.py
new file mode 100644
index 00000000..892ae0bd
--- /dev/null
+++ b/searx/botdetection/http_user_agent.py
@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``http_user_agent``
+--------------------------
+
+The ``http_user_agent`` method evaluates a request as the request of a bot if
+the User-Agent_ header is unset or matches the regular expression
+:py:obj:`USER_AGENT`.
+
+.. _User-Agent:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent
+
+"""
+
+from typing import Optional, Tuple
+import re
+import flask
+
+USER_AGENT = (
+    r'('
+    + r'unknown'
+    + r'|[Cc][Uu][Rr][Ll]|[wW]get|Scrapy|splash|JavaFX|FeedFetcher|python-requests|Go-http-client|Java|Jakarta|okhttp'
+    + r'|HttpClient|Jersey|Python|libwww-perl|Ruby|SynHttpClient|UniversalFeedParser|Googlebot|GoogleImageProxy'
+    + r'|bingbot|Baiduspider|yacybot|YandexMobileBot|YandexBot|Yahoo! Slurp|MJ12bot|AhrefsBot|archive.org_bot|msnbot'
+    + r'|MJ12bot|SeznamBot|linkdexbot|Netvibes|SMTBot|zgrab|James BOT|Sogou|Abonti|Pixray|Spinn3r|SemrushBot|Exabot'
+    + r'|ZmEu|BLEXBot|bitlybot'
+    # unmaintained Farside instances
+    + r'|'
+    + re.escape(r'Mozilla/5.0 (compatible; Farside/0.1.0; +https://farside.link)')
+    # other bots and client to block
+    + '|.*PetalBot.*'
+    + r')'
+)
+"""Regular expression that matches to User-Agent_ from known *bots*"""
+
+_regexp = None
+
+
+def regexp_user_agent():
+    global _regexp  # pylint: disable=global-statement
+    if not _regexp:
+        _regexp = re.compile(USER_AGENT)
+    return _regexp
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    user_agent = request.headers.get('User-Agent', 'unknown')
+    if regexp_user_agent().match(user_agent):
+        return (
+            429,
+            f"bot detected, HTTP header User-Agent: {user_agent}",
+        )
+    return None
diff --git a/searx/botdetection/ip_limit.py b/searx/botdetection/ip_limit.py
new file mode 100644
index 00000000..fce3f8b6
--- /dev/null
+++ b/searx/botdetection/ip_limit.py
@@ -0,0 +1,90 @@
+"""
+Method ``ip_limit``
+-------------------
+
+The ``ip_limit`` method counts request from an IP in *sliding windows*.  If
+there are to many requests in a sliding window, the request is evaluated as a
+bot request.  This method requires a redis DB and needs a HTTP X-Forwarded-For_
+header.  To take privacy only the hash value of an IP is stored in the redis DB
+and at least for a maximum of 10 minutes.
+
+The :py:obj:`link_token` method is used to investigate whether a request is
+*suspicious*.  If the :py:obj:`link_token` method is activated and a request is
+*suspicious* the request rates are reduced:
+
+- :py:obj:`BURST_MAX` -> :py:obj:`BURST_MAX_SUSPICIOUS`
+- :py:obj:`LONG_MAX` -> :py:obj:`LONG_MAX_SUSPICIOUS`
+
+.. _X-Forwarded-For:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For
+
+"""
+
+from typing import Optional, Tuple
+import flask
+
+from searx import redisdb
+from searx import logger
+from searx.redislib import incr_sliding_window
+
+from . import link_token
+
+logger = logger.getChild('botdetection.ip_limit')
+
+BURST_WINDOW = 20
+"""Time (sec) before sliding window for *burst* requests expires."""
+
+BURST_MAX = 15
+"""Maximum requests from one IP in the :py:obj:`BURST_WINDOW`"""
+
+BURST_MAX_SUSPICIOUS = 2
+"""Maximum of suspicious requests from one IP in the :py:obj:`BURST_WINDOW`"""
+
+LONG_WINDOW = 600
+"""Time (sec) before the longer sliding window expires."""
+
+LONG_MAX = 150
+"""Maximum requests from one IP in the :py:obj:`LONG_WINDOW`"""
+
+LONG_MAX_SUSPICIOUS = 10
+"""Maximum suspicious requests from one IP in the :py:obj:`LONG_WINDOW`"""
+
+API_WONDOW = 3600
+"""Time (sec) before sliding window for API requests (format != html) expires."""
+
+API_MAX = 4
+"""Maximum requests from one IP in the :py:obj:`API_WONDOW`"""
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+    redis_client = redisdb.client()
+
+    x_forwarded_for = request.headers.get('X-Forwarded-For', '')
+    if not x_forwarded_for:
+        logger.error("missing HTTP header X-Forwarded-For")
+
+    if request.args.get('format', 'html') != 'html':
+        c = incr_sliding_window(redis_client, 'IP limit - API_WONDOW:' + x_forwarded_for, API_WONDOW)
+        if c > API_MAX:
+            return 429, "BLOCK %s: API limit exceeded"
+
+    suspicious = link_token.is_suspicious(request)
+
+    if suspicious:
+        c = incr_sliding_window(redis_client, 'IP limit - BURST_WINDOW:' + x_forwarded_for, BURST_WINDOW)
+        if c > BURST_MAX_SUSPICIOUS:
+            return 429, f"bot detected, too many request from {x_forwarded_for} in BURST_MAX_SUSPICIOUS"
+
+        c = incr_sliding_window(redis_client, 'IP limit - LONG_WINDOW:' + x_forwarded_for, LONG_WINDOW)
+        if c > LONG_MAX_SUSPICIOUS:
+            return 429, f"bot detected, too many request from {x_forwarded_for} in LONG_MAX_SUSPICIOUS"
+
+    else:
+        c = incr_sliding_window(redis_client, 'IP limit - BURST_WINDOW:' + x_forwarded_for, BURST_WINDOW)
+        if c > BURST_MAX:
+            return 429, f"bot detected, too many request from {x_forwarded_for} in BURST_MAX"
+
+        c = incr_sliding_window(redis_client, 'IP limit - LONG_WINDOW:' + x_forwarded_for, LONG_WINDOW)
+        if c > LONG_MAX:
+            return 429, f"bot detected, too many request from {x_forwarded_for} in LONG_MAX"
+    return None
diff --git a/searx/botdetection/limiter.py b/searx/botdetection/limiter.py
new file mode 100644
index 00000000..71044c31
--- /dev/null
+++ b/searx/botdetection/limiter.py
@@ -0,0 +1,79 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+""".. _limiter src:
+
+Limiter
+=======
+
+.. sidebar:: info
+
+   The limiter requires a :ref:`Redis <settings redis>` database.
+
+Bot protection / IP rate limitation.  The intention of rate limitation is to
+limit suspicious requests from an IP.  The motivation behind this is the fact
+that SearXNG passes through requests from bots and is thus classified as a bot
+itself.  As a result, the SearXNG engine then receives a CAPTCHA or is blocked
+by the search engine (the origin) in some other way.
+
+To avoid blocking, the requests from bots to SearXNG must also be blocked, this
+is the task of the limiter.  To perform this task, the limiter uses the methods
+from the :py:obj:`searx.botdetection`.
+
+To enable the limiter activate:
+
+.. code:: yaml
+
+   server:
+     ...
+     limiter: true  # rate limit the number of request on the instance, block some bots
+
+and set the redis-url connection. Check the value, it depends on your redis DB
+(see :ref:`settings redis`), by example:
+
+.. code:: yaml
+
+   redis:
+     url: unix:///usr/local/searxng-redis/run/redis.sock?db=0
+
+"""
+
+from typing import Optional, Tuple
+import flask
+
+from searx.botdetection import (
+    http_accept,
+    http_accept_encoding,
+    http_accept_language,
+    http_connection,
+    http_user_agent,
+    ip_limit,
+)
+
+
+def filter_request(request: flask.Request) -> Optional[Tuple[int, str]]:
+
+    if request.path == '/healthz':
+        return None
+
+    for func in [
+        http_user_agent,
+    ]:
+        val = func.filter_request(request)
+        if val is not None:
+            return val
+
+    if request.path == '/search':
+
+        for func in [
+            http_accept,
+            http_accept_encoding,
+            http_accept_language,
+            http_connection,
+            http_user_agent,
+            ip_limit,
+        ]:
+            val = func.filter_request(request)
+            if val is not None:
+                return val
+
+    return None
diff --git a/searx/botdetection/link_token.py b/searx/botdetection/link_token.py
new file mode 100644
index 00000000..8ef215f6
--- /dev/null
+++ b/searx/botdetection/link_token.py
@@ -0,0 +1,126 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""
+Method ``link_token``
+---------------------
+
+The ``link_token`` method evaluates a request as :py:obj:`suspicious
+<is_suspicious>` if the URL ``/client<token>.css`` is not requested by the
+client.  By adding a random component (the token) in the URL a bot can not send
+a ping by request a static URL.
+
+.. note::
+
+   This method requires a redis DB and needs a HTTP X-Forwarded-For_ header.
+
+To get in use of this method a flask URL route needs to be added:
+
+.. code:: python
+
+   @app.route('/client<token>.css', methods=['GET', 'POST'])
+   def client_token(token=None):
+       link_token.ping(request, token)
+       return Response('', mimetype='text/css')
+
+And in the HTML template from flask a stylesheet link is needed (the value of
+``link_token`` comes from :py:obj:`get_token`):
+
+.. code:: html
+
+   <link rel="stylesheet"
+         href="{{ url_for('client_token', token=link_token) }}"
+         type="text/css" />
+
+.. _X-Forwarded-For:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For
+
+"""
+
+import string
+import random
+import flask
+
+from searx import logger
+from searx import redisdb
+from searx.redislib import secret_hash
+
+TOKEN_LIVE_TIME = 600
+"""Livetime (sec) of limiter's CSS token."""
+
+PING_KEY = 'SearXNG_limiter.ping'
+TOKEN_KEY = 'SearXNG_limiter.token'
+
+logger = logger.getChild('botdetection.link_token')
+
+
+def is_suspicious(request: flask.Request):
+    """Checks if there is a valid ping for this request, if not this request is
+    rated as *suspicious*"""
+    redis_client = redisdb.client()
+    if not redis_client:
+        return False
+
+    ping_key = get_ping_key(request)
+    if not redis_client.get(ping_key):
+        logger.warning(
+            "missing ping (IP: %s) / request: %s",
+            request.headers.get('X-Forwarded-For', ''),
+            ping_key,
+        )
+        return True
+
+    logger.debug("found ping for this request: %s", ping_key)
+    return False
+
+
+def ping(request: flask.Request, token: str):
+    """This function is called by a request to URL ``/client<token>.css``"""
+    redis_client = redisdb.client()
+    if not redis_client:
+        return
+    if not token_is_valid(token):
+        return
+    ping_key = get_ping_key(request)
+    logger.debug("store ping for: %s", ping_key)
+    redis_client.set(ping_key, 1, ex=TOKEN_LIVE_TIME)
+
+
+def get_ping_key(request: flask.Request):
+    """Generates a hashed key that fits (more or less) to a request.  At least
+    X-Forwarded-For_ is needed to be able to assign the request to an IP.
+
+    """
+    return secret_hash(
+        PING_KEY
+        + request.headers.get('X-Forwarded-For', '')
+        + request.headers.get('Accept-Language', '')
+        + request.headers.get('User-Agent', '')
+    )
+
+
+def token_is_valid(token) -> bool:
+    valid = token == get_token()
+    logger.debug("token is valid --> %s", valid)
+    return valid
+
+
+def get_token() -> str:
+    """Returns current token.  If there is no currently active token a new token
+    is generated randomly and stored in the redis DB.
+
+    - :py:obj:`TOKEN_LIVE_TIME`
+    - :py:obj:`TOKEN_KEY`
+
+    """
+    redis_client = redisdb.client()
+    if not redis_client:
+        # This function is also called when limiter is inactive / no redis DB
+        # (see render function in webapp.py)
+        return '12345678'
+    token = redis_client.get(TOKEN_KEY)
+    if token:
+        token = token.decode('UTF-8')
+    else:
+        token = ''.join(random.choice(string.ascii_lowercase + string.digits) for _ in range(16))
+        redis_client.set(TOKEN_KEY, token, ex=TOKEN_LIVE_TIME)
+    return token