link_token.py

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# SPDX-License-Identifier: AGPL-3.0-or-later
"""
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 valkey 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
 
"""
 
from ipaddress import (
    IPv4Network,
    IPv6Network,
    ip_address,
)
 
import string
import random
import flask
 
from searx.valkeylib import secret_hash
 
from ._helpers import (
    get_network,
    logger,
)
 
from . import config
from . import valkeydb
 
TOKEN_LIVE_TIME = 600
"""Lifetime (sec) of limiter's CSS token."""
 
PING_LIVE_TIME = 3600
"""Lifetime (sec) of the ping-key from a client (request)"""
 
PING_KEY = 'SearXNG_limiter.ping'
"""Prefix of all ping-keys generated by :py:obj:`get_ping_key`"""
 
TOKEN_KEY = 'SearXNG_limiter.token'
"""Key for which the current token is stored in the DB"""
 
logger = logger.getChild('botdetection.link_token')
 
 
def is_suspicious(network: IPv4Network | IPv6Network, request: flask.Request, renew: bool = False):
    """Checks whether a valid ping is exists for this (client) network, if not
    this request is rated as *suspicious*.  If a valid ping exists and argument
    ``renew`` is ``True`` the expire time of this ping is reset to
    :py:obj:`PING_LIVE_TIME`.
 
    """
    valkey_client = valkeydb.get_valkey_client()
    ping_key = get_ping_key(network, request)
    if not valkey_client.get(ping_key):
        logger.info("missing ping (IP: %s) / request: %s", network.compressed, ping_key)
        return True
 
    if renew:
        valkey_client.set(ping_key, 1, ex=PING_LIVE_TIME)
 
    logger.debug("found ping for (client) network %s -> %s", network.compressed, ping_key)
    return False
 
 
def ping(request: flask.Request, token: str):
    """This function is called by a request to URL ``/client<token>.css``.  If
    ``token`` is valid a :py:obj:`PING_KEY` for the client is stored in the DB.
    The expire time of this ping-key is :py:obj:`PING_LIVE_TIME`.
 
    """
    valkey_client = valkeydb.get_valkey_client()
    cfg = config.get_global_cfg()
 
    if not token_is_valid(token):
        return
 
    real_ip = ip_address(request.remote_addr)  # type: ignore
    network = get_network(real_ip, cfg)
 
    ping_key = get_ping_key(network, request)
    logger.debug(
        "store ping_key for (client) network %s (IP %s) -> %s", network.compressed, real_ip.compressed, ping_key
    )
    valkey_client.set(ping_key, 1, ex=PING_LIVE_TIME)
 
 
def get_ping_key(network: IPv4Network | IPv6Network, request: flask.Request) -> str:
    """Generates a hashed key that fits (more or less) to a *WEB-browser
    session* in a network."""
    return (
        PING_KEY
        + "["
        + secret_hash(
            network.compressed + 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 Valkey DB.  Without without a
    database connection, string "12345678" is returned.
 
    - :py:obj:`TOKEN_LIVE_TIME`
    - :py:obj:`TOKEN_KEY`
 
    """
    try:
        valkey_client = valkeydb.get_valkey_client()
    except ValueError:
        # This function is also called when limiter is inactive / no valkey DB
        # (see render function in webapp.py)
        return '12345678'
 
    token = valkey_client.get(TOKEN_KEY)
    if token:
        token = token.decode('UTF-8')  # type: ignore
    else:
        token = ''.join(random.choice(string.ascii_lowercase + string.digits) for _ in range(16))
        valkey_client.set(TOKEN_KEY, token, ex=TOKEN_LIVE_TIME)
    return token