[mod] limiter: trusted proxies (#4911)

Replaces `x_for` functionality with `trusted_proxies`. This allows defining
which IP / ranges to trust extracting the client IP address from X-Forwarded-For
and X-Real-IP headers.

We don't know if the proxy chain will give us the proper client
address (REMOTE_ADDR in the WSGI environment), so we rely on reading the headers
of the proxy before SearXNG (if there is one, in that case it must be added to
trusted_proxies) hoping it has done the proper checks. In case a proxy in the
chain does not check the client address correctly, integrity is compromised and
this should be fixed by whoever manages the proxy, not us.

Closes:

- https://github.com/searxng/searxng/issues/4940
- https://github.com/searxng/searxng/issues/4939
- https://github.com/searxng/searxng/issues/4907
- https://github.com/searxng/searxng/issues/3632
- https://github.com/searxng/searxng/issues/3191
- https://github.com/searxng/searxng/issues/1237

Related:

- https://github.com/searxng/searxng-docker/issues/386
- https://github.com/inetol-infrastructure/searxng-container/issues/81

searx/compat.py

@@ -8,6 +8,8 @@ __all__ = [
 ]
 
 import sys
+import warnings
+
 
 # TOML (lib) compatibility
 # ------------------------
@@ -16,3 +18,36 @@ if sys.version_info >= (3, 11):
     import tomllib
 else:
     import tomli as tomllib
+
+
+# limiter backward compatibility
+# ------------------------------
+
+LIMITER_CFG_DEPRECATED = {
+    "real_ip": "limiter: config section 'real_ip' is deprecated",
+    "real_ip.x_for": "real_ip.x_for has been replaced by botdetection.trusted_proxies",
+    "real_ip.ipv4_prefix": "real_ip.ipv4_prefix has been replaced by botdetection.ipv4_prefix",
+    "real_ip.ipv6_prefix": "real_ip.ipv6_prefix has been replaced by botdetection.ipv6_prefix'",
+}
+
+
+def limiter_fix_cfg(cfg, cfg_file):
+
+    kwargs = {
+        "category": DeprecationWarning,
+        "filename": str(cfg_file),
+        "lineno": 0,
+        "module": "searx.limiter",
+    }
+
+    for opt, msg in LIMITER_CFG_DEPRECATED.items():
+        try:
+            val = cfg.get(opt)
+        except KeyError:
+            continue
+
+        warnings.warn_explicit(msg, **kwargs)
+        if opt == "real_ip.ipv4_prefix":
+            cfg.set("botdetection.ipv4_prefix", val)
+        if opt == "real_ip.ipv6_prefix":
+            cfg.set("botdetection.ipv6_prefix", val)