[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/limiter.toml

@@ -1,8 +1,4 @@
-[real_ip]
-
-# Number of values to trust for X-Forwarded-For.
-
-x_for = 1
+[botdetection]
 
 # The prefix defines the number of leading bits in an address that are compared
 # to determine whether or not an address is part of a (client) network.
@@ -10,6 +6,19 @@ x_for = 1
 ipv4_prefix = 32
 ipv6_prefix = 48
 
+# If the request IP is in trusted_proxies list, the client IP address is
+# extracted from the X-Forwarded-For and X-Real-IP headers. This should be
+# used if SearXNG is behind a reverse proxy or load balancer.
+
+trusted_proxies = [
+  '127.0.0.0/8',
+  '::1',
+  # '192.168.0.0/16',
+  # '172.16.0.0/12',
+  # '10.0.0.0/8',
+  # 'fd00::/8',
+]
+
 [botdetection.ip_limit]
 
 # To get unlimited access in a local network, by default link-local addresses
@@ -37,4 +46,4 @@ pass_ip = [
 
 # Activate passlist of (hardcoded) IPs from the SearXNG organization,
 # e.g. `check.searx.space`.
-pass_searxng_org = true
+pass_searxng_org = true