rproxy, the proxy in front of each app, asks servewaf and fails open (closes #171)

`smallwebwaf rproxy` reads the first 64 KiB of each body, asks servewaf over
HTTP/1.1, and passes what servewaf allows to the app within its own size and
time limits. It reports each request servewaf answered. SWRP_DEFAULT_DISPOSITION
decides when servewaf gives no answer. A request it cannot put to servewaf as
sent, it refuses itself.

Duplication: internal/rproxy copies passing-on code from internal/proxy until
#172 removes the originals.
Rule suppressed: gosec G704 and G705, on servewaf's URL and the visitor's answer.
Rule suppressed: paralleltest, on the test of a switch rproxy refuses.
Judgement call: the SWRP_ size and time limits take off; SWRP_TIMEOUT_MS does not.
Judgement call: every target not starting with one / gets 400.

Model: opus-5-5

docs/admin-endpoints.md

@@ -51,3 +51,9 @@ curl -H "Authorization: Bearer $TOKEN" \
 
 `smallwebwaf servewaf` answers these endpoints at its own address. No check runs
 on those requests (see "`servewaf`" in [`SPEC.md`](../SPEC.md)).
+
+## `rproxy`
+
+`smallwebwaf rproxy` answers only the health check, at its own address. It asks
+`servewaf` about any other request under `/_smallwebwaf/` (see "`rproxy`" in
+[`SPEC.md`](../SPEC.md)).

docs/request-log.md

@@ -3,6 +3,7 @@
 The fields of the JSON object `smallwebwaf` writes for each request. "Request
 log" in [`SPEC.md`](../SPEC.md) says when a line is written and how the log is
 sent to a syslog server. A field that does not apply to a request is left out.
+`smallwebwaf rproxy` writes a shorter line, which "`rproxy`'s line" below gives.
 
 ## Example
 
@@ -12,7 +13,7 @@ sent to a syslog server. A field that does not apply to a request is left out.
 
 ## Every line
 
-Every line has:
+Every line but `rproxy`'s has:
 
 - `type`: `request` (the process's own messages have `process`);
 - the standard web log fields;
@@ -180,3 +181,36 @@ For an allowed request, the report's counting also changes these fields:
 - `counts`: the byte totals now include this request's;
 - `limit_hit`, `offence` and `ban_expires`: set when the report's bytes broke a
   byte limit.
+
+## `rproxy`'s line
+
+`smallwebwaf rproxy` writes one short line for each request. `request_id` joins
+it to `servewaf`'s two. It has these fields and no others:
+
+- `type`: `request`.
+- `time`: when the request arrived, RFC 3339 with milliseconds, in UTC.
+- `request_id`: the id `rproxy` made for the request when it arrived.
+- `client_ip`: the visitor's address.
+- `host` and `method`.
+- `path` and `query`: as the visitor sent them.
+- `status`: what the visitor got, `0` if nothing.
+- `request_bytes`: the bytes of the visitor's body that `rproxy` read, with what
+  a WebSocket carried from the visitor.
+- `response_bytes`: the bytes of the answer's body, with what a WebSocket
+  carried from the app.
+- `duration_request`: milliseconds from the request's headers being read until
+  the app had been sent the whole request, or the answer started if that was
+  sooner. It includes the wait for `servewaf`.
+- `duration_response`: milliseconds from then until the end of the answer.
+- `disposition`: what was done with the request: `allow`, `deny` or `close`. It
+  is `servewaf`'s answer, or the value of `SWRP_DEFAULT_DISPOSITION`. It is left
+  out when `servewaf` was not asked, as for the health check, and when the
+  visitor went away while it was being asked.
+- `default_disposition`: `true` when `SWRP_DEFAULT_DISPOSITION` gave
+  `disposition`, and otherwise left out.
+
+For example:
+
+```
+{"type":"request","time":"2026-10-03T12:00:00.123Z","request_id":"7Q2NHZ4KJ3VXW5YB6R3MEFTD2A","client_ip":"203.0.113.9","host":"app.example","method":"GET","path":"/","query":"","status":200,"request_bytes":0,"response_bytes":5120,"duration_request":1.204,"duration_response":3.104,"disposition":"allow"}
+```

docs/settings.md

@@ -340,11 +340,14 @@ A few limits are fixed rather than settings:
 - at most 1,000 DNS blocklist queries are under way at once;
 - at most 20,000 anomaly counters are kept;
 - each alert destination's queue holds at most 1,000 alerts;
-- `servewaf` holds at most 10,000 answered requests awaiting their reports.
+- `servewaf` holds at most 10,000 answered requests awaiting their reports;
+- `rproxy` takes a request line and headers of at most 32 KiB, closes a
+  kept-open connection idle for 120 seconds, and reads at most 1 MiB of the body
+  of `servewaf`'s answer.
 
 ## `servewaf`
 
-`smallwebwaf servewaf` uses every setting here but these, which it ignores:
+`smallwebwaf servewaf` uses every `SWWAF_` setting but these, which it ignores:
 
 - `SWWAF_UPSTREAM_URL` and `SWWAF_TRUSTED_PROXIES`;
 - the five timeouts under "Size and time limits";
@@ -358,3 +361,38 @@ at start still shows them.
 request to an app. `SWWAF_WAF_BODY_LIMIT` has an effect up to 64 KiB less one
 byte, whatever its value. A question carries no more of a body than 64 KiB (see
 "`servewaf`" in [`SPEC.md`](../SPEC.md)).
+
+## `rproxy`
+
+`smallwebwaf rproxy` reads the settings below and no others, and nothing else
+reads them. They are read once at start, and an invalid value stops the start.
+None can be given as a file, since `rproxy` never touches disk. "`rproxy`" in
+[`SPEC.md`](../SPEC.md) says what they do.
+
+A time here is a whole number of milliseconds above zero, such as `2000`.
+
+- `SWRP_WAF_BACKEND` (required, no default): `servewaf`'s URL. An `http` or
+  `https` URL with a host and an optional port, and nothing more. `rproxy`
+  speaks HTTP/1.1 to it.
+- `SWRP_TIMEOUT_MS` (default `2000`): how long `rproxy` waits for `servewaf`'s
+  answer, to a question and to a report. A time; never `off`.
+- `SWRP_DEFAULT_DISPOSITION` (default `allow`): what is done with a request
+  `servewaf` gives no answer about. `allow` passes it to the app, and `deny`
+  answers `503`.
+- `SWRP_MAX_REQUEST_SIZE` (default `100M`): the largest request body. A size or
+  `off`.
+- `SWRP_MAX_RESPONSE_SIZE` (default `1G`): the largest response body. A size or
+  `off`.
+- `SWRP_MAX_REQUEST_TIME_MS` (default `60000`): how long the visitor may take to
+  send its request line and headers. It then also bounds, from the end of the
+  headers, the time until the app has been sent the whole request. A time or
+  `off`.
+- `SWRP_MAX_RESPONSE_TIME_MS` (default `1800000`): how long the answer may take,
+  from the end of the request to its last byte reaching the visitor. A time or
+  `off`.
+- `SWRP_LISTEN_ADDR` (default `:8080`): where `rproxy` listens, such as `:9000`.
+- `SWRP_UPSTREAM_URL` (default `http://127.0.0.1:8081`): the app. An `http` or
+  `https` URL with a host and an optional port, and nothing more.
+- `SWRP_TRUSTED_PROXIES` (default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`):
+  the netblocks whose `X-Forwarded-For` is believed. Set but empty, it trusts
+  nothing.