Honour forwarded headers only from trusted proxies (closes #259)

A new ClientIP middleware sets request.RemoteAddr to the client's
address before the request is logged. X-Forwarded-For and X-Real-IP
count only when the peer is in TRUSTED_PROXIES, which defaults to the
RFC 1918 ranges. The login rate limit, the session's IP address and
hostname, and the request log all use that address.

Model: opus-5-5

README.md

@@ -1339,8 +1339,9 @@ Every session has a hostmask in IRC's form, `nick!username@hostname`:
   from, or that IP address when the lookup gives no name. The lookup waits up to
   three seconds.
 
-Over the HTTP API, the server takes the client's IP address from the first entry
-of the `X-Forwarded-For` header, then `X-Real-IP`, then the TCP connection (see
+Over the HTTP API, the client's IP address is the address of the TCP connection.
+When that connection comes from a trusted proxy, the server reads the address
+from `X-Forwarded-For` or `X-Real-IP` instead (see
 [Reverse proxy](#reverse-proxy)).
 
 Every user can see a hostmask: in `RPL_WHOISUSER` (`311`), `RPL_WHOREPLY`
@@ -1450,10 +1451,9 @@ over the limit gets `429` `too many login attempts, try again later` with
 attempt is more than 15 minutes old. Session creation has no rate limit; the
 hashcash stamp is its cost.
 
-The IP address comes from the first entry of `X-Forwarded-For`, else from
-`X-Real-IP`, else from the TCP connection. Without a reverse proxy that
-overwrites those headers, a client can set them itself and so pick any address
-(see [Reverse proxy](#reverse-proxy)).
+The IP address is the client's address as described in
+[Reverse proxy](#reverse-proxy). A client outside the trusted proxies cannot
+change it with `X-Forwarded-For` or `X-Real-IP`.
 
 ### Writing a client
 
@@ -1714,10 +1714,14 @@ Every setting is an environment variable. The server also reads a `.env` file in
 its working directory, and a YAML file `neoirc.yaml` in `/etc/neoirc/` or
 `$HOME/.config/neoirc/` with the same names as keys. A variable from the
 environment wins over one in `.env`, and both win over `neoirc.yaml`. An
-environment variable set to the empty string counts as unset, except
-`IRC_LISTEN_ADDR`, where the empty string turns the IRC listener off. A
-`neoirc.yaml` that is not valid YAML stops the server at startup. A setting that
-does not parse or is out of range also stops the server at startup, and the
+environment variable set to the empty string counts as unset, with two
+exceptions:
+
+- For `IRC_LISTEN_ADDR`, the empty string turns the IRC listener off.
+- For `TRUSTED_PROXIES`, the empty string trusts no proxy.
+
+A `neoirc.yaml` that is not valid YAML stops the server at startup. A setting
+that does not parse or is out of range also stops the server at startup, and the
 error names the setting.
 
 - `PORT` (default `8080`): the HTTP port. `0` lets the system pick a free port.
@@ -1747,6 +1751,11 @@ error names the setting.
   address. A fraction such as `0.5` is allowed. It must be above `0`.
 - `LOGIN_RATE_BURST` (default `5`): how many attempts an address may make at
   once before the rate applies. It must be at least `1`.
+- `TRUSTED_PROXIES` (default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`): the
+  reverse proxies whose `X-Forwarded-For` and `X-Real-IP` headers the server
+  believes. It is a comma-separated list of networks, such as `10.0.0.0/8`, and
+  single addresses, such as `192.0.2.1`. A list replaces the default entirely.
+  The empty string trusts nothing. See [Reverse proxy](#reverse-proxy).
 - `SESSION_IDLE_TIMEOUT` (default `720h`, 30 days): a duration with a unit, `h`
   for hours, `m` for minutes or `s` for seconds, such as `720h` or `36h30m`. A
   client with no request for this long is removed, and a session left with no
@@ -1804,14 +1813,30 @@ Run the server behind a reverse proxy that terminates TLS. The server itself
 speaks plain HTTP, and the auth cookie is `Secure`, so browsers send it only
 over HTTPS. The proxy must:
 
-- Set `X-Forwarded-For` to the client's address, replacing any value the client
-  sent. The server reads that header first and trusts it from anyone; it reads
-  `X-Real-IP` only when `X-Forwarded-For` is absent. Without a proxy that
-  overwrites the header, a client can choose its own address, which sets its
-  hostname and lets it dodge the login rate limit.
+- Connect from an address in `TRUSTED_PROXIES`. The default trusts the private
+  ranges `10.0.0.0/8`, `172.16.0.0/12` and `192.168.0.0/16`. A proxy on the same
+  host connects from `127.0.0.1`, which the default does not trust, so set
+  `TRUSTED_PROXIES=127.0.0.1` for it.
+- Add the client's address to `X-Forwarded-For`, or set `X-Real-IP`.
 - Wait at least 60 seconds for a response. A long poll can take 30 seconds, and
   the server's own write timeout is 60 seconds.
 
+The server finds the client's address in these steps:
+
+- A connection from outside `TRUSTED_PROXIES` is the client. The server ignores
+  its `X-Forwarded-For` and `X-Real-IP` headers.
+- For a connection from a trusted proxy, the server reads `X-Forwarded-For` from
+  right to left. It skips trusted addresses and takes the first untrusted one.
+  When every entry is trusted, it takes the leftmost.
+- Without `X-Forwarded-For`, the server takes `X-Real-IP`.
+- When an entry it reads is not an IP address, the server uses the proxy's
+  address.
+
+The client's address sets the session's hostname and counts toward the login
+rate limit. Any connection from a trusted address can choose its client address
+through `X-Forwarded-For`. If clients connect directly from private addresses,
+set `TRUSTED_PROXIES` to the proxy's address alone.
+
 Caddy:
 
 ```
@@ -1861,9 +1886,10 @@ The IRC listener has no TLS.
 
 The server logs with Go's `slog`, one line per event, to standard output. The
 lines are JSON, or plain text when standard output is a terminal. Every HTTP
-request is logged with its method, URL, protocol, status, latency, remote
-address, user agent, referrer and request ID. `DEBUG=true` adds debug-level
-lines.
+request is logged with its method, URL, protocol, status, latency, client
+address, user agent, referrer and request ID. The client address is the one
+found as described in [Reverse proxy](#reverse-proxy). `DEBUG=true` adds
+debug-level lines.
 
 ### The database