State what the default blocklist covers (closes #244)

The default blocklist covers private and reserved space, plus public
addresses that serve cloud credentials. A provider's other services on
public addresses, such as IBM Cloud's 161.26.0.0/16 and 166.8.0.0/14,
are deliberately not on it: they serve no credentials, reaching them
can be legitimate, and every cloud has some, so a partial list would
promise coverage it does not give.

The README's egress section and the comment above blockedNetworks now
state this rule, so nobody infers wider coverage and a future candidate
can be accepted or refused against it. No list change.

Model: opus-5-5

README.md

@@ -147,7 +147,7 @@ TTY detection, and security headers are always applied.
 | `RETENTION_SWEEP_INTERVAL` | How often the retention reaper and archive sweeper run (Go duration, must be positive) | `1h` |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint (10x that per IP across the route) | `120` |
-| `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted. A set value replaces the default. Under the default, any client with a private address, whether it connects directly or through the proxy, can choose its own rate-limit key by sending its own `X-Forwarded-For`; if any clients have private addresses, set it to the proxy's address alone. See [Trusted proxies](#trusted-proxies) | `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16` (RFC 1918) |
+| `TRUSTED_PROXIES`       | CIDRs whose forwarded headers are trusted (unset: all clients behind a proxy share one rate-limit bucket; a correct login password is never throttled either way) | `""` (none) |
 | `ALLOWED_EGRESS_CIDRS`  | CIDRs that delivery targets may reach despite the SSRF blocklist. Read [Allowing egress to your own network](#allowing-egress-to-your-own-network) before setting it | `""` (none) |
 
 #### Allowing egress to your own network
@@ -162,6 +162,14 @@ public cloud metadata addresses: currently only `168.63.129.16`, Azure's
 WireServer, which serves an Azure VM its credentials. Because it is a
 public address, listing it in `ALLOWED_EGRESS_CIDRS` reopens it.
 
+That is all the default blocklist covers: private and reserved space,
+plus public addresses that serve cloud credentials. A cloud provider's
+other services on public addresses are not refused — IBM Cloud's
+`161.26.0.0/16` and `166.8.0.0/14`, for example, which carry its DNS
+resolvers, time servers and package mirrors. They serve no credentials,
+reaching them can be a legitimate delivery, and every cloud has some, so
+a partial list would promise coverage it does not give.
+
 That default is also inconvenient for the thing webhooker is mostly
 for: taking a public webhook and forwarding it to something on your own
 network. A container on the same Docker network, a box on `10.x`, a
@@ -379,48 +387,41 @@ unlocked.
 `TRUSTED_PROXIES` is a comma-separated list of CIDR blocks (a bare
 address such as `192.168.1.7` is accepted and treated as a single
 host), for example `192.168.1.7, 2001:db8::5`. It decides whose
-`X-Forwarded-For` header the rate limiters believe, so it should cover
-the addresses of your reverse proxies.
+`X-Forwarded-For` header the rate limiters believe, so it should name
+the addresses of your reverse proxies and nothing else.
 
 `X-Forwarded-For` is honoured **only** when the connecting peer is
 inside one of these blocks; for every other peer the client identity is
-the connection's own address and the header is ignored. Unset (or
-empty), the list is the RFC 1918 private ranges: `10.0.0.0/8`,
-`172.16.0.0/12` and `192.168.0.0/16`. That covers a reverse proxy
-reaching webhooker over a Docker network or a private LAN without
-anything set. A set value replaces the default entirely. A set but
-unparseable value aborts startup.
+the connection's own address and the header is ignored. The default is
+the empty list, which trusts nobody — anything else would let any
+client pick its own rate limit bucket, minting a fresh one per request
+or draining someone else's. Set it to the address of your reverse
+proxy, and to nothing wider. A set but unparseable value aborts
+startup.
 
-Trusting those ranges has two consequences for clients with private
-addresses:
-
-- Any such client, whether it connects directly or through the proxy,
-  can choose its own rate-limit key by sending its own
-  `X-Forwarded-For`. A direct client's header is walked because the
-  client is itself trusted; behind the proxy, the client's own address
-  is skipped as a trusted hop when the chain is walked (below), so the
-  entry it wrote is taken as the client. If any of your clients have
-  private addresses, you must set `TRUSTED_PROXIES` to the proxy's
-  address alone.
-- A client behind the proxy that sends no `X-Forwarded-For` of its own
-  shares the proxy's bucket, because its own address is skipped too.
-  Setting the list to the proxy's address alone gives each its own
-  bucket.
-
-A proxy the list does not cover, such as nginx on the same host
-reaching webhooker over loopback, is not trusted: every request through
-it keys on the proxy's own address and all clients share a single
+That default is safe against forged headers, but leaving it unset in
+production has a cost you must know about. Production runs behind a
+TLS-terminating reverse proxy, so with `TRUSTED_PROXIES` unset every
+request keys on the proxy's own address and all clients share a single
 bucket per limit. The receiver limits become service-wide ceilings,
 and the login endpoint's failure counting collapses onto one key, so a
 stranger's wrong passwords throttle every other client's wrong
-passwords. Set `TRUSTED_PROXIES` to that proxy's address to restore
-per-client buckets.
+passwords.
 
 What it cannot do is lock the operator out. The login endpoint
 verifies credentials **before** it consults any limit and charges only
 failures, so a correct password is never throttled no matter how full
 the bucket is. See [Rate Limiting](#rate-limiting).
 
+The remedy is to set `TRUSTED_PROXIES` to your reverse proxy's
+address, which restores per-client buckets. webhooker logs a warning
+at startup whenever `TRUSTED_PROXIES` is empty, in every environment,
+because behind a proxy every client shares one bucket in `dev` and
+`prod` alike. The warning is informational when nothing proxies to the
+process: with no proxy in front, the peer address is the client's own
+and the buckets are already per-client. See
+[Rate Limiting](#rate-limiting) for what each limit shares.
+
 `X-Real-IP` and `True-Client-IP` are **never** read, from any peer.
 Reverse proxies append to `X-Forwarded-For` but forward other client
 headers verbatim, so a single-valued header is client-controlled even
@@ -442,14 +443,14 @@ Two operator requirements follow:
   (nginx `$proxy_add_x_forwarded_for`, HAProxy `option forwardfor`,
   Caddy and AWS ALB by default), and must append a bare address with
   no port.
-- Keep clients out of the list. Any address inside `TRUSTED_PROXIES`
+- List proxy hosts **only**. Any address inside `TRUSTED_PROXIES`
   chooses its own rate-limit key: its `X-Forwarded-For` is walked, so
   it can name a different address on every request to get a fresh
   bucket each time, or name another client's address to drain that
-  client's bucket. A block that also covers clients — the default, on
-  a network where clients have private addresses — makes all three
-  limits, including the unauthenticated webhook receiver, silently
-  bypassable by every client in the block.
+  client's bucket. Never list a block that also covers clients — a
+  broad `10.0.0.0/8` on a network where clients live in the same range
+  makes all three limits, including the unauthenticated webhook
+  receiver, silently bypassable by every client in the block.
 
 #### Sessions
 
@@ -771,16 +772,10 @@ repository's `Dockerfile` and runs it. The app needs:
 
 - **Environment variables:**
     - `WEBHOOKER_ENVIRONMENT=prod`
-    - `TRUSTED_PROXIES`: Docker networks use private addresses, so the
-      default covers your reverse proxy on that network. Under the
-      default, any client with a private address, whether it connects
-      directly or through the proxy, can choose its own rate-limit key
-      by sending its own `X-Forwarded-For`. If any clients have private
-      addresses, or the network's addresses are outside the RFC 1918
-      ranges, set it to the proxy's address there. The `remoteIP` field
-      of the `http request` log line for a request that came through
-      the proxy shows it; the health check's own lines show `::1`. See
-      [Trusted proxies](#trusted-proxies).
+    - `TRUSTED_PROXIES`: your reverse proxy's address on that Docker
+      network. The `remoteIP` field of the `http request` log line for a
+      request that came through the proxy shows it; the health check's
+      own lines show `::1`. See [Trusted proxies](#trusted-proxies).
     - Leave `BIND_ADDRESS` and `DATA_DIR` unset: the image sets
       `BIND_ADDRESS` to `0.0.0.0`, and `DATA_DIR` defaults to
       `/var/lib/webhooker`.
@@ -843,14 +838,12 @@ reports.
    behind a proxy means the `X-Forwarded-Proto` header. The block below
    sets it; without it every request is read as plaintext and cookies
    ship without `Secure`. See [Configuration](#configuration).
-3. **Make sure `TRUSTED_PROXIES` covers the proxy's address.** Unset,
-   it covers the RFC 1918 private ranges, so a proxy on a Docker
-   network or a private LAN is covered and one on loopback is not. For
-   a proxy it does not cover, every rate limiter keys on the connecting
-   peer, which is the proxy on every request: all clients collapse into
-   one global bucket per limit and the receiver's per-IP limits become
-   service-wide ceilings. See [Trusted proxies](#trusted-proxies). If
-   any clients have private addresses, list the proxy and nothing else.
+3. **Set `TRUSTED_PROXIES` to the proxy's address.** Unset, every rate
+   limiter keys on the connecting peer, which behind a proxy is the
+   proxy on every request: all clients collapse into one global bucket
+   per limit and the receiver's per-IP limits become service-wide
+   ceilings. See [Trusted proxies](#trusted-proxies). List the proxy
+   and nothing else.
 4. **Send `Host` as `$http_host`, not `$host`.** `$host` strips the
    port. webhooker's Origin/Referer check compares against the host it
    was given, so on any port other than 443 `$host` makes every form
@@ -1411,11 +1404,10 @@ It uses:
 - **[go-chi/httprate](https://github.com/go-chi/httprate)** for
   sliding-window rate limiting of the password-change and webhook
   receiver endpoints. The bucket is per client IP only when
-  `TRUSTED_PROXIES` covers the reverse proxy (by default it covers the
-  RFC 1918 private ranges); otherwise every client behind that proxy
-  shares one bucket per limit. The login endpoint counts failed
-  attempts itself instead, so that a correct password is never
-  throttled (see [Rate Limiting](#rate-limiting))
+  `TRUSTED_PROXIES` names the reverse proxy; unset, every client
+  behind that proxy shares one bucket per limit. The login endpoint
+  counts failed attempts itself instead, so that a correct password is
+  never throttled (see [Rate Limiting](#rate-limiting))
 - **[Prometheus](https://prometheus.io)** for metrics, served at
   `/metrics` behind basic auth
 - **[Sentry](https://sentry.io)** for optional error reporting
@@ -2596,30 +2588,30 @@ let one subscriber rotate source addresses and mint a fresh bucket per
 request, evading these limits at the network layer without spoofing
 anything; the cost is that distinct clients inside one `/64` share a
 bucket. IPv4-mapped addresses (`::ffff:1.2.3.4`) key as the IPv4 address
-they carry. See [Trusted proxies](#trusted-proxies). When that variable
-does not cover the reverse proxy, a client behind it shares one bucket
-with every other client behind the same proxy. Set `TRUSTED_PROXIES` to
-the proxy's address to get per-client limits back. What the shared bucket
+they carry. See [Trusted proxies](#trusted-proxies). Deployed without that
+variable set, a client behind a reverse proxy shares one bucket with
+every other client behind the same proxy. Set `TRUSTED_PROXIES` to the
+proxy's address to get per-client limits back. What the shared bucket
 costs is not the same for every limiter, and the two cases pull in
 opposite directions:
 
 - For the **receiver** limits it costs throughput, which is the safe
   direction to be wrong in: sharing can only make a limit bind sooner,
   never let a sender past it. It matters more for the aggregate limit
-  than for the per-entrypoint one: when `TRUSTED_PROXIES` does not
-  cover the reverse proxy a production deployment is required to run
-  behind, every request keys on the proxy, so the aggregate limit
-  becomes a service-wide ceiling of 1200 requests per minute across all
-  senders and all entrypoints, where the per-entrypoint limit's
-  capacity still grows with the number of entrypoints. Any deployment
-  with more than a handful of busy entrypoints must make sure
-  `TRUSTED_PROXIES` covers its proxy.
+  than for the per-entrypoint one: with `TRUSTED_PROXIES` unset behind
+  the reverse proxy a production deployment is required to run behind,
+  every request keys on the proxy, so the aggregate limit becomes a
+  service-wide ceiling of 1200 requests per minute across all senders
+  and all entrypoints, where the per-entrypoint limit's capacity still
+  grows with the number of entrypoints. Any deployment with more than a
+  handful of busy entrypoints must set `TRUSTED_PROXIES`.
 - For the **login and password-change** limits it costs precision, not
   availability. Login failures from every client land in one counter,
   so a stranger's wrong passwords make the operator's own wrong
   passwords answer `429` sooner; the operator's _correct_ password is
   never affected, because it is never counted. Production deployments
-  should still make sure `TRUSTED_PROXIES` covers their proxy.
+  should still set `TRUSTED_PROXIES`; webhooker warns at startup
+  whenever it is empty, in any environment.
 
 #### The login endpoint
 
@@ -2722,10 +2714,8 @@ re-fills both verification slots on its first two requests. The
 remedies are to block the source at the reverse proxy, or to
 rate-limit `POST /pages/login` there — the one place a limit can be
 applied without reintroducing the lockout, because the proxy sees the
-real client address. `TRUSTED_PROXIES` does not stop the saturation.
-The flood's source is in the proxy's access log: webhooker's own logs
-record the proxy's address, not the client's (see
-[Deployment behind a reverse proxy](#deployment-behind-a-reverse-proxy)).
+real client address. Setting `TRUSTED_PROXIES` does not stop the
+saturation, but it makes the source visible in the failure logs.
 
 Finer-grained per-webhook rate limits (configured in the web UI and
 enforced in the webhook handler) can layer on top of this env-level
@@ -3082,9 +3072,10 @@ check, see [The login endpoint](#the-login-endpoint).
   It runs behind session auth, so only a client already holding a
   valid session reaches it, and an operator throttled out of changing
   a password can still log in. The bucket is per client IP only when
-  `TRUSTED_PROXIES` covers the reverse proxy; otherwise every client
+  `TRUSTED_PROXIES` names the reverse proxy; unset, every client
   shares one bucket, which costs precision rather than availability
-  (see [Rate Limiting](#rate-limiting))
+  (see [Rate Limiting](#rate-limiting)). webhooker warns at startup
+  whenever `TRUSTED_PROXIES` is empty
 - Prometheus metrics behind basic auth
 - Static assets embedded in binary (no filesystem access needed at
   runtime)

internal/config/config.go

@@ -75,11 +75,6 @@ const (
 	// internet-exposed endpoint.
 	defaultReceiverRateLimit = 120
 
-	// defaultTrustedProxies is TRUSTED_PROXIES when it is unset: the
-	// RFC 1918 private ranges, which a reverse proxy reaching the
-	// process over a Docker network or a private LAN connects from.
-	defaultTrustedProxies = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
-
 	// maxPort is the highest valid TCP port number. The lower
 	// bound (at least 1) is enforced by envPositiveInt.
 	maxPort = 65535
@@ -177,14 +172,13 @@ type Config struct {
 
 	// TrustedProxies is the set of networks whose members are
 	// allowed to speak for the client with X-Forwarded-For, the
-	// only forwarded header read. Unless TRUSTED_PROXIES is set it
-	// is the RFC 1918 private ranges (defaultTrustedProxies).
-	// Other peers' forwarded headers are ignored and they are
-	// identified by the connection's own address. Under the
-	// default any client with a private address, directly or
-	// through a proxy, can choose its own rate-limit key, so
-	// where any clients have private addresses this must be set
-	// to the proxy hosts alone.
+	// only forwarded header read. It is empty unless
+	// TRUSTED_PROXIES is set, and empty means no peer is
+	// trusted: forwarded headers are then ignored entirely and
+	// clients are identified by the connection's own address.
+	// Members can choose their own rate-limit key, so this must
+	// name proxy hosts only, never a block that also covers
+	// clients.
 	TrustedProxies []netip.Prefix
 
 	// AllowedEgressCIDRs is the set of networks a delivery target
@@ -466,15 +460,14 @@ func parseCIDR(entry string) (netip.Prefix, error) {
 
 // envPrefixList returns the value of the named environment variable
 // parsed as a comma-separated list of CIDR blocks (bare addresses
-// allowed). An unset, empty, or blank value is read as defaultValue
-// instead. A set value containing an unparseable entry is a hard
-// error naming the key and the bad entry, so startup fails loudly
-// rather than silently running with a list the operator did not
-// intend.
-func envPrefixList(key, defaultValue string) ([]netip.Prefix, error) {
+// allowed). An unset, empty, or blank value yields an empty list. A
+// set value containing an unparseable entry is a hard error naming
+// the key and the bad entry, so startup fails loudly rather than
+// silently running with a list the operator did not intend.
+func envPrefixList(key string) ([]netip.Prefix, error) {
 	v := strings.TrimSpace(os.Getenv(key))
 	if v == "" {
-		v = defaultValue
+		return nil, nil
 	}
 
 	var prefixes []netip.Prefix
@@ -688,12 +681,12 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
-	trustedProxies, err := envPrefixList("TRUSTED_PROXIES", defaultTrustedProxies)
+	trustedProxies, err := envPrefixList("TRUSTED_PROXIES")
 	if err != nil {
 		return nil, err
 	}
 
-	allowedEgressCIDRs, err := envPrefixList("ALLOWED_EGRESS_CIDRS", "")
+	allowedEgressCIDRs, err := envPrefixList("ALLOWED_EGRESS_CIDRS")
 	if err != nil {
 		return nil, err
 	}
@@ -767,6 +760,50 @@ func (c *Config) warnEgressAllowlist(log *slog.Logger) {
 	)
 }
 
+// warnSharedRateLimitBucket logs a startup warning whenever
+// TRUSTED_PROXIES is empty, in any environment.
+//
+// With no trusted proxies every rate limiter keys on the connecting
+// peer's address. Whether that is harmless or dangerous depends on
+// what is in front of the process, which this code cannot observe:
+// with nothing in front, the peer is the client and the limits are
+// per-client as intended; behind a reverse proxy the peer is the proxy
+// for every request, so all clients share one bucket per limiter.
+//
+// The login endpoint no longer spends budget on arrival — it verifies
+// credentials first and charges only failures — so a shared bucket
+// cannot deny the operator a correct password. What it does collapse
+// is the failure counting: one client's wrong passwords throttle
+// everyone else's wrong passwords, and the receiver's limits become
+// service-wide ceilings.
+//
+// The warning is deliberately not gated on WEBHOOKER_ENVIRONMENT:
+// behind a proxy every client shares one bucket in dev and prod alike.
+//
+// The default of trusting nobody is deliberate — trusting forwarded
+// headers from arbitrary peers lets any client choose its own bucket —
+// so this warns rather than failing startup or changing the key.
+func (c *Config) warnSharedRateLimitBucket(log *slog.Logger) {
+	if len(c.TrustedProxies) > 0 {
+		return
+	}
+
+	log.Warn(
+		"TRUSTED_PROXIES is empty: every rate limit keys on the "+
+			"connecting peer's address. With nothing proxying to "+
+			"this process that is the client itself and the limits "+
+			"are per-client as intended. Behind a reverse proxy the "+
+			"peer is the proxy on every request, so all clients "+
+			"share one bucket per limit: the receiver limits become "+
+			"service-wide ceilings, and one client's failed logins "+
+			"throttle every other client's failed logins — a "+
+			"correct password still gets in. If anything proxies to "+
+			"this process, set TRUSTED_PROXIES to its address.",
+		"environment", c.Environment,
+		"trustedProxies", len(c.TrustedProxies),
+	)
+}
+
 // New creates a Config by reading environment variables.
 //
 //nolint:revive // lc parameter is required by fx even if unused.
@@ -812,6 +849,7 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"hasMetricsAuth", s.MetricsAuthEnabled(),
 	)
 
+	s.warnSharedRateLimitBucket(log)
 	s.warnEgressAllowlist(log)
 
 	return s, nil

internal/config/config_test.go

@@ -551,11 +551,6 @@ func testReceiverRateLimitSuccess(
 }
 
 func TestTrustedProxies(t *testing.T) {
-	// Unset, the RFC 1918 private ranges are trusted, so a reverse
-	// proxy on a Docker network or a private LAN is covered without
-	// configuration.
-	defaultProxies := []string{cidrPrivateV4, "172.16.0.0/12", "192.168.0.0/16"}
-
 	tests := []struct {
 		name        string
 		set         bool
@@ -564,21 +559,18 @@ func TestTrustedProxies(t *testing.T) {
 		expected    []string
 	}{
 		{
+			// The default must be "trust nobody": an empty list
+			// means forwarded headers are ignored, never that
+			// every peer may speak for the client.
 			name:     caseUnsetUsesDefault,
 			set:      false,
-			expected: defaultProxies,
+			expected: []string{},
 		},
 		{
-			name:     "blank value uses default",
+			name:     "blank value trusts nothing",
 			set:      true,
 			value:    "  ",
-			expected: defaultProxies,
-		},
-		{
-			name:     "set value replaces the default entirely",
-			set:      true,
-			value:    "203.0.113.7",
-			expected: []string{"203.0.113.7/32"},
+			expected: []string{},
 		},
 		{
 			name:  caseValidValueParsed,
@@ -853,6 +845,101 @@ func TestEgressAllowlistWarning(t *testing.T) {
 	}
 }
 
+// TestSharedRateLimitBucketWarning covers the startup warning that
+// tells an operator a deployment behind a reverse proxy shares one
+// rate-limit bucket between every client, which turns the receiver
+// limits into service-wide ceilings and collapses login failure
+// counting. It must fire whenever TRUSTED_PROXIES is empty, in any
+// environment, because behind a proxy every client shares one bucket
+// in dev and prod alike. It stays quiet once proxies are named.
+func TestSharedRateLimitBucketWarning(t *testing.T) {
+	tests := []struct {
+		name           string
+		environment    string
+		trustedProxies string
+		expectWarning  bool
+	}{
+		{
+			name:          "prod without trusted proxies warns",
+			environment:   config.EnvironmentProd,
+			expectWarning: true,
+		},
+		{
+			name:           "prod with trusted proxies is quiet",
+			environment:    config.EnvironmentProd,
+			trustedProxies: cidrPrivateV4,
+			expectWarning:  false,
+		},
+		{
+			name:          "dev without trusted proxies warns",
+			environment:   config.EnvironmentDev,
+			expectWarning: true,
+		},
+		{
+			name:           "dev with trusted proxies is quiet",
+			environment:    config.EnvironmentDev,
+			trustedProxies: cidrPrivateV4,
+			expectWarning:  false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			t.Setenv("WEBHOOKER_ENVIRONMENT", tt.environment)
+
+			if tt.trustedProxies == "" {
+				require.NoError(
+					t, os.Unsetenv("TRUSTED_PROXIES"),
+				)
+			} else {
+				t.Setenv("TRUSTED_PROXIES", tt.trustedProxies)
+			}
+
+			var buf bytes.Buffer
+
+			log := slog.New(slog.NewJSONHandler(
+				&buf, &slog.HandlerOptions{
+					Level: slog.LevelDebug,
+				},
+			))
+
+			require.NoError(
+				t,
+				config.WarnSharedRateLimitBucketForTest(log),
+			)
+
+			if !tt.expectWarning {
+				assert.Empty(t, buf.String())
+
+				return
+			}
+
+			logged := buf.String()
+
+			assert.Contains(t, logged, `"level":"WARN"`)
+			assert.Contains(t, logged, "TRUSTED_PROXIES")
+			assert.Contains(t, logged, "share one bucket")
+			assert.Contains(
+				t, logged, "throttle every other client's failed logins",
+			)
+			// The warning must not claim a lockout the login
+			// endpoint no longer permits: credentials are verified
+			// before any budget is spent.
+			assert.Contains(
+				t, logged, "a correct password still gets in",
+			)
+			// The text must stay accurate for a developer with
+			// nothing in front of the process, where an empty
+			// list costs nothing.
+			assert.Contains(
+				t, logged, "nothing proxying to this process",
+			)
+		})
+	}
+}
+
 // metricsEnv describes what one subtest below puts in the
 // environment for a single METRICS_ variable. A variable that is
 // set to the empty string and one that is not set at all are

internal/config/export_test.go

@@ -6,6 +6,21 @@ import "log/slog"
 // the external config_test package so each helper can be covered by
 // its own table-driven test without weakening the package API.
 
+// WarnSharedRateLimitBucketForTest loads a Config from the current
+// environment and emits its startup warnings to log. The real logger
+// writes to stdout, so this lets the warning's firing condition be
+// asserted against a handler the test controls.
+func WarnSharedRateLimitBucketForTest(log *slog.Logger) error {
+	c, err := loadFromEnv()
+	if err != nil {
+		return err
+	}
+
+	c.warnSharedRateLimitBucket(log)
+
+	return nil
+}
+
 // WarnEgressAllowlistForTest loads a Config from the current
 // environment and emits its egress-allowlist startup warning to
 // log, so a test can assert both that the warning fires only when

internal/delivery/ssrf.go

@@ -43,6 +43,12 @@ var (
 // permit specific blocks out of this set with
 // ALLOWED_EGRESS_CIDRS; see Guard.
 //
+// A public address belongs here only if it serves cloud
+// credentials; a provider's other services on public addresses,
+// such as its DNS resolvers or package mirrors, stay out, since
+// reaching them can be legitimate and no list of them could be
+// complete.
+//
 //nolint:gochecknoglobals // package-level network list is appropriate here
 var blockedNetworks []*net.IPNet
 

internal/handlers/auth.go

@@ -103,10 +103,9 @@ func (h *Handlers) renderLoginError(
 // The credential check runs BEFORE any rate-limit budget is
 // consulted, and only a failed check spends budget. That is what
 // keeps the single administrative path reachable: behind the reverse
-// proxy this deployment requires, when TRUSTED_PROXIES does not cover
-// it, every client shares one bucket, so a limiter spent on arrival
-// lets any stranger deny the operator's own correct password
-// indefinitely.
+// proxy this deployment requires, with TRUSTED_PROXIES unset, every
+// client shares one bucket, so a limiter spent on arrival lets any
+// stranger deny the operator's own correct password indefinitely.
 //
 // Verifying first means every login POST costs an Argon2id hash, so
 // the work is taken under a bounded number of verification slots.

internal/handlers/auth_test.go

@@ -25,7 +25,7 @@ const (
 
 	// sharedProxyPeer is the whole point of this file. Production is
 	// required to run behind a TLS-terminating reverse proxy, and
-	// when TRUSTED_PROXIES does not cover it every client — attacker
+	// TRUSTED_PROXIES defaults to empty, so every client — attacker
 	// and operator alike — reaches the process from the proxy's
 	// address and shares one rate-limit bucket. Both parties in
 	// these tests therefore use the same RemoteAddr.
@@ -115,11 +115,11 @@ func floodFailures(
 // done-criterion of https://git.eeqj.de/sneak/webhooker/issues/150.
 //
 // The attacker and the operator share one rate-limit bucket, because
-// behind the mandated reverse proxy, when TRUSTED_PROXIES does not
-// cover it, every client keys on the proxy's address. The attacker
-// floods the operator's own username — a single-admin product has a
-// predictable one — far past the failure limit. The operator must
-// still be able to log in with the correct password.
+// behind the mandated reverse proxy with TRUSTED_PROXIES unset every
+// client keys on the proxy's address. The attacker floods the
+// operator's own username — a single-admin product has a predictable
+// one — far past the failure limit. The operator must still be able
+// to log in with the correct password.
 //
 // This fails if credentials stop being verified ahead of the limiter.
 func TestLogin_StrangersFloodCannotLockOutTheOperator(t *testing.T) {

internal/middleware/loginguard.go

@@ -108,10 +108,10 @@ type failureWindow struct {
 //
 // A limiter that spends budget on arrival cannot protect a
 // single-admin product: behind the reverse proxy the deployment
-// requires, when TRUSTED_PROXIES does not cover it, every client
-// keys on the proxy, so a stranger trickling five POSTs a minute
-// keeps the one bucket full and the operator's own correct password
-// is answered 429 forever. There is no second administrative path.
+// requires, with TRUSTED_PROXIES unset, every client keys on the
+// proxy, so a stranger trickling five POSTs a minute keeps the one
+// bucket full and the operator's own correct password is answered 429
+// forever. There is no second administrative path.
 //
 // So budget is spent only by a FAILED verification. A correct
 // password is never throttled, whatever the counters say, which is

internal/middleware/ratelimit.go

@@ -123,8 +123,9 @@ func bucketKey(addr netip.Addr) string {
 	return prefix.String()
 }
 
-// isTrustedProxy reports whether addr belongs to a network in
-// TRUSTED_PROXIES, which by default is the RFC 1918 private ranges.
+// isTrustedProxy reports whether addr belongs to a network the
+// operator listed in TRUSTED_PROXIES. The list is empty by default,
+// so by default nothing is trusted.
 func (m *Middleware) isTrustedProxy(addr netip.Addr) bool {
 	for _, prefix := range m.params.Config.TrustedProxies {
 		if prefix.Contains(addr) {

internal/middleware/ratelimit_test.go

@@ -426,8 +426,8 @@ func assertSharedBucket(
 }
 
 // TestRateLimitKey_SpoofedForwardedFromUntrustedPeer is the test
-// this gating exists for: from a peer that is not a trusted
-// proxy, a client that rotates a forwarded header on every
+// this gating exists for: with no trusted proxies configured (the
+// default), a client that rotates a forwarded header on every
 // request must stay in one bucket. If forwarded headers were
 // trusted unconditionally, each spoofed value would mint a fresh
 // bucket and the limit would stop no one.

internal/server/routes.go

@@ -140,12 +140,11 @@ func (s *Server) setupPageRoutes() {
 		r.Use(s.mw.NoCache())
 
 		// The login POST carries no pre-emptive rate limiter. Behind
-		// the reverse proxy production requires, when TRUSTED_PROXIES
-		// does not cover it, every client shares one bucket, so a
-		// limiter spent on arrival lets any stranger deny the operator
-		// the only administrative path. The handler verifies
-		// credentials first and charges only failures; see
-		// Handlers.authenticateUser.
+		// the reverse proxy production requires, with TRUSTED_PROXIES
+		// unset, every client shares one bucket, so a limiter spent
+		// on arrival lets any stranger deny the operator the only
+		// administrative path. The handler verifies credentials first
+		// and charges only failures; see Handlers.authenticateUser.
 		r.Get("/login", s.h.HandleLoginPage())
 		r.Post("/login", s.h.HandleLoginSubmit())