build: stop installing golangci-lint on the host

The linter runs only inside the pinned golangci-lint container image
(Dockerfile.lint, and the Dockerfile lint stage). Remove the host
install that was still left behind: script/bootstrap's version and
sha256 pins, the release download, the ensure/verify helpers, and the
curl dependency they were the only user of; and drop golangci-lint from
the nix-shell package lists in the Makefile and script/test, which build
and test but never lint.

Model: opus-4-8

Dockerfile

@@ -13,9 +13,12 @@ RUN go mod download
 # Copy source code
 COPY . .
 
-# Run formatting check and linter
+# Run formatting check and linter. The linter is invoked directly, not
+# via `make lint`: `make lint` now builds Dockerfile.lint, and there is
+# no Docker inside a Docker build. This is the same linter, image, and
+# config that Dockerfile.lint and script/lint run.
 RUN make fmt-check
-RUN make lint
+RUN golangci-lint run --config .golangci.yml ./...
 
 # Build stage
 # golang:1.25.4-alpine, 2026-02-25

Dockerfile.lint

@@ -0,0 +1,41 @@
+# Dockerfile.lint: the one and only path that runs golangci-lint.
+#
+# golangci-lint is never installed on the host; it runs only inside this
+# build. A clean build of this file therefore IS a clean lint over the
+# whole tree. It runs the same linter and config as Dockerfile's lint
+# stage, pinned to the same image so the two cannot drift to different
+# linter versions.
+#
+# golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
+FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60
+
+# pixa is CGO/libvips: the type-aware linters compile every package, so
+# this image needs the same C libraries the build does.
+RUN apk add --no-cache build-base vips-dev libheif-dev pkgconfig
+
+WORKDIR /src
+
+# Modules first for layer caching; go.mod/go.sum settle this layer's
+# result, so it may safely be reused between runs.
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# Caching is deliberately waived for the lint step: an unchanged tree
+# must still run the linter, not return a cached success in well under a
+# second having linted nothing. CACHEBUST carries a value that differs
+# on every run (script/lint supplies it and refuses to build without
+# one). The lint RUN below references it, so BuildKit cannot serve that
+# step from cache. Keep the ${CACHEBUST} reference on that step: dropping
+# it lets the linter cache again and report a green that linted nothing.
+ARG CACHEBUST
+RUN test -n "${CACHEBUST}" || { \
+        echo "Dockerfile.lint requires the CACHEBUST build-arg; build it via script/lint." >&2; \
+        exit 1; }
+
+# `golangci-lint config verify` is deliberately not run: it fetches its
+# JSON schema over an unpinned live HTTPS call, which REPO_POLICIES.md
+# forbids for external references.
+RUN echo "pixa-lint: running golangci-lint (${CACHEBUST})" && \
+    golangci-lint run --config .golangci.yml ./...

Makefile

@@ -10,7 +10,7 @@ ifdef HAS_PKGCONFIG
   NIX_RUN_PREFIX =
   NIX_RUN_SUFFIX =
 else
-  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif golangci-lint git --run '
+  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif git --run '
   NIX_RUN_SUFFIX = '
 endif
 

README.md

@@ -122,6 +122,10 @@ Configured via YAML file (`--config`). Key settings:
 
 - `access_control_allow_origin` — CORS origin
 - `allowlist_hosts` — list of allowed upstream hosts
+- `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
+  added to the always-enforced built-in ranges (loopback, private,
+  link-local, CGNAT, benchmark, NAT64, and the like); an invalid CIDR
+  aborts startup
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout

TODO.md

@@ -25,21 +25,31 @@ The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P1: implement blocked networks configuration to extend SSRF protection
+P1: rate limit global concurrent upstream fetches to prevent resource
+exhaustion
 
 # Completed Steps
 
-- 2026-09-21 validate dimensions and fit mode on the encrypted-URL
+- 2026-09-21 blocked networks configuration extending SSRF protection: a
-  route and the token generator (closes #62): added a shared
+  `blocked_networks` config key taking a list of CIDRs (parsed with
-  `ValidateImageRequest` in `internal/imgcache` enforcing the
+  `net/netip`, an invalid entry aborts startup naming the key and value),
-  `MaxDimension` bound and `ValidateFitMode`, applied by both the
+  added to the built-in blocklist rather than replacing it; the built-in
-  `/v1/image/` and `/v1/e/` routes, so an over-limit size or an unknown
+  ranges extended to CGNAT `100.64.0.0/10`, IETF protocol assignments
-  fit mode is a 400 rather than an out-of-memory or a 500 from the
+  `192.0.0.0/24`, benchmark `198.18.0.0/15`, and NAT64 `64:ff9b::/96`
-  processor; the URL generator now checks every numeric form field and
+  (IPv4-mapped forms covered); enforcement stays in the dial-time
-  rejects a non-numeric or out-of-range `width`, `height`, `quality`,
+  re-resolution so the DNS-rebinding window remains closed; documented in
-  or `ttl` with a 400 naming the field instead of coercing it to `0`,
+  `README.md` and `config.example.yml`.
-  and `width`/`height` are bounds-checked so an unusable token cannot be
+- 2026-09-21 run all linting in Docker via `Dockerfile.lint` +
-  minted
+  `script/lint` (closes #104): `script/lint` builds a hash-pinned root
+  `Dockerfile.lint`, and no host or nix-shell `golangci-lint` path
+  remains (`script/bootstrap` installs no linter, and the Makefile and
+  `script/test` nix-shell package lists carry only build/test deps); a
+  per-run `CACHEBUST` build-arg forces the lint step to execute every
+  run, so an unchanged tree cannot return a cached green that linted
+  nothing; `Dockerfile`'s lint stage runs `golangci-lint` directly,
+  since `make lint` now builds a container and there is no Docker inside
+  a build; `golangci-lint config verify` stays out, as it fetches its
+  schema over an unpinned live HTTPS call
 - 2026-09-21 http.Server hardening (closes #92): added
   `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
   `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the
@@ -141,8 +151,6 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Future Steps
 
-- P1: rate limit global concurrent upstream fetches to prevent
-  resource exhaustion
 - P1: strip EXIF and other metadata from processed images (privacy)
 - P2: security
     - referer blacklist

config.example.yml

@@ -22,6 +22,15 @@ allowlist_hosts:
   - github.com
   - user-images.githubusercontent.com
 
+# Additional CIDR ranges to refuse when fetching upstream, extending the
+# SSRF protection. These are added to the always-enforced built-in ranges
+# (loopback, RFC 1918 private, link-local, CGNAT, benchmark, NAT64, and
+# similar), never replacing them. Each entry must be a valid CIDR in IPv4
+# or IPv6 form; an invalid entry aborts startup.
+# blocked_networks:
+#   - 100.64.0.0/10
+#   - 2001:db8::/32
+
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
 allow_http: false
 

internal/config/blocked_networks_internal_test.go

@@ -0,0 +1,90 @@
+package config
+
+import (
+	"testing"
+)
+
+// TestBlockedNetworksParsed loads a valid blocked_networks list and checks
+// each CIDR is parsed into the resolved prefixes in order.
+func TestBlockedNetworksParsed(t *testing.T) {
+	t.Parallel()
+
+	yamlContent := signingKeyLine + `blocked_networks:
+  - 203.0.113.0/24
+  - 2001:db8::/32
+`
+
+	c, err := configFromYAML(t, yamlContent)
+	if err != nil {
+		t.Fatalf("valid blocked_networks should load, got error: %v", err)
+	}
+
+	want := []string{"203.0.113.0/24", "2001:db8::/32"}
+	if len(c.BlockedNetworks) != len(want) {
+		t.Fatalf("BlockedNetworks = %v, want %d entries", c.BlockedNetworks, len(want))
+	}
+
+	for i, w := range want {
+		if got := c.BlockedNetworks[i].String(); got != w {
+			t.Errorf("BlockedNetworks[%d] = %q, want %q", i, got, w)
+		}
+	}
+}
+
+// TestBlockedNetworksOmittedIsEmpty confirms an omitted key leaves the
+// operator list empty; the built-in defaults still apply in the fetcher.
+func TestBlockedNetworksOmittedIsEmpty(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine)
+	if err != nil {
+		t.Fatalf("minimal config should be valid, got error: %v", err)
+	}
+
+	if len(c.BlockedNetworks) != 0 {
+		t.Errorf("BlockedNetworks = %v, want empty", c.BlockedNetworks)
+	}
+}
+
+// TestBlockedNetworksInvalidAbortsStartup checks that malformed values abort
+// startup with an error naming the key and the offending value.
+func TestBlockedNetworksInvalidAbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, []abortCase{
+		{
+			name: "not-a-cidr",
+			yaml: signingKeyLine + `blocked_networks:
+  - not-a-cidr
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, "not-a-cidr"},
+		},
+		{
+			name: "bare-address-without-prefix",
+			yaml: signingKeyLine + `blocked_networks:
+  - 10.0.0.1
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, "10.0.0.1"},
+		},
+		{
+			name: "empty-entry",
+			yaml: signingKeyLine + `blocked_networks:
+  - ""
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks},
+		},
+		{
+			name: "non-string-entry",
+			yaml: signingKeyLine + `blocked_networks:
+  - 42
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks},
+		},
+		{
+			name: "null-value",
+			yaml: signingKeyLine + `blocked_networks:
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, nullValueText},
+		},
+	})
+}

internal/config/config.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"math"
+	"net/netip"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -42,6 +43,7 @@ const (
 	keyAllowHTTP                  = "allow_http"
 	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
 	keyCacheMaxBytes              = "cache_max_bytes"
+	keyBlockedNetworks            = "blocked_networks"
 )
 
 // placeholderSigningKey is the dummy signing_key shipped in
@@ -60,6 +62,7 @@ var (
 	errNotAnInteger      = errors.New("not an integer")
 	errNotABoolean       = errors.New("not a boolean")
 	errNotAStringList    = errors.New("not a list of strings")
+	errNotAValidCIDR     = errors.New("not a valid CIDR network")
 	errNotAMetricsMap    = errors.New("not a map of metrics settings")
 	errEmptyListEntry    = errors.New("list contains an empty entry")
 	errEmptyEntry        = errors.New("contains an empty entry")
@@ -109,6 +112,11 @@ type Config struct {
 	AllowHTTP                  bool     // Allow non-TLS upstream (testing only)
 	UpstreamConnectionsPerHost int      // Max concurrent connections per upstream host
 
+	// BlockedNetworks are operator-supplied CIDR ranges to refuse in
+	// addition to the built-in SSRF blocklist. Enforced by the upstream
+	// fetcher's dialer; the built-in ranges always apply.
+	BlockedNetworks []netip.Prefix
+
 	// CacheMaxBytes is the disk cache size limit in bytes. Zero
 	// disables the disk cache entirely. When cache_max_bytes is
 	// omitted from the configuration, this holds the computed default
@@ -177,6 +185,11 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		}
 	}
 
+	blockedNetworks, err := getBlockedNetworks(sc)
+	if err != nil {
+		return nil, err
+	}
+
 	loader := &strictLoader{sc: sc}
 
 	c := &Config{
@@ -193,6 +206,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		UpstreamConnectionsPerHost: loader.intVal(
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
 		CacheMaxBytes:   loader.int64Val(keyCacheMaxBytes, 0),
+		BlockedNetworks: blockedNetworks,
 	}
 
 	// The computed default for cache_max_bytes needs a validated
@@ -224,7 +238,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		return nil, loader.err
 	}
 
-	err := c.validate()
+	err = c.validate()
 	if err != nil {
 		return nil, err
 	}
@@ -308,7 +322,7 @@ func isKnownConfigKey(key string) bool {
 	switch key {
 	case keyDebug, keyMaintenanceMode, keyPort, keyStateDir, keySentryDSN,
 		keyDBURL, keyMetrics, keySigningKey, keyAllowlistHosts, keyAllowHTTP,
-		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, "env":
+		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, keyBlockedNetworks, "env":
 		return true
 	}
 
@@ -802,3 +816,87 @@ func getStringSlice(sc *smartconfig.Config) []string {
 
 	return nil
 }
+
+// getBlockedNetworks parses the blocked_networks value into CIDR prefixes,
+// or returns nil if the key is omitted. It accepts a YAML list of strings
+// or a comma-separated string. An explicitly null value, a wrong type, an
+// empty entry, a non-string entry, or an unparseable CIDR aborts startup
+// naming the key and the offending value; a default (the built-in
+// blocklist alone) applies only to an omitted key.
+func getBlockedNetworks(sc *smartconfig.Config) ([]netip.Prefix, error) {
+	if sc == nil {
+		return nil, nil
+	}
+
+	raw, ok := sc.Get(keyBlockedNetworks)
+	if !ok {
+		return nil, nil
+	}
+
+	if raw == nil {
+		return nil, errNullConfigValue(keyBlockedNetworks)
+	}
+
+	entries, err := blockedNetworkEntries(raw)
+	if err != nil {
+		return nil, err
+	}
+
+	prefixes := make([]netip.Prefix, 0, len(entries))
+
+	for _, entry := range entries {
+		prefix, err := netip.ParsePrefix(entry)
+		if err != nil {
+			return nil, fmt.Errorf("config key %q: value %q is %w",
+				keyBlockedNetworks, entry, errNotAValidCIDR)
+		}
+
+		prefixes = append(prefixes, prefix)
+	}
+
+	return prefixes, nil
+}
+
+// blockedNetworkEntries extracts the raw blocked_networks entries as
+// trimmed, non-empty strings, from either a YAML list of strings or a
+// comma-separated string. Any other shape is a configuration error.
+func blockedNetworkEntries(raw any) ([]string, error) {
+	switch val := raw.(type) {
+	case []any:
+		entries := make([]string, 0, len(val))
+
+		for _, item := range val {
+			str, ok := item.(string)
+			if !ok {
+				return nil, fmt.Errorf("config key %q: list entry %v (%T) is %w",
+					keyBlockedNetworks, item, item, errNotAString)
+			}
+
+			if strings.TrimSpace(str) == "" {
+				return nil, fmt.Errorf("config key %q: %w",
+					keyBlockedNetworks, errEmptyListEntry)
+			}
+
+			entries = append(entries, strings.TrimSpace(str))
+		}
+
+		return entries, nil
+	case string:
+		entries := make([]string, 0)
+
+		for part := range strings.SplitSeq(val, ",") {
+			trimmed := strings.TrimSpace(part)
+			if trimmed == "" {
+				return nil, fmt.Errorf("config key %q: value %q %w",
+					keyBlockedNetworks, val, errEmptyEntry)
+			}
+
+			entries = append(entries, trimmed)
+		}
+
+		return entries, nil
+	default:
+		return nil, fmt.Errorf("config key %q: value %v (%T) is %w",
+			keyBlockedNetworks, raw, raw, errNotAStringList)
+	}
+}

internal/handlers/auth.go

@@ -2,8 +2,6 @@ package handlers
 
 import (
 	"crypto/subtle"
-	"errors"
-	"fmt"
 	"html/template"
 	"net/http"
 	"net/url"
@@ -15,11 +13,6 @@ import (
 	"sneak.berlin/go/pixa/internal/templates"
 )
 
-// errInvalidFormField reports a generator form field whose value is
-// non-numeric or out of range. The offending field name is wrapped in so the
-// response can name it.
-var errInvalidFormField = errors.New("invalid")
-
 // HandleRoot serves the login page or generator page based on authentication state.
 func (s *Handlers) HandleRoot() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
@@ -105,26 +98,18 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 		// Validate source URL
 		parsed, err := url.Parse(sourceURL)
 		if err != nil || parsed.Host == "" {
-			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form,
+			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form)
-				http.StatusBadRequest)
 
 			return
 		}
 
-		payload, expiresAt, ttl, err := buildGeneratePayload(parsed, r.Form)
+		payload, expiresAt, ttl := buildGeneratePayload(parsed, r.Form)
-		if err != nil {
-			s.renderGeneratorWithForm(w, r, err.Error(), r.Form,
-				http.StatusBadRequest)
-
-			return
-		}
 
 		// Generate encrypted token
 		token, err := s.encGen.Generate(payload)
 		if err != nil {
 			s.log.Error("failed to generate encrypted URL", "error", err)
-			s.renderGeneratorWithForm(w, r, "Failed to generate URL", r.Form,
+			s.renderGeneratorWithForm(w, r, "Failed to generate URL", r.Form)
-				http.StatusInternalServerError)
 
 			return
 		}
@@ -152,38 +137,17 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 }
 
 // buildGeneratePayload parses the numeric form fields and assembles the
-// encrypted URL payload. ttl=0 means never expires (ExpiresAt stays 0). A
+// encrypted URL payload. ttl=0 means never expires (ExpiresAt stays 0).
-// non-numeric or out-of-range field, or an unrecognized fit mode, is a client
-// error naming the offending field, so an unusable token is never minted.
 func buildGeneratePayload(
 	parsed *url.URL, form url.Values,
-) (*encurl.Payload, time.Time, int, error) {
+) (*encurl.Payload, time.Time, int) {
-	width, err := parseFormDimension(form, "width")
+	width, _ := strconv.Atoi(form.Get("width"))
-	if err != nil {
+	height, _ := strconv.Atoi(form.Get("height"))
-		return nil, time.Time{}, 0, err
+	quality, _ := strconv.Atoi(form.Get("quality"))
-	}
+	ttl, _ := strconv.Atoi(form.Get("ttl"))
 
-	height, err := parseFormDimension(form, "height")
+	if quality <= 0 {
-	if err != nil {
+		quality = 85
-		return nil, time.Time{}, 0, err
-	}
-
-	quality, err := parseFormCount(form, "quality", encurl.DefaultQuality)
-	if err != nil {
-		return nil, time.Time{}, 0, err
-	}
-
-	ttl, err := parseFormCount(form, "ttl", 0)
-	if err != nil {
-		return nil, time.Time{}, 0, err
-	}
-
-	fitMode := imgcache.FitMode(form.Get("fit"))
-
-	err = imgcache.ValidateFitMode(fitMode)
-	if err != nil {
-		return nil, time.Time{}, 0,
-			fmt.Errorf("%w: %s", imgcache.ErrInvalidFitMode, form.Get("fit"))
 	}
 
 	var (
@@ -204,45 +168,11 @@ func buildGeneratePayload(
 		Height:      height,
 		Format:      imgcache.ImageFormat(form.Get("format")),
 		Quality:     quality,
-		FitMode:     fitMode,
+		FitMode:     imgcache.FitMode(form.Get("fit")),
 		ExpiresAt:   expiresAtUnix,
 	}
 
-	return payload, expiresAt, ttl, nil
+	return payload, expiresAt, ttl
-}
-
-// parseFormDimension reads an optional width or height form field. An empty
-// value means "original size" (0). A non-numeric, negative, or over-limit
-// value is rejected with an error naming the field.
-func parseFormDimension(form url.Values, field string) (int, error) {
-	raw := form.Get(field)
-	if raw == "" {
-		return 0, nil
-	}
-
-	value, err := strconv.Atoi(raw)
-	if err != nil || value < 0 || value > imgcache.MaxDimension {
-		return 0, fmt.Errorf("%w %s", errInvalidFormField, field)
-	}
-
-	return value, nil
-}
-
-// parseFormCount reads an optional non-negative integer form field, returning
-// def when the field is empty and an error naming the field when the value is
-// non-numeric or negative.
-func parseFormCount(form url.Values, field string, def int) (int, error) {
-	raw := form.Get(field)
-	if raw == "" {
-		return def, nil
-	}
-
-	value, err := strconv.Atoi(raw)
-	if err != nil || value < 0 {
-		return 0, fmt.Errorf("%w %s", errInvalidFormField, field)
-	}
-
-	return value, nil
 }
 
 // generatorData holds template data for the generator page.
@@ -282,15 +212,6 @@ func (s *Handlers) renderLogin(
 
 func (s *Handlers) renderGenerator(
 	w http.ResponseWriter, r *http.Request, data *generatorData,
-) {
-	s.renderGeneratorStatus(w, r, data, http.StatusOK)
-}
-
-// renderGeneratorStatus renders the generator page with an explicit HTTP
-// status. The status is written before the body so both it and the
-// Content-Type header take effect; a rejected form uses 400.
-func (s *Handlers) renderGeneratorStatus(
-	w http.ResponseWriter, r *http.Request, data *generatorData, status int,
 ) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 
@@ -300,19 +221,17 @@ func (s *Handlers) renderGeneratorStatus(
 
 	data.CSRFField = csrfField(r)
 
-	w.WriteHeader(status)
-
 	err := templates.Render(w, "generator.html", data)
 	if err != nil {
 		s.log.Error("failed to render generator template", "error", err)
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
 	}
 }
 
 func (s *Handlers) renderGeneratorWithForm(
-	w http.ResponseWriter, r *http.Request, errorMsg string,
+	w http.ResponseWriter, r *http.Request, errorMsg string, form url.Values,
-	form url.Values, status int,
 ) {
-	s.renderGeneratorStatus(w, r, &generatorData{
+	s.renderGenerator(w, r, &generatorData{
 		Error:       errorMsg,
 		FormURL:     form.Get("url"),
 		FormWidth:   form.Get("width"),
@@ -321,7 +240,7 @@ func (s *Handlers) renderGeneratorWithForm(
 		FormQuality: form.Get("quality"),
 		FormFit:     form.Get("fit"),
 		FormTTL:     form.Get("ttl"),
-	}, status)
+	})
 }
 
 func (s *Handlers) buildGeneratedURL(r *http.Request, token, format string) string {

internal/handlers/auth_generate_internal_test.go

@@ -1,66 +0,0 @@
-package handlers
-
-import (
-	"maps"
-	"net/http"
-	"net/http/httptest"
-	"net/url"
-	"strings"
-	"testing"
-)
-
-// generatePost submits the /generate form with a valid session and CSRF token
-// plus the caller's extra fields, returning the recorder.
-func generatePost(
-	t *testing.T, extra url.Values,
-) *httptest.ResponseRecorder {
-	t.Helper()
-
-	h, srv := newCSRFTestRouter(t)
-
-	sessionCookie := newSessionCookie(t, h)
-	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
-	cookies = append(cookies, sessionCookie)
-
-	form := url.Values{
-		sourceURLField: {testSourceURL},
-		csrfTokenField: {token},
-	}
-	maps.Copy(form, extra)
-
-	return postForm(srv, "/generate", cookies, form)
-}
-
-// TestGeneratePostRejectsNonNumericWidth verifies that a non-numeric width is
-// rejected with 400 naming the field rather than being coerced to 0 and
-// minting a 0-width token.
-func TestGeneratePostRejectsNonNumericWidth(t *testing.T) {
-	t.Parallel()
-
-	rec := generatePost(t, url.Values{"width": {"abc"}})
-
-	if rec.Code != http.StatusBadRequest {
-		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
-	}
-
-	if strings.Contains(rec.Body.String(), "/v1/e/") {
-		t.Error("a token was generated for non-numeric width")
-	}
-}
-
-// TestGeneratePostRejectsOverLimitWidth verifies that a width beyond
-// MaxDimension is rejected at generation time so an unusable token cannot be
-// minted.
-func TestGeneratePostRejectsOverLimitWidth(t *testing.T) {
-	t.Parallel()
-
-	rec := generatePost(t, url.Values{"width": {"100000"}})
-
-	if rec.Code != http.StatusBadRequest {
-		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
-	}
-
-	if strings.Contains(rec.Body.String(), "/v1/e/") {
-		t.Error("a token was generated for an over-limit width")
-	}
-}

internal/handlers/handlers.go

@@ -111,6 +111,8 @@ func (s *Handlers) initImageService() error {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
 	}
 
+	fetcherCfg.BlockedNetworks = s.config.BlockedNetworks
+
 	// Create the service
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
 		Cache:         cache,

internal/handlers/image.go

@@ -110,6 +110,13 @@ func (s *Handlers) parseImageRequest(
 
 	if fit := query.Get("fit"); fit != "" {
 		req.FitMode = imgcache.FitMode(fit)
+
+		fitErr := imgcache.ValidateFitMode(req.FitMode)
+		if fitErr != nil {
+			s.respondError(w, "invalid fit mode: "+fit, http.StatusBadRequest)
+
+			return nil, false
+		}
 	}
 
 	// Default quality if not set
@@ -122,18 +129,6 @@ func (s *Handlers) parseImageRequest(
 		req.FitMode = imgcache.FitCover
 	}
 
-	// Enforce dimension and fit-mode bounds, shared with the encrypted-URL
-	// route. Dimensions are already bounded by the path parser above; this
-	// also rejects an unrecognized fit mode with 400 instead of letting it
-	// reach the processor as a 500.
-	err = imgcache.ValidateImageRequest(req)
-	if err != nil {
-		s.respondError(w, "invalid image request: "+err.Error(),
-			http.StatusBadRequest)
-
-		return nil, false
-	}
-
 	return req, true
 }
 

internal/handlers/imageenc.go

@@ -50,19 +50,6 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		// Convert payload to ImageRequest
 		req := payload.ToImageRequest()
 
-		// Apply the same dimension and fit-mode bounds as the plain image
-		// route: a sealed payload is trusted for its origin, not for staying
-		// within limits, so an over-limit size or unknown fit mode is a 400
-		// here rather than an out-of-memory or a 500 from the processor.
-		err = imgcache.ValidateImageRequest(req)
-		if err != nil {
-			s.log.Debug("encrypted URL failed validation", "error", err)
-			s.respondError(w, "invalid encrypted URL: "+err.Error(),
-				http.StatusBadRequest)
-
-			return
-		}
-
 		// Log the request
 		s.log.Debug("encrypted image request",
 			"host", req.SourceHost,

internal/handlers/imageenc_internal_test.go

@@ -1,98 +0,0 @@
-package handlers
-
-import (
-	"context"
-	"log/slog"
-	"net/http"
-	"net/http/httptest"
-	"testing"
-
-	"github.com/go-chi/chi/v5"
-
-	"sneak.berlin/go/pixa/internal/encurl"
-	"sneak.berlin/go/pixa/internal/imgcache"
-)
-
-// newEncTestServer builds a router serving the encrypted-URL route with a
-// generator seeded by the shared test signing key. The image service is left
-// nil: these tests exercise validation that rejects a token before any image
-// is fetched, so the handler must never reach the service.
-func newEncTestServer(t *testing.T) (*encurl.Generator, http.Handler) {
-	t.Helper()
-
-	encGen, err := encurl.NewGenerator(testSigningKey)
-	if err != nil {
-		t.Fatalf("encurl.NewGenerator() error = %v", err)
-	}
-
-	h := &Handlers{
-		log:    slog.New(slog.DiscardHandler),
-		encGen: encGen,
-	}
-
-	r := chi.NewRouter()
-	r.Get("/v1/e/{token}/*", h.HandleImageEnc())
-
-	return encGen, r
-}
-
-// getEncToken issues a GET for the given token and returns the recorder.
-func getEncToken(srv http.Handler, token string) *httptest.ResponseRecorder {
-	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodGet, "/v1/e/"+token+"/img.jpg", nil)
-	rec := httptest.NewRecorder()
-	srv.ServeHTTP(rec, req)
-
-	return rec
-}
-
-// TestHandleImageEnc_OverLimitDimension_Returns400 verifies that a decrypted
-// token requesting a dimension beyond MaxDimension is rejected with 400
-// instead of reaching the image processor and libvips.
-func TestHandleImageEnc_OverLimitDimension_Returns400(t *testing.T) {
-	t.Parallel()
-
-	encGen, srv := newEncTestServer(t)
-
-	token, err := encGen.Generate(&encurl.Payload{
-		SourceHost: "cdn.example.com",
-		SourcePath: "/photo.jpg",
-		Width:      100000,
-		Height:     100000,
-	})
-	if err != nil {
-		t.Fatalf("Generate() error = %v", err)
-	}
-
-	rec := getEncToken(srv, token)
-
-	if rec.Code != http.StatusBadRequest {
-		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
-	}
-}
-
-// TestHandleImageEnc_InvalidFitMode_Returns400 verifies that a decrypted token
-// carrying an unrecognized fit mode is rejected with 400 rather than surfacing
-// as a 500 from the image processor's default branch.
-func TestHandleImageEnc_InvalidFitMode_Returns400(t *testing.T) {
-	t.Parallel()
-
-	encGen, srv := newEncTestServer(t)
-
-	token, err := encGen.Generate(&encurl.Payload{
-		SourceHost: "cdn.example.com",
-		SourcePath: "/photo.jpg",
-		Width:      800,
-		Height:     600,
-		FitMode:    imgcache.FitMode("bogus"),
-	})
-	if err != nil {
-		t.Fatalf("Generate() error = %v", err)
-	}
-
-	rec := getEncToken(srv, token)
-
-	if rec.Code != http.StatusBadRequest {
-		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
-	}
-}

internal/httpfetcher/blocked_networks_internal_test.go

@@ -0,0 +1,111 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"net/netip"
+	"testing"
+)
+
+// TestIsPrivateIPBlocksSpecialRanges covers the internal and special-use
+// ranges added to the built-in blocklist, in IPv4, IPv6, and IPv4-mapped
+// forms, alongside public controls that must stay reachable.
+func TestIsPrivateIPBlocksSpecialRanges(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		ip   string
+		want bool
+	}{
+		{"cgnat-low", "100.64.0.1", true},
+		{"cgnat-high", "100.127.255.254", true},
+		{"ietf-protocol", "192.0.0.1", true},
+		{"benchmark-low", "198.18.0.1", true},
+		{"benchmark-high", "198.19.255.254", true},
+		{"nat64", "64:ff9b::1", true},
+		{"nat64-embeds-private", "64:ff9b::a00:1", true}, // maps 10.0.0.1
+		{"ipv4-mapped-private", "::ffff:10.0.0.1", true},
+		{"cloud-metadata", "169.254.169.254", true},
+		{"public-v4", "8.8.8.8", false},
+		{"test-net-1-public", testPublicHost, false}, // TEST-NET-1, stays public
+		{"public-v6", "2001:4860:4860::8888", false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ip := net.ParseIP(tc.ip)
+			if ip == nil {
+				t.Fatalf("failed to parse IP %q", tc.ip)
+			}
+
+			got := isPrivateIP(ip)
+			if got != tc.want {
+				t.Errorf("isPrivateIP(%q) = %v, want %v", tc.ip, got, tc.want)
+			}
+		})
+	}
+}
+
+// transportOf returns the *http.Transport backing a fetcher, so a test can
+// exercise the SSRF-safe dialer New installed with the operator blocklist.
+func transportOf(t *testing.T, f *HTTPFetcher) *http.Transport {
+	t.Helper()
+
+	transport, ok := f.client.Transport.(*http.Transport)
+	if !ok {
+		t.Fatalf("transport is %T, want *http.Transport", f.client.Transport)
+	}
+
+	return transport
+}
+
+// TestDialerEnforcesBlockedNetworks proves an operator-supplied
+// blocked_networks entry is enforced by the dialer, in addition to the
+// built-in ranges, while an address outside both stays dialable.
+func TestDialerEnforcesBlockedNetworks(t *testing.T) {
+	t.Parallel()
+
+	cfg := DefaultConfig()
+	// TEST-NET-2 (198.51.100.0/24) is public to the built-in check, so
+	// blocking it can only come from the operator-supplied list.
+	cfg.BlockedNetworks = []netip.Prefix{netip.MustParsePrefix("198.51.100.0/24")}
+
+	transport := transportOf(t, New(cfg))
+
+	blocked := []string{
+		"198.51.100.5:80", // operator-supplied range
+		"10.0.0.5:80",     // built-in RFC 1918, still enforced
+		"100.64.0.1:80",   // built-in CGNAT range
+	}
+
+	for _, addr := range blocked {
+		t.Run("blocked/"+addr, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := transport.DialContext(context.Background(), "tcp", addr)
+			if !errors.Is(err, ErrSSRFBlocked) {
+				t.Errorf("DialContext(%q) = %v, want ErrSSRFBlocked", addr, err)
+			}
+		})
+	}
+
+	t.Run("public-not-blocked", func(t *testing.T) {
+		t.Parallel()
+
+		// A cancelled context makes the dial fail without touching the
+		// network; the point is only that a public literal outside every
+		// blocked range is not SSRF-blocked.
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+
+		_, err := transport.DialContext(ctx, "tcp", testPublicHost+":80")
+		if errors.Is(err, ErrSSRFBlocked) {
+			t.Errorf("public target SSRF-blocked with operator list set: %v", err)
+		}
+	})
+}

internal/httpfetcher/httpfetcher.go

@@ -11,6 +11,7 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptrace"
+	"net/netip"
 	neturl "net/url"
 	"slices"
 	"strings"
@@ -46,6 +47,20 @@ const (
 	localhostIPv6 = "::1"
 )
 
+// builtinBlockedPrefixes are internal or special-use ranges that Go's
+// net.IP predicates (IsPrivate, IsLinkLocalUnicast, and the like) do not
+// already cover. They are always blocked, in addition to any
+// operator-supplied networks. IPv4-mapped IPv6 addresses are unmapped
+// before matching, so these IPv4 ranges are caught in both forms.
+//
+//nolint:gochecknoglobals // immutable built-in blocklist
+var builtinBlockedPrefixes = []netip.Prefix{
+	netip.MustParsePrefix("100.64.0.0/10"), // RFC 6598 CGNAT / carrier-grade NAT
+	netip.MustParsePrefix("192.0.0.0/24"),  // RFC 6890 IETF protocol assignments
+	netip.MustParsePrefix("198.18.0.0/15"), // RFC 2544 benchmarking range
+	netip.MustParsePrefix("64:ff9b::/96"),  // RFC 6052 NAT64 (maps onto IPv4)
+}
+
 // Fetcher errors.
 var (
 	ErrSSRFBlocked        = errors.New("request blocked: private or internal IP")
@@ -107,6 +122,9 @@ type Config struct {
 	AllowHTTP bool
 	// MaxConnectionsPerHost limits concurrent connections to each upstream host.
 	MaxConnectionsPerHost int
+	// BlockedNetworks are operator-supplied CIDR ranges refused by the
+	// dialer, in addition to the always-enforced built-in ranges.
+	BlockedNetworks []netip.Prefix
 }
 
 // DefaultConfig returns a Config with sensible defaults.
@@ -142,9 +160,13 @@ func New(config *Config) *HTTPFetcher {
 		config = DefaultConfig()
 	}
 
-	// Create transport with SSRF-safe dialer
+	// Create transport with SSRF-safe dialer. The dialer re-resolves and
+	// re-checks at connect time (closing the DNS-rebinding window) against
+	// both the built-in ranges and the operator-supplied blocklist.
 	transport := &http.Transport{
-		DialContext:         ssrfSafeDialer,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			return dialSSRFSafe(ctx, network, addr, config.BlockedNetworks)
+		},
 		TLSHandshakeTimeout: DefaultTLSTimeout,
 		MaxIdleConns:        DefaultMaxIdleConns,
 		IdleConnTimeout:     DefaultIdleConnTimeout,
@@ -451,11 +473,53 @@ func isPrivateIP(ip net.IP) bool {
 		}
 	}
 
-	return false
+	// Special-use ranges the net.IP predicates above do not cover.
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return true
+	}
+
+	addr = addr.Unmap()
+
+	return slices.ContainsFunc(builtinBlockedPrefixes, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr)
+	})
 }
 
-// ssrfSafeDialer is a custom dialer that validates IP addresses before connecting.
+// isBlockedIP reports whether ip is refused, either by the built-in
+// internal-range check or by one of the operator-supplied prefixes.
+func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
+	if isPrivateIP(ip) {
+		return true
+	}
+
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return true
+	}
+
+	addr = addr.Unmap()
+
+	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr)
+	})
+}
+
+// ssrfSafeDialer validates IP addresses against the built-in blocked ranges
+// before connecting. New wraps dialSSRFSafe with the operator-supplied
+// blocklist; this entry point enforces the built-in ranges alone.
 func ssrfSafeDialer(ctx context.Context, network, addr string) (net.Conn, error) {
+	return dialSSRFSafe(ctx, network, addr, nil)
+}
+
+// dialSSRFSafe re-resolves addr and refuses to connect to any built-in
+// internal range or operator-supplied blocked prefix, closing the
+// DNS-rebinding window at connect time.
+func dialSSRFSafe(
+	ctx context.Context,
+	network, addr string,
+	blocked []netip.Prefix,
+) (net.Conn, error) {
 	host, port, err := net.SplitHostPort(addr)
 	if err != nil {
 		return nil, err
@@ -468,9 +532,11 @@ func ssrfSafeDialer(ctx context.Context, network, addr string) (net.Conn, error)
 	}
 
 	// Check all resolved IPs
-	if slices.ContainsFunc(ips, isPrivateIP) {
+	for _, ip := range ips {
+		if isBlockedIP(ip, blocked) {
 			return nil, ErrSSRFBlocked
 		}
+	}
 
 	// Connect using the first valid IP
 	var dialer net.Dialer

internal/imgcache/imgcache.go

@@ -59,23 +59,6 @@ func ValidateFitMode(fit FitMode) error {
 	}
 }
 
-// ValidateImageRequest checks that a request's dimensions are within
-// MaxDimension and its fit mode is recognized. Both the plain /v1/image/
-// route and the encrypted /v1/e/ route validate through this function so a
-// request from either source enforces identical bounds, regardless of how it
-// was constructed. A width or height of 0 means "original size" and is valid.
-func ValidateImageRequest(req *ImageRequest) error {
-	if req.Size.Width < 0 || req.Size.Height < 0 {
-		return ErrInvalidSize
-	}
-
-	if req.Size.Width > MaxDimension || req.Size.Height > MaxDimension {
-		return ErrDimensionTooLarge
-	}
-
-	return ValidateFitMode(req.FitMode)
-}
-
 // ImageRequest represents a request for a processed image
 type ImageRequest struct {
 	// SourceHost is the origin host (e.g., "cdn.example.com")

internal/imgcache/validate_internal_test.go

@@ -1,64 +0,0 @@
-package imgcache
-
-import (
-	"errors"
-	"testing"
-)
-
-func TestValidateImageRequest(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		name    string
-		req     ImageRequest
-		wantErr error
-	}{
-		{
-			name: "within bounds",
-			req:  ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: FitCover},
-		},
-		{
-			name: "original size and empty fit",
-			req:  ImageRequest{Size: Size{Width: 0, Height: 0}},
-		},
-		{
-			name:    "width over limit",
-			req:     ImageRequest{Size: Size{Width: MaxDimension + 1, Height: 600}},
-			wantErr: ErrDimensionTooLarge,
-		},
-		{
-			name:    "height over limit",
-			req:     ImageRequest{Size: Size{Width: 800, Height: MaxDimension + 1}},
-			wantErr: ErrDimensionTooLarge,
-		},
-		{
-			name:    "negative width",
-			req:     ImageRequest{Size: Size{Width: -1, Height: 600}},
-			wantErr: ErrInvalidSize,
-		},
-		{
-			name:    "invalid fit mode",
-			req:     ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: "bogus"},
-			wantErr: ErrInvalidFitMode,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			err := ValidateImageRequest(&tt.req)
-			if tt.wantErr == nil {
-				if err != nil {
-					t.Fatalf("ValidateImageRequest() error = %v, want nil", err)
-				}
-
-				return
-			}
-
-			if !errors.Is(err, tt.wantErr) {
-				t.Fatalf("ValidateImageRequest() error = %v, want %v", err, tt.wantErr)
-			}
-		})
-	}
-}

script/bootstrap

@@ -3,20 +3,13 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go). golangci-lint is packaged in nix, brew, and apk; on apt
+# make, or go). The linter is never installed on the host: golangci-lint
-# it is installed from a hash-verified GitHub release archive (never
+# runs only inside Dockerfile.lint (see script/lint). CGO image libraries
-# curl | sh). CGO image libraries (pkg-config, vips, libheif) are
+# (pkg-config, vips, libheif) are installed for the govips bindings.
-# installed for the govips bindings.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-08-07. Never "latest"; exact versions only.
-GOLANGCI_LINT_VERSION="2.12.2"
-# sha256 of golangci-lint-2.12.2-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="8df580d2670fed8fa984aac0507099af8df275e665215f5c7a2ae3943893a553"
-GOLANGCI_LINT_SHA256_ARM64="44cd40a8c76c86755375adfeea52cfd3533cb43d7bd647771e0ae065e166df3a"
-
 PKGMGR=""
 SUDO=""
 
@@ -57,52 +50,6 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# verify_sha256 <file> <expected-hash>
-verify_sha256() {
-    if command -v sha256sum >/dev/null 2>&1; then
-        actual="$(sha256sum "$1" | cut -d' ' -f1)"
-    else
-        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
-    fi
-    if [ "$actual" != "$2" ]; then
-        echo "bootstrap: sha256 mismatch for $1" >&2
-        echo "  expected: $2" >&2
-        echo "  actual:   $actual" >&2
-        exit 1
-    fi
-}
-
-# apt has no golangci-lint package: install a pinned release archive
-# from GitHub, verified by hardcoded sha256 (never curl | sh).
-install_golangci_lint_release() {
-    case "$(uname -m)" in
-        x86_64) goarch="amd64"; sha="$GOLANGCI_LINT_SHA256_AMD64" ;;
-        aarch64|arm64) goarch="arm64"; sha="$GOLANGCI_LINT_SHA256_ARM64" ;;
-        *)
-            echo "bootstrap: unsupported architecture $(uname -m)" >&2
-            exit 1
-            ;;
-    esac
-    if missing curl; then pkg_install curl curl curl curl; fi
-    name="golangci-lint-${GOLANGCI_LINT_VERSION}-linux-${goarch}"
-    tmp="$(mktemp -d)"
-    curl -fsSL -o "$tmp/$name.tar.gz" \
-        "https://github.com/golangci/golangci-lint/releases/download/v${GOLANGCI_LINT_VERSION}/${name}.tar.gz"
-    verify_sha256 "$tmp/$name.tar.gz" "$sha"
-    tar -xzf "$tmp/$name.tar.gz" -C "$tmp"
-    $SUDO install -m 0755 "$tmp/$name/golangci-lint" /usr/local/bin/golangci-lint
-    rm -rf "$tmp"
-}
-
-ensure_golangci_lint() {
-    if ! missing golangci-lint; then return 0; fi
-    detect_pkgmgr
-    case "$PKGMGR" in
-        apt) install_golangci_lint_release ;;
-        *) pkg_install golangci-lint golangci-lint golangci-lint golangci-lint ;;
-    esac
-}
-
 # CGO dependencies for govips (image processing)
 ensure_cgo_deps() {
     if missing pkg-config; then
@@ -123,9 +70,8 @@ main() {
     if missing git; then pkg_install git git git git; fi
     if missing make; then pkg_install gnumake make make make; fi
 
-    # Go toolchain and linter
+    # Go toolchain
     if missing go; then pkg_install go golang go go; fi
-    ensure_golangci_lint
 
     # CGO image libraries
     ensure_cgo_deps

script/lint

@@ -1,23 +1,55 @@
 #!/bin/sh
-# script/lint: run the linter. CGO dependencies (pkg-config, vips,
+# script/lint: run golangci-lint over the whole tree.
-# libheif) come from nix-shell when not already available (e.g. inside
+#
-# a Docker build or an existing nix-shell).
+# The linter is never installed on the host: it runs only inside the
+# Dockerfile.lint build, one way, everywhere. A clean build is a clean
+# lint. See Dockerfile.lint for why the lint step cannot be cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-run_with_cgo_deps() {
+main() {
-    if command -v pkg-config >/dev/null 2>&1; then
+    cd "$ROOT"
-        sh -c "$1"
+
-    else
+    # A value no other run repeats. Dockerfile.lint folds it into the
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+    # lint step's cache key, so the linter re-executes every run instead
+    # of an unchanged tree returning a cached success having linted
+    # nothing.
+    cachebust="$(date +%s)-$$"
+
+    tmp="$(mktemp -d "${TMPDIR:-/tmp}/pixa-lint.XXXXXX")"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+
+    # --progress=plain so the lint step's own output reaches the log we
+    # check below; --output=type=cacheonly because we want the linter's
+    # verdict, not an image left in the local store. The build status
+    # travels through a file: a pipeline's exit status is tee's, not the
+    # build's.
+    (
+        set +e
+        docker build \
+            --progress=plain \
+            --build-arg CACHEBUST="$cachebust" \
+            --output=type=cacheonly \
+            -f Dockerfile.lint . 2>&1
+        echo "$?" >"$tmp/status"
+    ) | tee "$tmp/build.log"
+
+    status="$(cat "$tmp/status" 2>/dev/null || echo 1)"
+    [ "${status:-1}" -eq 0 ] || exit "${status:-1}"
+
+    # The linter's start line must appear as build output, not only in
+    # the build's echo of the RUN instruction. A step served from cache
+    # prints the instruction and none of its output; a step that runs
+    # prints a "#<n> <elapsed> ..." output line. Requiring that output
+    # line means a future edit dropping the CACHEBUST reference from
+    # Dockerfile.lint fails here rather than passing having linted
+    # nothing.
+    if ! grep -Eq '^#[0-9]+ +[0-9]+\.[0-9]+ +pixa-lint: running golangci-lint' \
+            "$tmp/build.log"; then
+        echo "script/lint: golangci-lint did not execute (cached step?)." >&2
+        exit 1
     fi
 }
 
-main() {
-    cd "$ROOT"
-    echo "Running linter..."
-    run_with_cgo_deps "golangci-lint run"
-}
-
 main "$@"

script/test

@@ -10,7 +10,7 @@ run_with_cgo_deps() {
     if command -v pkg-config >/dev/null 2>&1; then
         sh -c "$1"
     else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+        nix-shell -p pkg-config vips libheif git --run "$1"
     fi
 }