From 3b69b58ba32f77907fa1779e7fb96dd884219751 Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 12:39:02 +1100
Subject: [PATCH 1/6] Harden nuget per-request rewrite and fix HEAD length

- set Cache-Control no-store and Vary on rewritten responses
- accept only http/https X-Forwarded-Proto
- reject invalid Host headers and JSON-escape the injected base URL
- route remote HEAD and serve rewritten HEAD via the GET path
- classify snupkg, readme and icon as immutable
---
 internal/api/v1/proxy.go              | 77 ++++++++++++++++++++++++---
 internal/api/v1/rewrite_test.go       | 74 +++++++++++++++++++++++++
 internal/api/v1/scheme_test.go        |  6 +++
 internal/provider/nuget/nuget.go      | 22 ++++++--
 internal/provider/nuget/nuget_test.go | 26 +++++++++
 5 files changed, 194 insertions(+), 11 deletions(-)
 create mode 100644 internal/api/v1/rewrite_test.go

diff --git a/internal/api/v1/proxy.go b/internal/api/v1/proxy.go
index 7429007..94a7a4f 100644
--- a/internal/api/v1/proxy.go
+++ b/internal/api/v1/proxy.go
@@ -6,7 +6,10 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net"
 	"net/http"
+	"strconv"
+	"strings"
 
 	"github.com/go-chi/chi/v5"
 
@@ -16,6 +19,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/proxy"
 	"git.unkin.net/unkin/artifactapi/internal/storage"
 	"git.unkin.net/unkin/artifactapi/internal/virtual"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
 type ProxyHandler struct {
@@ -41,6 +45,7 @@ func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *da
 func (h *ProxyHandler) Routes() chi.Router {
 	r := chi.NewRouter()
 	r.Get("/remote/{remoteName}/*", h.handleProxy)
+	r.Head("/remote/{remoteName}/*", h.handleProxyHead)
 	r.Get("/local/{localName}/*", h.handleLocal)
 	r.Get("/virtual/{virtualName}/*", h.handleVirtual)
 	return r
@@ -107,13 +112,11 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 	defer result.Reader.Close()
 
 	var body io.Reader = result.Reader
-	if sr, ok := prov.(provider.ServeRewriter); ok && prov.Classify(path) == provider.Mutable {
-		raw, err := io.ReadAll(result.Reader)
-		if err != nil {
-			http.Error(w, "bad gateway", http.StatusBadGateway)
+	if sr, ok := serveRewriter(prov, path); ok {
+		raw, ok := rewriteServed(w, r, sr, *remote, result.Reader)
+		if !ok {
 			return
 		}
-		raw = sr.RewriteServed(raw, *remote, fmt.Sprintf("%s://%s", scheme(r), r.Host))
 		body, result.Size = bytes.NewReader(raw), int64(len(raw))
 	}
 
@@ -142,6 +145,13 @@ func (h *ProxyHandler) handleProxyHead(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Rewritten bodies differ in size from the cached object, so HEAD takes the
+	// GET path; net/http discards the body for HEAD.
+	if _, ok := serveRewriter(prov, path); ok {
+		h.handleProxy(w, r)
+		return
+	}
+
 	result, err := h.engine.Head(r.Context(), *remote, path, prov)
 	if err != nil {
 		var proxyErr *proxy.ProxyError
@@ -253,8 +263,63 @@ func scheme(r *http.Request) string {
 	if r.TLS != nil {
 		return "https"
 	}
-	if fwd := r.Header.Get("X-Forwarded-Proto"); fwd != "" {
+	if fwd := r.Header.Get("X-Forwarded-Proto"); fwd == "http" || fwd == "https" {
 		return fwd
 	}
 	return "http"
 }
+
+// rewriteServed rewrites a cached body for the requesting host and marks the
+// response uncacheable by shared caches, since it varies per Host.
+func rewriteServed(w http.ResponseWriter, r *http.Request, sr provider.ServeRewriter, remote models.Remote, src io.Reader) ([]byte, bool) {
+	if !validHost(r.Host) {
+		http.Error(w, "invalid host header", http.StatusBadRequest)
+		return nil, false
+	}
+	raw, err := io.ReadAll(src)
+	if err != nil {
+		http.Error(w, "bad gateway", http.StatusBadGateway)
+		return nil, false
+	}
+	raw = sr.RewriteServed(raw, remote, fmt.Sprintf("%s://%s", scheme(r), r.Host))
+	w.Header().Set("Cache-Control", "no-store")
+	w.Header().Set("Vary", "Host, X-Forwarded-Proto")
+	w.Header().Set("Content-Length", strconv.Itoa(len(raw)))
+	return raw, true
+}
+
+func serveRewriter(prov provider.Provider, path string) (provider.ServeRewriter, bool) {
+	sr, ok := prov.(provider.ServeRewriter)
+	return sr, ok && prov.Classify(path) == provider.Mutable
+}
+
+// validHost accepts host[:port] where host is a DNS name or IP literal.
+func validHost(hostport string) bool {
+	host, port := hostport, ""
+	if h, p, err := net.SplitHostPort(hostport); err == nil {
+		host, port = h, p
+	} else if strings.HasPrefix(hostport, "[") {
+		return strings.HasSuffix(hostport, "]") && net.ParseIP(hostport[1:len(hostport)-1]) != nil
+	}
+	if port != "" {
+		if n, err := strconv.Atoi(port); err != nil || n < 1 || n > 65535 {
+			return false
+		}
+	}
+	if net.ParseIP(host) != nil {
+		return true
+	}
+	if strings.HasPrefix(hostport, "[") {
+		return false
+	}
+	if host == "" || len(host) > 253 {
+		return false
+	}
+	for _, label := range strings.Split(strings.TrimSuffix(host, "."), ".") {
+		if label == "" || len(label) > 63 || label[0] == '-' || label[len(label)-1] == '-' ||
+			strings.Trim(strings.ToLower(label), "abcdefghijklmnopqrstuvwxyz0123456789-_") != "" {
+			return false
+		}
+	}
+	return true
+}
diff --git a/internal/api/v1/rewrite_test.go b/internal/api/v1/rewrite_test.go
new file mode 100644
index 0000000..c2c30a9
--- /dev/null
+++ b/internal/api/v1/rewrite_test.go
@@ -0,0 +1,74 @@
+package v1
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider/nuget"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestValidHost(t *testing.T) {
+	for _, h := range []string{"aa.example", "aa.example:8443", "artifactapi.k8s.syd1.au.unkin.net", "10.0.0.1:80", "[::1]:8080", "[::1]", "::1", "localhost", "my_host.local"} {
+		if !validHost(h) {
+			t.Errorf("validHost(%q) = false, want true", h)
+		}
+	}
+	for _, h := range []string{"", "evil\".example", "a b", "aa.example:0", "aa.example:99999", "aa.example:x", "-bad.example", "a..b", "[::1", "[nope]:80", "a/b", "a<script>", strings.Repeat("a", 64) + ".example"} {
+		if validHost(h) {
+			t.Errorf("validHost(%q) = true, want false", h)
+		}
+	}
+}
+
+var nugetOrg = models.Remote{Name: "nuget-org", BaseURL: "https://api.nuget.org/v3/index.json"}
+
+const serviceIndex = `{"resources":[{"@id":"https://api.nuget.org/v3-flatcontainer/"}]}`
+
+func rewrite(t *testing.T, host, proto string) (*httptest.ResponseRecorder, string, bool) {
+	t.Helper()
+	r := httptest.NewRequest(http.MethodGet, "/api/v1/remote/nuget-org/index.json", nil)
+	r.Host = host
+	if proto != "" {
+		r.Header.Set("X-Forwarded-Proto", proto)
+	}
+	w := httptest.NewRecorder()
+	raw, ok := rewriteServed(w, r, &nuget.Provider{}, nugetOrg, strings.NewReader(serviceIndex))
+	return w, string(raw), ok
+}
+
+func TestRewriteServedHeaders(t *testing.T) {
+	w, body, ok := rewrite(t, "aa.example:8443", "https")
+	if !ok {
+		t.Fatal("valid request rejected")
+	}
+	if want := `{"resources":[{"@id":"https://aa.example:8443/api/v1/remote/nuget-org/v3-flatcontainer/"}]}`; body != want {
+		t.Errorf("body = %s, want %s", body, want)
+	}
+	if got := w.Header().Get("Cache-Control"); got != "no-store" {
+		t.Errorf("Cache-Control = %q, want no-store", got)
+	}
+	if got := w.Header().Get("Vary"); got != "Host, X-Forwarded-Proto" {
+		t.Errorf("Vary = %q", got)
+	}
+	if got := w.Header().Get("Content-Length"); got != "91" || len(body) != 91 {
+		t.Errorf("Content-Length = %q, body len %d", got, len(body))
+	}
+}
+
+func TestRewriteServedIgnoresBogusProto(t *testing.T) {
+	_, plain, _ := rewrite(t, "aa.example", "")
+	_, bogus, _ := rewrite(t, "aa.example", "javascript")
+	if bogus != plain || !strings.Contains(bogus, `"http://aa.example/`) {
+		t.Errorf("bogus X-Forwarded-Proto changed the body: %s", bogus)
+	}
+}
+
+func TestRewriteServedRejectsInvalidHost(t *testing.T) {
+	w, _, ok := rewrite(t, `evil"},{"x":"`, "")
+	if ok || w.Code != http.StatusBadRequest {
+		t.Errorf("invalid host: ok=%v code=%d, want rejected with 400", ok, w.Code)
+	}
+}
diff --git a/internal/api/v1/scheme_test.go b/internal/api/v1/scheme_test.go
index e74ce67..cbf1889 100644
--- a/internal/api/v1/scheme_test.go
+++ b/internal/api/v1/scheme_test.go
@@ -14,6 +14,12 @@ func TestScheme(t *testing.T) {
 	if got := scheme(r); got != "https" {
 		t.Errorf("X-Forwarded-Proto scheme = %q, want https", got)
 	}
+	for _, bogus := range []string{"javascript", "HTTPS", "https, http", "ftp"} {
+		r := &http.Request{Header: http.Header{"X-Forwarded-Proto": {bogus}}}
+		if got := scheme(r); got != "http" {
+			t.Errorf("X-Forwarded-Proto %q scheme = %q, want http", bogus, got)
+		}
+	}
 	if got := scheme(&http.Request{Header: http.Header{}}); got != "http" {
 		t.Errorf("default scheme = %q, want http", got)
 	}
diff --git a/internal/provider/nuget/nuget.go b/internal/provider/nuget/nuget.go
index 4857173..7685d4f 100644
--- a/internal/provider/nuget/nuget.go
+++ b/internal/provider/nuget/nuget.go
@@ -1,5 +1,6 @@
 // Package nuget proxies a NuGet v3 feed: the service index, registration pages
-// and flat container indexes are mutable, .nupkg/.nuspec downloads immutable.
+// and flat container indexes are mutable; package, symbol, nuspec, readme and
+// icon blobs are immutable.
 package nuget
 
 import (
@@ -7,6 +8,7 @@ import (
 	"encoding/json"
 	"net/http"
 	"net/url"
+	pathpkg "path"
 	"strings"
 
 	"git.unkin.net/unkin/artifactapi/internal/auth"
@@ -34,10 +36,12 @@ func (p *Provider) Classify(path string) provider.Mutability {
 
 func (p *Provider) ContentType(path string) string {
 	switch {
-	case strings.HasSuffix(path, ".nupkg"):
+	case strings.HasSuffix(path, ".nupkg"), strings.HasSuffix(path, ".snupkg"), pathpkg.Base(path) == "icon":
 		return "application/octet-stream"
 	case strings.HasSuffix(path, ".nuspec"):
 		return "application/xml"
+	case pathpkg.Base(path) == "readme":
+		return "text/markdown"
 	}
 	return "application/json"
 }
@@ -64,8 +68,8 @@ func (p *Provider) RewriteServed(body []byte, remote models.Remote, proxyBaseURL
 	if from == "" || !json.Valid(body) {
 		return body
 	}
-	to := strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.Name
-	return []byte(strings.ReplaceAll(string(body), from, to))
+	to, _ := json.Marshal(strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.Name)
+	return []byte(strings.ReplaceAll(string(body), from, string(to[1:len(to)-1])))
 }
 
 func (p *Provider) AuthHeaders(_ context.Context, remote models.Remote) (http.Header, error) {
@@ -80,6 +84,14 @@ func origin(baseURL string) string {
 	return u.Scheme + "://" + u.Host
 }
 
+// isPackage matches flat container blobs: {id}.{ver}.(s)nupkg, {id}.nuspec and
+// the {id}/{ver}/readme and {id}/{ver}/icon files.
 func isPackage(path string) bool {
-	return strings.HasSuffix(path, ".nupkg") || strings.HasSuffix(path, ".nuspec")
+	for _, ext := range []string{".nupkg", ".snupkg", ".nuspec"} {
+		if strings.HasSuffix(path, ext) {
+			return true
+		}
+	}
+	base := pathpkg.Base(path)
+	return (base == "readme" || base == "icon") && strings.Count(strings.Trim(path, "/"), "/") >= 2
 }
diff --git a/internal/provider/nuget/nuget_test.go b/internal/provider/nuget/nuget_test.go
index 688ca06..5aab56b 100644
--- a/internal/provider/nuget/nuget_test.go
+++ b/internal/provider/nuget/nuget_test.go
@@ -19,7 +19,11 @@ func TestClassify(t *testing.T) {
 	}{
 		{"v3-flatcontainer/newtonsoft.json/13.0.3/newtonsoft.json.13.0.3.nupkg", provider.Immutable},
 		{"v3-flatcontainer/newtonsoft.json/13.0.3/newtonsoft.json.nuspec", provider.Immutable},
+		{"v3-flatcontainer/newtonsoft.json/13.0.3/newtonsoft.json.13.0.3.snupkg", provider.Immutable},
+		{"v3-flatcontainer/newtonsoft.json/13.0.3/readme", provider.Immutable},
+		{"v3-flatcontainer/newtonsoft.json/13.0.3/icon", provider.Immutable},
 		{"v3-flatcontainer/newtonsoft.json/index.json", provider.Mutable},
+		{"readme", provider.Mutable},
 		{"v3/registration5-gz-semver2/newtonsoft.json/index.json", provider.Mutable},
 		{"index.json", provider.Mutable},
 	}
@@ -30,6 +34,28 @@ func TestClassify(t *testing.T) {
 	}
 }
 
+func TestContentType(t *testing.T) {
+	p := &Provider{}
+	for path, want := range map[string]string{
+		"v3-flatcontainer/a/1.0.0/a.1.0.0.snupkg": "application/octet-stream",
+		"v3-flatcontainer/a/1.0.0/icon":           "application/octet-stream",
+		"v3-flatcontainer/a/1.0.0/readme":         "text/markdown",
+		"v3-flatcontainer/a/index.json":           "application/json",
+	} {
+		if got := p.ContentType(path); got != want {
+			t.Errorf("ContentType(%q) = %q, want %q", path, got, want)
+		}
+	}
+}
+
+func TestRewriteServedEscapesJSON(t *testing.T) {
+	body := `{"@id":"https://api.nuget.org/v3-flatcontainer/"}`
+	got := (&Provider{}).RewriteServed([]byte(body), nugetOrg, `http://evil"<x>`)
+	if want := `{"@id":"http://evil\"\u003cx\u003e/api/v1/remote/nuget-org/v3-flatcontainer/"}`; string(got) != want {
+		t.Errorf("RewriteServed = %s, want %s", got, want)
+	}
+}
+
 func TestUpstreamURL(t *testing.T) {
 	p := &Provider{}
 	tests := []struct {
-- 
2.47.3


From 3dd4acd233318967d7d87b1e29d34157cec994a2 Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 12:40:33 +1100
Subject: [PATCH 2/6] Add HEAD route for remotes matching GET headers

---
 e2e/head_test.go               | 100 +++++++++++++++++++++++++++++++++
 internal/api/v1/proxy.go       |  66 +++++++++++++---------
 internal/api/v1/routes_test.go |  17 ++++++
 internal/proxy/engine.go       |   2 +-
 internal/proxy/engine_test.go  |  14 +++++
 5 files changed, 173 insertions(+), 26 deletions(-)
 create mode 100644 e2e/head_test.go
 create mode 100644 internal/api/v1/routes_test.go

diff --git a/e2e/head_test.go b/e2e/head_test.go
new file mode 100644
index 0000000..f7b9ac7
--- /dev/null
+++ b/e2e/head_test.go
@@ -0,0 +1,100 @@
+//go:build e2e
+
+package e2e
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+)
+
+// assertHeadMatchesGet checks HEAD returns GET's status, type and length with no body.
+func assertHeadMatchesGet(t *testing.T, url string) {
+	t.Helper()
+	get, err := http.Get(url)
+	if err != nil {
+		t.Fatalf("GET: %v", err)
+	}
+	body, _ := io.ReadAll(get.Body)
+	get.Body.Close()
+	if get.StatusCode != http.StatusOK {
+		t.Fatalf("GET %s: %d %s", url, get.StatusCode, body)
+	}
+
+	head, err := http.Head(url)
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	hb, _ := io.ReadAll(head.Body)
+	head.Body.Close()
+	if head.StatusCode != http.StatusOK {
+		t.Fatalf("HEAD %s: got %d, want 200", url, head.StatusCode)
+	}
+	if len(hb) != 0 {
+		t.Errorf("HEAD returned a %d-byte body", len(hb))
+	}
+	if head.Header.Get("Content-Type") != get.Header.Get("Content-Type") {
+		t.Errorf("HEAD Content-Type %q, GET %q", head.Header.Get("Content-Type"), get.Header.Get("Content-Type"))
+	}
+	if head.ContentLength != int64(len(body)) {
+		t.Errorf("HEAD Content-Length %d, GET body %d bytes", head.ContentLength, len(body))
+	}
+}
+
+func TestProxyHeadNuGetRewritten(t *testing.T) {
+	var upstream *httptest.Server
+	upstream = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]any{"version": "3.0.0", "resources": []map[string]string{
+			{"@id": upstream.URL + "/v3-flatcontainer/", "@type": "PackageBaseAddress/3.0.0"},
+		}})
+	}))
+	defer upstream.Close()
+
+	createRemote(t, mustFormat(`{"name": "head-nuget", "package_type": "nuget", "base_url": %q}`, upstream.URL+"/v3/index.json"))
+	defer deleteRemote(t, "head-nuget")
+
+	assertHeadMatchesGet(t, apiURL("/api/v1/remote/head-nuget/index.json"))
+}
+
+func TestProxyHeadGitHubRPMRepodata(t *testing.T) {
+	rpm := testsupport.MinimalRPM("demo", "1.2", "3", "x86_64")
+	var gh *httptest.Server
+	mux := http.NewServeMux()
+	mux.HandleFunc("/repos/acme/tools/releases", func(w http.ResponseWriter, r *http.Request) {
+		if p := r.URL.Query().Get("page"); p != "" && p != "1" {
+			w.Write([]byte("[]"))
+			return
+		}
+		json.NewEncoder(w).Encode([]map[string]any{{"tag_name": "v1.2-3", "assets": []map[string]any{{
+			"name":                 "demo-1.2-3.x86_64.rpm",
+			"size":                 len(rpm),
+			"browser_download_url": gh.URL + "/acme/tools/releases/download/v1.2-3/demo-1.2-3.x86_64.rpm",
+		}}}})
+	})
+	mux.HandleFunc("/acme/tools/releases/download/", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Length", strconv.Itoa(len(rpm)))
+		w.Write(rpm)
+	})
+	gh = httptest.NewServer(mux)
+	defer gh.Close()
+
+	createRemote(t, mustFormat(`{"name": "head-ghrpm", "package_type": "github_rpm", "base_url": %q, "releases_remote": "github"}`, gh.URL+"/repos/acme/tools"))
+	defer deleteRemote(t, "head-ghrpm")
+
+	url := apiURL("/api/v1/remote/head-ghrpm/repodata/repomd.xml")
+	deadline := time.Now().Add(30 * time.Second)
+	for {
+		if _, status := getBody(t, url); status == http.StatusOK || time.Now().After(deadline) {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+	assertHeadMatchesGet(t, url)
+}
diff --git a/internal/api/v1/proxy.go b/internal/api/v1/proxy.go
index 7429007..43ba060 100644
--- a/internal/api/v1/proxy.go
+++ b/internal/api/v1/proxy.go
@@ -16,6 +16,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/proxy"
 	"git.unkin.net/unkin/artifactapi/internal/storage"
 	"git.unkin.net/unkin/artifactapi/internal/virtual"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
 type ProxyHandler struct {
@@ -41,6 +42,7 @@ func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *da
 func (h *ProxyHandler) Routes() chi.Router {
 	r := chi.NewRouter()
 	r.Get("/remote/{remoteName}/*", h.handleProxy)
+	r.Head("/remote/{remoteName}/*", h.handleProxyHead)
 	r.Get("/local/{localName}/*", h.handleLocal)
 	r.Get("/virtual/{virtualName}/*", h.handleVirtual)
 	return r
@@ -68,31 +70,43 @@ func (h *ProxyHandler) handleDockerPing(w http.ResponseWriter, r *http.Request)
 	w.WriteHeader(http.StatusOK)
 }
 
-func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
+// remoteFor resolves the remote and provider named in the request, writing the
+// error response itself when either is missing.
+func (h *ProxyHandler) remoteFor(w http.ResponseWriter, r *http.Request) (*models.Remote, provider.Provider, bool) {
 	remoteName := chi.URLParam(r, "remoteName")
-	path := chi.URLParam(r, "*")
-
 	remote, err := h.db.GetRemote(r.Context(), remoteName)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("remote %q not found", remoteName), http.StatusNotFound)
-		return
+		return nil, nil, false
 	}
-
 	prov, err := provider.Get(remote.PackageType)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("no provider for %q", remote.PackageType), http.StatusInternalServerError)
+		return nil, nil, false
+	}
+	return remote, prov, true
+}
+
+// serveRemote lets metadata-only remotes (e.g. github_rpm) synthesize their own
+// responses and redirect package downloads instead of proxying bytes.
+func (h *ProxyHandler) serveRemote(w http.ResponseWriter, r *http.Request, remote *models.Remote, prov provider.Provider, path string) bool {
+	rs, ok := prov.(provider.RemoteServer)
+	return ok && rs.ServeRemote(w, r, *remote, path, fmt.Sprintf("%s://%s", scheme(r), r.Host), h.db)
+}
+
+func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
+	remote, prov, ok := h.remoteFor(w, r)
+	if !ok {
 		return
 	}
-
-	// Metadata-only remotes (e.g. github_rpm) synthesize their own responses and
-	// redirect package downloads to a backend remote instead of proxying bytes.
-	if rs, ok := prov.(provider.RemoteServer); ok {
-		proxyBaseURL := fmt.Sprintf("%s://%s", scheme(r), r.Host)
-		if rs.ServeRemote(w, r, *remote, path, proxyBaseURL, h.db) {
-			return
-		}
+	path := chi.URLParam(r, "*")
+	if h.serveRemote(w, r, remote, prov, path) {
+		return
 	}
+	h.serveFetch(w, r, remote, prov, path)
+}
 
+func (h *ProxyHandler) serveFetch(w http.ResponseWriter, r *http.Request, remote *models.Remote, prov provider.Provider, path string) {
 	result, err := h.engine.Fetch(r.Context(), *remote, path, prov, r.Header)
 	if err != nil {
 		var proxyErr *proxy.ProxyError
@@ -100,7 +114,7 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, proxyErr.Message, proxyErr.Status)
 			return
 		}
-		slog.Error("proxy fetch failed", "remote", remoteName, "path", path, "error", err)
+		slog.Error("proxy fetch failed", "remote", remote.Name, "path", path, "error", err)
 		http.Error(w, "bad gateway", http.StatusBadGateway)
 		return
 	}
@@ -120,25 +134,27 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", result.ContentType)
 	w.Header().Set("X-Artifact-Source", result.Source)
 	if result.Size > 0 {
+		w.Header().Set("Content-Length", fmt.Sprintf("%d", result.Size))
 		w.Header().Set("X-Artifact-Size", fmt.Sprintf("%d", result.Size))
 	}
 	w.WriteHeader(http.StatusOK)
 	io.Copy(w, body)
 }
 
+// handleProxyHead answers HEAD like GET without a body. Responses a provider
+// synthesizes or rewrites per request go through the GET path so their headers
+// match; everything else is answered from store metadata or an upstream HEAD.
 func (h *ProxyHandler) handleProxyHead(w http.ResponseWriter, r *http.Request) {
-	remoteName := chi.URLParam(r, "remoteName")
-	path := chi.URLParam(r, "*")
-
-	remote, err := h.db.GetRemote(r.Context(), remoteName)
-	if err != nil {
-		http.Error(w, fmt.Sprintf("remote %q not found", remoteName), http.StatusNotFound)
+	remote, prov, ok := h.remoteFor(w, r)
+	if !ok {
 		return
 	}
-
-	prov, err := provider.Get(remote.PackageType)
-	if err != nil {
-		http.Error(w, fmt.Sprintf("no provider for %q", remote.PackageType), http.StatusInternalServerError)
+	path := chi.URLParam(r, "*")
+	if h.serveRemote(w, r, remote, prov, path) {
+		return
+	}
+	if _, ok := prov.(provider.ServeRewriter); ok && prov.Classify(path) == provider.Mutable {
+		h.serveFetch(w, r, remote, prov, path)
 		return
 	}
 
@@ -149,7 +165,7 @@ func (h *ProxyHandler) handleProxyHead(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, proxyErr.Message, proxyErr.Status)
 			return
 		}
-		slog.Error("proxy head failed", "remote", remoteName, "path", path, "error", err)
+		slog.Error("proxy head failed", "remote", remote.Name, "path", path, "error", err)
 		http.Error(w, "bad gateway", http.StatusBadGateway)
 		return
 	}
diff --git a/internal/api/v1/routes_test.go b/internal/api/v1/routes_test.go
new file mode 100644
index 0000000..4493a5c
--- /dev/null
+++ b/internal/api/v1/routes_test.go
@@ -0,0 +1,17 @@
+package v1
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+)
+
+func TestRoutesRemoteHead(t *testing.T) {
+	r := (&ProxyHandler{}).Routes()
+	for _, m := range []string{http.MethodGet, http.MethodHead} {
+		if !r.Match(chi.NewRouteContext(), m, "/remote/myremote/some/path") {
+			t.Errorf("%s /remote/myremote/some/path not routed", m)
+		}
+	}
+}
diff --git a/internal/proxy/engine.go b/internal/proxy/engine.go
index 45a1b87..cc267cc 100644
--- a/internal/proxy/engine.go
+++ b/internal/proxy/engine.go
@@ -353,7 +353,7 @@ func (e *Engine) headUpstreamOnce(ctx context.Context, remote models.Remote, pat
 				req.Header.Set(k, v)
 			}
 		}
-		return http.DefaultClient.Do(req)
+		return clientForRemote(remote).Do(req)
 	}
 
 	resp, err := doHead(nil)
diff --git a/internal/proxy/engine_test.go b/internal/proxy/engine_test.go
index 87ce690..2374aec 100644
--- a/internal/proxy/engine_test.go
+++ b/internal/proxy/engine_test.go
@@ -216,6 +216,20 @@ func TestHead(t *testing.T) {
 	}
 }
 
+func TestHeadUpstreamUsesRemoteTimeout(t *testing.T) {
+	slow := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(3 * time.Second)
+	}))
+	defer slow.Close()
+	r := models.Remote{Name: "slow-head", PackageType: models.PackageGeneric, BaseURL: slow.URL, UpstreamResponseHeaderTimeout: 1}
+
+	start := time.Now()
+	_, err := (&Engine{}).headUpstreamOnce(context.Background(), r, "blob.bin", prov(t, models.PackageGeneric))
+	if err == nil || time.Since(start) > 2*time.Second {
+		t.Fatalf("HEAD ignored the remote's response header timeout: err=%v after %s", err, time.Since(start))
+	}
+}
+
 func TestStaleOnError(t *testing.T) {
 	requireStack(t)
 	ctx := context.Background()
-- 
2.47.3


From a7aa37982a637e6db7eec9e5e27f7e2b8e87de51 Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 12:43:01 +1100
Subject: [PATCH 3/6] Leave remote HEAD routing to the generic HEAD fix

---
 internal/api/v1/proxy.go | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/internal/api/v1/proxy.go b/internal/api/v1/proxy.go
index 94a7a4f..dbcfbef 100644
--- a/internal/api/v1/proxy.go
+++ b/internal/api/v1/proxy.go
@@ -45,7 +45,6 @@ func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *da
 func (h *ProxyHandler) Routes() chi.Router {
 	r := chi.NewRouter()
 	r.Get("/remote/{remoteName}/*", h.handleProxy)
-	r.Head("/remote/{remoteName}/*", h.handleProxyHead)
 	r.Get("/local/{localName}/*", h.handleLocal)
 	r.Get("/virtual/{virtualName}/*", h.handleVirtual)
 	return r
@@ -112,7 +111,7 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 	defer result.Reader.Close()
 
 	var body io.Reader = result.Reader
-	if sr, ok := serveRewriter(prov, path); ok {
+	if sr, ok := prov.(provider.ServeRewriter); ok && prov.Classify(path) == provider.Mutable {
 		raw, ok := rewriteServed(w, r, sr, *remote, result.Reader)
 		if !ok {
 			return
@@ -145,13 +144,6 @@ func (h *ProxyHandler) handleProxyHead(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Rewritten bodies differ in size from the cached object, so HEAD takes the
-	// GET path; net/http discards the body for HEAD.
-	if _, ok := serveRewriter(prov, path); ok {
-		h.handleProxy(w, r)
-		return
-	}
-
 	result, err := h.engine.Head(r.Context(), *remote, path, prov)
 	if err != nil {
 		var proxyErr *proxy.ProxyError
@@ -288,11 +280,6 @@ func rewriteServed(w http.ResponseWriter, r *http.Request, sr provider.ServeRewr
 	return raw, true
 }
 
-func serveRewriter(prov provider.Provider, path string) (provider.ServeRewriter, bool) {
-	sr, ok := prov.(provider.ServeRewriter)
-	return sr, ok && prov.Classify(path) == provider.Mutable
-}
-
 // validHost accepts host[:port] where host is a DNS name or IP literal.
 func validHost(hostport string) bool {
 	host, port := hostport, ""
-- 
2.47.3


From fe10a50d27cadcc9a9b1dbf9ae4361cfdc00b49b Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 12:56:29 +1100
Subject: [PATCH 4/6] Share strict X-Forwarded-Proto parsing and reject empty
 Host ports

---
 e2e/head_test.go                   | 20 ++++++++--
 e2e/scheme_test.go                 | 59 ++++++++++++++++++++++++++++++
 internal/api/terraform/registry.go | 10 +----
 internal/api/v1/proxy.go           | 16 +++++---
 internal/api/v1/rewrite_test.go    |  2 +-
 internal/api/v1/scheme_test.go     | 22 +++++++----
 6 files changed, 105 insertions(+), 24 deletions(-)
 create mode 100644 e2e/scheme_test.go

diff --git a/e2e/head_test.go b/e2e/head_test.go
index f7b9ac7..487a909 100644
--- a/e2e/head_test.go
+++ b/e2e/head_test.go
@@ -39,8 +39,10 @@ func assertHeadMatchesGet(t *testing.T, url string) {
 	if len(hb) != 0 {
 		t.Errorf("HEAD returned a %d-byte body", len(hb))
 	}
-	if head.Header.Get("Content-Type") != get.Header.Get("Content-Type") {
-		t.Errorf("HEAD Content-Type %q, GET %q", head.Header.Get("Content-Type"), get.Header.Get("Content-Type"))
+	for _, h := range []string{"Content-Type", "Cache-Control", "Vary"} {
+		if head.Header.Get(h) != get.Header.Get(h) {
+			t.Errorf("HEAD %s %q, GET %q", h, head.Header.Get(h), get.Header.Get(h))
+		}
 	}
 	if head.ContentLength != int64(len(body)) {
 		t.Errorf("HEAD Content-Length %d, GET body %d bytes", head.ContentLength, len(body))
@@ -60,7 +62,19 @@ func TestProxyHeadNuGetRewritten(t *testing.T) {
 	createRemote(t, mustFormat(`{"name": "head-nuget", "package_type": "nuget", "base_url": %q}`, upstream.URL+"/v3/index.json"))
 	defer deleteRemote(t, "head-nuget")
 
-	assertHeadMatchesGet(t, apiURL("/api/v1/remote/head-nuget/index.json"))
+	url := apiURL("/api/v1/remote/head-nuget/index.json")
+	assertHeadMatchesGet(t, url)
+
+	req, _ := http.NewRequest(http.MethodHead, url, nil)
+	req.Host = "evil.example:"
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("HEAD with invalid Host: got %d, want 400", resp.StatusCode)
+	}
 }
 
 func TestProxyHeadGitHubRPMRepodata(t *testing.T) {
diff --git a/e2e/scheme_test.go b/e2e/scheme_test.go
new file mode 100644
index 0000000..4ab636f
--- /dev/null
+++ b/e2e/scheme_test.go
@@ -0,0 +1,59 @@
+//go:build e2e
+
+package e2e
+
+import (
+	"encoding/json"
+	"net/http"
+	"strings"
+	"testing"
+)
+
+func getForwardedHTTPS(t *testing.T, url string) *http.Response {
+	t.Helper()
+	req, _ := http.NewRequest(http.MethodGet, url, nil)
+	req.Header.Set("X-Forwarded-Proto", "https")
+	client := &http.Client{CheckRedirect: func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse }}
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("GET %s: %v", url, err)
+	}
+	return resp
+}
+
+func TestForwardedProtoCargoConfig(t *testing.T) {
+	createRemote(t, `{"name": "scheme-cargo", "package_type": "cargo", "base_url": "https://index.crates.io"}`)
+	defer deleteRemote(t, "scheme-cargo")
+
+	resp := getForwardedHTTPS(t, apiURL("/api/v1/remote/scheme-cargo/config.json"))
+	defer resp.Body.Close()
+	var cfg map[string]string
+	if err := json.NewDecoder(resp.Body).Decode(&cfg); err != nil {
+		t.Fatal(err)
+	}
+	if want := "https://" + strings.TrimPrefix(baseURL, "http://") + "/api/v1/remote/scheme-cargo/"; !strings.HasPrefix(cfg["dl"], want) {
+		t.Errorf("dl = %q, want prefix %q", cfg["dl"], want)
+	}
+}
+
+func TestForwardedProtoVirtualRedirect(t *testing.T) {
+	createRemote(t, `{"name": "scheme-rpm", "package_type": "rpm", "base_url": "https://rpm.example.com"}`)
+	defer deleteRemote(t, "scheme-rpm")
+	resp, err := http.Post(apiURL("/api/v2/virtuals"), "application/json",
+		strings.NewReader(`{"name": "scheme-virt", "package_type": "rpm", "members": ["scheme-rpm"]}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("create virtual: status %d", resp.StatusCode)
+	}
+	defer deleteRequest(t, apiURL("/api/v2/virtuals/scheme-virt"))
+
+	resp = getForwardedHTTPS(t, apiURL("/api/v1/virtual/scheme-virt/scheme-rpm/Packages/a.rpm"))
+	resp.Body.Close()
+	loc := resp.Header.Get("Location")
+	if want := "https://" + strings.TrimPrefix(baseURL, "http://") + "/api/v1/remote/scheme-rpm/Packages/a.rpm"; resp.StatusCode != http.StatusFound || loc != want {
+		t.Errorf("redirect = %d %q, want 302 %q", resp.StatusCode, loc, want)
+	}
+}
diff --git a/internal/api/terraform/registry.go b/internal/api/terraform/registry.go
index 7aee9de..cd69aca 100644
--- a/internal/api/terraform/registry.go
+++ b/internal/api/terraform/registry.go
@@ -13,6 +13,7 @@ import (
 
 	"github.com/go-chi/chi/v5"
 
+	v1 "git.unkin.net/unkin/artifactapi/internal/api/v1"
 	"git.unkin.net/unkin/artifactapi/internal/database"
 	tfprov "git.unkin.net/unkin/artifactapi/internal/provider/terraform"
 	"git.unkin.net/unkin/artifactapi/internal/tfsign"
@@ -290,12 +291,5 @@ func writeJSON(w http.ResponseWriter, v any) {
 }
 
 func baseURL(r *http.Request) string {
-	scheme := "http"
-	if r.TLS != nil {
-		scheme = "https"
-	}
-	if fwd := r.Header.Get("X-Forwarded-Proto"); fwd != "" {
-		scheme = fwd
-	}
-	return scheme + "://" + r.Host
+	return v1.Scheme(r) + "://" + r.Host
 }
diff --git a/internal/api/v1/proxy.go b/internal/api/v1/proxy.go
index 8a6e81e..9015105 100644
--- a/internal/api/v1/proxy.go
+++ b/internal/api/v1/proxy.go
@@ -94,7 +94,7 @@ func (h *ProxyHandler) remoteFor(w http.ResponseWriter, r *http.Request) (*model
 // responses and redirect package downloads instead of proxying bytes.
 func (h *ProxyHandler) serveRemote(w http.ResponseWriter, r *http.Request, remote *models.Remote, prov provider.Provider, path string) bool {
 	rs, ok := prov.(provider.RemoteServer)
-	return ok && rs.ServeRemote(w, r, *remote, path, fmt.Sprintf("%s://%s", scheme(r), r.Host), h.db)
+	return ok && rs.ServeRemote(w, r, *remote, path, fmt.Sprintf("%s://%s", Scheme(r), r.Host), h.db)
 }
 
 func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
@@ -189,7 +189,7 @@ func (h *ProxyHandler) handleVirtual(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	proxyBaseURL := fmt.Sprintf("%s://%s", scheme(r), r.Host)
+	proxyBaseURL := fmt.Sprintf("%s://%s", Scheme(r), r.Host)
 
 	loc, ok, err := h.virtualEngine.MemberRedirect(r.Context(), *virt, path, r.URL.RawQuery, proxyBaseURL)
 	if err != nil {
@@ -266,11 +266,14 @@ func (h *ProxyHandler) serveLocalFile(w http.ResponseWriter, r *http.Request, re
 	io.Copy(w, reader)
 }
 
-func scheme(r *http.Request) string {
+// Scheme returns the client-facing scheme: TLS, else the first
+// X-Forwarded-Proto value when it is http or https, else http.
+func Scheme(r *http.Request) string {
 	if r.TLS != nil {
 		return "https"
 	}
-	if fwd := r.Header.Get("X-Forwarded-Proto"); fwd == "http" || fwd == "https" {
+	fwd, _, _ := strings.Cut(r.Header.Get("X-Forwarded-Proto"), ",")
+	if fwd = strings.ToLower(strings.TrimSpace(fwd)); fwd == "https" {
 		return fwd
 	}
 	return "http"
@@ -288,7 +291,7 @@ func rewriteServed(w http.ResponseWriter, r *http.Request, sr provider.ServeRewr
 		http.Error(w, "bad gateway", http.StatusBadGateway)
 		return nil, false
 	}
-	raw = sr.RewriteServed(raw, remote, fmt.Sprintf("%s://%s", scheme(r), r.Host))
+	raw = sr.RewriteServed(raw, remote, fmt.Sprintf("%s://%s", Scheme(r), r.Host))
 	w.Header().Set("Cache-Control", "no-store")
 	w.Header().Set("Vary", "Host, X-Forwarded-Proto")
 	w.Header().Set("Content-Length", strconv.Itoa(len(raw)))
@@ -299,6 +302,9 @@ func rewriteServed(w http.ResponseWriter, r *http.Request, sr provider.ServeRewr
 func validHost(hostport string) bool {
 	host, port := hostport, ""
 	if h, p, err := net.SplitHostPort(hostport); err == nil {
+		if p == "" {
+			return false
+		}
 		host, port = h, p
 	} else if strings.HasPrefix(hostport, "[") {
 		return strings.HasSuffix(hostport, "]") && net.ParseIP(hostport[1:len(hostport)-1]) != nil
diff --git a/internal/api/v1/rewrite_test.go b/internal/api/v1/rewrite_test.go
index c2c30a9..24fd732 100644
--- a/internal/api/v1/rewrite_test.go
+++ b/internal/api/v1/rewrite_test.go
@@ -16,7 +16,7 @@ func TestValidHost(t *testing.T) {
 			t.Errorf("validHost(%q) = false, want true", h)
 		}
 	}
-	for _, h := range []string{"", "evil\".example", "a b", "aa.example:0", "aa.example:99999", "aa.example:x", "-bad.example", "a..b", "[::1", "[nope]:80", "a/b", "a<script>", strings.Repeat("a", 64) + ".example"} {
+	for _, h := range []string{"", "evil\".example", "a b", "aa.example:0", "aa.example:99999", "aa.example:x", "aa.example:", ":8080", "-bad.example", "a..b", "[::1", "[nope]:80", "a/b", "a<script>", strings.Repeat("a", 64) + ".example"} {
 		if validHost(h) {
 			t.Errorf("validHost(%q) = true, want false", h)
 		}
diff --git a/internal/api/v1/scheme_test.go b/internal/api/v1/scheme_test.go
index cbf1889..6b451c8 100644
--- a/internal/api/v1/scheme_test.go
+++ b/internal/api/v1/scheme_test.go
@@ -7,20 +7,28 @@ import (
 )
 
 func TestScheme(t *testing.T) {
-	if got := scheme(&http.Request{TLS: &tls.ConnectionState{}}); got != "https" {
+	if got := Scheme(&http.Request{TLS: &tls.ConnectionState{}}); got != "https" {
 		t.Errorf("TLS request scheme = %q, want https", got)
 	}
 	r := &http.Request{Header: http.Header{"X-Forwarded-Proto": {"https"}}}
-	if got := scheme(r); got != "https" {
+	if got := Scheme(r); got != "https" {
 		t.Errorf("X-Forwarded-Proto scheme = %q, want https", got)
 	}
-	for _, bogus := range []string{"javascript", "HTTPS", "https, http", "ftp"} {
-		r := &http.Request{Header: http.Header{"X-Forwarded-Proto": {bogus}}}
-		if got := scheme(r); got != "http" {
-			t.Errorf("X-Forwarded-Proto %q scheme = %q, want http", bogus, got)
+	for fwd, want := range map[string]string{
+		"https, http": "https",
+		"http, https": "http",
+		"HTTPS":       "https",
+		" https ":     "https",
+		"javascript":  "http",
+		"ftp":         "http",
+		"https;evil":  "http",
+	} {
+		r := &http.Request{Header: http.Header{"X-Forwarded-Proto": {fwd}}}
+		if got := Scheme(r); got != want {
+			t.Errorf("X-Forwarded-Proto %q scheme = %q, want %s", fwd, got, want)
 		}
 	}
-	if got := scheme(&http.Request{Header: http.Header{}}); got != "http" {
+	if got := Scheme(&http.Request{Header: http.Header{}}); got != "http" {
 		t.Errorf("default scheme = %q, want http", got)
 	}
 }
-- 
2.47.3


From 776990253060d2a00ea57408c89011110b730afd Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 12:59:33 +1100
Subject: [PATCH 5/6] Fetch rewritten mutable responses on HEAD to match GET
 length

---
 e2e/head_test.go                         | 68 ++++++++++++++++++++++++
 internal/api/v1/proxy.go                 |  8 ++-
 internal/api/v1/routes_test.go           | 24 +++++++++
 internal/provider/helm/helm.go           |  2 +
 internal/provider/npm/npm.go             |  2 +
 internal/provider/provider.go            |  6 +++
 internal/provider/puppet/puppet.go       |  2 +
 internal/provider/pypi/pypi.go           |  2 +
 internal/provider/terraform/terraform.go |  2 +
 9 files changed, 115 insertions(+), 1 deletion(-)

diff --git a/e2e/head_test.go b/e2e/head_test.go
index f7b9ac7..9c43be3 100644
--- a/e2e/head_test.go
+++ b/e2e/head_test.go
@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -98,3 +99,70 @@ func TestProxyHeadGitHubRPMRepodata(t *testing.T) {
 	}
 	assertHeadMatchesGet(t, url)
 }
+
+func TestProxyHeadNpmUncachedPackument(t *testing.T) {
+	const abbreviated = "application/vnd.npm.install-v1+json"
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		doc := map[string]any{"name": "left-pad", "versions": map[string]any{"1.3.0": map[string]any{}}}
+		if r.Header.Get("Accept") != abbreviated {
+			doc["readme"] = strings.Repeat("full packument ", 200)
+		}
+		json.NewEncoder(w).Encode(doc)
+	}))
+	defer upstream.Close()
+
+	createRemote(t, mustFormat(`{"name": "head-npm", "package_type": "npm", "base_url": %q}`, upstream.URL))
+	defer deleteRemote(t, "head-npm")
+
+	url := apiURL("/api/v1/remote/head-npm/left-pad")
+	do := func(method string) (*http.Response, []byte) {
+		req, _ := http.NewRequest(method, url, nil)
+		req.Header.Set("Accept", abbreviated)
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatalf("%s: %v", method, err)
+		}
+		defer resp.Body.Close()
+		b, _ := io.ReadAll(resp.Body)
+		return resp, b
+	}
+	head, _ := do(http.MethodHead)
+	get, body := do(http.MethodGet)
+	if head.StatusCode != http.StatusOK || get.StatusCode != http.StatusOK {
+		t.Fatalf("HEAD %d, GET %d", head.StatusCode, get.StatusCode)
+	}
+	if head.ContentLength != int64(len(body)) {
+		t.Errorf("uncached HEAD Content-Length %d, GET body %d bytes", head.ContentLength, len(body))
+	}
+}
+
+func TestProxyHeadNonRewritingUsesUpstreamHead(t *testing.T) {
+	methods := make(chan string, 4)
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		methods <- r.Method
+		w.Header().Set("Content-Type", "text/xml")
+		w.Write([]byte("<repomd/>"))
+	}))
+	defer upstream.Close()
+
+	createRemote(t, mustFormat(`{"name": "head-rpm", "package_type": "rpm", "base_url": %q}`, upstream.URL))
+	defer deleteRemote(t, "head-rpm")
+
+	resp, err := http.Head(apiURL("/api/v1/remote/head-rpm/repodata/repomd.xml"))
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusOK || resp.Header.Get("X-Artifact-Source") != "remote" {
+		t.Fatalf("HEAD %d source %q", resp.StatusCode, resp.Header.Get("X-Artifact-Source"))
+	}
+	close(methods)
+	var seen []string
+	for m := range methods {
+		seen = append(seen, m)
+	}
+	if len(seen) != 1 || seen[0] != http.MethodHead {
+		t.Errorf("upstream saw %v, want a single HEAD", seen)
+	}
+}
diff --git a/internal/api/v1/proxy.go b/internal/api/v1/proxy.go
index 43ba060..39c2481 100644
--- a/internal/api/v1/proxy.go
+++ b/internal/api/v1/proxy.go
@@ -141,6 +141,12 @@ func (h *ProxyHandler) serveFetch(w http.ResponseWriter, r *http.Request, remote
 	io.Copy(w, body)
 }
 
+func rewritesMutable(prov provider.Provider, path string) bool {
+	_, sr := prov.(provider.ServeRewriter)
+	_, rr := prov.(provider.ResponseRewriter)
+	return (sr || rr) && prov.Classify(path) == provider.Mutable
+}
+
 // handleProxyHead answers HEAD like GET without a body. Responses a provider
 // synthesizes or rewrites per request go through the GET path so their headers
 // match; everything else is answered from store metadata or an upstream HEAD.
@@ -153,7 +159,7 @@ func (h *ProxyHandler) handleProxyHead(w http.ResponseWriter, r *http.Request) {
 	if h.serveRemote(w, r, remote, prov, path) {
 		return
 	}
-	if _, ok := prov.(provider.ServeRewriter); ok && prov.Classify(path) == provider.Mutable {
+	if rewritesMutable(prov, path) {
 		h.serveFetch(w, r, remote, prov, path)
 		return
 	}
diff --git a/internal/api/v1/routes_test.go b/internal/api/v1/routes_test.go
index 4493a5c..e29d8cf 100644
--- a/internal/api/v1/routes_test.go
+++ b/internal/api/v1/routes_test.go
@@ -5,6 +5,11 @@ import (
 	"testing"
 
 	"github.com/go-chi/chi/v5"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	"git.unkin.net/unkin/artifactapi/internal/provider/npm"
+	"git.unkin.net/unkin/artifactapi/internal/provider/nuget"
+	"git.unkin.net/unkin/artifactapi/internal/provider/rpm"
 )
 
 func TestRoutesRemoteHead(t *testing.T) {
@@ -15,3 +20,22 @@ func TestRoutesRemoteHead(t *testing.T) {
 		}
 	}
 }
+
+func TestRewritesMutable(t *testing.T) {
+	tests := []struct {
+		name string
+		prov provider.Provider
+		path string
+		want bool
+	}{
+		{"npm packument", &npm.Provider{}, "left-pad", true},
+		{"npm tarball", &npm.Provider{}, "left-pad/-/left-pad-1.3.0.tgz", false},
+		{"nuget index", &nuget.Provider{}, "index.json", true},
+		{"rpm repodata", &rpm.Provider{}, "repodata/repomd.xml", false},
+	}
+	for _, tc := range tests {
+		if got := rewritesMutable(tc.prov, tc.path); got != tc.want {
+			t.Errorf("%s: rewritesMutable = %v, want %v", tc.name, got, tc.want)
+		}
+	}
+}
diff --git a/internal/provider/helm/helm.go b/internal/provider/helm/helm.go
index d37765e..07d81a6 100644
--- a/internal/provider/helm/helm.go
+++ b/internal/provider/helm/helm.go
@@ -39,6 +39,8 @@ func (p *Provider) UpstreamURL(remote models.Remote, path string) string {
 	return strings.TrimRight(remote.BaseURL, "/") + "/" + strings.TrimLeft(path, "/")
 }
 
+func (p *Provider) RewritesResponse() {}
+
 func (p *Provider) RewriteResponse(body []byte, remote models.Remote, proxyBaseURL string) ([]byte, error) {
 	if proxyBaseURL == "" {
 		return nil, nil
diff --git a/internal/provider/npm/npm.go b/internal/provider/npm/npm.go
index 38cf32d..8bc098d 100644
--- a/internal/provider/npm/npm.go
+++ b/internal/provider/npm/npm.go
@@ -37,6 +37,8 @@ func (p *Provider) UpstreamURL(remote models.Remote, path string) string {
 	return strings.TrimRight(remote.BaseURL, "/") + "/" + strings.TrimLeft(path, "/")
 }
 
+func (p *Provider) RewritesResponse() {}
+
 func (p *Provider) RewriteResponse(body []byte, remote models.Remote, proxyBaseURL string) ([]byte, error) {
 	if proxyBaseURL == "" || !json.Valid(body) {
 		return nil, nil
diff --git a/internal/provider/provider.go b/internal/provider/provider.go
index 4c005f3..ca21e8c 100644
--- a/internal/provider/provider.go
+++ b/internal/provider/provider.go
@@ -82,6 +82,12 @@ type ServeRewriter interface {
 	RewriteServed(body []byte, remote models.Remote, proxyBaseURL string) []byte
 }
 
+// ResponseRewriter marks providers whose RewriteResponse changes mutable
+// bodies, so the served length is only known after a fetch.
+type ResponseRewriter interface {
+	RewritesResponse()
+}
+
 // RemoteMetadataStore is the persistence surface a RemoteServer needs to cache
 // and read the metadata it derives per upstream asset. *database.DB satisfies it.
 type RemoteMetadataStore interface {
diff --git a/internal/provider/puppet/puppet.go b/internal/provider/puppet/puppet.go
index 142a431..03bf1d8 100644
--- a/internal/provider/puppet/puppet.go
+++ b/internal/provider/puppet/puppet.go
@@ -39,6 +39,8 @@ func (p *Provider) UpstreamURL(remote models.Remote, path string) string {
 	return strings.TrimRight(remote.BaseURL, "/") + "/" + strings.TrimLeft(path, "/")
 }
 
+func (p *Provider) RewritesResponse() {}
+
 func (p *Provider) RewriteResponse(body []byte, remote models.Remote, proxyBaseURL string) ([]byte, error) {
 	if proxyBaseURL == "" {
 		return nil, nil
diff --git a/internal/provider/pypi/pypi.go b/internal/provider/pypi/pypi.go
index e87e9c1..373e584 100644
--- a/internal/provider/pypi/pypi.go
+++ b/internal/provider/pypi/pypi.go
@@ -52,6 +52,8 @@ func (p *Provider) UpstreamURL(remote models.Remote, path string) string {
 	return strings.TrimRight(remote.BaseURL, "/") + "/" + strings.TrimLeft(path, "/")
 }
 
+func (p *Provider) RewritesResponse() {}
+
 func (p *Provider) RewriteResponse(body []byte, remote models.Remote, proxyBaseURL string) ([]byte, error) {
 	if proxyBaseURL == "" {
 		return nil, nil
diff --git a/internal/provider/terraform/terraform.go b/internal/provider/terraform/terraform.go
index f29c33d..022d094 100644
--- a/internal/provider/terraform/terraform.go
+++ b/internal/provider/terraform/terraform.go
@@ -73,6 +73,8 @@ func (p *Provider) UpstreamURL(remote models.Remote, path string) string {
 	return strings.TrimRight(remote.BaseURL, "/") + "/v1/providers/" + strings.TrimLeft(path, "/")
 }
 
+func (p *Provider) RewritesResponse() {}
+
 func (p *Provider) RewriteResponse(body []byte, remote models.Remote, proxyBaseURL string) ([]byte, error) {
 	if remote.ReleasesRemote == "" {
 		return nil, nil
-- 
2.47.3


From 29f335c3642baf47ab1b2cf5f9bad04192f9688a Mon Sep 17 00:00:00 2001
From: unkin-agent <unkin-agent@unkin.net>
Date: Sat, 17 Oct 2026 17:12:46 +1100
Subject: [PATCH 6/6] Document new nuget immutable blob classes

---
 docs/nuget/remote.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/nuget/remote.md b/docs/nuget/remote.md
index ba0109a..7e6ed99 100644
--- a/docs/nuget/remote.md
+++ b/docs/nuget/remote.md
@@ -4,4 +4,4 @@ Shared classification, TTL and eviction rules: [caching](../common/caching.md).
 
 | Type | Mutable (auto-detected) | Immutable (auto-detected) |
 |---|---|---|
-| `nuget` | `index.json` (service index), registration and flat container indexes | `.nupkg`, `.nuspec` |
+| `nuget` | `index.json` (service index), registration and flat container indexes | `.nupkg`, `.snupkg`, `.nuspec`, `readme`, `icon` |
-- 
2.47.3