docs/ARCHITECTURE.md

@@ -208,7 +208,21 @@ automatic** (D-021).
 
 - Dinners are grounded in real recipe pages found through recipe search
   (`searxng` or `brave`, per `recipe_search_settings`).
-- Fetching is **SSRF-safe**.
+- Fetching is **SSRF-safe** (`recipes.Fetcher`): https:// only, at every
+  redirect hop. Its dialer resolves the host once and refuses unless every
+  answer is public — every non-globally-reachable IPv4 special-purpose
+  range (loopback, RFC 1918, link-local, carrier-grade NAT, documentation
+  TEST-NETs, benchmarking 198.18/15, multicast, reserved and more) is
+  refused, IPv4-mapped forms included, and IPv6 must be global unicast
+  (2000::/3) outside documentation, 6to4, Teredo and other special ranges —
+  then dials the address it checked, so a
+  rebinding DNS answer cannot swap in a private one. Keep-alives and
+  environment proxies are off, so every request dials and checks afresh. At
+  most 3 redirects; a body over 2 MB is an error, never truncated; 10 s for
+  the whole fetch; a descriptive `User-Agent`; requests to one host at least
+  1 s apart, counted from requests actually sent (an abandoned wait holds no
+  slot); 2xx pages cached by URL for an hour (64 pages, oldest evicted), and
+  callers get their own copy of a cached body.
 - Recipe data is parsed from schema.org `Recipe` JSON-LD.
 - Photos are cached locally (`photos`) and processed with
   `golang.org/x/image`.

internal/recipes/fetcher.go

@@ -0,0 +1,523 @@
+// Package recipes fetches and holds real recipe pages that ground dinners.
+package recipes
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/netip"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+)
+
+const (
+	// UserAgent identifies MealQuest to the recipe sites it fetches.
+	UserAgent = "MealQuest/1.0 (household meal planner; recipe fetcher)"
+	// MaxBodyBytes caps a fetched body. A larger page is an error, never
+	// silently truncated: a cut-off recipe page would parse as a wrong one.
+	MaxBodyBytes = 2 << 20
+	// MaxRedirects is how many redirects a fetch follows; each hop is
+	// re-checked for scheme and address.
+	MaxRedirects = 3
+	// Timeout bounds a whole fetch: rate-limit waits, every redirect hop
+	// and reading the body.
+	Timeout = 10 * time.Second
+	// HostInterval is the least time between two requests to one host.
+	HostInterval = time.Second
+	// CacheTTL is how long a fetched page is served from the cache.
+	CacheTTL = time.Hour
+	// CacheSize is how many pages the cache holds; the oldest is evicted
+	// first.
+	CacheSize = 64
+)
+
+var (
+	// ErrNotHTTPS is returned for a URL, or a redirect, that is not
+	// https://.
+	ErrNotHTTPS = errors.New("recipes: only https:// URLs are fetched")
+	// ErrBlockedAddress is returned when a host resolves to an address
+	// that is not public, such as loopback or a private network.
+	ErrBlockedAddress = errors.New("recipes: address is not public")
+	// ErrTooManyRedirects is returned after MaxRedirects redirects.
+	ErrTooManyRedirects = errors.New("recipes: too many redirects")
+	// ErrTooLarge is returned for a body over MaxBodyBytes.
+	ErrTooLarge = errors.New("recipes: body exceeds size cap")
+	// ErrTimeout is returned when a fetch runs past Timeout.
+	ErrTimeout = errors.New("recipes: fetch timed out")
+)
+
+// StatusError is returned for a final response whose status is not 2xx.
+type StatusError struct {
+	URL    string
+	Status int
+}
+
+func (e *StatusError) Error() string {
+	return fmt.Sprintf("recipes: GET %s: HTTP %d", e.URL, e.Status)
+}
+
+// Page is a fetched page.
+type Page struct {
+	// URL is the final URL, after redirects.
+	URL         string
+	ContentType string
+	Body        []byte
+}
+
+// Resolver looks up a host's addresses; *net.Resolver satisfies it.
+type Resolver interface {
+	LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error)
+}
+
+// dialFunc dials an address that has already passed the public-address
+// check.
+type dialFunc func(ctx context.Context, network, address string) (net.Conn, error)
+
+// Fetcher retrieves recipe pages without reaching private networks. Its
+// dialer resolves each host itself, refuses the connection unless every
+// answer is a public address, and dials the checked address, so a second DNS
+// answer cannot swap in a private one. Keep-alives are off: every request,
+// redirect hops included, dials and checks afresh. Proxies from the
+// environment are not used, since a proxy would dial on the Fetcher's
+// behalf. Requests are HTTP/1.1, so each connection carries exactly one
+// request and its first write is that request. A Fetcher is safe for
+// concurrent use.
+type Fetcher struct {
+	client    *http.Client
+	resolver  Resolver
+	dial      dialFunc
+	tlsConfig *tls.Config
+	// timeout bounds a fetch; tests shorten it.
+	timeout time.Duration
+	// now is the clock; tests replace it.
+	now func() time.Time
+	// sleep waits d or until ctx is done; tests replace it.
+	sleep func(ctx context.Context, d time.Duration) error
+
+	mu sync.Mutex
+	// lastSent is when each host's latest request began to be written to
+	// its connection; a request whose write fails before any byte is sent
+	// hands its slot back.
+	lastSent map[string]time.Time
+	// released is closed, then replaced, whenever a slot is handed back,
+	// waking requests waiting for their turn.
+	released chan struct{}
+	cache    map[string]cacheEntry
+}
+
+type cacheEntry struct {
+	page    Page
+	fetched time.Time
+}
+
+// NewFetcher returns a Fetcher that resolves through the system resolver.
+func NewFetcher() *Fetcher {
+	return newFetcher(net.DefaultResolver, (&net.Dialer{}).DialContext, nil)
+}
+
+// newFetcher returns a Fetcher over resolver and dial; tlsConfig, when not
+// nil, replaces the default TLS settings so tests can trust their servers.
+func newFetcher(resolver Resolver, dial dialFunc, tlsConfig *tls.Config) *Fetcher {
+	f := &Fetcher{
+		resolver:  resolver,
+		dial:      dial,
+		tlsConfig: tlsConfig,
+		timeout:   Timeout,
+		now:       time.Now,
+		sleep:     sleep,
+		lastSent:  map[string]time.Time{},
+		released:  make(chan struct{}),
+		cache:     map[string]cacheEntry{},
+	}
+	transport := &http.Transport{
+		Proxy:             nil,
+		DialTLSContext:    f.dialTLS,
+		DisableKeepAlives: true,
+	}
+	f.client = &http.Client{
+		Transport:     &requestTransport{next: transport},
+		CheckRedirect: checkRedirect,
+	}
+	return f
+}
+
+// Fetch returns the page at rawURL, from the cache when it was fetched
+// within CacheTTL. Only 2xx responses are returned and cached. The
+// returned Body is the caller's own copy.
+func (f *Fetcher) Fetch(ctx context.Context, rawURL string) (Page, error) {
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return Page{}, fmt.Errorf("recipes: parse URL: %w", err)
+	}
+	if err := checkURL(u); err != nil {
+		return Page{}, err
+	}
+	key := u.String()
+	if page, ok := f.cached(key); ok {
+		return page, nil
+	}
+
+	fetchCtx, cancel := context.WithTimeout(ctx, f.timeout)
+	defer cancel()
+	page, err := f.get(fetchCtx, key)
+	if err != nil {
+		if ctx.Err() == nil && errors.Is(fetchCtx.Err(), context.DeadlineExceeded) {
+			return Page{}, fmt.Errorf("%w after %s: %s", ErrTimeout, f.timeout, key)
+		}
+		return Page{}, err
+	}
+	f.store(key, page)
+	return page, nil
+}
+
+func (f *Fetcher) get(ctx context.Context, rawURL string) (Page, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, rawURL, nil)
+	if err != nil {
+		return Page{}, fmt.Errorf("recipes: build request: %w", err)
+	}
+	req.Header.Set("User-Agent", UserAgent)
+	req.Header.Set("Accept", "text/html,application/xhtml+xml")
+	resp, err := f.client.Do(req)
+	if err != nil {
+		return Page{}, err
+	}
+	defer resp.Body.Close()
+	final := resp.Request.URL.String()
+	if resp.StatusCode < 200 || resp.StatusCode > 299 {
+		return Page{}, &StatusError{URL: final, Status: resp.StatusCode}
+	}
+	if resp.ContentLength > MaxBodyBytes {
+		return Page{}, fmt.Errorf("%w: %s declares %d bytes", ErrTooLarge, final, resp.ContentLength)
+	}
+	body, err := io.ReadAll(io.LimitReader(resp.Body, MaxBodyBytes+1))
+	if err != nil {
+		return Page{}, fmt.Errorf("recipes: read %s: %w", final, err)
+	}
+	if len(body) > MaxBodyBytes {
+		return Page{}, fmt.Errorf("%w: %s is over %d bytes", ErrTooLarge, final, MaxBodyBytes)
+	}
+	return Page{URL: final, ContentType: resp.Header.Get("Content-Type"), Body: body}, nil
+}
+
+// cached returns the page cached under key if it is younger than CacheTTL.
+func (f *Fetcher) cached(key string) (Page, bool) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	e, ok := f.cache[key]
+	if !ok {
+		return Page{}, false
+	}
+	if f.now().Sub(e.fetched) >= CacheTTL {
+		delete(f.cache, key)
+		return Page{}, false
+	}
+	page := e.page
+	page.Body = bytes.Clone(page.Body)
+	return page, true
+}
+
+// store caches a copy of page under key, evicting the oldest entry when
+// full.
+func (f *Fetcher) store(key string, page Page) {
+	page.Body = bytes.Clone(page.Body)
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if _, ok := f.cache[key]; !ok && len(f.cache) >= CacheSize {
+		var oldest string
+		var oldestAt time.Time
+		for k, e := range f.cache {
+			if oldest == "" || e.fetched.Before(oldestAt) {
+				oldest, oldestAt = k, e.fetched
+			}
+		}
+		delete(f.cache, oldest)
+	}
+	f.cache[key] = cacheEntry{page: page, fetched: f.now()}
+}
+
+// waitTurn blocks until a request to host may be sent, keeping requests to
+// one host at least HostInterval apart. A wait holds no reservation, and a
+// request ending with ctx before its turn takes no slot. The request let
+// through takes the host's slot and must be sent at once, or call release
+// if sending fails, so that it delays no other request. Fetch calls it from
+// turnConn's first Write, so the slot is taken after DNS, dialing and the
+// TLS handshake, as the request is written.
+func (f *Fetcher) waitTurn(ctx context.Context, host string) (release func(), err error) {
+	for {
+		if err := ctx.Err(); err != nil {
+			return nil, err
+		}
+		f.mu.Lock()
+		now := f.now()
+		last, sent := f.lastSent[host]
+		wait := last.Add(HostInterval).Sub(now)
+		if !sent || wait <= 0 {
+			f.lastSent[host] = now
+			f.mu.Unlock()
+			return func() { f.release(host, now, last, sent) }, nil
+		}
+		released := f.released
+		f.mu.Unlock()
+		if err := f.waitOrRelease(ctx, wait, released); err != nil {
+			return nil, err
+		}
+	}
+}
+
+// release hands back the slot taken at taken for a request that was not
+// sent, restoring the host's previous send time, and wakes waiting requests.
+func (f *Fetcher) release(host string, taken, prev time.Time, hadPrev bool) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if !f.lastSent[host].Equal(taken) {
+		// A later request already holds the slot.
+		return
+	}
+	if hadPrev {
+		f.lastSent[host] = prev
+	} else {
+		delete(f.lastSent, host)
+	}
+	close(f.released)
+	f.released = make(chan struct{})
+}
+
+// waitOrRelease sleeps for d, returning early without error when a slot is
+// handed back so the caller can look again.
+func (f *Fetcher) waitOrRelease(ctx context.Context, d time.Duration, released <-chan struct{}) error {
+	sleepCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	go func() {
+		select {
+		case <-released:
+			cancel()
+		case <-sleepCtx.Done():
+		}
+	}()
+	err := f.sleep(sleepCtx, d)
+	select {
+	case <-released:
+		return nil
+	default:
+		return err
+	}
+}
+
+// dialPublic resolves address's host once, refuses unless every answer is
+// public, then dials the checked addresses in turn.
+func (f *Fetcher) dialPublic(ctx context.Context, network, address string) (net.Conn, error) {
+	host, port, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, fmt.Errorf("recipes: dial address %q: %w", address, err)
+	}
+	var addrs []netip.Addr
+	if ip, err := netip.ParseAddr(host); err == nil {
+		addrs = []netip.Addr{ip}
+	} else {
+		answers, err := f.resolver.LookupIPAddr(ctx, host)
+		if err != nil {
+			return nil, fmt.Errorf("recipes: resolve %s: %w", host, err)
+		}
+		for _, a := range answers {
+			ip, ok := netip.AddrFromSlice(a.IP)
+			if !ok {
+				return nil, fmt.Errorf("recipes: resolve %s: malformed address %v", host, a.IP)
+			}
+			addrs = append(addrs, ip)
+		}
+	}
+	if len(addrs) == 0 {
+		return nil, fmt.Errorf("recipes: resolve %s: no addresses", host)
+	}
+	for _, ip := range addrs {
+		if !IsPublic(ip) {
+			return nil, fmt.Errorf("%w: %s resolves to %s", ErrBlockedAddress, host, ip)
+		}
+	}
+	var errs []error
+	for _, ip := range addrs {
+		conn, err := f.dial(ctx, network, net.JoinHostPort(ip.Unmap().WithZone("").String(), port))
+		if err == nil {
+			return conn, nil
+		}
+		errs = append(errs, err)
+	}
+	return nil, errors.Join(errs...)
+}
+
+// requestCtxKey carries a request's own context into its dial. The
+// transport dials with a context that keeps the request's values but not
+// its cancellation; the connection and the rate-limit wait must end with
+// the request.
+type requestCtxKey struct{}
+
+// requestTransport hands each request's context, redirect hops included,
+// to dialTLS.
+type requestTransport struct {
+	next http.RoundTripper
+}
+
+func (t *requestTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	ctx := req.Context()
+	return t.next.RoundTrip(req.WithContext(context.WithValue(ctx, requestCtxKey{}, ctx)))
+}
+
+// dialTLS connects to address through dialPublic and completes the TLS
+// handshake, all under the request's context, and returns a turnConn whose
+// first write waits for the host's rate-limit slot. ALPN offers only
+// HTTP/1.1, so the connection carries this one request.
+func (f *Fetcher) dialTLS(dialCtx context.Context, network, address string) (net.Conn, error) {
+	ctx, ok := dialCtx.Value(requestCtxKey{}).(context.Context)
+	if !ok {
+		return nil, fmt.Errorf("recipes: dial %s outside a request", address)
+	}
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, fmt.Errorf("recipes: dial address %q: %w", address, err)
+	}
+	conn, err := f.dialPublic(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	cfg := &tls.Config{}
+	if f.tlsConfig != nil {
+		cfg = f.tlsConfig.Clone()
+	}
+	cfg.ServerName = host
+	cfg.NextProtos = []string{"http/1.1"}
+	tc := tls.Client(conn, cfg)
+	if err := tc.HandshakeContext(ctx); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("recipes: TLS handshake with %s: %w", host, err)
+	}
+	return &turnConn{Conn: tc, f: f, ctx: ctx, host: strings.ToLower(host)}, nil
+}
+
+// turnConn is a connection ready to carry one request. Its first Write —
+// the start of the request — waits for the host's rate-limit slot, so two
+// requests to one host are written at least HostInterval apart however
+// long each spent resolving, dialing and in the TLS handshake. A first
+// Write that sends nothing hands the slot back.
+type turnConn struct {
+	net.Conn
+	f    *Fetcher
+	ctx  context.Context
+	host string
+	// started is set by the first Write; the transport writes a request
+	// from one goroutine, so it needs no lock.
+	started bool
+}
+
+func (c *turnConn) Write(p []byte) (int, error) {
+	if c.started {
+		return c.Conn.Write(p)
+	}
+	release, err := c.f.waitTurn(c.ctx, c.host)
+	if err != nil {
+		return 0, err
+	}
+	c.started = true
+	n, err := c.Conn.Write(p)
+	if n == 0 && err != nil {
+		release()
+	}
+	return n, err
+}
+
+// checkURL refuses anything but an absolute https:// URL with a host.
+func checkURL(u *url.URL) error {
+	if u.Scheme != "https" {
+		return fmt.Errorf("%w: %q", ErrNotHTTPS, u.Redacted())
+	}
+	if u.Hostname() == "" {
+		return fmt.Errorf("recipes: URL %q has no host", u.Redacted())
+	}
+	return nil
+}
+
+// checkRedirect applies checkURL to every hop and stops after
+// MaxRedirects. The hop's address is checked when it is dialed.
+func checkRedirect(req *http.Request, via []*http.Request) error {
+	if len(via) > MaxRedirects {
+		return fmt.Errorf("%w: more than %d", ErrTooManyRedirects, MaxRedirects)
+	}
+	return checkURL(req.URL)
+}
+
+// blockedIPv4 are the IPv4 special-purpose ranges that are not globally
+// reachable (IANA IPv4 Special-Purpose Address Registry). IPv4-mapped IPv6
+// addresses are unmapped before the check, so these cover them too.
+var blockedIPv4 = []netip.Prefix{
+	netip.MustParsePrefix("0.0.0.0/8"),       // "this network"
+	netip.MustParsePrefix("10.0.0.0/8"),      // RFC 1918
+	netip.MustParsePrefix("100.64.0.0/10"),   // carrier-grade NAT
+	netip.MustParsePrefix("127.0.0.0/8"),     // loopback
+	netip.MustParsePrefix("169.254.0.0/16"),  // link-local
+	netip.MustParsePrefix("172.16.0.0/12"),   // RFC 1918
+	netip.MustParsePrefix("192.0.0.0/24"),    // IETF protocol assignments
+	netip.MustParsePrefix("192.0.2.0/24"),    // documentation, TEST-NET-1
+	netip.MustParsePrefix("192.88.99.0/24"),  // 6to4 relay anycast (deprecated)
+	netip.MustParsePrefix("192.168.0.0/16"),  // RFC 1918
+	netip.MustParsePrefix("198.18.0.0/15"),   // benchmarking
+	netip.MustParsePrefix("198.51.100.0/24"), // documentation, TEST-NET-2
+	netip.MustParsePrefix("203.0.113.0/24"),  // documentation, TEST-NET-3
+	netip.MustParsePrefix("224.0.0.0/4"),     // multicast
+	netip.MustParsePrefix("240.0.0.0/4"),     // reserved, broadcast
+}
+
+// globalIPv6 is the only IPv6 range dialed: global unicast. Everything
+// outside it is refused, including ::, ::1, IPv4-compatible and
+// IPv4-translated forms, NAT64 (64:ff9b::/96), discard-only (100::/64),
+// unique local (fc00::/7), link-local (fe80::/10) and multicast (ff00::/8).
+var globalIPv6 = netip.MustParsePrefix("2000::/3")
+
+// blockedIPv6 are the special-purpose ranges inside global unicast that are
+// not public or embed an arbitrary IPv4 address.
+var blockedIPv6 = []netip.Prefix{
+	netip.MustParsePrefix("2001::/23"),     // IETF protocol assignments, Teredo, benchmarking
+	netip.MustParsePrefix("2001:db8::/32"), // documentation
+	netip.MustParsePrefix("2002::/16"),     // 6to4, embeds any IPv4
+	netip.MustParsePrefix("3fff::/20"),     // documentation
+}
+
+// IsPublic reports whether ip is a public unicast address the Fetcher may
+// dial. IPv4 addresses are public unless in a special-purpose range; IPv6
+// addresses must be global unicast and outside its special-purpose ranges.
+func IsPublic(ip netip.Addr) bool {
+	if !ip.IsValid() {
+		return false
+	}
+	ip = ip.Unmap().WithZone("")
+	blocked := blockedIPv4
+	if ip.Is6() {
+		if !globalIPv6.Contains(ip) {
+			return false
+		}
+		blocked = blockedIPv6
+	}
+	for _, p := range blocked {
+		if p.Contains(ip) {
+			return false
+		}
+	}
+	return true
+}
+
+// sleep waits d or until ctx is done, returning ctx's error if it is.
+func sleep(ctx context.Context, d time.Duration) error {
+	t := time.NewTimer(d)
+	defer t.Stop()
+	select {
+	case <-t.C:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}

internal/recipes/fetcher_test.go

@@ -0,0 +1,721 @@
+package recipes
+
+import (
+	"context"
+	"crypto/tls"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// stubResolver answers each host from a list of answers, one per lookup;
+// the last answer repeats.
+type stubResolver struct {
+	mu      sync.Mutex
+	answers map[string][][]string
+	calls   map[string]int
+}
+
+func (r *stubResolver) LookupIPAddr(_ context.Context, host string) ([]net.IPAddr, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	seq, ok := r.answers[host]
+	if !ok {
+		return nil, fmt.Errorf("stub: no such host %s", host)
+	}
+	n := r.calls[host]
+	r.calls[host]++
+	if n >= len(seq) {
+		n = len(seq) - 1
+	}
+	var out []net.IPAddr
+	for _, s := range seq[n] {
+		out = append(out, net.IPAddr{IP: net.ParseIP(s)})
+	}
+	return out, nil
+}
+
+func (r *stubResolver) lookups(host string) int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.calls[host]
+}
+
+// testEnv is a Fetcher whose public test addresses all reach one httptest
+// TLS server, and which records every address it dials.
+type testEnv struct {
+	f        *Fetcher
+	resolver *stubResolver
+	server   *httptest.Server
+
+	mu     sync.Mutex
+	dialed []string
+}
+
+// publicIP, publicIP2 and publicIPv6 are public addresses; the test dialer
+// routes them to the test server and never reaches the real hosts.
+const (
+	publicIP   = "93.184.215.14"
+	publicIP2  = "93.184.215.20"
+	publicIPv6 = "2606:2800:21f::10"
+)
+
+func newTestEnv(t *testing.T, handler http.Handler, answers map[string][][]string) *testEnv {
+	t.Helper()
+	server := httptest.NewTLSServer(handler)
+	t.Cleanup(server.Close)
+	pool := x509.NewCertPool()
+	pool.AddCert(server.Certificate())
+	env := &testEnv{
+		resolver: &stubResolver{answers: answers, calls: map[string]int{}},
+		server:   server,
+	}
+	dial := func(ctx context.Context, network, address string) (net.Conn, error) {
+		env.mu.Lock()
+		env.dialed = append(env.dialed, address)
+		env.mu.Unlock()
+		host, _, err := net.SplitHostPort(address)
+		if err != nil {
+			return nil, err
+		}
+		if host == publicIP || host == publicIP2 || host == publicIPv6 {
+			var d net.Dialer
+			return d.DialContext(ctx, network, server.Listener.Addr().String())
+		}
+		return nil, fmt.Errorf("test dial: no route to %s", address)
+	}
+	env.f = newFetcher(env.resolver, dial, &tls.Config{RootCAs: pool})
+	// Real requests in these tests are not rate-limited unless a test
+	// installs a fake clock.
+	env.f.sleep = func(context.Context, time.Duration) error { return nil }
+	return env
+}
+
+func (e *testEnv) dials() []string {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	return append([]string(nil), e.dialed...)
+}
+
+func publicAnswers() map[string][][]string {
+	return map[string][][]string{"example.com": {{publicIP}}}
+}
+
+func TestFetchReturnsPage(t *testing.T) {
+	t.Parallel()
+	var gotUA string
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		w.Header().Set("Content-Type", "text/html")
+		fmt.Fprint(w, "<html>tacos</html>")
+	}), publicAnswers())
+
+	page, err := env.f.Fetch(t.Context(), "https://example.com/tacos")
+	if err != nil {
+		t.Fatalf("Fetch: %v", err)
+	}
+	if string(page.Body) != "<html>tacos</html>" || page.ContentType != "text/html" || page.URL != "https://example.com/tacos" {
+		t.Errorf("page = %+v", page)
+	}
+	if gotUA != UserAgent {
+		t.Errorf("User-Agent = %q, want %q", gotUA, UserAgent)
+	}
+	if got := env.dials(); len(got) != 1 || got[0] != publicIP+":443" {
+		t.Errorf("dialed %v, want [%s:443]", got, publicIP)
+	}
+}
+
+func TestFetchDialsPublicIPv6(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "ok")
+	}), map[string][][]string{"example.com": {{publicIPv6}}})
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/"); err != nil {
+		t.Fatalf("Fetch: %v", err)
+	}
+	if got := env.dials(); len(got) != 1 || got[0] != "["+publicIPv6+"]:443" {
+		t.Errorf("dialed %v, want [[%s]:443]", got, publicIPv6)
+	}
+}
+
+func TestFetchRejectsNonHTTPS(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.NotFoundHandler(), publicAnswers())
+	for _, raw := range []string{"http://example.com/tacos", "ftp://example.com/", "example.com/tacos", "https:///tacos"} {
+		_, err := env.f.Fetch(t.Context(), raw)
+		if err == nil {
+			t.Errorf("Fetch(%q) succeeded", raw)
+		}
+		if strings.HasPrefix(raw, "http://") && !errors.Is(err, ErrNotHTTPS) {
+			t.Errorf("Fetch(%q) error = %v, want ErrNotHTTPS", raw, err)
+		}
+	}
+	if got := env.dials(); len(got) != 0 {
+		t.Errorf("dialed %v, want nothing", got)
+	}
+	if n := env.resolver.lookups("example.com"); n != 0 {
+		t.Errorf("resolved example.com %d times, want 0", n)
+	}
+}
+
+// blockedIPs has an address in every refused range, including IPv4-mapped
+// forms of the IPv4 ranges.
+var blockedIPs = []string{
+	"127.0.0.1", "127.255.255.254", "::1", "::ffff:127.0.0.1", // loopback
+	"10.0.0.1", "172.16.0.1", "172.31.255.255", "192.168.1.1", // RFC 1918
+	"::ffff:10.1.2.3", "::ffff:172.20.0.1", "::ffff:192.168.0.10",
+	"169.254.169.254", "::ffff:169.254.169.254", "fe80::1", // link-local
+	"100.64.0.1", "100.127.255.254", "::ffff:100.100.0.1", // carrier-grade NAT
+	"224.0.0.1", "239.255.255.250", "ff02::1", "::ffff:224.0.0.251", // multicast
+	"0.0.0.0", "0.1.2.3", "::", "::ffff:0.0.0.0", // unspecified
+	"fc00::1", "fd12:3456:789a::1", // unique local
+	"255.255.255.255", "240.0.0.1", "fec0::1", // reserved, site-local
+	"192.0.0.1", "192.0.0.255", // IETF protocol assignments
+	"192.0.2.1", "198.51.100.7", "203.0.113.10", "::ffff:203.0.113.10", // documentation
+	"192.88.99.1",                                       // 6to4 relay anycast
+	"198.18.0.1", "198.19.255.255", "::ffff:198.18.0.1", // benchmarking
+	"2001:db8::1", "3fff::1", // IPv6 documentation
+	"2001:2::1", "100::1", // IPv6 benchmarking, discard-only
+	"::ffff:0:a00:1", // IPv4-translated
+	"64:ff9b::a00:1", "64:ff9b:1::a00:1", "2002:a00:1::1", "2001:0:a00:1::1", "::a00:1",
+}
+
+func TestIsPublic(t *testing.T) {
+	t.Parallel()
+	for _, s := range blockedIPs {
+		if IsPublic(netip.MustParseAddr(s)) {
+			t.Errorf("IsPublic(%s) = true, want false", s)
+		}
+	}
+	for _, s := range []string{
+		"8.8.8.8", publicIP, "::ffff:8.8.8.8", "100.63.255.255", "100.128.0.1", "172.32.0.1",
+		"192.0.1.255", "192.0.3.1", "192.88.98.255", "198.17.255.255", "198.20.0.1",
+		"198.51.99.255", "203.0.112.255", "203.0.114.1", "223.255.255.254",
+		"2606:4700::1111", publicIPv6, "2001:4860:4860::8888", "2a00:1450::1",
+	} {
+		if !IsPublic(netip.MustParseAddr(s)) {
+			t.Errorf("IsPublic(%s) = false, want true", s)
+		}
+	}
+	if IsPublic(netip.Addr{}) {
+		t.Error("IsPublic(zero Addr) = true, want false")
+	}
+}
+
+func TestFetchRefusesEachBlockedRange(t *testing.T) {
+	t.Parallel()
+	for _, ip := range blockedIPs {
+		t.Run(ip, func(t *testing.T) {
+			t.Parallel()
+			env := newTestEnv(t, http.NotFoundHandler(), map[string][][]string{"example.com": {{ip}}})
+			_, err := env.f.Fetch(t.Context(), "https://example.com/")
+			if !errors.Is(err, ErrBlockedAddress) {
+				t.Fatalf("Fetch error = %v, want ErrBlockedAddress", err)
+			}
+			if got := env.dials(); len(got) != 0 {
+				t.Errorf("dialed %v, want nothing", got)
+			}
+		})
+	}
+}
+
+func TestFetchRefusesBlockedLiteralHost(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.NotFoundHandler(), nil)
+	for _, raw := range []string{"https://127.0.0.1/", "https://[::1]/", "https://[::ffff:10.0.0.1]/", "https://169.254.169.254/latest/meta-data/", "https://[fe80::1%25eth0]/"} {
+		if _, err := env.f.Fetch(t.Context(), raw); !errors.Is(err, ErrBlockedAddress) {
+			t.Errorf("Fetch(%q) error = %v, want ErrBlockedAddress", raw, err)
+		}
+	}
+	if got := env.dials(); len(got) != 0 {
+		t.Errorf("dialed %v, want nothing", got)
+	}
+}
+
+func TestFetchRefusesMixedAnswer(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.NotFoundHandler(), map[string][][]string{"example.com": {{publicIP, "10.0.0.1"}}})
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/"); !errors.Is(err, ErrBlockedAddress) {
+		t.Fatalf("Fetch error = %v, want ErrBlockedAddress", err)
+	}
+	if got := env.dials(); len(got) != 0 {
+		t.Errorf("dialed %v, want nothing", got)
+	}
+}
+
+func TestFetchRefusesRedirectToPrivateHost(t *testing.T) {
+	t.Parallel()
+	answers := map[string][][]string{
+		"example.com":      {{publicIP}},
+		"evil.example.com": {{"10.0.0.5"}},
+	}
+	for _, target := range []string{"https://evil.example.com/admin", "https://127.0.0.1/admin", "https://[::ffff:192.168.0.1]/"} {
+		t.Run(target, func(t *testing.T) {
+			t.Parallel()
+			env := newTestEnv(t, http.RedirectHandler(target, http.StatusFound), answers)
+			_, err := env.f.Fetch(t.Context(), "https://example.com/tacos")
+			if !errors.Is(err, ErrBlockedAddress) {
+				t.Fatalf("Fetch error = %v, want ErrBlockedAddress", err)
+			}
+			if got := env.dials(); len(got) != 1 || got[0] != publicIP+":443" {
+				t.Errorf("dialed %v, want only the first hop", got)
+			}
+		})
+	}
+}
+
+func TestFetchRefusesRedirectToHTTP(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.RedirectHandler("http://example.com/plain", http.StatusMovedPermanently), publicAnswers())
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/"); !errors.Is(err, ErrNotHTTPS) {
+		t.Fatalf("Fetch error = %v, want ErrNotHTTPS", err)
+	}
+}
+
+// chainHandler redirects /hop/N to /hop/N-1 until /hop/0, which serves a
+// page.
+var chainHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	var n int
+	if _, err := fmt.Sscanf(r.URL.Path, "/hop/%d", &n); err != nil {
+		http.NotFound(w, r)
+		return
+	}
+	if n == 0 {
+		fmt.Fprint(w, "arrived")
+		return
+	}
+	http.Redirect(w, r, fmt.Sprintf("https://recipes.example.com/hop/%d", n-1), http.StatusFound)
+})
+
+func TestFetchFollowsAtMostThreeRedirects(t *testing.T) {
+	t.Parallel()
+	answers := map[string][][]string{
+		"example.com":         {{publicIP}},
+		"recipes.example.com": {{publicIP2}},
+	}
+	env := newTestEnv(t, chainHandler, answers)
+	page, err := env.f.Fetch(t.Context(), "https://example.com/hop/3")
+	if err != nil {
+		t.Fatalf("Fetch with 3 redirects: %v", err)
+	}
+	if page.URL != "https://recipes.example.com/hop/0" || string(page.Body) != "arrived" {
+		t.Errorf("page = %+v", page)
+	}
+	// Every hop is dialed, so every hop's address is checked.
+	if got := env.dials(); len(got) != 4 || got[0] != publicIP+":443" || got[3] != publicIP2+":443" {
+		t.Errorf("dialed %v, want 4 checked hops", got)
+	}
+
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/hop/4"); !errors.Is(err, ErrTooManyRedirects) {
+		t.Fatalf("Fetch with 4 redirects error = %v, want ErrTooManyRedirects", err)
+	}
+}
+
+// TestFetchRefusesDNSSwitchToPrivate covers DNS rebinding: the host's first
+// answer is public and every later one private. The first fetch dials the
+// address it checked; the next lookup's private answer is refused, and the
+// private address is never dialed.
+func TestFetchRefusesDNSSwitchToPrivate(t *testing.T) {
+	t.Parallel()
+	answers := map[string][][]string{"example.com": {{publicIP}, {"127.0.0.1"}}}
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "ok")
+	}), answers)
+
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/one"); err != nil {
+		t.Fatalf("first Fetch: %v", err)
+	}
+	if n := env.resolver.lookups("example.com"); n != 1 {
+		t.Fatalf("first Fetch resolved %d times, want 1 (checked IP must be dialed IP)", n)
+	}
+	_, err := env.f.Fetch(t.Context(), "https://example.com/two")
+	if !errors.Is(err, ErrBlockedAddress) {
+		t.Fatalf("second Fetch error = %v, want ErrBlockedAddress", err)
+	}
+	if got := env.dials(); len(got) != 1 || got[0] != publicIP+":443" {
+		t.Errorf("dialed %v, want only %s:443", got, publicIP)
+	}
+}
+
+func TestFetchSizeCap(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		size := MaxBodyBytes
+		if r.URL.Path != "/exact" {
+			size++
+		}
+		if r.URL.Path == "/declared" {
+			w.Header().Set("Content-Length", fmt.Sprint(size))
+		}
+		w.Write([]byte(strings.Repeat("a", size)))
+	}), publicAnswers())
+
+	page, err := env.f.Fetch(t.Context(), "https://example.com/exact")
+	if err != nil {
+		t.Fatalf("Fetch at the cap: %v", err)
+	}
+	if len(page.Body) != MaxBodyBytes {
+		t.Errorf("body is %d bytes, want %d", len(page.Body), MaxBodyBytes)
+	}
+	for _, path := range []string{"/declared", "/chunked"} {
+		if _, err := env.f.Fetch(t.Context(), "https://example.com"+path); !errors.Is(err, ErrTooLarge) {
+			t.Errorf("Fetch %s error = %v, want ErrTooLarge", path, err)
+		}
+	}
+}
+
+func TestFetchTimeout(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/slow-body" {
+			w.Write([]byte("start"))
+			w.(http.Flusher).Flush()
+		}
+		<-r.Context().Done()
+	}), publicAnswers())
+	env.f.timeout = 100 * time.Millisecond
+
+	for _, path := range []string{"/slow-headers", "/slow-body"} {
+		start := time.Now()
+		_, err := env.f.Fetch(t.Context(), "https://example.com"+path)
+		if !errors.Is(err, ErrTimeout) {
+			t.Errorf("Fetch %s error = %v, want ErrTimeout", path, err)
+		}
+		if elapsed := time.Since(start); elapsed > 5*time.Second {
+			t.Errorf("Fetch %s took %s", path, elapsed)
+		}
+	}
+	if Timeout != 10*time.Second {
+		t.Errorf("Timeout = %s, want 10s", Timeout)
+	}
+}
+
+func TestFetchCallerCancelIsNotTimeout(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.NotFoundHandler(), publicAnswers())
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	_, err := env.f.Fetch(ctx, "https://example.com/")
+	if !errors.Is(err, context.Canceled) || errors.Is(err, ErrTimeout) {
+		t.Fatalf("Fetch error = %v, want context.Canceled", err)
+	}
+}
+
+// fakeClock is a clock whose sleep advances it and records the wait.
+type fakeClock struct {
+	mu     sync.Mutex
+	t      time.Time
+	sleeps []time.Duration
+}
+
+func (c *fakeClock) now() time.Time {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.t
+}
+
+func (c *fakeClock) sleep(_ context.Context, d time.Duration) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.sleeps = append(c.sleeps, d)
+	c.t = c.t.Add(d)
+	return nil
+}
+
+func (c *fakeClock) advance(d time.Duration) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.t = c.t.Add(d)
+}
+
+func (c *fakeClock) waits() []time.Duration {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return append([]time.Duration(nil), c.sleeps...)
+}
+
+func installClock(f *Fetcher) *fakeClock {
+	c := &fakeClock{t: time.Date(2026, 10, 5, 18, 0, 0, 0, time.UTC)}
+	f.now = c.now
+	f.sleep = c.sleep
+	return c
+}
+
+func TestFetchCachesByURL(t *testing.T) {
+	t.Parallel()
+	var mu sync.Mutex
+	hits := map[string]int{}
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		mu.Lock()
+		hits[r.URL.Path]++
+		n := hits[r.URL.Path]
+		mu.Unlock()
+		if r.URL.Path == "/missing" {
+			http.NotFound(w, r)
+			return
+		}
+		fmt.Fprintf(w, "%s #%d", r.URL.Path, n)
+	}), publicAnswers())
+	clock := installClock(env.f)
+
+	fetch := func(path string) string {
+		t.Helper()
+		page, err := env.f.Fetch(t.Context(), "https://example.com"+path)
+		if err != nil {
+			t.Fatalf("Fetch %s: %v", path, err)
+		}
+		return string(page.Body)
+	}
+	if got := fetch("/a"); got != "/a #1" {
+		t.Errorf("first /a = %q", got)
+	}
+	if got := fetch("/a"); got != "/a #1" {
+		t.Errorf("cached /a = %q, want the first response", got)
+	}
+	if got := fetch("/b"); got != "/b #1" {
+		t.Errorf("/b = %q, want its own response", got)
+	}
+	clock.advance(CacheTTL)
+	if got := fetch("/a"); got != "/a #2" {
+		t.Errorf("/a after CacheTTL = %q, want a refetch", got)
+	}
+
+	for range 2 {
+		var se *StatusError
+		if _, err := env.f.Fetch(t.Context(), "https://example.com/missing"); !errors.As(err, &se) || se.Status != http.StatusNotFound {
+			t.Fatalf("Fetch /missing error = %v, want StatusError 404", err)
+		}
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	if hits["/missing"] != 2 {
+		t.Errorf("/missing was requested %d times, want 2 (errors are not cached)", hits["/missing"])
+	}
+}
+
+func TestCacheEvictsOldest(t *testing.T) {
+	t.Parallel()
+	f := newFetcher(&stubResolver{}, nil, nil)
+	clock := installClock(f)
+	for i := range CacheSize + 1 {
+		f.store(fmt.Sprintf("https://example.com/%d", i), Page{})
+		clock.advance(time.Second)
+	}
+	if len(f.cache) != CacheSize {
+		t.Errorf("cache holds %d pages, want %d", len(f.cache), CacheSize)
+	}
+	if _, ok := f.cached("https://example.com/0"); ok {
+		t.Error("oldest page was not evicted")
+	}
+	if _, ok := f.cached(fmt.Sprintf("https://example.com/%d", CacheSize)); !ok {
+		t.Error("newest page is missing")
+	}
+}
+
+func TestFetchRateLimitsPerHost(t *testing.T) {
+	t.Parallel()
+	answers := map[string][][]string{
+		"example.com":         {{publicIP}},
+		"recipes.example.com": {{publicIP2}},
+	}
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "ok")
+	}), answers)
+	clock := installClock(env.f)
+
+	for _, raw := range []string{"https://example.com/1", "https://recipes.example.com/1", "https://example.com/2", "https://example.com/3"} {
+		if _, err := env.f.Fetch(t.Context(), raw); err != nil {
+			t.Fatalf("Fetch %s: %v", raw, err)
+		}
+	}
+	// example.com/2 waits a full interval; example.com/3 waits another.
+	// recipes.example.com has its own slot and never waits.
+	got := clock.waits()
+	if len(got) != 2 || got[0] != HostInterval || got[1] != HostInterval {
+		t.Errorf("waits = %v, want [%s %s]", got, HostInterval, HostInterval)
+	}
+}
+
+func TestFetchCacheIsNotSharedWithCallers(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "tacos")
+	}), publicAnswers())
+
+	for range 2 {
+		page, err := env.f.Fetch(t.Context(), "https://example.com/tacos")
+		if err != nil {
+			t.Fatalf("Fetch: %v", err)
+		}
+		if string(page.Body) != "tacos" {
+			t.Fatalf("body = %q, want the cached page unchanged", page.Body)
+		}
+		// Overwrite the returned body: neither the fetched nor the cached
+		// copy may reach the cache.
+		copy(page.Body, "XXXXX")
+	}
+}
+
+// TestFetchCanceledWaitHoldsNoSlot covers a burst of fetches whose
+// rate-limit waits end with their context: they must not push back the
+// next request that is allowed through.
+func TestFetchCanceledWaitHoldsNoSlot(t *testing.T) {
+	t.Parallel()
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "ok")
+	}), publicAnswers())
+	clock := installClock(env.f)
+
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/first"); err != nil {
+		t.Fatalf("first Fetch: %v", err)
+	}
+	env.f.sleep = func(context.Context, time.Duration) error { return context.DeadlineExceeded }
+	for i := range 5 {
+		if _, err := env.f.Fetch(t.Context(), fmt.Sprintf("https://example.com/burst/%d", i)); !errors.Is(err, context.DeadlineExceeded) {
+			t.Fatalf("burst Fetch %d error = %v, want context.DeadlineExceeded", i, err)
+		}
+	}
+	env.f.sleep = clock.sleep
+	clock.advance(HostInterval)
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/after"); err != nil {
+		t.Fatalf("Fetch after the burst: %v", err)
+	}
+	if got := clock.waits(); len(got) != 0 {
+		t.Errorf("waits = %v, want none: canceled waits held slots", got)
+	}
+}
+
+// TestFetchUnsentRequestHoldsNoSlot covers requests to a host with no
+// previous request that are never sent — canceled before Fetch, or failing
+// to connect — and checks that the next request is not delayed.
+func TestFetchUnsentRequestHoldsNoSlot(t *testing.T) {
+	t.Parallel()
+	const unroutableIP = "93.184.215.99"
+	answers := map[string][][]string{"example.com": {{unroutableIP}, {publicIP}}}
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, "ok")
+	}), answers)
+	clock := installClock(env.f)
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	if _, err := env.f.Fetch(ctx, "https://example.com/canceled"); !errors.Is(err, context.Canceled) {
+		t.Fatalf("canceled Fetch error = %v, want context.Canceled", err)
+	}
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/unroutable"); err == nil || !strings.Contains(err.Error(), "no route") {
+		t.Fatalf("unroutable Fetch error = %v, want a dial failure", err)
+	}
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/next"); err != nil {
+		t.Fatalf("next Fetch: %v", err)
+	}
+	if got := clock.waits(); len(got) != 0 {
+		t.Errorf("waits = %v, want none: unsent requests held slots", got)
+	}
+	if got := env.dials(); len(got) != 2 {
+		t.Errorf("dials = %v, want the unroutable and the next request only", got)
+	}
+}
+
+// TestReleaseWakesWaiter checks that a request waiting behind a slot that
+// is handed back goes ahead at once instead of sleeping out the interval.
+func TestReleaseWakesWaiter(t *testing.T) {
+	t.Parallel()
+	f := newFetcher(&stubResolver{}, nil, nil)
+	release, err := f.waitTurn(t.Context(), "example.com")
+	if err != nil {
+		t.Fatalf("first waitTurn: %v", err)
+	}
+	done := make(chan error, 1)
+	start := time.Now()
+	go func() {
+		_, err := f.waitTurn(t.Context(), "example.com")
+		done <- err
+	}()
+	time.Sleep(50 * time.Millisecond)
+	release()
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("waiting waitTurn: %v", err)
+		}
+		if waited := time.Since(start); waited >= HostInterval {
+			t.Errorf("waiter took %s, want less than %s", waited, HostInterval)
+		}
+	case <-time.After(HostInterval / 2):
+		t.Fatal("waiter still waiting after the slot was handed back")
+	}
+}
+
+// stallingResolver holds its first lookup until unstall is closed, then
+// answers from the stub.
+type stallingResolver struct {
+	*stubResolver
+	once    sync.Once
+	stalled chan struct{}
+	unstall chan struct{}
+}
+
+func (r *stallingResolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) {
+	first := false
+	r.once.Do(func() { first = true })
+	if first {
+		close(r.stalled)
+		<-r.unstall
+	}
+	return r.stubResolver.LookupIPAddr(ctx, host)
+}
+
+// TestFetchRateLimitsSendsNotSetup covers a request that stalls in DNS
+// while a later request to the same host is sent: the stalled request must
+// still be sent a full HostInterval after the other one, because the slot
+// is taken when the request is written, not when it starts.
+func TestFetchRateLimitsSendsNotSetup(t *testing.T) {
+	t.Parallel()
+	var clock *fakeClock
+	var mu sync.Mutex
+	sentAt := map[string]time.Time{}
+	env := newTestEnv(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		mu.Lock()
+		sentAt[r.URL.Path] = clock.now()
+		mu.Unlock()
+		fmt.Fprint(w, "ok")
+	}), publicAnswers())
+	resolver := &stallingResolver{
+		stubResolver: env.resolver,
+		stalled:      make(chan struct{}),
+		unstall:      make(chan struct{}),
+	}
+	env.f.resolver = resolver
+	clock = installClock(env.f)
+
+	slow := make(chan error, 1)
+	go func() {
+		_, err := env.f.Fetch(t.Context(), "https://example.com/slow")
+		slow <- err
+	}()
+	<-resolver.stalled
+	if _, err := env.f.Fetch(t.Context(), "https://example.com/fast"); err != nil {
+		t.Fatalf("fast Fetch: %v", err)
+	}
+	close(resolver.unstall)
+	if err := <-slow; err != nil {
+		t.Fatalf("slow Fetch: %v", err)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+	if gap := sentAt["/slow"].Sub(sentAt["/fast"]); gap < HostInterval {
+		t.Errorf("slow request sent %s after fast, want at least %s", gap, HostInterval)
+	}
+	if got := clock.waits(); len(got) != 1 || got[0] != HostInterval {
+		t.Errorf("waits = %v, want [%s]: only the stalled request waits", got, HostInterval)
+	}
+}