fix(nano): cap upstream response headers at 16 KiB

The hasJoined and name-lookup clients limited the body to 64 KiB but
left headers at the transport default of 1 MiB. A configured root could
answer with a megabyte of headers and stall the body, holding a few MiB
of heap per in-flight login for the full five seconds; enough parallel
logins take down the host, and every source's logins with it.

Both clients now share a transport with MaxResponseHeaderBytes set to
16 KiB. Real roots come nowhere near it: Mojang's sessionserver sends
338 bytes of headers, LittleSkin 752, api.mojang.com 327. A source over
the cap fails the request and the resolver moves on to the next one.

The new subtest puts a source with 64 KiB of headers and a valid profile
ahead of an honest one and expects the honest player. Without the cap
the padded source wins.

internal/api/handlers_hasjoined.go

@@ -45,6 +45,7 @@ var felisAuthNS = uuid.NewSHA1(uuid.NameSpaceURL, []byte("nano.felis.lolicon.bes
 // wasted Mojang round-trip; add parallel fan-out only if login latency bites.
 var authHTTPClient = &http.Client{
 	Timeout:   5 * time.Second,
+	Transport: upstreamTransport,
 	// A redirect is not a hasJoined answer. Following one would let a configured root point
 	// this host at any URL it can reach — this listener included, where each hop re-runs the
 	// whole source scan inside the same login's timeout. The 3xx is returned as-is and the
@@ -52,6 +53,16 @@ var authHTTPClient = &http.Client{
 	CheckRedirect: func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse },
 }
 
+// upstreamTransport caps response headers, which the 64 KiB body limit does not cover. The
+// default allows 1 MiB, so a root that sends that much and then stalls the body pins a few
+// MiB per in-flight login for the whole timeout, and enough parallel logins OOM the host
+// for every source. Real roots answer in well under 1 KiB of headers.
+var upstreamTransport = func() *http.Transport {
+	t := http.DefaultTransport.(*http.Transport).Clone()
+	t.MaxResponseHeaderBytes = 16 << 10
+	return t
+}()
+
 // AuthSource is one upstream Yggdrasil root in the multiplexer's priority list (config
 // order = priority). URL is the full hasJoined endpoint the query string is appended to.
 // Identity marks the authoritative source (Mojang) whose UUIDs are trusted as-is; every
@@ -193,7 +204,7 @@ var mojangProfileAPI = "https://api.mojang.com/users/profiles/minecraft/"
 // SECOND Mojang round-trip on a third-party login (the identity leg already spent one), and
 // api.mojang.com is exactly what is unreliable from the networks these servers sit on. A
 // slow answer falls back to the cache instead of holding the login open.
-var profileHTTPClient = &http.Client{Timeout: 2 * time.Second}
+var profileHTTPClient = &http.Client{Timeout: 2 * time.Second, Transport: upstreamTransport}
 
 // A name's premium status changes on human timescales, not per login, so it is cached — but
 // asymmetrically, because the two directions have very different costs. "Taken" is nearly

internal/api/handlers_hasjoined_test.go

@@ -247,6 +247,29 @@ func TestHasJoined(t *testing.T) {
 		}
 	})
 
+	// A root whose headers blow past the cap is dropped like any failed source, even when the
+	// body behind them is a well-formed profile.
+	t.Run("oversized response headers skip the source", func(t *testing.T) {
+		stubMojangNames(t)
+		bloated := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("X-Padding", strings.Repeat("a", 64<<10))
+			_ = json.NewEncoder(w).Encode(map[string]any{"id": notchMojangID, "name": "Steve0"})
+		}))
+		t.Cleanup(bloated.Close)
+		honest := fakeYgg(t, "0123456789abcdef0123456789abcdef", "Steve0")
+		api := newTestAPI(newFakeRepo(), newFakeCluster())
+		api.AuthSources = []AuthSource{
+			{Tag: "evil", Prefix: "EV", URL: bloated.URL},
+			{Tag: "littleskin", Prefix: "LS", URL: honest.URL},
+		}
+
+		w := getHasJoined(api.InternalHandler(), "Steve0", "abc")
+		want := undashed(uuid.NewMD5(felisAuthNS, []byte("littleskin:0123456789abcdef0123456789abcdef")))
+		if w.Code != http.StatusOK || profileOf(t, w).ID != want {
+			t.Fatalf("code = %d body = %q, want the next source's player", w.Code, w.Body.String())
+		}
+	})
+
 	// The reused bar gate: a barred CANONICAL UUID is rejected at the resolver, so a
 	// reclaimed squatter stays out even on a consumer with no limbo plugin. Keyed on the
 	// dashed canonical (post-rewrite), the same form Repo.ReclaimUsername stores.