middleware: take the client address from the right of X-Forwarded-For (closes #181)

realIP took the first X-Forwarded-For entry, which the client itself
can write, so behind a proxy that appends to the header a client chose
the address dnswatcher logs and the /metrics rate limit counts. It now
walks the entries from the right past trusted proxies, using the
existing trusted-proxy check, and takes the first that is not one; the
leftmost when all are. All X-Forwarded-For header lines are read as one
list, since a proxy may add its own line instead of appending to the
client's. An empty entry where the client address belongs falls back
to the peer address, as an empty first entry did before. X-Real-IP is
unchanged.

Model: opus-5-5

internal/middleware/middleware_test.go

@@ -482,3 +482,99 @@ func TestMetricsRateLimitKeysOnClientAddress(t *testing.T) {
 		})
 	}
 }
+
+// TestRealIP checks which address realIP takes as the client's. Each
+// element of forwardedFor is sent as an X-Forwarded-For header line of
+// its own, and 198.51.100.9 is always an entry the client wrote itself.
+func TestRealIP(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		remoteAddr   string
+		xRealIP      string
+		forwardedFor []string
+		want         string
+	}{
+		{
+			"untrusted peer, both headers ignored",
+			"198.51.100.1:4000",
+			"203.0.113.1",
+			[]string{"203.0.113.2"},
+			"198.51.100.1",
+		},
+		{
+			"X-Real-IP from a trusted proxy wins",
+			"10.0.0.1:4000",
+			"203.0.113.1",
+			[]string{"203.0.113.2"},
+			"203.0.113.1",
+		},
+		{
+			"client's own entry, then the one the proxy added",
+			"10.0.0.1:4000",
+			"",
+			[]string{"198.51.100.9, 203.0.113.1"},
+			"203.0.113.1",
+		},
+		{
+			"several trusted proxies",
+			"10.0.0.1:4000",
+			"",
+			[]string{"198.51.100.9, 203.0.113.1, 10.0.0.3, 10.0.0.2"},
+			"203.0.113.1",
+		},
+		{
+			"proxy adds a header line of its own",
+			"10.0.0.1:4000",
+			"",
+			[]string{"198.51.100.9", "203.0.113.1"},
+			"203.0.113.1",
+		},
+		{
+			"every entry a trusted proxy",
+			"10.0.0.1:4000",
+			"",
+			[]string{"10.0.0.3, 10.0.0.2"},
+			"10.0.0.3",
+		},
+		{
+			"empty where the client address belongs",
+			"10.0.0.1:4000",
+			"",
+			[]string{"203.0.113.1, , 10.0.0.2"},
+			"10.0.0.1",
+		},
+		{
+			"no headers from a trusted proxy",
+			"10.0.0.1:4000",
+			"",
+			nil,
+			"10.0.0.1",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/", nil,
+			)
+			req.RemoteAddr = tt.remoteAddr
+
+			if tt.xRealIP != "" {
+				req.Header.Set("X-Real-IP", tt.xRealIP)
+			}
+
+			for _, line := range tt.forwardedFor {
+				req.Header.Add("X-Forwarded-For", line)
+			}
+
+			got := middleware.RealIP(req)
+			if got != tt.want {
+				t.Errorf("realIP = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}