core: fix ParseNetworkAddress port-range span off-by-one (#7975)

A port range is inclusive on both ends, so a range from start to end
spans (end - start + 1) ports. The old (end - start) > maxPortSpan
check let 0-65535 through even though that spans 65536 ports, one
above the 65535 cap. Include the trailing port in the count.

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>

listeners.go

@@ -361,7 +361,7 @@ func ParseNetworkAddressWithDefaults(addr, defaultNetwork string, defaultPort ui
 		if end < start {
 			return NetworkAddress{}, fmt.Errorf("end port must not be less than start port")
 		}
-		if (end - start) > maxPortSpan {
+		if (end-start)+1 > maxPortSpan {
 			return NetworkAddress{}, fmt.Errorf("port range exceeds %d ports", maxPortSpan)
 		}
 	}

listeners_port_span_test.go

@@ -0,0 +1,60 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddy
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestParseNetworkAddress_PortSpanBoundary is the regression test for
+// the off-by-one in the port-range span check. A range like "0-65535"
+// spans 65536 ports (inclusive on both ends) and should be rejected,
+// but the old (end-start)>maxPortSpan check let it through. The maximum
+// valid full-16-bit range "1-65535" (65535 ports) must still be accepted.
+func TestParseNetworkAddress_PortSpanBoundary(t *testing.T) {
+	for _, tt := range []struct {
+		name    string
+		addr    string
+		wantErr bool
+	}{
+		{
+			name:    "0-65535 spans 65536 ports and is rejected",
+			addr:    "tcp/:0-65535",
+			wantErr: true,
+		},
+		{
+			name:    "1-65535 spans 65535 ports and is accepted",
+			addr:    "tcp/:1-65535",
+			wantErr: false,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := ParseNetworkAddress(tt.addr)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected error, got nil")
+				}
+				if !strings.Contains(err.Error(), "port range exceeds") {
+					t.Errorf("expected 'port range exceeds' error, got: %v", err)
+				}
+				return
+			}
+			if err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}