firewall: add default value for file based zones

roles/firewall/files/firewall

@@ -5,13 +5,21 @@ from dataclasses import dataclass
 from enum import Enum
 from graphlib import TopologicalSorter
 from ipaddress import IPv4Address, IPv4Network, IPv6Network
+from pathlib import Path
 from typing import Generic, Iterator, TypeAlias, TypeVar
 
 import nft
 from nftables import Nftables
-from pydantic import (BaseModel, Extra, FilePath, IPvAnyNetwork,
-                      ValidationError, conint, parse_obj_as, root_validator,
-                      validator)
+from pydantic import (
+    BaseModel,
+    Extra,
+    IPvAnyNetwork,
+    ValidationError,
+    conint,
+    parse_obj_as,
+    root_validator,
+    validator,
+)
 from yaml import safe_load
 
 # ==========[ PYDANTIC ]========================================================
@@ -75,9 +83,14 @@ class PortRange(str):
 ZoneName: TypeAlias = str
 
 
+class ZoneEntryFile(RestrictiveBaseModel):
+    path: Path
+    default: None | AutoSet[IPvAnyNetwork] = None
+
+
 class ZoneEntry(RestrictiveBaseModel):
     addrs: AutoSet[IPvAnyNetwork] = AutoSet()
-    file: FilePath | None = None
+    file: ZoneEntryFile | None = None
     negate: bool = False
     zones: AutoSet[ZoneName] = AutoSet()
 
@@ -189,10 +202,6 @@ class Firewall(RestrictiveBaseModel):
 # ==========[ ZONES ]===========================================================
 
 
-class ZoneFile(RestrictiveBaseModel):
-    __root__: AutoSet[IPvAnyNetwork]
-
-
 @dataclass(eq=True, frozen=True)
 class ResolvedZone:
     addrs: set[IPvAnyNetwork]
@@ -213,17 +222,24 @@ def resolve_zones(yaml_zones: dict[ZoneName, ZoneEntry]) -> Zones:
             )
 
         elif yaml_zones[name].file is not None:
-            with open(yaml_zones[name].file, "r") as file:
-                try:
-                    yaml_addrs = ZoneFile(__root__=safe_load(file))
-                except Exception as e:
-                    raise Exception(
-                        f"YAML parsing of the included file '{yaml_zones[name].file}' failed: {e}"
-                    )
+            file_entry = yaml_zones[name].file
 
-            zones[name] = ResolvedZone(
-                yaml_addrs.__root__, yaml_zones[name].negate
+            try:
+                with open(file_entry.path, "r") as file:
+                    try:
+                        addrs = parse_obj_as(
+                            AutoSet[IPvAnyNetwork], safe_load(file)
                         )
+                    except ValidationError as e:
+                        raise ValueError(
+                            f"parsing of '{yaml_zones[name].file}' failed: {e}"
+                        )
+            except OSError as e:
+                if file_entry.default is None:
+                    raise e
+                addrs = file_entry.default
+
+            zones[name] = ResolvedZone(addrs, yaml_zones[name].negate)
 
         elif yaml_zones[name].zones:
             addrs: set[IPvAnyNetwork] = set()
@@ -231,7 +247,7 @@ def resolve_zones(yaml_zones: dict[ZoneName, ZoneEntry]) -> Zones:
             for subzone in yaml_zones[name].zones:
                 if yaml_zones[subzone].negate:
                     raise ValueError(
-                        f"subzone '{subzone}' of zone '{name}' cannot be negated"
+                        f"subzone '{subzone}' of '{name}' cannot be negated"
                     )
                 addrs.update(yaml_zones[subzone].addrs)