Allow raw ip addresses

app/config.py

@@ -5,6 +5,8 @@ from typing import List, Optional
 
 import yaml
 
+from . import dns_resolver
+
 
 class ConfigError(Exception):
     """Raised for any problem with the config file's contents."""
@@ -16,9 +18,12 @@ class Endpoint:
     host: str
     scheme: str  # "http" or "https"
     port: int
-    dns_server: str  # DNS server used to resolve `host` — the system/router
-    # resolver is never consulted, see app/dns_resolver.py
     expected_status: int
+    # DNS server used to resolve `host` — the system/router resolver is
+    # never consulted, see app/dns_resolver.py. Optional (and unused) when
+    # `host` is already a literal IP address, since there's nothing to
+    # resolve in that case.
+    dns_server: Optional[str] = None
     path: str = "/"
     timeout_seconds: Optional[float] = None
     verify_tls: bool = True  # set False for endpoints with self-signed certs
@@ -103,14 +108,22 @@ def load_config(path: str) -> AppConfig:
                 f"{ctx} ('{name}'): scheme must be 'http' or 'https', got '{scheme}'"
             )
 
+        host = _require(ep, "host", ctx)
+        dns_server = ep.get("dns_server") or None
+        if not dns_resolver.is_ip_literal(host) and not dns_server:
+            raise ConfigError(
+                f"{ctx} ('{name}'): 'dns_server' is required when 'host' is not "
+                "a literal IP address"
+            )
+
         timeout = ep.get("timeout_seconds")
         endpoints.append(
             Endpoint(
                 name=name,
-                host=_require(ep, "host", ctx),
+                host=host,
                 scheme=scheme,
                 port=int(_require(ep, "port", ctx)),
-                dns_server=_require(ep, "dns_server", ctx),
+                dns_server=dns_server,
                 expected_status=int(_require(ep, "expected_status", ctx)),
                 path=ep.get("path", "/"),
                 timeout_seconds=float(timeout) if timeout else None,