fix: DNS check timeout with IPv6-broken authoritative NS

Add IPv4 fallback for authoritative NS queries in query_dns().
When dig @ns returns no useful answer (e.g. IPv6 transport to
the NS times out), retry with -4 to force IPv4. Also limits
timeout and tries (+timeout=10 +tries=2) to avoid hanging.

Fixes #851

cmdeploy/src/cmdeploy/remote/rdns.py

@@ -64,11 +64,13 @@ def get_dkim_entry(mail_domain, pre_command, dkim_selector):
     )
 
 
-def query_dns(typ, domain):
+def query_dns(typ, domain, shell_exec=None):
+    if shell_exec is None:
+        shell_exec = shell
     # Get autoritative nameserver from the SOA record.
     soa_answers = [
         x.split()
-        for x in shell(
+        for x in shell_exec(
             f"dig -r -q {domain} -t SOA +noall +authority +answer", print=log_progress
         ).split("\n")
     ]
@@ -78,8 +80,27 @@ def query_dns(typ, domain):
     ns = soa[0][4]
 
     # Query authoritative nameserver directly to bypass DNS cache.
-    res = shell(f"dig @{ns} -r -q {domain} -t {typ} +short", print=log_progress)
-    return next((line for line in res.split("\n") if not line.startswith(";")), "")
+    return _dig_authoritative(ns, domain, typ, shell_exec=shell_exec)
+
+
+def _parse_dig_result(output):
+    """Return first non-comment, non-empty line from dig output, or empty string."""
+    lines = [line for line in output.split("\n") if not line.startswith(";")]
+    return next((line for line in lines if line.strip()), "")
+
+
+def _dig_authoritative(ns, domain, typ, shell_exec=None):
+    """Query authoritative NS, falling back to IPv4-only if default fails."""
+    if shell_exec is None:
+        shell_exec = shell
+
+    # limit timeout and tries to not hang on a broken default NS
+    cmd = f"dig @{ns} -r -q {domain} -t {typ} +short +timeout=10 +tries=2"
+    result = _parse_dig_result(shell_exec(cmd, print=log_progress))
+    if result:
+        return result
+    # Fallback: force IPv4 transport (handles broken IPv6 to NS)
+    return _parse_dig_result(shell_exec(cmd + " -4", print=log_progress))
 
 
 def check_zonefile(zonefile, verbose=True):

cmdeploy/src/cmdeploy/tests/test_dns.py

@@ -214,3 +214,59 @@ class TestZonefileChecks:
         assert not mockout.captured_red
         assert "correct" in mockout.captured_green[0]
         assert not mockout.captured_red
+
+
+class TestDigAuthoritative:
+    @pytest.fixture
+    def dig_auth(self):
+        """Helper that calls _dig_authoritative with a recording shell function."""
+        calls = []
+
+        def run(first_result, ipv4_result=None):
+            def shell(cmd, print=print):
+                calls.append(cmd)
+                if "-4" in cmd:
+                    return ipv4_result or ""
+                return first_result
+
+            result = remote.rdns._dig_authoritative(
+                "ns1.example.", "example.com", "A", shell_exec=shell
+            )
+            return result, calls
+
+        return run
+
+    def test_ipv4_fallback_on_error(self, dig_auth):
+        """Fallback to -4 when first query returns only error lines."""
+        result, calls = dig_auth(
+            first_result=(
+                ";; communications error to 2a01:4f8:10a:1044::80#53: timed out\n"
+                ";; communications error to 2a01:4f8:10a:1044::80#53: timed out\n"
+            ),
+            ipv4_result="1.2.3.4",
+        )
+        assert len(calls) == 2
+        assert "-4" not in calls[0]
+        assert "-4" in calls[1]
+        assert result == "1.2.3.4"
+
+    def test_no_fallback_on_success(self, dig_auth):
+        """No -4 fallback when first query returns a valid answer."""
+        result, calls = dig_auth(first_result="1.2.3.4")
+        assert result == "1.2.3.4"
+        assert len(calls) == 1
+
+    def test_fallback_with_mixed_error_lines(self, dig_auth):
+        """Fallback triggers when output has only error lines mixed with empty."""
+        result, calls = dig_auth(
+            first_result=(
+                ";; communications error to 2a01:4f8::80#53: timed out\n"
+                "\n"
+                ";; communications error to 2a01:4f8::80#53: timed out\n"
+            ),
+            ipv4_result=";; some warning\n1.2.3.4",
+        )
+        assert len(calls) == 2
+        assert result == "1.2.3.4"
+
+