feat(config): add swarmctl-host to INI, env, CLI; drop hardcoded bind from systemd unit

[swarm] swarmctl-host → DECNET_SWARMCTL_HOST so operators set the bind
address once in decnet.ini; `decnet swarmctl` and the systemd unit both
resolve it via envvar — no --host/--port pinned on ExecStart.

decnet/cli/init.py

@@ -74,6 +74,7 @@ _CONFIG_PLACEHOLDER = """\
 # master-host = 10.0.0.1
 # syslog-port = 6514
 # swarmctl-port = 8770
+# swarmctl-host = 127.0.0.1
 
 # [logging]
 # system-log = /var/log/decnet/decnet.system.log

decnet/cli/swarmctl.py

@@ -16,8 +16,16 @@ from .utils import console, log
 def register(app: typer.Typer) -> None:
     @app.command()
     def swarmctl(
-        port: int = typer.Option(8770, "--port", help="Port for the swarm controller"),
-        host: str = typer.Option("127.0.0.1", "--host", help="Bind address for the swarm controller"),
+        port: int = typer.Option(
+            8770, "--port",
+            envvar="DECNET_SWARMCTL_PORT",
+            help="Port for the swarm controller. Defaults to [swarm] swarmctl-port from /etc/decnet/decnet.ini, else 8770.",
+        ),
+        host: str = typer.Option(
+            "127.0.0.1", "--host",
+            envvar="DECNET_SWARMCTL_HOST",
+            help="Bind address for the swarm controller. Defaults to [swarm] swarmctl-host from /etc/decnet/decnet.ini, else 127.0.0.1.",
+        ),
         daemon: bool = typer.Option(False, "--daemon", "-d", help="Detach to background as a daemon process"),
         no_listener: bool = typer.Option(False, "--no-listener", help="Do not auto-spawn the syslog-TLS listener alongside swarmctl"),
         tls: bool = typer.Option(False, "--tls", help="Serve over HTTPS with mTLS (required for cross-host worker heartbeats)"),

decnet/config_ini.py

@@ -39,6 +39,7 @@ Shape::
     master-host = 10.0.0.1        # required on agents
     syslog-port = 6514
     swarmctl-port = 8770
+    swarmctl-host = 127.0.0.1     # bind address for `decnet swarmctl`
 
     [logging]
     system-log = /var/log/decnet/decnet.system.log
@@ -120,6 +121,7 @@ _DOMAIN_MAP: dict[str, dict[str, str]] = {
         "master-host": "DECNET_SWARM_MASTER_HOST",
         "syslog-port": "DECNET_SWARM_SYSLOG_PORT",
         "swarmctl-port": "DECNET_SWARMCTL_PORT",
+        "swarmctl-host": "DECNET_SWARMCTL_HOST",
     },
     "logging": {
         "system-log": "DECNET_SYSTEM_LOGS",

decnet/env.py

@@ -114,6 +114,11 @@ DECNET_SWARM_MASTER_HOST: str | None = os.environ.get("DECNET_SWARM_MASTER_HOST"
 DECNET_HOST_UUID: str | None = os.environ.get("DECNET_HOST_UUID")
 DECNET_MASTER_HOST: str | None = os.environ.get("DECNET_MASTER_HOST")
 DECNET_SWARMCTL_PORT: int = _port("DECNET_SWARMCTL_PORT", 8770)
+# Bind address for the master-side swarm controller. Loopback by default —
+# operators flip to 0.0.0.0 (or a specific NIC) on production masters where
+# workers heartbeat in over mTLS from other hosts. Seeded by [swarm]
+# swarmctl-host in /etc/decnet/decnet.ini.
+DECNET_SWARMCTL_HOST: str = os.environ.get("DECNET_SWARMCTL_HOST", "127.0.0.1")
 
 # Ingester batching: how many log rows to accumulate per commit, and the
 # max wait (ms) before flushing a partial batch. Larger batches reduce

deploy/decnet-swarmctl.service.j2

@@ -10,10 +10,12 @@ User={{ user }}
 Group={{ group }}
 WorkingDirectory={{ install_dir }}
 EnvironmentFile=-{{ install_dir }}/.env.local
-# Default bind is loopback — the controller is a master-local orchestrator
-# reached by the CLI and the web dashboard, not by workers.
+# Bind/port resolved from /etc/decnet/decnet.ini ([swarm] swarmctl-host /
+# swarmctl-port) — falls back to 127.0.0.1:8770 when the keys are absent.
+# Pass --host/--port on the ExecStart line only if you want to pin them
+# regardless of what the INI says.
 Environment=DECNET_SYSTEM_LOGS=/var/log/decnet/decnet.swarmctl.log
-ExecStart={{ venv_dir }}/bin/decnet swarmctl --host 127.0.0.1 --port 8770
+ExecStart={{ venv_dir }}/bin/decnet swarmctl
 StandardOutput=append:/var/log/decnet/decnet.swarmctl.log
 StandardError=append:/var/log/decnet/decnet.swarmctl.log
 

tests/config/test_config_ini.py

@@ -302,3 +302,18 @@ swarmctl-port = 9999
     load_ini_config(ini)
     # [master] loaded first, [swarm] lost via setdefault
     assert os.environ["DECNET_SWARMCTL_PORT"] == "9001"
+
+
+def test_swarm_section_seeds_swarmctl_host(monkeypatch, tmp_path):
+    """[swarm] swarmctl-host → DECNET_SWARMCTL_HOST so the systemd unit and
+    `decnet swarmctl` CLI both pick up the operator's bind choice from the
+    INI without anyone passing --host on ExecStart."""
+    _scrub(monkeypatch, "DECNET_MODE", "DECNET_SWARMCTL_HOST", "DECNET_SWARMCTL_PORT")
+    ini = _write_ini(tmp_path, """
+[swarm]
+swarmctl-host = 0.0.0.0
+swarmctl-port = 9000
+""")
+    load_ini_config(ini)
+    assert os.environ["DECNET_SWARMCTL_HOST"] == "0.0.0.0"
+    assert os.environ["DECNET_SWARMCTL_PORT"] == "9000"