diff --git a/modules/mosquitto/client.ts b/modules/mosquitto/client.ts
index 4658662..ae8ed2e 100644
--- a/modules/mosquitto/client.ts
+++ b/modules/mosquitto/client.ts
@@ -18,6 +18,7 @@ import { randomBytes } from "node:crypto";
 import { connect as tcpConnect } from "node:net";
 import { readFileSync } from "node:fs";
 import { execFile } from "node:child_process";
+import { basename, dirname } from "node:path";
 import { promisify } from "node:util";
 
 import { missingAcls, parseRoleAcls, staleAcls, wantedAcls } from "./topics.js";
@@ -30,6 +31,14 @@ export interface MqttConn {
   /** The Dynamic Security admin client the runtime authenticates as. */
   readonly adminUser: string;
   readonly adminPassword: string;
+  /**
+   * The broker's own container, when `mosquitto_ctrl` is run inside it rather than from this
+   * machine's packages. The broker's image carries the tool at the broker's version, and inside it
+   * the broker listens on 127.0.0.1:1883 whatever port the machine publishes.
+   */
+  readonly container?: string;
+  /** The broker's image, to seed the security file before the broker has ever started. */
+  readonly image?: string;
 }
 
 export class MosquittoClient {
@@ -53,7 +62,11 @@ export class MosquittoClient {
         "mosquitto host or admin password is not set — mosquitto's own code cannot reach the broker",
       );
     }
-    return new MosquittoClient({ host, port, adminUser, adminPassword: adminPassword ?? "" });
+    return new MosquittoClient({
+      host, port, adminUser, adminPassword: adminPassword ?? "",
+      container: env.MESH_MQTT_CTRL_CONTAINER || undefined,
+      image: env.MESH_MQTT_CTRL_IMAGE || undefined,
+    });
   }
 
   get host(): string {
@@ -84,16 +97,18 @@ export class MosquittoClient {
    * to this single-purpose runtime container; see the module README.
    */
   async ctl(...args: string[]): Promise<string> {
+    const inside = this.conn.container !== undefined;
     const base = [
-      "-h", this.conn.host,
-      "-p", String(this.conn.port),
+      "-h", inside ? "127.0.0.1" : this.conn.host,
+      "-p", inside ? "1883" : String(this.conn.port),
       "-u", this.conn.adminUser,
       "-P", this.conn.adminPassword,
     ];
     let stdout: string;
     let stderr: string;
     try {
-      ({ stdout, stderr } = await run("mosquitto_ctrl", [...base, "dynsec", ...args], {
+      const [command, argv] = this.ctrl([...base, "dynsec", ...args]);
+      ({ stdout, stderr } = await run(command, argv, {
         maxBuffer: 16 << 20,
         timeout: 30_000,
       }));
@@ -240,10 +255,27 @@ export class MosquittoClient {
   async initBootstrapFile(configFile: string): Promise<void> {
     // `dynsec init <file> <admin-username> [admin-password]` is an offline file operation — it does
     // not connect to the broker. The password is a positional argument (omitting it prompts).
+    if (this.conn.image) {
+      // Before the broker has ever started there is no container to enter: a throwaway one from the
+      // broker's own image writes the file into the directory the broker will mount.
+      await run("docker", [
+        "run", "--rm", "--entrypoint", "mosquitto_ctrl",
+        "-v", `${dirname(configFile)}:/mosquitto/data`,
+        this.conn.image,
+        "dynsec", "init", `/mosquitto/data/${basename(configFile)}`, this.conn.adminUser, this.conn.adminPassword,
+      ], { maxBuffer: 16 << 20 });
+      return;
+    }
     await run("mosquitto_ctrl", ["dynsec", "init", configFile, this.conn.adminUser, this.conn.adminPassword], {
       maxBuffer: 16 << 20,
     });
   }
+
+  /** How `mosquitto_ctrl` is run here: inside the broker's container when one is named. */
+  ctrl(argv: string[]): [string, string[]] {
+    if (this.conn.container) return ["docker", ["exec", this.conn.container, "mosquitto_ctrl", ...argv]];
+    return ["mosquitto_ctrl", argv];
+  }
 }
 
 /** Generate a URL-safe password with no argv- or MQTT-hostile characters. */
diff --git a/modules/mosquitto/module.json b/modules/mosquitto/module.json
index 88be665..9308199 100644
--- a/modules/mosquitto/module.json
+++ b/modules/mosquitto/module.json
@@ -92,7 +92,7 @@
       "type": "file",
       "path": "${dir:state}/bootstrap.env",
       "mode": "0600",
-      "content": "MESH_PROVISION_MQTT=127.0.0.1:${port:1883}\nMESH_PROVISION_ADMIN_USER=mesh-admin\nMESH_PROVISION_PASSWORD_FILE=${dir:mesh-state}/admin\nMESH_DYNSEC_FILE=${dir:data}/dynamic-security.json\n"
+      "content": "MESH_PROVISION_MQTT=127.0.0.1:${port:1883}\nMESH_PROVISION_ADMIN_USER=mesh-admin\nMESH_PROVISION_PASSWORD_FILE=${dir:mesh-state}/admin\nMESH_DYNSEC_FILE=${dir:data}/dynamic-security.json\nMESH_MQTT_CTRL_IMAGE=eclipse-mosquitto@sha256:38c0da4f2ef84284d47b3b3eeea1cb3bdeabe81ee10caf0cd5c5ff61ee3ea408\n"
     },
     {
       "id": "bootstrap",
@@ -125,11 +125,6 @@
         "${dir:data}:/mosquitto/data",
         "${dir:state}/mosquitto.conf:/mosquitto/config/mosquitto.conf:ro"
       ]
-    },
-    {
-      "id": "client",
-      "type": "package",
-      "package": "mosquitto"
     }
   ],
   "build": {
@@ -153,7 +148,8 @@
           "MESH_RECEIVES": "${dir:grants}/mesh.json",
           "MESH_PROVISION_MQTT": "127.0.0.1:${port:1883}",
           "MESH_PROVISION_ADMIN_USER": "mesh-admin",
-          "MESH_PROVISION_PASSWORD_FILE": "${dir:mesh-state}/admin"
+          "MESH_PROVISION_PASSWORD_FILE": "${dir:mesh-state}/admin",
+          "MESH_MQTT_CTRL_CONTAINER": "mosquitto"
         }
       }
     ]
diff --git a/modules/mosquitto/test/ctrl.test.ts b/modules/mosquitto/test/ctrl.test.ts
new file mode 100644
index 0000000..495c6b1
--- /dev/null
+++ b/modules/mosquitto/test/ctrl.test.ts
@@ -0,0 +1,19 @@
+// Run after `npm run build`.
+// mosquitto_ctrl runs inside the broker's own container when the manifest names it, so a machine
+// needs no mosquitto package (whose index may be too stale to install from) and the tool always
+// matches the broker's version.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { MosquittoClient } from "../dist/client.js"; // compiled: client.ts uses parameter properties, which type stripping cannot run
+
+const env = { MESH_PROVISION_MQTT: "127.0.0.1:21883", MESH_MQTT_PASSWORD: "pw" };
+
+test("named, the broker's container runs mosquitto_ctrl", () => {
+  const c = MosquittoClient.fromEnv({ ...env, MESH_MQTT_CTRL_CONTAINER: "mosquitto" });
+  assert.deepEqual(c.ctrl(["dynsec", "listClients"]), ["docker", ["exec", "mosquitto", "mosquitto_ctrl", "dynsec", "listClients"]]);
+});
+
+test("unnamed, this machine's mosquitto_ctrl runs", () => {
+  const c = MosquittoClient.fromEnv(env);
+  assert.deepEqual(c.ctrl(["dynsec", "listClients"]), ["mosquitto_ctrl", ["dynsec", "listClients"]]);
+});