diff --git a/README.md b/README.md
index ba07340..4dd480d 100644
--- a/README.md
+++ b/README.md
@@ -387,6 +387,7 @@ The following configuration settings may be applied (a `Y` in column `$` means a
 | `OTR_USER`            |  Y    |               | MQTT username
 | `OTR_PASS`            |  Y    |               | MQTT password
 | `OTR_QOS`             |       | `2`           | MQTT QoS
+| `OTR_CLIENTID`        |       | hostname+pid  | MQTT ClientID (override with -i)
 | `OTR_HTTPHOST`        |       | `localhost`   | Address for the HTTP module to bind to
 | `OTR_HTTPPORT`        |       | `8083`        | Port number of the HTTP module to bind to
 | `OTR_LUASCRIPT`       |       |               | Path to the Lua script
diff --git a/etc/ot-recorder.default b/etc/ot-recorder.default
index 34502d2..86a466b 100644
--- a/etc/ot-recorder.default
+++ b/etc/ot-recorder.default
@@ -40,6 +40,12 @@
 
 # OTR_QOS=2
 
+# -----------------------------------------------------
+# MQTT clientid (default is constant+hostname+pid)
+#
+
+# OTR_CLIENTID=""
+
 # -----------------------------------------------------
 # Address for the HTTP module to bind to (default: localhost)
 #
diff --git a/misc.c b/misc.c
index ac584a9..6deff8e 100644
--- a/misc.c
+++ b/misc.c
@@ -125,6 +125,11 @@ void get_defaults(char *filename, struct udata *ud)
 
 	if (config_lookup_string(cf, "OTR_STORAGEDIR", &value) != CONFIG_FALSE)
 		strcpy(STORAGEDIR, value);
+
+	if (ud == NULL) {
+		/* being invoked by ocat; return */
+		return;
+	}
 #if WITH_MQTT
 	if (config_lookup_string(cf, "OTR_HOST", &value) != CONFIG_FALSE) {
 		if (ud->hostname) free(ud->hostname);
@@ -144,6 +149,10 @@ void get_defaults(char *filename, struct udata *ud)
 	if (config_lookup_int(cf, "OTR_QOS", &ival) != CONFIG_FALSE) {
 		ud->qos = ival;
 	}
+	if (config_lookup_string(cf, "OTR_CLIENTID", &value) != CONFIG_FALSE) {
+		if (ud->clientid) free(ud->clientid);
+		ud->clientid = strdup(value);
+	}
 
 	/* Topics is a blank-separated string of words; split and add to JSON array */
 	if (config_lookup_string(cf, "OTR_TOPICS", &value) != CONFIG_FALSE) {
diff --git a/recorder.c b/recorder.c
index 5c3dc47..978a2f1 100644
--- a/recorder.c
+++ b/recorder.c
@@ -1136,6 +1136,7 @@ int main(int argc, char **argv)
 	udata.password		= NULL;
 	udata.hostname		= strdup("localhost");
 	udata.port		= 1883;
+	udata.clientid		= NULL;
 	udata.topics		= NULL;
 #endif
 	udata.ignoreretained	= TRUE;
@@ -1161,6 +1162,15 @@ int main(int argc, char **argv)
 
 	openlog("ot-recorder", LOG_PID | LOG_PERROR, syslog_facility_code(logfacility));
 
+	utstring_new(clientid);
+	utstring_printf(clientid, "ot-recorder");
+	if (uname(&uts) == 0) {
+		utstring_printf(clientid, "-%s", uts.nodename);
+	}
+	utstring_printf(clientid, "-%d", getpid());
+
+	ud->clientid = strdup(UB(clientid));
+
 	get_defaults(CONFIGFILE, &udata);
 
 
@@ -1190,12 +1200,6 @@ int main(int argc, char **argv)
 		ud->password = strdup(p);
 	}
 
-	utstring_new(clientid);
-	utstring_printf(clientid, "ot-recorder");
-	if (uname(&uts) == 0) {
-		utstring_printf(clientid, "-%s", uts.nodename);
-	}
-	utstring_printf(clientid, "-%d", getpid());
 #endif
 
 	while (1) {
@@ -1267,6 +1271,8 @@ int main(int argc, char **argv)
 			case 'i':
 				utstring_clear(clientid);
 				utstring_printf(clientid, "%s", optarg);
+				free(ud->clientid);
+				ud->clientid = strdup(UB(clientid));
 				break;
 			case 'P':
 				udata.pubprefix = strdup(optarg);	/* TODO: do we want this? */
@@ -1467,7 +1473,7 @@ int main(int argc, char **argv)
 #ifdef WITH_MQTT
 	mosquitto_lib_init();
 
-	mosq = mosquitto_new(UB(clientid), CLEAN_SESSION, (void *)&udata);
+	mosq = mosquitto_new(ud->clientid, CLEAN_SESSION, (void *)&udata);
 	if (!mosq) {
 		fprintf(stderr, "Error: Out of memory.\n");
 		mosquitto_lib_cleanup();
@@ -1515,7 +1521,7 @@ int main(int argc, char **argv)
 
 	olog(LOG_INFO, "connecting to MQTT on %s:%d as clientID %s %s TLS",
 		ud->hostname, ud->port,
-		UB(clientid),
+		ud->clientid,
 		(cafile) ? "with" : "without");
 
 	rc = mosquitto_connect(mosq, ud->hostname, ud->port, 60);
@@ -1613,6 +1619,7 @@ int main(int argc, char **argv)
 	mosquitto_destroy(mosq);
 	mosquitto_lib_cleanup();
 	free(ud->hostname);
+	free(ud->clientid);
 #endif
 
 	return (0);
diff --git a/udata.h b/udata.h
index 4033187..f325b6d 100644
--- a/udata.h
+++ b/udata.h
@@ -19,6 +19,7 @@ struct udata {
 	int port;			/* MQTT port */
 	char *username;			/* MQTT user */
 	char *password;			/* MQTT password */
+	char *clientid;			/* MQTT clientid */
 #endif
 	int skipdemo;			/* True if _demo users are to be skipped */
 	int revgeo;			/* True (default) if we should do reverse Geo lookups */