diff --git a/test/src/Makefile.am b/test/src/Makefile.am
index 226c37bd..a54bbae1 100644
--- a/test/src/Makefile.am
+++ b/test/src/Makefile.am
@@ -1,4 +1,4 @@
 noinst_PROGRAMS = serv
 
-serv_SOURCES    = serv.c
+serv_SOURCES    = serv.c sd-daemon.c sd-daemon.h
 serv_CPPFLAGS   = -D_XOPEN_SOURCE=600 -D_BSD_SOURCE -D_GNU_SOURCE -D_DEFAULT_SOURCE
diff --git a/test/src/sd-daemon.c b/test/src/sd-daemon.c
new file mode 100644
index 00000000..67bea2d5
--- /dev/null
+++ b/test/src/sd-daemon.c
@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: MIT-0 */
+
+/*
+ * systemd notify protocol, supporting readiness notification on startup
+ * and reloading, according to the protocol defined at:
+ * https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
+ *
+ * This protocol is guaranteed to be stable as per:
+ * https://systemd.io/PORTABILITY_AND_STABILITY/
+ */
+
+#define _GNU_SOURCE 1
+#include <errno.h>
+#include <inttypes.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <time.h>
+#include <unistd.h>
+
+/*
+ * NOTE: As per spec., this function does not return POSIX OK(0) but
+ *       rather "a positive value", i.e., 1.
+ */
+int sd_notify(int unset_environment, const char *state)
+{
+	struct sockaddr_un sun = {
+		.sun_family = AF_UNIX,
+	};
+	size_t len, sun_len;
+	const char *path;
+	ssize_t written;
+	int sd;
+
+	if (!state)
+		return -EINVAL;
+
+	len = strlen(state);
+	if (len == 0)
+		return -EINVAL;
+
+	/* If the variable is not set, the protocol is a noop */
+	path = getenv("NOTIFY_SOCKET");
+	if (!path)
+		return 0;	/* Not set? Nothing to do */
+
+	/* Only AF_UNIX is supported, with path or abstract sockets */
+	if (path[0] != '/' && path[0] != '@')
+		return -EAFNOSUPPORT;
+
+	/* Ensure there is room for NUL byte */
+	sun_len = strlen(path);
+	if (sun_len >= sizeof(sun.sun_path))
+		return -E2BIG;
+
+	strlcpy(sun.sun_path, path, sizeof(sun.sun_path));
+	if (sun.sun_path[0] == '@')
+		sun.sun_path[0] = 0;
+
+	sd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+	if (sd == -1)
+		return -errno;
+
+	if (connect(sd, (struct sockaddr *)&sun, offsetof(struct sockaddr_un, sun_path) + sun_len) == -1) {
+		close(sd);
+		return -errno;
+	}
+
+	if (unset_environment)
+		unsetenv("NOTIFY_SOCKET");
+
+	written = write(sd, state, len);
+	if (written != (ssize_t) len) {
+		close(sd);
+		return written < 0 ? -errno : -EPROTO;
+	}
+
+	return 1;
+}
diff --git a/test/src/sd-daemon.h b/test/src/sd-daemon.h
new file mode 100644
index 00000000..b9ed4a0d
--- /dev/null
+++ b/test/src/sd-daemon.h
@@ -0,0 +1,6 @@
+#ifndef SD_DAEMON_H_
+#define SD_DAEMON_H_
+
+int sd_notify(int unset_environment, const char *state);
+
+#endif	/* SD_DAEMON_H_ */
diff --git a/test/src/serv.c b/test/src/serv.c
index fbb7031c..b8ece7b9 100644
--- a/test/src/serv.c
+++ b/test/src/serv.c
@@ -16,6 +16,7 @@
 #include <sysexits.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
+#include "sd-daemon.h"
 
 #define PROGNM "serv"
 
@@ -160,7 +161,7 @@ static int usage(int rc)
 		" -h       Show help text (this)\n"
 		" -i IDENT Change process identity, incl. logs, pidfile, etc.\n"
 		" -n       Run in foreground\n"
-		" -N SOCK  Send 'READY=1\\n' on $NOTIFY_SOCKET or SOCK\n"
+		" -N SOCK  Send '\\n' on SOCK (integer), for s6 readiness\n"
 		" -p       Create PID file despite running in foreground\n"
 		" -P FILE  Create PID file using FILE\n"
 		" -r SVC   Call initctl to restart service SVC (self)\n"
@@ -170,6 +171,8 @@ static int usage(int rc)
 		"to let the rest of the system know it's done.  When the program runs\n"
 		"in the foreground it does not create a PID file by default.\n"
 		"\n"
+		"When acting as a systemd daemon, this program expects NOTIFY_SOCKET\n"
+		"\n"
 		"Regardless of how This daemon is started it provides a single service\n"
 		"to others, spice ... guarded by sandworms.\n",
 		ident);
@@ -256,13 +259,13 @@ int main(int argc, char *argv[])
 	if (do_pidfile > 0)
 		pidfile(pidfn);
 
-	if (!do_notify) {
-		const char *sock = getenv("NOTIFY_SOCKET");
+	if (!do_notify && getenv("NOTIFY_SOCKET"))
+		do_notify = 1;
 
-		/* systemd style */
-		if (sock)
-			do_notify = atoi(sock);
-	}
+	if (do_notify)
+		inf("Will notify %s ...", notify_s6 ? "s6" : "systemd");
+	else
+		inf("No notify socket ...");
 
 	if (do_crash) {
 		inf("Simulating crash, exiting with code %d", EX_SOFTWARE);
@@ -279,15 +282,24 @@ int main(int argc, char *argv[])
 
 		if (reloading) {
 			if (do_notify > 0) {
+				/* Issue #343: see notify.sh test for more details */
 				inf("Delaying notify by 3 seconds for notify.sh ...");
 				sleep(3);
-				inf("Notifying Finit on socket %d, READY=1", do_notify);
-				if (write(do_notify, "READY=1\n", 8) == -1)
-					err(1, "Failed sending ready notification to Finit");
+
 				if (notify_s6) {
+					inf("Notifying Finit on socket %d", do_notify);
+					if (write(do_notify, "\n", 1) < 1)
+						err(1, "Failed sending ready notification to Finit");
 					inf("s6 notify, closing socket %d ...", do_notify);
-					close(do_notify);
+					if (close(do_notify))
+						err(1, "Failed closing notify socket");
 					do_notify = 0;
+				} else {
+					int rc;
+
+					inf("Notifying Finit on NOTIFY_SOCKET");
+					rc = sd_notify(0, "READY=1");
+					inf("sd_notify () => %d", rc);
 				}
 			}
 			if (do_pidfile > 0)