diff --git a/share/man/man4/ng_source.4 b/share/man/man4/ng_source.4
--- a/share/man/man4/ng_source.4
+++ b/share/man/man4/ng_source.4
@@ -32,7 +32,7 @@
 .\" Author: Dave Chapeskie
 .\" $FreeBSD$
 .\"
-.Dd March 1, 2007
+.Dd January 18, 2021
 .Dt NG_SOURCE 4
 .Os
 .Sh NAME
@@ -91,7 +91,14 @@
 .Xr ng_ether 4
 node manually.
 .Pp
-Once interface is configured, upon receipt of a
+If the node is connected to a netgraph network, which does not
+terminate in a real
+.Xr ng_ether 4
+interface, limit the packet injection rate explicitly with the
+.Va NGM_SOURCE_SETPPS
+control message.
+.Pp
+Upon receipt of a
 .Dv NGM_SOURCE_START
 control message the node starts sending
 the previously queued packets out the
diff --git a/sys/netgraph/ng_source.c b/sys/netgraph/ng_source.c
--- a/sys/netgraph/ng_source.c
+++ b/sys/netgraph/ng_source.c
@@ -683,8 +683,8 @@
 static int
 ng_source_start(sc_p sc, uint64_t packets)
 {
-	if (sc->output_ifp == NULL) {
-		printf("ng_source: start without iface configured\n");
+	if (sc->output_ifp == NULL && sc->stats.maxPps == 0) {
+		printf("ng_source: start without iface or pps configured\n");
 		return (ENXIO);
 	}