Index: lib/libc/sys/getsockopt.2
===================================================================
--- lib/libc/sys/getsockopt.2
+++ lib/libc/sys/getsockopt.2
@@ -311,6 +311,9 @@
 The default value for
 .Dv SO_SNDLOWAT
 is set to a convenient size for network efficiency, often 1024.
+This socket option is applicable only to
+.Dv SOCK_STREAM
+type sockets.
 .Pp
 .Dv SO_RCVLOWAT
 is an option to set the minimum count for input operations.
@@ -328,6 +331,9 @@
 Receive calls may still return less than the low water mark if an error
 occurs, a signal is caught, or the type of data next in the receive queue
 is different from that which was returned.
+This socket option is applicable only to
+.Dv SOCK_STREAM
+type sockets.
 .Pp
 .Dv SO_SNDTIMEO
 is an option to set a timeout value for output operations.
@@ -583,9 +589,10 @@
 .Fa optlen
 is not in a valid part of the process address space.
 .It Bq Er EINVAL
-Installing an
+Use of a socket option not applicable to this kind of socket was attempted.
+This includes, but is not limited to: attempt to install an
 .Xr accept_filter 9
-on a non-listening socket was attempted.
+on a non-listening socket and attempt to set watermarks on non-stream sockets.
 .It Bq Er ENOMEM
 A memory allocation failed that was required to service the request.
 .El
Index: sys/kern/uipc_socket.c
===================================================================
--- sys/kern/uipc_socket.c
+++ sys/kern/uipc_socket.c
@@ -3137,10 +3137,15 @@
 			so->so_user_cookie = val32;
 			break;
 
-		case SO_SNDBUF:
-		case SO_RCVBUF:
 		case SO_SNDLOWAT:
 		case SO_RCVLOWAT:
+			if (so->so_type != SOCK_STREAM) {
+				error = EINVAL;
+				goto bad;
+			}
+			/* FALLTHROUGH */
+		case SO_SNDBUF:
+		case SO_RCVBUF:
 			error = sooptcopyin(sopt, &optval, sizeof optval,
 			    sizeof optval);
 			if (error)