Index: bin/stty/modes.c
===================================================================
--- bin/stty/modes.c
+++ bin/stty/modes.c
@@ -91,6 +91,8 @@
 	{ "-rtsflow",	0, CRTS_IFLOW },
 	{ "mdmbuf",	MDMBUF, 0 },
 	{ "-mdmbuf",	0, MDMBUF },
+	{ "rtsdtr",	0, CNO_RTSDTR },
+	{ "-rtsdtr",	CNO_RTSDTR, 0 },
 	{ NULL,		0, 0 },
 };
 
Index: bin/stty/print.c
===================================================================
--- bin/stty/print.c
+++ bin/stty/print.c
@@ -184,6 +184,12 @@
 	put("-dsrflow", CDSR_OFLOW, 0);
 	put("-dtrflow", CDTR_IFLOW, 0);
 	put("-mdmbuf", MDMBUF, 0);	/* XXX mdmbuf ==  dtrflow */
+	if (on(CNO_RTSDTR))
+		bput("-rtsdtr");
+	else {
+		if (fmt >= BSD)
+			bput("rtsdtr");
+	}
 
 	/* special control characters */
 	cc = tp->c_cc;
Index: bin/stty/stty.1
===================================================================
--- bin/stty/stty.1
+++ bin/stty/stty.1
@@ -145,6 +145,8 @@
 control.
 .It Cm crtscts Pq Fl crtscts
 Enable (disable) RTS/CTS flow control.
+.It Cm rtsdtr Pq Fl -rtsdtr
+Enable (disable) asserting RTS/DTR on open.
 .El
 .Ss Input Modes:
 This corresponds to the c_iflag in the termios structure.
Index: share/man/man4/termios.4
===================================================================
--- share/man/man4/termios.4
+++ share/man/man4/termios.4
@@ -1185,6 +1185,8 @@
 /* RTS flow control of input */
 .It Dv MDMBUF
 /* flow control output via Carrier */
+.It Dv CNO_RTSDTR
+/* Do not assert RTS or DTR automatically */
 .El
 .Pp
 The
@@ -1266,6 +1268,12 @@
 .Dv MDMBUF
 is set then output flow control is controlled by the state
 of Carrier Detect.
+.Pp
+If
+.Dv CNO_RTSDTR
+is set then the RTS and DTR lines will not be asserted when the device
+is opened.
+As a result, this flag is only useful on initial-state devices.
 .Pp
 If the object for which the control modes are set is not an asynchronous
 serial connection, some of the modes may be ignored; for example, if an
Index: sys/dev/uart/uart_tty.c
===================================================================
--- sys/dev/uart/uart_tty.c
+++ sys/dev/uart/uart_tty.c
@@ -285,13 +285,16 @@
 		parity = UART_PARITY_NONE;
 	if (UART_PARAM(sc, t->c_ospeed, databits, stopbits, parity) != 0)
 		return (EINVAL);
-	UART_SETSIG(sc, SER_DDTR | SER_DTR);
+	if ((t->c_cflag & CNO_RTSDTR) == 0)
+		UART_SETSIG(sc, SER_DDTR | SER_DTR);
 	/* Set input flow control state. */
 	if (!sc->sc_hwiflow) {
 		if ((t->c_cflag & CRTS_IFLOW) && sc->sc_isquelch)
 			UART_SETSIG(sc, SER_DRTS);
-		else
-			UART_SETSIG(sc, SER_DRTS | SER_RTS);
+		else {
+			if ((t->c_cflag & CNO_RTSDTR) == 0)
+				UART_SETSIG(sc, SER_DRTS | SER_RTS);
+		}
 	} else
 		UART_IOCTL(sc, UART_IOCTL_IFLOW, (t->c_cflag & CRTS_IFLOW));
 	/* Set output flow control state. */
Index: sys/dev/usb/serial/umcs.c
===================================================================
--- sys/dev/usb/serial/umcs.c
+++ sys/dev/usb/serial/umcs.c
@@ -499,7 +499,9 @@
 	 * Enable DTR/RTS on modem control, enable modem interrupts --
 	 * documented
 	 */
-	sc->sc_ports[pn].sc_mcr = MCS7840_UART_MCR_DTR | MCS7840_UART_MCR_RTS | MCS7840_UART_MCR_IE;
+	sc->sc_ports[pn].sc_mcr = MCS7840_UART_MCR_IE;
+	if (ucom->sc_tty == NULL || (ucom->sc_tty->t_termios.c_cflag & CNO_RTSDTR) == 0)
+		sc->sc_ports[pn].sc_mcr |= MCS7840_UART_MCR_DTR | MCS7840_UART_MCR_RTS;
 	if (umcs7840_set_UART_reg_sync(sc, pn, MCS7840_UART_REG_MCR, sc->sc_ports[pn].sc_mcr))
 		return;
 
Index: sys/dev/usb/serial/usb_serial.c
===================================================================
--- sys/dev/usb/serial/usb_serial.c
+++ sys/dev/usb/serial/usb_serial.c
@@ -796,7 +796,8 @@
 	    &sc->sc_start_task[0].hdr, 
 	    &sc->sc_start_task[1].hdr);
 
-	ucom_modem(tp, SER_DTR | SER_RTS, 0);
+	if (sc->sc_tty == NULL || (sc->sc_tty->t_termios.c_cflag & CNO_RTSDTR) == 0)
+		ucom_modem(tp, SER_DTR | SER_RTS, 0);
 
 	ucom_ring(sc, 0);
 
Index: sys/kern/tty.c
===================================================================
--- sys/kern/tty.c
+++ sys/kern/tty.c
@@ -93,7 +93,7 @@
 			FLUSHO|NOKERNINFO|NOFLSH)
 #define TTYSUP_CFLAG	(CIGNORE|CSIZE|CSTOPB|CREAD|PARENB|PARODD|\
 			HUPCL|CLOCAL|CCTS_OFLOW|CRTS_IFLOW|CDTR_IFLOW|\
-			CDSR_OFLOW|CCAR_OFLOW)
+			CDSR_OFLOW|CCAR_OFLOW|CNO_RTSDTR)
 
 #define	TTY_CALLOUT(tp,d) (dev2unit(d) & TTYUNIT_CALLOUT)
 
@@ -332,7 +332,8 @@
 		if (TTY_CALLOUT(tp, dev) || dev == dev_console)
 			tp->t_termios.c_cflag |= CLOCAL;
 
-		ttydevsw_modem(tp, SER_DTR|SER_RTS, 0);
+		if ((tp->t_termios.c_cflag & CNO_RTSDTR) == 0)
+			ttydevsw_modem(tp, SER_DTR|SER_RTS, 0);
 
 		error = ttydevsw_open(tp);
 		if (error != 0)
Index: sys/sys/_termios.h
===================================================================
--- sys/sys/_termios.h
+++ sys/sys/_termios.h
@@ -143,6 +143,7 @@
 #define	CDTR_IFLOW	0x00040000	/* DTR flow control of input */
 #define	CDSR_OFLOW	0x00080000	/* DSR flow control of output */
 #define	CCAR_OFLOW	0x00100000	/* DCD flow control of output */
+#define	CNO_RTSDTR	0x00200000	/* Do not assert RTS or DTR automatically */
 #endif