Write page about serial access (#120)

source/manual/how-tos/serial_access.rst

@@ -0,0 +1,42 @@
+=============
+Serial Access
+=============
+
+Besides the web frontend, SSH and a locally connected monitor (if your device supports it), OPNsense can also be
+controlled via serial. Accessing OPNsense via serial is similar to accessing via SSH, but unlike SSH, the system can
+be accessed at any time, even when OPNsense is not running. This makes it especially useful for installing OPNsense,
+as well as for emergency troubleshooting when you accidentally cut off internet access.
+
+--------------------------------
+Connecting to the serial console
+--------------------------------
+
+If you already installed OPNsense via a non-serial installer, serial access needs to be turned on. To do this, open
+the web interface, navigate to **System->Settings->Administration**, scroll down to 'Console' and set the primary or
+secondary console to 'Serial console'. Note: this is **only** necessary if you already installed OPNsense, and did not
+use the serial installer to do so. In all other cases (accessing BIOS, running the serial installer, connecting to an
+installation that was done via serial), serial access is already available.
+
+On Unix-like systems, you can connect to the serial console using the ``screen`` program, with a baud rate of 115200.
+The device name can differ per system and per serial device. Examples of names are:
+
+* /dev/ttyS0 (serial port, Linux)
+* /dev/ttyUSB0 (usb-to-serial, Linux)
+* /dev/cuau0 (serial port, FreeBSD)
+* /dev/cuaU0 (usb-to-serial, FreeBSD)
+* /dev/tty.usbmodem1112421 (usb-to-serial, macOS)
+* COM1, COM2, ... (Windows)
+
+For example, on the Deciso DEC630, accessed from macOS, the device is named ``/dev/tty.usbmodem1112421``. Entering
+the serial console thus involves opening a terminal and executing the following instruction:
+
+::
+
+  screen /dev/tty.usbmodem1112421 115200
+
+If OPNsense is running, you will now be asked for your username and password. The credentials are the same as those
+used for SSH.
+
+A thing to note is that the screen won't always auto-update. If you connect and see no output, try pressing `Enter`
+first before checking the other (more complex) possibilities. Another thing is that, when connecting via ``screen``,
+you might not be able to scroll (but you can still pipe the output through a pager like ``more`` or ``less``).

source/manual/howtos.rst

@@ -7,6 +7,7 @@ How-tos
    :titlesonly:
    :glob:
 
+   how-tos/serial_access
    how-tos/accounting
    how-tos/bind
    how-tos/c-icap

source/manual/install.rst

@@ -240,7 +240,8 @@ Download the installation image from one of the mirrors listed on the `OPNsense
 The easiest method of installation is the USB-memstick installer. If
 your target platform has a serial interface choose the "serial image.
 64-bit and 32-bit install images are provided. The following examples
-apply to both.
+apply to both. If you need to know more about using the serial interface,
+consult the :doc:`serial access how-to<how-tos/serial_access>`.
 
 Write the image to a USB flash drive (>=1 GB) or an IDE hard disk,
 either with dd under FreeBSD or under Windows with physdiskwrite