console-server: add documentation

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diff --git a/docs/services/console-server.rst b/docs/services/console-server.rst
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+.. _console_server:
+
+##############
+Console-Server
+##############
+
+Starting of with VyOS 1.3 (equuleus) we added support for running VyOS as an
+Out-of-Band Management device which provides remote access by means of SSH to
+directly attached serial interfaces.
+
+Serial interfaces can be any interface which is directly connected to the CPU
+or chipset (mostly known as a ttyS interface in Linux) or any other USB to
+serial converter (Prolific PL2303 or FTDI FT232/FT4232 based chips).
+
+If you happened to use a Cisco NM-16A - Sixteen Port Async Network Module or
+NM-32A - Thirty-two Port Async Network Module - this is your VyOS replacement.
+
+Setup
+=====
+
+In the past serial interface have been defined as ttySx and ttyUSBx where x was
+an instance number of the serial interface. It was discovered that from system
+boot to system boot the mapping of USB based serial interfaces will differ,
+depending which driver was loaded first by the operating system. This will become
+rather painful if you not only have serial interfaces for a console server
+connected but in addition also a serial backed :ref:`wwan-interface`.
+
+To overcome this issue and the fact that in almost 50% of all cheap USB to serial
+converters there is no serial number programmed, the USB to serial interface is
+now directly identified by the USB root bridge and bus it connects to. This
+somehow mimics the new network interface definitions we see in recend Linux
+distributions.
+
+For additional details you can refer to https://phabricator.vyos.net/T2490.
+
+.. opcmd:: show system usb
+
+  Retrieve a tree like representation of all connected USB devices.
+
+  .. note:: If a device is unplugged and re-plugged it will receive a new
+    Port, Dev, If identification.
+
+  .. code-block:: none
+
+    vyos@vyos:~$ show system usb
+    /:  Bus 03.Port 1: Dev 1, Class=root_hub, Driver=ehci-pci/2p, 480M
+        |__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/4p, 480M
+            |__ Port 3: Dev 4, If 0, Class=Vendor Specific Class, Driver=qcserial, 480M
+            |__ Port 3: Dev 4, If 2, Class=Vendor Specific Class, Driver=qcserial, 480M
+            |__ Port 3: Dev 4, If 3, Class=Vendor Specific Class, Driver=qcserial, 480M
+            |__ Port 3: Dev 4, If 8, Class=Vendor Specific Class, Driver=qmi_wwan, 480M
+    /:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 5000M
+    /:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 480M
+        |__ Port 1: Dev 2, If 0, Class=Vendor Specific Class, Driver=pl2303, 12M
+        |__ Port 2: Dev 3, If 0, Class=Hub, Driver=hub/4p, 480M
+            |__ Port 4: Dev 5, If 2, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+            |__ Port 4: Dev 5, If 0, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+            |__ Port 4: Dev 5, If 3, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+            |__ Port 4: Dev 5, If 1, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+            |__ Port 3: Dev 4, If 0, Class=Hub, Driver=hub/4p, 480M
+                |__ Port 3: Dev 6, If 0, Class=Hub, Driver=hub/4p, 480M
+                    |__ Port 4: Dev 8, If 2, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                    |__ Port 4: Dev 8, If 0, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                    |__ Port 4: Dev 8, If 3, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                    |__ Port 4: Dev 8, If 1, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                |__ Port 4: Dev 7, If 3, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                |__ Port 4: Dev 7, If 1, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                |__ Port 4: Dev 7, If 2, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+                |__ Port 4: Dev 7, If 0, Class=Vendor Specific Class, Driver=ftdi_sio, 480M
+
+
+.. opcmd:: show system usb
+
+  Retrieve a list and description of all connected USB serial devices. The device name
+  displayed, e.g. `usb0b2.4p1.0` can be directly used when accessing the serial console
+  as console-server device.
+
+  .. code-block:: none
+
+    vyos@vyos$ show system usb serial
+    Device           Model               Vendor
+    ------           ------              ------
+    usb0b1.3p1.0     MC7710              Sierra Wireless, Inc.
+    usb0b1.3p1.2     MC7710              Sierra Wireless, Inc.
+    usb0b1.3p1.3     MC7710              Sierra Wireless, Inc.
+    usb0b1p1.0       USB-Serial_Controller_D Prolific Technology, Inc.
+    usb0b2.3.3.4p1.0 Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.3.4p1.1 Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.3.4p1.2 Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.3.4p1.3 Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.4p1.0   Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.4p1.1   Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.4p1.2   Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.3.4p1.3   Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.4p1.0     Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.4p1.1     Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.4p1.2     Quad_RS232-HS       Future Technology Devices International, Ltd
+    usb0b2.4p1.3     Quad_RS232-HS       Future Technology Devices International, Ltd
+
+
+Configuration
+=============
+
+Between computers, the most common configuration used was "8N1": eight bit
+characters, with one start bit, one stop bit, and no parity bit. Thus 10 Baud
+times are used to send a single character, and so dividing the signalling
+bit-rate by ten results in the overall transmission speed in characters per
+second. This is also the default setting if none of those options are defined.
+
+.. cfgcmd:: service console-server <device> data-bits
+
+  Configure either seven or eight data bits. This defaults to eight data
+  bits if left unconfigured.
+
+.. cfgcmd:: service console-server <device> description
+
+  A user friendly description identifying the connected peripheral.
+
+.. cfgcmd:: service console-server <device> parity [even | odd | none]
+
+  Set the parity option for the console. If unset this will default to none.
+
+.. cfgcmd:: service console-server <device> stop-bits
+
+  Configure either one or two stop bits. This defaults to one stop bits if
+  left unconfigured.
+
+.. cfgcmd:: service console-server <device> speed <baudrate>
+
+  .. note:: USB to serial converters will handle most of their work in software
+     so you should be carefull with the selected baudrate as some times they
+     can't cope with the expected speed.
+
+Remote Access
+-------------
+
+Each individual configured console-server device can be directly exposed to
+the outside world. A user can directly connect via SSH to the configured
+port.
+
+.. cfgcmd:: service console-server <device> ssh port <port>
+
+  Accept SSH connections for the given `<device>` on TCP port `<port>`.
+  After successfull authentication the user will be directly dropped to
+  the connected serial device.
+
+  .. hint:: Multiple users can connect to the same serial device but only
+     one is allowed to write to the console port.
+
+Operation
+=========
+
+.. opcmd:: show console-server ports
+
+  Show configured serial ports and their respective interface configuration.
+
+  .. code-block:: none
+
+    vyos@vyos:~$ show console-server ports
+     usb0b2.4p1.0             on /dev/serial/by-bus/usb0b2.4p1.0@ at   9600n
+
+.. opcmd:: show console-server user
+
+  Show currently connected users.
+
+  .. code-block::
+
+    vyos@vyos:~$ show console-server user
+     usb0b2.4p1.0               up   vyos@localhost
+
+
+.. opcmd:: connect console-server <device>
+
+  Locally connect to serial port identified by `<device>`.
+
+  .. code-block:: none
+
+    vyos@vyos-r1:~$ connect console-server usb0b2.4p1.0
+    [Enter `^Ec?' for help]
+    [-- MOTD -- VyOS Console Server]
+
+    vyos-r2 login:
+
+  .. hint:: Multiple users can connect to the same serial device but only
+     one is allowed to write to the console port.
+
+  .. hint:: The sequence ``^Ec?`` translates to: ``Ctrl+E c ?``. To quit
+     the session use: ``Ctrl+E c .``
diff --git a/docs/services/index.rst b/docs/services/index.rst
index cc6b96ae..76520b52 100644
--- a/docs/services/index.rst
+++ b/docs/services/index.rst
@@ -10,6 +10,7 @@ This chapter describes the available system/network services provided by VyOS.
    :maxdepth: 1
 
    conntrack
+   console-server
    dhcp
    dns-forwarding
    dynamic-dns