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VirtualBox COM Port Passthrough: Definitive Guide

Last updated Apr 12, 2021

Nowadays there isn’t as much use for COM ports as it once was. And yet, they still can find their practical application and do you a great deal of good.

For example, you’ve got to have at least one serial port or you won’t be able to interact with the vast majority of the kernel debugging tools. In actual practice, it doesn’t have to be a physical COM-port. With the right software tool, you might as well do kernel debugging through a virtual serial port on a virtual machine.

This tutorial will show you how to redirect a serial port from the host to a guest VM in VirtualBox and how to create virtual serial ports in VirtualBox.

Access serial ports in VirtualBox

FlexiHub

Windows, macOS, Linux, Android
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A very easy method of implementing VirtualBox serial port passthrough is with the software tool FlexiHub.
FlexiHub

FlexiHub offers users a simple path to enable serial ports in VirtualBox or any other virtualization platform by redirecting serial port devices to guest virtual machines over the network.

FlexiHub works by creating virtual connections and redirecting serial port data over a network to the VirtualBox virtual machines. There are no complex parameters to configure. With FlexiHub, you can add a VirtualBox serial port with just one click.

How to share COM ports to VirtualBox

1
Register a FlexiHub account and start a free demo evaluation.
 flexihub registration
2
Install FlexiHub on the VirtualBox host with a physical serial port that will be redirected. After that, install FlexiHub on the VirtualBox virtual machine that will connect to the host’s serial port.
 install flexihub
3
Launch the app both on the host and guest VM and log into your account.
 log into flexihub
4
The serial devices that are available for connection will be displayed in the FlexiHub’s interface.
 virtualbox com port to network
5
On the guest machine, click ‘Connect’ in the app’s interface to access a serial port in VirtualBox.
 access serial port in virtualbox

From this point on, serial devices connected to the VirtualBox serial ports will be redirected to the virtual environment without any extra configuration.

For the video instructions on how to share serial ports to VirtualBox, you can check out this brief guide:

share serial ports to VirtualBox

How to configure a virtual serial port on VirtualBox


Another method of setting up COM port forwarding on VirtualBox can be done with capabilities built into the platform. Virtual serial ports are enabled with the guest OS presented with a standard UART device to ensure compatibility. The connection is configurable and provides the ability to send and receive data. Configuration details vary based on the host OS being used.

virtualbox serial port

VirtualBox serial port setup can be accomplished through the Settings tab or the VBox Manage command. This allows you to create up to four virtual serial ports in each virtual machine. You are required to configure the Port Mode and Port Number.

You should use standard values when defining the Port Number as it describes the serial port that the virtual machine will use. You also have the option of creating a user-defined serial port by providing an IRQ and I/O base address.

Port Mode defines how the virtual port is connected. The Port Mode options are:

  1. Disconnected;
  2. Connected to Host Device;
  3. Connected to Host Pipe;
  4. RAW file;
  5. TCP socket.
  • Disconnected indicates that the virtual session can see the device but cannot use it. It is as if there is no cabled connection to the peripheral.
  • The Host Device Connection requires establishing a physical connection between the virtual port and the hardware port on the host machine. On a Windows system, this will usually be a name such as COM1. Conversely, on a Linux machine, you will be connecting to /dev/ttyS0 or something similar. VirtualBox will then redirect all data transmitted through the virtual port to the physical device.
  • When using a Host Pipe connection to enable VirtualBox to share COM ports, the configuration must be done to connect the virtual port and a software pipe on the host operating system.
    virtualbox serial port
The specifics vary depending on the type of physical host on which VirtualBox is running.
Windows
Windows
The data gets transmitted through a named pipe. The pipe name needs to be registered with the following format \\.\pipe\<name> where <name> is used as a machine identifier. It can be a user-defined name chosen for simplicity or any reason.
 
macOS
macOS
The connection is made through a local domain socket. Using tmp as the socket filename is a common practice that provides the VirtualBox user with privileges such as writing to the pipe.
 
Linux
Linux
Linux users can make use of multiple tools that establish a connection to a local domain socket. One that is widely used and available in many Linux distros is Socat.
VirtualBox operates under the premise that the socket or pipe it references exists.
When you are implementing a direct connection between two virtual machines, one machine creates a pipe or socket and the second VM attaches to it.
 
  1. Raw File: The output of a virtual serial port can be directed to a file. This can be useful when gathering diagnostic or troubleshooting information. Any file can be used as long as the VirtualBox user has the required level of permission to create and write to it.

  2. TCP Socket: A TCP socket can act as a TP client or server and facilitates sending VirtualBox COM port traffic to networks using the TCP/IP protocol. This method enables a remote machine to be directly connected to a guest serial port over a TCP network.

    • TCP Server: The current Pipe/Socket checkbox needs to be deselected on the TCP server and the address/path field is used to specify the port number. This number can be 23 or 2023, and on UNIX systems needs to be more than 1024.
    • TCP Client: The "Connect to Existing Pipe/Socket" box needs to be checked to put the TCP socket into client mode. When creating a virtual null-modem cable connection the other end connects over the TCP network by designating the hostname’s port in the Path/Address field.

You can configure four serial ports on each machine and multiple port numbers can be chosen.

We hope the details in this guide demonstrate how to share a COM port in VirtualBox. FlexiHub may present the simplest way to add COM ports in VirtualBox, but as you can see there are other options. Once configured, you can enjoy the full functionality of serial devices from within a virtual session as if you were directly connected to the peripheral equipment.


FlexiHub

Requirements: Windows 7/8/10, Server 2008 R2/2012/2016/2019
Size: 8.82MB
Version: 5.1.13941 ()
User rating: (4.8 based on 78+ users)