Proxmox VE : accessing COM port from the host in a VM


Proxmox 使用 qemu/kvm manager (qm) 工具管理 qemu server,「qm」是它的主要用來管理的命令。由 qm 的手冊得知「args」是 VM CONFIGURATION 中的一條屬性設定,其目的是將參數傳遞至「kvm」。

因此在 qemu-server 設定檔中加上一行「args: -serial /dev/ttyS0」,則表示在「kvm」依設定檔配置虛擬機會跟據「args」裏的參數,也虛擬一個serial port 裝置,虛擬的裝置仍映射至 Host 端 /dev/ttyS0 的裝置上。

args: ...
    Note: this option is for experts only. It allows you to pass arbitrary arguments to kvm, for example:

    args: -no-reboot -no-hpet

而 kvm 配置虛擬機硬體 serial port 裝置時,也有多樣的設定方式不是只能從 Host 端直接映射。完整設定方式可由 man  qemu 查詢得知。

       -serial dev
           Redirect the virtual serial port to host character device dev. The default device is "vc" in
           graphical mode and "stdio" in non graphical mode.

           This option can be used several times to simulate up to 4 serial ports.

           Use "-serial none" to disable all serial ports.

           Available character devices are:

               Virtual console. Optionally, a width and height can be given in pixel with


               It is also possible to specify width or height in characters:


           pty [Linux only] Pseudo TTY (a new PTY is automatically allocated)

               No device is allocated.

               void device

               [Linux only] Use host tty, e.g. /dev/ttyS0. The host serial port parameters are set according
               to the emulated ones.

               [Linux only, parallel port only] Use host parallel port N. Currently SPP and EPP parallel port
               features can be used.

               Write output to filename. No character can be read.

               [Unix only] standard input/output

               name pipe filename

               [Windows only] Use host serial port n

               This implements UDP Net Console.  When remote_host or src_ip are not specified they default to
       When not using a specified src_port a random port is automatically chosen.

               If you just want a simple readonly console you can use "netcat" or "nc", by starting QEMU
               with: "-serial udp::4555" and nc as: "nc -u -l -p 4555". Any time QEMU writes something to
               that port it will appear in the netconsole session.

               If you plan to send characters back via netconsole or you want to stop and start QEMU a lot of
               times, you should have QEMU use the same source port each time by using something like
               "-serial udp::4555@4556" to QEMU. Another approach is to use a patched version of netcat which
               can listen to a TCP port and send and receive characters via udp.  If you have a patched
               version of netcat which activates telnet remote echo and single char transfer, then you can
               use the following options to step up a netcat redirector to allow telnet on port 5555 to
               access the QEMU port.

               "QEMU Options:"
                   -serial udp::4555@4556

               "netcat options:"
                   -u -P 4555 -L -t -p 5555 -I -T

               "telnet options:"
                   localhost 5555

               The TCP Net Console has two modes of operation.  It can send the serial I/O to a location or
               wait for a connection from a location.  By default the TCP Net Console is sent to host at the
               port.  If you use the server option QEMU will wait for a client socket application to connect
               to the port before continuing, unless the "nowait" option was specified.  The "nodelay" option
               disables the Nagle buffering algorithm.  If host is omitted, is assumed. Only one TCP
               connection at a time is accepted. You can use "telnet" to connect to the corresponding
               character device.

               "Example to send tcp console to port 4444"
                   -serial tcp:

               "Example to listen and wait on port 4444 for connection"
                   -serial tcp::4444,server

               "Example to not wait and listen on ip port 4444"
                   -serial tcp:,server,nowait

               The telnet protocol is used instead of raw tcp sockets.  The options work the same as if you
               had specified "-serial tcp".  The difference is that the port acts like a telnet server or
               client using telnet option negotiation.  This will also allow you to send the MAGIC_SYSRQ
               sequence if you use a telnet that supports sending the break sequence.  Typically in unix
               telnet you do it with Control-] and then type "send break" followed by pressing the enter key.

               A unix domain socket is used instead of a tcp socket.  The option works the same as if you had
               specified "-serial tcp" except the unix domain socket path is used for connections.

               This is a special option to allow the monitor to be multiplexed onto another serial port.  The
               monitor is accessed with key sequence of Control-a and then pressing c. See monitor access
               pcsys_keys in the -nographic section for more keys.  dev_string should be any one of the
               serial devices specified above.  An example to multiplex the monitor onto a telnet server
               listening on port 4444 would be:

               "-serial mon:telnet::4444,server,nowait"
               Braille device.  This will use BrlAPI to display the braille output on a real or fake device.

               Three button serial mouse. Configure the guest to use Microsoft protocol.

Proxmox VE : accessing COM port from the host in a VM

Posted on January 5, 2010

If you want to access the COM/serial port of your host machine from a KVM virtual machine in Proxmox VE, simply do the following :

vim /etc/qemu-server/104.conf
where 104 is the ID of the VM

add “args: -serial /dev/ttyS0″ to the end of the file

It should look like this :

name: testVM
ide2: debian-500-i386-netinst.iso,media=cdrom
smp: 1
vlan0: rtl8139=XX:XX:XX:XX:XX:XX
bootdisk: ide0
ide0: vm-104-disk.qcow2
ostype: other
memory: 256
args: -serial /dev/ttyS0

If the VM is already running, you may have to stop the VM completely and start it again for it to see the COM port.

If you simply reboot the machine, it may not see it.

Proxmox VE :



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