LibvirtConsoleManagement

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Libvirt Console Management

This page describes an approach to managing virtual machine text consoles, which leverages the conserver daemon

Background information

With Xen, LXC, KVM, etc hypervisors there is typically a serial console or a paravirtualized log for each virtual machine. Typically this is exposed on the host as a dynamically allocated psuedo TTY, but they can also be configured to use a UNIX or TCP socket, a FIFO pipe or (output only) to a plain file.

The consoles are typically used as a text-mode interactive shell, or to log all guest console messages, or both. There are a number of limitations with the console functionality that is exposed natively by the hypervisors

  • It is impossible to configure a console to both provide an interactive pTTY, and log all data to a file concurrently.
  • If configured to log to a file, the file will grow without any bounds
  • The console TTY names are unstable across restarts of the virtual machine, since they are dynamically allocated by the host kernel
  • The console TTYs only exist while the VM is running
  • There is typically no native, secure, remote TCP access

Architecture design

The problems described with the native hypervisor console configuration can broadly be addressed by leveraging the conserver daemon, which is distributed as a standard part of most Linux distributions. The main integration pain point is that the conserver daemon uses a static configuration file, while virtual machines can come & go at any moment.

There is a need, therefore, to have a way of automatically generating/updating the conserver configuration file on the fly to deal with dynamic virtual machines. This can be achieved by having a process which listens out for libvirt domain lifecycle events and triggers an update of the conserver configuration file at appropriate times.