Connection URIs

Since libvirt supports many different kinds of virtualization (often referred to as "drivers" or "hypervisors"), we need a way to be able to specify which driver a connection refers to. Additionally we may want to refer to a driver on a remote machine over the network.

To this end, libvirt uses URIs as used on the Web and as defined in RFC 2396. This page documents libvirt URIs.

Specifying URIs to libvirt

The URI is passed as the name parameter to virConnectOpen or virConnectOpenReadOnly. For example:

virConnectPtr conn = virConnectOpenReadOnly ("test:///default");

Configuring URI aliases

To simplify life for administrators, it is possible to setup URI aliases in a libvirt client configuration file. The configuration file is /etc/libvirt/libvirt.conf for the root user, or $XDG_CONFIG_HOME/libvirt/libvirt.conf for any unprivileged user. In this file, the following syntax can be used to setup aliases

uri_aliases = [
  "hail=qemu+ssh://root@hail.cloud.example.com/system",
  "sleet=qemu+ssh://root@sleet.cloud.example.com/system",
]

A URI alias should be a string made up from the characters a-Z, 0-9, _, -. Following the = can be any libvirt URI string, including arbitrary URI parameters. URI aliases will apply to any application opening a libvirt connection, unless it has explicitly passed the VIR_CONNECT_NO_ALIASES parameter to virConnectOpenAuth. If the passed in URI contains characters outside the allowed alias character set, no alias lookup will be attempted.

Default URI choice

If the URI passed to virConnectOpen* is NULL, then libvirt will use the following logic to determine what URI to use.

  1. The environment variable LIBVIRT_DEFAULT_URI
  2. The client configuration file uri_default parameter
  3. Probe each hypervisor in turn until one that works is found

Specifying URIs to virsh, virt-manager and virt-install

In virsh use the -c or --connect option:

virsh -c test:///default list

If virsh finds the environment variable VIRSH_DEFAULT_CONNECT_URI set, it will try this URI by default. Use of this environment variable is, however, deprecated now that libvirt supports LIBVIRT_DEFAULT_URI itself.

When using the interactive virsh shell, you can also use the connect URI command to reconnect to another hypervisor.

In virt-manager use the -c or --connect=URI option:

virt-manager -c test:///default

In virt-install use the --connect=URI option:

virt-install --connect=test:///default [other options]

xen:///system URI

This section describes a feature which is new in libvirt > 0.2.3. For libvirt ≤ 0.2.3 use "xen".

To access a Xen hypervisor running on the local machine use the URI xen:///system.

qemu:///... QEMU and KVM URIs

To use QEMU support in libvirt you must be running the libvirtd daemon (named libvirt_qemud in releases prior to 0.3.0). The purpose of this daemon is to manage qemu instances.

The libvirtd daemon should be started by the init scripts when the machine boots. It should appear as a process libvirtd --daemon running as root in the background and will handle qemu instances on behalf of all users of the machine (among other things).

So to connect to the daemon, one of two different URIs is used:

(If you do libvirtd --help, the daemon will print out the paths of the Unix domain socket(s) that it listens on in the various different modes).

KVM URIs are identical. You select between qemu, qemu accelerated and KVM guests in the guest XML as described here.

Remote URIs

Remote URIs have the general form ("[...]" meaning an optional part):

driver[+transport]://[username@][hostname][:port]/[path][?extraparameters]

Either the transport or the hostname must be given in order to distinguish this from a local URI.

Some examples:

Extra parameters

Extra parameters can be added to remote URIs as part of the query string (the part following ?). Remote URIs understand the extra parameters shown below. Any others are passed unmodified through to the back end. Note that parameter values must be URI-escaped.

Name Transports Meaning
name any transport The name passed to the remote virConnectOpen function. The name is normally formed by removing transport, hostname, port number, username and extra parameters from the remote URI, but in certain very complex cases it may be better to supply the name explicitly.
Example: name=qemu:///system
tls_priority tls A vaid GNUTLS priority string
Example: tls_priority=NORMAL:-VERS-SSL3.0
mode unix, ssh, libssh, libssh2
auto
automatically determine the daemon
direct
connect to per-driver daemons
legacy
connect to libvirtd
Can also be set in libvirt.conf as remote_mode
Example: mode=direct
command ssh, ext The external command. For ext transport this is required. For ssh the default is ssh. The PATH is searched for the command.
Example: command=/opt/openssh/bin/ssh
socket unix, ssh, libssh2, libssh The path to the Unix domain socket, which overrides the compiled-in default. For ssh transport, this is passed to the remote netcat command (see next).
Example: socket=/opt/libvirt/run/libvirt/libvirt-sock
netcat ssh, libssh2, libssh The name of the netcat command on the remote machine. The default is nc. For ssh transport, libvirt constructs an ssh command which looks like:
command -p port [-l username] hostname netcat -U socket
where port, username, hostname can be specified as part of the remote URI, and command, netcat and socket come from extra parameters (or sensible defaults).
Example: netcat=/opt/netcat/bin/nc
keyfile ssh, libssh2, libssh The name of the private key file to use to authentication to the remote machine. If this option is not used the default keys are used.
Example: keyfile=/root/.ssh/example_key
no_verify ssh, tls SSH: If set to a non-zero value, this disables client's strict host key checking making it auto-accept new host keys. Existing host keys will still be validated.

TLS: If set to a non-zero value, this disables client checks of the server's certificate. Note that to disable server checks of the client's certificate or IP address you must change the libvirtd configuration.
Example: no_verify=1
no_tty ssh If set to a non-zero value, this stops ssh from asking for a password if it cannot log in to the remote machine automatically (eg. using ssh-agent etc.). Use this when you don't have access to a terminal - for example in graphical programs which use libvirt.
Example: no_tty=1
pkipath tls Specifies x509 certificates path for the client. If any of the CA certificate, client certificate, or client key is missing, the connection will fail with a fatal error.
Example: pkipath=/tmp/pki/client
known_hosts libssh2, libssh Path to the known_hosts file to verify the host key against. LibSSH2 and libssh support OpenSSH-style known_hosts files, although LibSSH2 does not support all key types, so using files created by the OpenSSH binary may result into truncating the known_hosts file. Thus, with LibSSH2 it's recommended to use the default known_hosts file is located in libvirt's client local configuration directory e.g.: ~/.config/libvirt/known_hosts. Note: Use absolute paths.
Example: known_hosts=/root/.ssh/known_hosts
sshauth libssh2, libssh A comma separated list of authentication methods to use. Default (is "agent,privkey,password,keyboard-interactive". The order of the methods is preserved. Some methods may require additional parameters.
Example: sshauth=privkey,agent

test:///... Test URIs

The test driver is a dummy hypervisor for test purposes. The URIs supported are:

Other & legacy URI formats

NULL and empty string URIs

Libvirt allows you to pass a NULL pointer to virConnectOpen*. Empty string ("") acts in the same way. Traditionally this has meant connect to the local Xen hypervisor. However in future this may change to mean connect to the best available hypervisor.

The theory is that if, for example, Xen is unavailable but the machine is running an OpenVZ kernel, then we should not try to connect to the Xen hypervisor since that is obviously the wrong thing to do.

In any case applications linked to libvirt can continue to pass NULL as a default choice, but should always allow the user to override the URI, either by constructing one or by allowing the user to type a URI in directly (if that is appropriate). If your application wishes to connect specifically to a Xen hypervisor, then for future proofing it should choose a full xen:///system URI.

Legacy: "xen"

Another legacy URI is to specify name as the string "xen". This will continue to refer to the Xen hypervisor. However you should prefer a full xen:///system URI in all future code.