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.
The URI is passed as the name parameter to virConnectOpen or virConnectOpenReadOnly. For example:
virConnectPtr conn = virConnectOpenReadOnly ("test:///default");
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.
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]
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:///.
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:
qemu:///system connects to a system mode daemon. qemu:///session connects to a session mode daemon.
(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 are formed by taking ordinary local URIs and adding a hostname and/or transport name. For example:
| Local URI | Remote URI | Meaning |
|---|---|---|
xen:///
|
xen://oirase/
|
Connect to the Xen hypervisor running on host oirase
using TLS. |
xen:///
|
xen+ssh://oirase/
|
Connect to the Xen hypervisor running on host oirase
by going over an ssh connection. |
test:///default
|
test+tcp://oirase/default
|
Connect to the test driver on host oirase
using an unsecured TCP connection. |
Remote URIs in libvirt offer a rich syntax and many features. We refer you to the libvirt remote URI reference and full documentation for libvirt remote support.
The test driver is a dummy hypervisor for test purposes. The URIs supported are:
test:///default connects to a default set of
host definitions built into the driver. test:///path/to/host/definitions connects to
a set of host definitions held in the named file.
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:/// URI.
If XenD is running and configured in /etc/xen/xend-config.sxp:
(xend-unix-server yes)
then it listens on a Unix domain socket, usually at
/var/lib/xend/xend-socket. You may pass a different path
using a file URI such as:
virsh -c ///var/run/xend/xend-socket
http://... (xend-http-server)
If XenD is running and configured in /etc/xen/xend-config.sxp:
(xend-http-server yes)
then it listens on TCP port 8000. libvirt allows you to
try to connect to xend running on remote machines by passing
http://hostname[:port]/, for example:
virsh -c http://oirase/ list
This method is unencrypted and insecure and is definitely not recommended for production use. Instead use libvirt's remote support.
Notes:
"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:/// URI in all future code.
Libvirt continues to support connections to a separately running Xen proxy daemon. This provides a way to allow non-root users to make a safe (read-only) subset of queries to the hypervisor.
There is no specific "Xen proxy" URI. However if a Xen URI of any of
the ordinary or legacy forms is used (eg. NULL,
"", "xen", ...) which fails, and the
user is not root, and the Xen proxy socket can be connected to
(/tmp/libvirt_proxy_conn), then libvirt will use a proxy
connection.
You should consider using libvirt remote support in future.