Remote support

Libvirt allows you to access hypervisors running on remote machines through authenticated and encrypted connections.

Basic usage

On the remote machine, libvirtd should be running in general. See the section on configuring libvirtd for more information.

Not all hypervisors supported by libvirt require a running libvirtd. If you want to connect to a VMware ESX/ESXi or GSX server then libvirtd is not necessary. See the VMware ESX page for details.

To tell libvirt that you want to access a remote resource, you should supply a hostname in the normal URI that is passed to virConnectOpen (or virsh -c ...). For example, if you normally use qemu:///system to access the system-wide QEMU daemon, then to access the system-wide QEMU daemon on a remote machine called compute1.libvirt.org you would use qemu://compute1.libvirt.org/system.

The section on remote URIs describes in more detail these remote URIs.

From an API point of view, apart from the change in URI, the API should behave the same. For example, ordinary calls are routed over the remote connection transparently, and values or errors from the remote side are returned to you as if they happened locally. Some differences you may notice:

Additional errors can be generated, specifically ones relating to failures in the remote transport itself.
Remote calls are handled synchronously, so they will be much slower than, say, direct hypervisor calls.

Transports

Remote libvirt supports a range of transports:

tls: TLS 1.0 (SSL 3.1) authenticated and encrypted TCP/IP socket, usually listening on a public port number. To use this you will need to generate client and server certificates. The standard port is 16514.
unix: Unix domain socket. Since this is only accessible on the local machine, it is not encrypted, and uses Unix permissions or SELinux for authentication. The standard socket names are /var/run/libvirt/libvirt-sock and /var/run/libvirt/libvirt-sock-ro (the latter for read-only connections).
ssh: Transported over an ordinary ssh (secure shell) connection. Requires Netcat (nc) installed and libvirtd should be running on the remote machine. You should use some sort of ssh key management (eg. ssh-agent) otherwise programs which use this transport will stop to ask for a password.
ext: Any external program which can make a connection to the remote machine by means outside the scope of libvirt.
tcp: Unencrypted TCP/IP socket. Not recommended for production use, this is normally disabled, but an administrator can enable it for testing or use over a trusted network. The standard port is 16509.
libssh2: Transport over the SSH protocol using libssh2 instead of the OpenSSH binary. This transport uses the libvirt authentication callback for all ssh authentication calls and therefore supports keyboard-interactive authentication even with graphical management applications. As with the classic ssh transport netcat is required on the remote side.
libssh: Transport over the SSH protocol using libssh instead of the OpenSSH binary. This transport uses the libvirt authentication callback for all ssh authentication calls and therefore supports keyboard-interactive authentication even with graphical management applications. As with the classic ssh transport netcat is required on the remote side.

The default transport, if no other is specified, is tls.

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:

xen+ssh://rjones@towada/system
— Connect to a remote Xen hypervisor on host towada using ssh transport and ssh username rjones.
xen://towada/system
— Connect to a remote Xen hypervisor on host towada using TLS.
xen://towada/system?no_verify=1
— Connect to a remote Xen hypervisor on host towada using TLS. Do not verify the server's certificate.
qemu+unix:///system?socket=/opt/libvirt/run/libvirt/libvirt-sock
— Connect to the local qemu instances over a non-standard Unix socket (the full path to the Unix socket is supplied explicitly in this case).
test+tcp://localhost:5000/default
— Connect to a libvirtd daemon offering unencrypted TCP/IP connections on localhost port 5000 and use the test driver with default settings.
qemu+libssh2://user@host/system?known_hosts=/home/user/.ssh/known_hosts
— Connect to a remote host using a ssh connection with the libssh2 driver and use a different known_hosts file.
qemu+libssh://user@host/system?known_hosts=/home/user/.ssh/known_hosts
— Connect to a remote host using a ssh connection with the libssh driver and use a different known_hosts file.

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`
`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`

Generating TLS certificates

Public Key Infrastructure set up

If you are unsure how to create TLS certificates, skip to the next section.

Location	Machine	Description	Required fields
`/etc/pki/CA/cacert.pem`	Installed on the client and server	CA's certificate (more info)	n/a
`$HOME/.pki/cacert.pem`	Installed on the client	CA's certificate (more info)	n/a
`/etc/pki/libvirt/private/serverkey.pem`	Installed on the server	Server's private key (more info)	n/a
`/etc/pki/libvirt/servercert.pem`	Installed on the server	Server's certificate signed by the CA. (more info)	CommonName (CN) must be the hostname of the server as it is seen by clients. All hostname and IP address variants that might be used to reach the server should be listed in Subject Alt Name fields.
`/etc/pki/libvirt/private/clientkey.pem`	Installed on the client	Client's private key. (more info)	n/a
`/etc/pki/libvirt/clientcert.pem`	Installed on the client	Client's certificate signed by the CA (more info)	Distinguished Name (DN) can be checked against an access control list (`tls_allowed_dn_list`).
`$HOME/.pki/libvirt/clientkey.pem`	Installed on the client	Client's private key. (more info)	n/a
`$HOME/.pki/libvirt/clientcert.pem`	Installed on the client	Client's certificate signed by the CA (more info)	Distinguished Name (DN) can be checked against an access control list (`tls_allowed_dn_list`).

If 'pkipath' is specified in URI, then all the client certificates must be found in the path specified, otherwise the connection will fail with a fatal error. If 'pkipath' is not specified:

For a non-root user, libvirt tries to find the certificates in $HOME/.pki/libvirt first. If the required CA certificate cannot be found, then the global default location (/etc/pki/CA/cacert.pem) will be used. Likewise, if either the client certificate or the client key cannot be found, then the global default locations (/etc/pki/libvirt/clientcert.pem, /etc/pki/libvirt/private/clientkey.pem) will be used.
For the root user, the global default locations will always be used.

Background to TLS certificates

Libvirt supports TLS certificates for verifying the identity of the server and clients. There are two distinct checks involved:

The client should know that it is connecting to the right server. Checking done by client by matching the certificate that the server sends to the server's hostname. May be disabled by adding ?no_verify=1 to the remote URI.
The server should know that only permitted clients are connecting. This can be done based on client's IP address, or on client's IP address and client's certificate. Checking done by the server. May be enabled and disabled in the libvirtd.conf file.

For full certificate checking you will need to have certificates issued by a recognised Certificate Authority (CA) for your server(s) and all clients. To avoid the expense of getting certificates from a commercial CA, you can set up your own CA and tell your server(s) and clients to trust certificates issues by your own CA. Follow the instructions in the next section.

Be aware that the default configuration for libvirtd allows any client to connect provided they have a valid certificate issued by the CA for their own IP address. You may want to change this to make it less (or more) permissive, depending on your needs.

Setting up a Certificate Authority (CA)

You will need the GnuTLS certtool program documented here. In Fedora, it is in the gnutls-utils package.

Create a private key for your CA:

certtool --generate-privkey > cakey.pem

and self-sign it by creating a file with the signature details called ca.info containing:

cn = Name of your organization
ca
cert_signing_key

certtool --generate-self-signed --load-privkey cakey.pem \
  --template ca.info --outfile cacert.pem

(You can delete ca.info file now if you want).

Now you have two files which matter:

cakey.pem - Your CA's private key (keep this very secret!)
cacert.pem - Your CA's certificate (this is public).

cacert.pem has to be installed on clients and server(s) to let them know that they can trust certificates issued by your CA.

The normal installation directory for cacert.pem is /etc/pki/CA/cacert.pem on all clients and servers.

To see the contents of this file, do:

certtool -i --infile cacert.pem

X.509 certificate info:

Version: 3
Serial Number (hex): 00
Subject: CN=Libvirt Project
Issuer: CN=Libvirt Project
Signature Algorithm: RSA-SHA
Validity:
        Not Before: Mon Jun 18 16:22:18 2007
        Not After: Tue Jun 17 16:22:18 2008
[etc]

This is all that is required to set up your CA. Keep the CA's private key carefully as you will need it when you come to issue certificates for your clients and servers.

Issuing server certificates

For each server (libvirtd) you need to issue a certificate containing one or more hostnames and/or IP addresses. Historically the CommonName (CN) field would contain the hostname of the server and would match the hostname used in the URI that clients pass to libvirt. In most TLS implementations the CN field is considered legacy data. The preferential mechanism is to use Subject Alt Name (SAN) extension fields to validate against. In the future use of the CN field for validation may be discontinued entirely, so it is strongly recommended to include the SAN fields.

In the example below, clients will be connecting to the server using a URI of qemu://compute1.libvirt.org/system, so the CN must be "compute1.libvirt.org".

Make a private key for the server:

certtool --generate-privkey > serverkey.pem

and sign that key with the CA's private key by first creating a template file called server.info. The template file will contain a number of fields to define the server as follows:

organization = Name of your organization
cn = compute1.libvirt.org
dns_name = compute1
dns_name = compute1.libvirt.org
ip_address = 10.0.0.74
ip_address = 192.168.1.24
ip_address = 2001:cafe::74
ip_address = fe20::24
tls_www_server
encryption_key
signing_key

The 'cn' field should refer to the fully qualified public hostname of the server. For the SAN extension data, there must also be one or more 'dns_name' fields that contain all possible hostnames that can be reasonably used by clients to reach the server, both with and without domain name qualifiers. If clients are likely to connect to the server by IP address, then one or more 'ip_address' fields should also be added.

Use the template file as input to a certtool command to sign the server certificate:

certtool --generate-certificate --load-privkey serverkey.pem \
  --load-ca-certificate cacert.pem --load-ca-privkey cakey.pem \
  --template server.info --outfile servercert.pem

This gives two files:

serverkey.pem - The server's private key.
servercert.pem - The server's public key.

We can examine this certificate and its signature:

certtool -i --infile servercert.pem
X.509 certificate info:

Version: 3
Serial Number (hex): 00
Subject: O=Libvirt Project,CN=compute1.libvirt.org
Issuer: CN=Libvirt Project
Signature Algorithm: RSA-SHA
Validity:
        Not Before: Wed Oct 04 09:09:44 UTC 2017
        Not After: Thu Oct 04 09:09:44 UTC 2018
Extensions:
        Basic Constraints (critical):
                Certificate Authority (CA): FALSE
        Subject Alternative Name (not critical):
                DNSname: compute1
                DNSname: compute1.libvirt.org
                IPAddress: 10.0.0.74
                IPAddress: 192.168.1.24
                IPAddress: 2001:cafe::74
                IPAddress: fe20::24

Note the "Issuer" CN is "Libvirt Project" (the CA) and the "Subject" CN is "compute1.libvirt.org" (the server). Notice that the hostname listed in the CN must also be duplicated as a DNSname entry

Finally we have two files to install:

serverkey.pem is the server's private key which should be copied to the server only as /etc/pki/libvirt/private/serverkey.pem.
servercert.pem is the server's certificate which can be installed on the server as /etc/pki/libvirt/servercert.pem.

Issuing client certificates

For each client (ie. any program linked with libvirt, such as virt-manager) you need to issue a certificate with the X.509 Distinguished Name (DN) set to a suitable name. You can decide this on a company / organisation policy. For example:

C=GB,ST=London,L=London,O=Libvirt Project,CN=name_of_client

The process is the same as for setting up the server certificate so here we just briefly cover the steps.

Make a private key:

certtool --generate-privkey > clientkey.pem

Act as CA and sign the certificate. Create client.info containing:

country = GB
state = London
locality = London
organization = Libvirt Project
cn = client1
tls_www_client
encryption_key
signing_key

and sign by doing:

certtool --generate-certificate --load-privkey clientkey.pem \
  --load-ca-certificate cacert.pem --load-ca-privkey cakey.pem \
  --template client.info --outfile clientcert.pem

Install the certificates on the client machine:

cp clientkey.pem /etc/pki/libvirt/private/clientkey.pem
cp clientcert.pem /etc/pki/libvirt/clientcert.pem

Troubleshooting TLS certificate problems

failed to verify client's certificate: On the server side, run the libvirtd server with the '--listen' and '--verbose' options while the client is connecting. The verbose log messages should tell you enough to diagnose the problem.

You can use the virt-pki-validate shell script to analyze the setup on the client or server machines, preferably as root. It will try to point out the possible problems and provide solutions to fix the set up up to a point where you have secure remote access.

libvirtd configuration file

Libvirtd (the remote daemon) is configured from a file called /etc/libvirt/libvirtd.conf, or specified on the command line using -f filename or --config filename.

This file should contain lines of the form below. Blank lines and comments beginning with # are ignored.

setting = value

The following settings, values and default are:

Line	Default	Meaning
listen_tls [0\|1]	1 (on)	Listen for secure TLS connections on the public TCP/IP port. Note: it is also necessary to start the server in listening mode by running it with --listen or editing /etc/sysconfig/libvirtd by uncommenting the LIBVIRTD_ARGS="--listen" line to cause the server to come up in listening mode whenever it is started.
listen_tcp [0\|1]	0 (off)	Listen for unencrypted TCP connections on the public TCP/IP port. Note: it is also necessary to start the server in listening mode.
tls_port "service"	"16514"	The port number or service name to listen on for secure TLS connections.
tcp_port "service"	"16509"	The port number or service name to listen on for unencrypted TCP connections.
unix_sock_group "groupname"	"root"	The UNIX group to own the UNIX domain socket. If the socket permissions allow group access, then applications running under matching group can access the socket. Only valid if running as root
unix_sock_ro_perms "octal-perms"	"0777"	The permissions for the UNIX domain socket for read-only client connections. The default allows any user to monitor domains.
unix_sock_rw_perms "octal-perms"	"0700"	The permissions for the UNIX domain socket for read-write client connections. The default allows only root to manage domains.
tls_no_verify_certificate [0\|1]	0 (certificates are verified)	If set to 1 then if a client certificate check fails, it is not an error.
tls_no_verify_address [0\|1]	0 (addresses are verified)	If set to 1 then if a client IP address check fails, it is not an error.
key_file "filename"	"/etc/pki/libvirt/ private/serverkey.pem"	Change the path used to find the server's private key. If you set this to an empty string, then no private key is loaded.
cert_file "filename"	"/etc/pki/libvirt/ servercert.pem"	Change the path used to find the server's certificate. If you set this to an empty string, then no certificate is loaded.
ca_file "filename"	"/etc/pki/CA/cacert.pem"	Change the path used to find the trusted CA certificate. If you set this to an empty string, then no trusted CA certificate is loaded.
crl_file "filename"	(no CRL file is used)	Change the path used to find the CA certificate revocation list (CRL) file. If you set this to an empty string, then no CRL is loaded.
tls_allowed_dn_list ["DN1", "DN2"]	(none - DNs are not checked)	Enable an access control list of client certificate Distinguished Names (DNs) which can connect to the TLS port on this server. The default is that DNs are not checked. This list may contain wildcards such as `"C=GB,ST=London,L=London,O=Libvirt Project,CN="` See the POSIX `fnmatch` function for the format of the wildcards. Note that if this is an empty list, no client can connect*. Note also that GnuTLS returns DNs without spaces after commas between the fields (and this is what we check against), but the `openssl x509` tool shows spaces.

IPv6 support

The libvirtd service and libvirt remote client driver both use the getaddrinfo() functions for name resolution and are thus fully IPv6 enabled. ie, if a server has IPv6 address configured the daemon will listen for incoming connections on both IPv4 and IPv6 protocols. If a client has an IPv6 address configured and the DNS address resolved for a service is reachable over IPv6, then an IPv6 connection will be made, otherwise IPv4 will be used. In summary it should just 'do the right thing(tm)'.

Limitations

Fine-grained authentication: libvirt in general, but in particular the remote case should support more fine-grained authentication for operations, rather than just read-write/read-only as at present.

Please come and discuss these issues and more on the mailing list.