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. See the section on configuring libvirtd for more information.

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 oirase you would use qemu://oirase/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:

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.

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

Remote URIs

See also: documentation on ordinary ("local") 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
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 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 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
no_verify 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

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 all clients and servers 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.
/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).

Background to TLS certificates

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

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:

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=Red Hat Emerging Technologies
Issuer: CN=Red Hat Emerging Technologies
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 with the X.509 CommonName (CN) field set to the hostname of the server. The CN must match the hostname which clients will be using to connect to the server.

In the example below, clients will be connecting to the server using a URI of xen://oirase/, so the CN must be "oirase".

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 (only the CN field matters, which as explained above must be the server's hostname): 

organization = Name of your organization
cn = oirase
tls_www_server
encryption_key
signing_key

and sign: 

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:

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=Red Hat Emerging Technologies,CN=oirase
Issuer: CN=Red Hat Emerging Technologies
Signature Algorithm: RSA-SHA
Validity:
        Not Before: Mon Jun 18 16:34:49 2007
        Not After: Tue Jun 17 16:34:49 2008

Note the "Issuer" CN is "Red Hat Emerging Technologies" (the CA) and the "Subject" CN is "oirase" (the server).

Finally we have two files to install:

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, I use: 

C=GB,ST=London,L=London,O=Red Hat,CN=name_of_client

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

  1. Make a private key: 
    certtool --generate-privkey > clientkey.pem
  2. Act as CA and sign the certificate. Create client.info containing: 
    country = GB
state = London
locality = London
organization = Red Hat
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
  3. 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 pki_check.sh 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.
listen_tcp [0|1] 0 (off) Listen for unencrypted TCP connections on the public TCP/IP port.
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.
mdns_adv [0|1] 1 (advertise with mDNS) If set to 1 then the virtualization service will be advertised over mDNS to hosts on the local LAN segment.
mdns_name "name" "Virtualization Host HOSTNAME" The name to advertise for this host with Avahi mDNS. The default includes the machine's short hostname. This must be unique to the local LAN segment.
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=Red Hat,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.

tls_allowed_ip_list ["ip1", "ip2", "ip3"] (none - clients can connect from anywhere)

Enable an access control list of the IP addresses of clients who can connect to the TLS or TCP ports on this server.

The default is that clients can connect from any IP address.

This list may contain wildcards such as 192.168.* See the POSIX fnmatch function for the format of the wildcards.

Note that if this is an empty list, no client can connect.

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

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

Implementation notes

The current implementation uses XDR-encoded packets with a simple remote procedure call implementation which also supports asynchronous messaging and asynchronous and out-of-order replies, although these latter features are not used at the moment.

The implementation should be considered strictly internal to libvirt and subject to change at any time without notice. If you wish to talk to libvirtd, link to libvirt. If there is a problem that means you think you need to use the protocol directly, please first discuss this on the mailing list.

The messaging protocol is described in qemud/remote_protocol.x.

Authentication and encryption (for TLS) is done using GnuTLS and the RPC protocol is unaware of this layer.

Protocol messages are sent using a simple 32 bit length word (encoded XDR int) followed by the message header (XDR remote_message_header) followed by the message body. The length count includes the length word itself, and is measured in bytes. Maximum message size is REMOTE_MESSAGE_MAX and to avoid denial of services attacks on the XDR decoders strings are individually limited to REMOTE_STRING_MAX bytes. In the TLS case, messages may be split over TLS records, but a TLS record cannot contain parts of more than one message. In the common RPC case a single REMOTE_CALL message is sent from client to server, and the server then replies synchronously with a single REMOTE_REPLY message, but other forms of messaging are also possible.

The protocol contains support for multiple program types and protocol versioning, modelled after SunRPC.