Remote support

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

• Basic usage
• Transports
• Remote URIs
  • Extra parameters
• Generating TLS certificates
  • Public Key Infrastructure set up
  • Background to TLS certificates
  • Setting up a Certificate Authority (CA)
  • Issuing server certificates
  • Issuing client certificates
  • Troubleshooting TLS certificate problems
• libvirtd configuration file
• IPv6 support
• Limitations
• Implementation notes

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 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:

• 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.

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:

• xen+ssh://rjones@towada/
  — Connect to a remote Xen hypervisor on host towada using ssh transport and ssh username rjones.
• xen://towada/
  — Connect to a remote Xen hypervisor on host towada using TLS.
• xen://towada/?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.

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.

command -p port [-l username] hostname netcat -U socket

Generating TLS certificates

Public Key Infrastructure set up

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

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. If any of the required certificates can not be found, then the global default locations (/etc/pki/CA/cacert.pem, /etc/pki/libvirt/private/clientkey, /etc/pki/libvirt/clientcert.pem) will be used.
• For the root user, the global default locations will 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=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:

• 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=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:

• 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, 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 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:

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.

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.