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<h1>Remote support</h1>
<p>
Libvirt allows you to access hypervisors running on remote
machines through authenticated and encrypted connections.
</p>
<ul><li>
<a href="#Remote_basic_usage">Basic usage</a>
</li><li>
<a href="#Remote_transports">Transports</a>
</li><li>
<a href="#Remote_URI_reference">Remote URIs</a>
<ul><li>
<a href="#Remote_URI_parameters">Extra parameters</a>
</li></ul></li><li>
<a href="#Remote_certificates">Generating TLS certificates</a>
<ul><li>
<a href="#Remote_PKI">Public Key Infrastructure set up</a>
</li><li>
<a href="#Remote_TLS_background">Background to TLS certificates</a>
</li><li>
<a href="#Remote_TLS_CA">Setting up a Certificate Authority (CA)</a>
</li><li>
<a href="#Remote_TLS_server_certificates">Issuing server certificates</a>
</li><li>
<a href="#Remote_TLS_client_certificates">Issuing client certificates</a>
</li><li>
<a href="#Remote_TLS_troubleshooting">Troubleshooting TLS certificate problems</a>
</li></ul></li><li>
<a href="#Remote_libvirtd_configuration">libvirtd configuration file</a>
</li><li>
<a href="#Remote_IPv6">IPv6 support</a>
</li><li>
<a href="#Remote_limitations">Limitations</a>
</li><li>
<a href="#Remote_implementation_notes">Implementation notes</a>
</li></ul>
<h3>
<a name="Remote_basic_usage" id="Remote_basic_usage">Basic usage</a>
</h3>
<p>
On the remote machine, <code>libvirtd</code> should be running.
See <a href="#Remote_libvirtd_configuration">the section
on configuring libvirtd</a> for more information.
</p>
<p>
To tell libvirt that you want to access a remote resource,
you should supply a hostname in the normal <a href="uri.html">URI</a> that is passed
to <code>virConnectOpen</code> (or <code>virsh -c ...</code>).
For example, if you normally use <code>qemu:///system</code>
to access the system-wide QEMU daemon, then to access
the system-wide QEMU daemon on a remote machine called
<code>oirase</code> you would use <code>qemu://oirase/system</code>.
</p>
<p>
The <a href="#Remote_URI_reference">section on remote URIs</a>
describes in more detail these remote URIs.
</p>
<p>
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:
</p>
<ul><li> Additional errors can be generated, specifically ones
relating to failures in the remote transport itself. </li><li> Remote calls are handled synchronously, so they will be
much slower than, say, direct hypervisor calls. </li></ul>
<h3>
<a name="Remote_transports" id="Remote_transports">Transports</a>
</h3>
<p>
Remote libvirt supports a range of transports:
</p>
<dl><dt> tls </dt><dd><a href="http://en.wikipedia.org/wiki/Transport_Layer_Security" title="Transport Layer Security">TLS</a>
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
<a href="#Remote_certificates" title="Generating TLS certificates">generate client and
server certificates</a>.
The standard port is 16514.
</dd><dt> unix </dt><dd> 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
<code>/var/run/libvirt/libvirt-sock</code> and
<code>/var/run/libvirt/libvirt-sock-ro</code> (the latter
for read-only connections).
</dd><dt> ssh </dt><dd> Transported over an ordinary
<a href="http://www.openssh.com/" title="OpenSSH homepage">ssh
(secure shell)</a> connection.
Requires <a href="http://netcat.sourceforge.net/">Netcat (nc)</a>
installed and libvirtd should be running
on the remote machine. You should use some sort of
ssh key management (eg.
<a href="http://mah.everybody.org/docs/ssh" title="Using ssh-agent with ssh">ssh-agent</a>)
otherwise programs which use
this transport will stop to ask for a password. </dd><dt> ext </dt><dd> Any external program which can make a connection to the
remote machine by means outside the scope of libvirt. </dd><dt> tcp </dt><dd> 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.
</dd></dl>
<p>
The default transport, if no other is specified, is <code>tls</code>.
</p>
<h3>
<a name="Remote_URI_reference" id="Remote_URI_reference">Remote URIs</a>
</h3>
<p>
See also: <a href="uri.html">documentation on ordinary ("local") URIs</a>.
</p>
<p>
Remote URIs have the general form ("[...]" meaning an optional part):
</p>
<p><code>driver</code>[<code>+transport</code>]<code>://</code>[<code>username@</code>][<code>hostname</code>][<code>:port</code>]<code>/</code>[<code>path</code>][<code>?extraparameters</code>]
</p>
<p>
Either the transport or the hostname must be given in order
to distinguish this from a local URI.
</p>
<p>
Some examples:
</p>
<ul><li><code>xen+ssh://rjones@towada/</code><br /> — Connect to a
remote Xen hypervisor on host <code>towada</code> using ssh transport and ssh
username <code>rjones</code>.
</li><li><code>xen://towada/</code><br /> — Connect to a
remote Xen hypervisor on host <code>towada</code> using TLS.
</li><li><code>xen://towada/?no_verify=1</code><br /> — Connect to a
remote Xen hypervisor on host <code>towada</code> using TLS. Do not verify
the server's certificate.
</li><li><code>qemu+unix:///system?socket=/opt/libvirt/run/libvirt/libvirt-sock</code><br /> —
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).
</li><li><code>test+tcp://localhost:5000/default</code><br /> —
Connect to a libvirtd daemon offering unencrypted TCP/IP connections
on localhost port 5000 and use the test driver with default
settings.
</li></ul>
<h4>
<a name="Remote_URI_parameters" id="Remote_URI_parameters">Extra parameters</a>
</h4>
<p>
Extra parameters can be added to remote URIs as part
of the query string (the part following <q><code>?</code></q>).
Remote URIs understand the extra parameters shown below.
Any others are passed unmodified through to the back end.
Note that parameter values must be
<a href="http://xmlsoft.org/html/libxml-uri.html#xmlURIEscapeStr">URI-escaped</a>.
</p>
<table class="top_table"><tr><th> Name </th><th> Transports </th><th> Meaning </th></tr><tr><td>
<code>name</code>
</td><td>
<i>any transport</i>
</td><td>
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.
</td></tr><tr><td colspan="2"></td><td> Example: <code>name=qemu:///system</code> </td></tr><tr><td>
<code>command</code>
</td><td> ssh, ext </td><td>
The external command. For ext transport this is required.
For ssh the default is <code>ssh</code>.
The PATH is searched for the command.
</td></tr><tr><td colspan="2"></td><td> Example: <code>command=/opt/openssh/bin/ssh</code> </td></tr><tr><td>
<code>socket</code>
</td><td> unix, ssh </td><td>
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).
</td></tr><tr><td colspan="2"></td><td> Example: <code>socket=/opt/libvirt/run/libvirt/libvirt-sock</code> </td></tr><tr><td>
<code>netcat</code>
</td><td> ssh </td><td>
The name of the netcat command on the remote machine.
The default is <code>nc</code>. For ssh transport, libvirt
constructs an ssh command which looks like:
<pre><i>command</i> -p <i>port</i> [-l <i>username</i>] <i>hostname</i> <i>netcat</i> -U <i>socket</i>
</pre>
where <i>port</i>, <i>username</i>, <i>hostname</i> can be
specified as part of the remote URI, and <i>command</i>, <i>netcat</i>
and <i>socket</i> come from extra parameters (or
sensible defaults).
</td></tr><tr><td colspan="2"></td><td> Example: <code>netcat=/opt/netcat/bin/nc</code> </td></tr><tr><td>
<code>no_verify</code>
</td><td> tls </td><td>
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
<a href="#Remote_libvirtd_configuration">change the libvirtd
configuration</a>.
</td></tr><tr><td colspan="2"></td><td> Example: <code>no_verify=1</code> </td></tr><tr><td>
<code>no_tty</code>
</td><td> ssh </td><td>
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.
</td></tr><tr><td colspan="2"></td><td> Example: <code>no_tty=1</code> </td></tr></table>
<h3>
<a name="Remote_certificates" id="Remote_certificates">Generating TLS certificates</a>
</h3>
<h4>
<a name="Remote_PKI" id="Remote_PKI">Public Key Infrastructure set up</a>
</h4>
<p>
If you are unsure how to create TLS certificates, skip to the
next section.
</p>
<table class="top_table"><tr><th> Location </th><th> Machine </th><th> Description </th><th> Required fields </th></tr><tr><td>
<code>/etc/pki/CA/cacert.pem</code>
</td><td> Installed on all clients and servers </td><td> CA's certificate (<a href="#Remote_TLS_CA">more info</a>)</td><td> n/a </td></tr><tr><td>
<code>/etc/pki/libvirt/ private/serverkey.pem</code>
</td><td> Installed on the server </td><td> Server's private key (<a href="#Remote_TLS_server_certificates">more info</a>)</td><td> n/a </td></tr><tr><td>
<code>/etc/pki/libvirt/ servercert.pem</code>
</td><td> Installed on the server </td><td> Server's certificate signed by the CA.
(<a href="#Remote_TLS_server_certificates">more info</a>) </td><td> CommonName (CN) must be the hostname of the server as it
is seen by clients. </td></tr><tr><td>
<code>/etc/pki/libvirt/ private/clientkey.pem</code>
</td><td> Installed on the client </td><td> Client's private key. (<a href="#Remote_TLS_client_certificates">more info</a>) </td><td> n/a </td></tr><tr><td>
<code>/etc/pki/libvirt/ clientcert.pem</code>
</td><td> Installed on the client </td><td> Client's certificate signed by the CA
(<a href="#Remote_TLS_client_certificates">more info</a>) </td><td> Distinguished Name (DN) can be checked against an access
control list (<code>tls_allowed_dn_list</code>).
</td></tr></table>
<h4>
<a name="Remote_TLS_background" id="Remote_TLS_background">Background to TLS certificates</a>
</h4>
<p>
Libvirt supports TLS certificates for verifying the identity
of the server and clients. There are two distinct checks involved:
</p>
<ul><li> 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
<code>?no_verify=1</code> to the
<a href="#Remote_URI_parameters">remote URI</a>.
</li><li> 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 <a href="#Remote_libvirtd_configuration">libvirtd.conf file</a>.
</li></ul>
<p>
For full certificate checking you will need to have certificates
issued by a recognised <a href="http://en.wikipedia.org/wiki/Certificate_authority">Certificate
Authority (CA)</a> 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.
</p>
<p>
Be aware that the <a href="#Remote_libvirtd_configuration">default
configuration for libvirtd</a> 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.
</p>
<h4>
<a name="Remote_TLS_CA" id="Remote_TLS_CA">Setting up a Certificate Authority (CA)</a>
</h4>
<p>
You will need the <a href="http://www.gnu.org/software/gnutls/manual/html_node/Invoking-certtool.html">GnuTLS
certtool program documented here</a>. In Fedora, it is in the
<code>gnutls-utils</code> package.
</p>
<p>
Create a private key for your CA:
</p>
<pre>
certtool --generate-privkey > cakey.pem
</pre>
<p>
and self-sign it by creating a file with the
signature details called
<code>ca.info</code> containing:
</p>
<pre>
cn = <i>Name of your organization</i>
ca
cert_signing_key
</pre>
<pre>
certtool --generate-self-signed --load-privkey cakey.pem \
--template ca.info --outfile cacert.pem
</pre>
<p>
(You can delete <code>ca.info</code> file now if you
want).
</p>
<p>
Now you have two files which matter:
</p>
<ul><li><code>cakey.pem</code> - Your CA's private key (keep this very secret!)
</li><li><code>cacert.pem</code> - Your CA's certificate (this is public).
</li></ul>
<p><code>cacert.pem</code> has to be installed on clients and
server(s) to let them know that they can trust certificates issued by
your CA.
</p>
<p>
The normal installation directory for <code>cacert.pem</code>
is <code>/etc/pki/CA/cacert.pem</code> on all clients and servers.
</p>
<p>
To see the contents of this file, do:
</p>
<pre><b>certtool -i --infile cacert.pem</b>
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
<i>[etc]</i>
</pre>
<p>
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.
</p>
<h4>
<a name="Remote_TLS_server_certificates" id="Remote_TLS_server_certificates">Issuing server certificates</a>
</h4>
<p>
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.
</p>
<p>
In the example below, clients will be connecting to the
server using a <a href="#Remote_URI_reference">URI</a> of
<code>xen://oirase/</code>, so the CN must be "<code>oirase</code>".
</p>
<p>
Make a private key for the server:
</p>
<pre>
certtool --generate-privkey > serverkey.pem
</pre>
<p>
and sign that key with the CA's private key by first
creating a template file called <code>server.info</code>
(only the CN field matters, which as explained above must
be the server's hostname):
</p>
<pre>
organization = <i>Name of your organization</i>
cn = oirase
tls_www_server
encryption_key
signing_key
</pre>
<p>
and sign:
</p>
<pre>
certtool --generate-certificate --load-privkey serverkey.pem \
--load-ca-certificate cacert.pem --load-ca-privkey cakey.pem \
--template server.info --outfile servercert.pem
</pre>
<p>
This gives two files:
</p>
<ul><li><code>serverkey.pem</code> - The server's private key.
</li><li><code>servercert.pem</code> - The server's public key.
</li></ul>
<p>
We can examine this certificate and its signature:
</p>
<pre><b>certtool -i --infile servercert.pem</b>
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
</pre>
<p>
Note the "Issuer" CN is "Red Hat Emerging Technologies" (the CA) and
the "Subject" CN is "oirase" (the server).
</p>
<p>
Finally we have two files to install:
</p>
<ul><li><code>serverkey.pem</code> is
the server's private key which should be copied to the
server <i>only</i> as
<code>/etc/pki/libvirt/private/serverkey.pem</code>.
</li><li><code>servercert.pem</code> is the server's certificate
which can be installed on the server as
<code>/etc/pki/libvirt/servercert.pem</code>.
</li></ul>
<h4>
<a name="Remote_TLS_client_certificates" id="Remote_TLS_client_certificates">Issuing client certificates</a>
</h4>
<p>
For each client (ie. any program linked with libvirt, such as
<a href="http://virt-manager.et.redhat.com/">virt-manager</a>)
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:
</p>
<pre>
C=GB,ST=London,L=London,O=Red Hat,CN=<i>name_of_client</i>
</pre>
<p>
The process is the same as for
<a href="#Remote_TLS_server_certificates">setting up the
server certificate</a> so here we just briefly cover the
steps.
</p>
<ol><li>
Make a private key:
<pre>
certtool --generate-privkey > clientkey.pem
</pre>
</li><li>
Act as CA and sign the certificate. Create client.info containing:
<pre>
country = GB
state = London
locality = London
organization = Red Hat
cn = client1
tls_www_client
encryption_key
signing_key
</pre>
and sign by doing:
<pre>
certtool --generate-certificate --load-privkey clientkey.pem \
--load-ca-certificate cacert.pem --load-ca-privkey cakey.pem \
--template client.info --outfile clientcert.pem
</pre>
</li><li>
Install the certificates on the client machine:
<pre>
cp clientkey.pem /etc/pki/libvirt/private/clientkey.pem
cp clientcert.pem /etc/pki/libvirt/clientcert.pem
</pre>
</li></ol>
<h4>
<a name="Remote_TLS_troubleshooting" id="Remote_TLS_troubleshooting">Troubleshooting TLS certificate problems</a>
</h4>
<dl><dt> failed to verify client's certificate </dt><dd>
<p>
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.
</p>
</dd></dl>
<p> You can use the <a href="pki_check.sh">pki_check.sh</a> 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.</p>
<h3>
<a name="Remote_libvirtd_configuration" id="Remote_libvirtd_configuration">libvirtd configuration file</a>
</h3>
<p>
Libvirtd (the remote daemon) is configured from a file called
<code>/etc/libvirt/libvirtd.conf</code>, or specified on
the command line using <code>-f filename</code> or
<code>--config filename</code>.
</p>
<p>
This file should contain lines of the form below.
Blank lines and comments beginning with <code>#</code> are ignored.
</p>
<pre>setting = value</pre>
<p>The following settings, values and default are:</p>
<table class="top_table"><tr><th> Line </th><th> Default </th><th> Meaning </th></tr><tr><td> listen_tls <i>[0|1]</i> </td><td> 1 (on) </td><td>
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.
</td></tr><tr><td> listen_tcp <i>[0|1]</i> </td><td> 0 (off) </td><td>
Listen for unencrypted TCP connections on the public TCP/IP port.
Note: it is also necessary to start the server in listening mode.
</td></tr><tr><td> tls_port <i>"service"</i> </td><td> "16514" </td><td>
The port number or service name to listen on for secure TLS connections.
</td></tr><tr><td> tcp_port <i>"service"</i> </td><td> "16509" </td><td>
The port number or service name to listen on for unencrypted TCP connections.
</td></tr><tr><td> mdns_adv <i>[0|1]</i> </td><td> 1 (advertise with mDNS) </td><td>
If set to 1 then the virtualization service will be advertised over
mDNS to hosts on the local LAN segment.
</td></tr><tr><td> mdns_name <i>"name"</i> </td><td> "Virtualization Host HOSTNAME" </td><td>
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.
</td></tr><tr><td> unix_sock_group <i>"groupname"</i> </td><td> "root" </td><td>
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
</td></tr><tr><td> unix_sock_ro_perms <i>"octal-perms"</i> </td><td> "0777" </td><td>
The permissions for the UNIX domain socket for read-only client connections.
The default allows any user to monitor domains.
</td></tr><tr><td> unix_sock_rw_perms <i>"octal-perms"</i> </td><td> "0700" </td><td>
The permissions for the UNIX domain socket for read-write client connections.
The default allows only root to manage domains.
</td></tr><tr><td> tls_no_verify_certificate <i>[0|1]</i> </td><td> 0 (certificates are verified) </td><td>
If set to 1 then if a client certificate check fails, it is not an error.
</td></tr><tr><td> tls_no_verify_address <i>[0|1]</i> </td><td> 0 (addresses are verified) </td><td>
If set to 1 then if a client IP address check fails, it is not an error.
</td></tr><tr><td> key_file <i>"filename"</i> </td><td> "/etc/pki/libvirt/ private/serverkey.pem" </td><td>
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.
</td></tr><tr><td> cert_file <i>"filename"</i> </td><td> "/etc/pki/libvirt/ servercert.pem" </td><td>
Change the path used to find the server's certificate.
If you set this to an empty string, then no certificate is loaded.
</td></tr><tr><td> ca_file <i>"filename"</i> </td><td> "/etc/pki/CA/cacert.pem" </td><td>
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.
</td></tr><tr><td> crl_file <i>"filename"</i> </td><td> (no CRL file is used) </td><td>
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.
</td></tr><tr><td> tls_allowed_dn_list ["DN1", "DN2"] </td><td> (none - DNs are not checked) </td><td>
<p>
Enable an access control list of client certificate Distinguished
Names (DNs) which can connect to the TLS port on this server.
</p>
<p>
The default is that DNs are not checked.
</p>
<p>
This list may contain wildcards such as <code>"C=GB,ST=London,L=London,O=Red Hat,CN=*"</code>
See the POSIX <code>fnmatch</code> function for the format
of the wildcards.
</p>
<p>
Note that if this is an empty list, <i>no client can connect</i>.
</p>
<p>
Note also that GnuTLS returns DNs without spaces
after commas between the fields (and this is what we check against),
but the <code>openssl x509</code> tool shows spaces.
</p>
</td></tr></table>
<h3>
<a name="Remote_IPv6" id="Remote_IPv6">IPv6 support</a>
</h3>
<p>
The libvirtd service and libvirt remote client driver both use the
<code>getaddrinfo()</code> 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)'.
</p>
<h3>
<a name="Remote_limitations" id="Remote_limitations">Limitations</a>
</h3>
<ul><li> Remote storage: To be fully useful, particularly for
creating new domains, it should be possible to enumerate
and provision storage on the remote machine. This is currently
in the design phase. </li><li> Migration: We expect libvirt will support migration,
and obviously remote support is what makes migration worthwhile.
This is also in the design phase. Issues <a href="https://www.redhat.com/mailman/listinfo/libvir-list" title="libvir-list mailing list">to discuss</a> include
which path the migration data should follow (eg. client to
client direct, or client to server to client) and security.
</li><li> 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.
</li></ul>
<p>
Please come and discuss these issues and more on <a href="https://www.redhat.com/mailman/listinfo/libvir-list" title="libvir-list mailing list">the mailing list</a>.
</p>
<h3>
<a name="Remote_implementation_notes" id="Remote_implementation_notes">Implementation notes</a>
</h3>
<p>
The current implementation uses <a href="http://en.wikipedia.org/wiki/External_Data_Representation" title="External Data Representation">XDR</a>-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.
</p>
<p>
The implementation should be considered <b>strictly internal</b> to
libvirt and <b>subject to change at any time without notice</b>. 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 <a href="https://www.redhat.com/mailman/listinfo/libvir-list" title="libvir-list mailing list">the mailing list</a>.
</p>
<p>
The messaging protocol is described in
<code>qemud/remote_protocol.x</code>.
</p>
<p>
Authentication and encryption (for TLS) is done using <a href="http://www.gnu.org/software/gnutls/" title="GnuTLS project page">GnuTLS</a> and the RPC protocol is unaware of this layer.
</p>
<p>
Protocol messages are sent using a simple 32 bit length word (encoded
XDR int) followed by the message header (XDR
<code>remote_message_header</code>) followed by the message body. The
length count includes the length word itself, and is measured in
bytes. Maximum message size is <code>REMOTE_MESSAGE_MAX</code> and to
avoid denial of services attacks on the XDR decoders strings are
individually limited to <code>REMOTE_STRING_MAX</code> 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 <code>REMOTE_CALL</code> message is sent from client to
server, and the server then replies synchronously with a single
<code>REMOTE_REPLY</code> message, but other forms of messaging are
also possible.
</p>
<p>
The protocol contains support for multiple program types and protocol
versioning, modelled after SunRPC.
</p>
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