12 Connecting and authorizing #

To grant access for non-root accounts, configure the sockets to be owned and accessible by a certain group (libvirt in the following example). This authentication method can be used for local and remote SSH connections.

Access control for Unix sockets with Polkit is the default authentication method on SUSE Linux Enterprise Server for non-remote connections. Therefore, no libvirt configuration changes are needed. With Polkit authorization enabled, permissions on both sockets default to 0777 and each application trying to access a socket needs to authenticate via Polkit.

Important: Polkit authentication for local connections only

Authentication with Polkit can only be used for local connections on the VM Host Server itself, since Polkit does not handle remote authentication.

Two policies for accessing libvirt's sockets exist:

By default, the policy for accessing the read-write socket is to authenticate with the root password once and grant the privilege for the current and for future sessions.

To grant users access to a socket without having to provide the root password, you need to create a rule in /etc/polkit-1/rules.d. Create the file /etc/polkit-1/rules.d/10-grant-libvirt with the following content to grant access to the read-write socket to all members of the group libvirt:

polkit.addRule(function(action, subject) {
  if (action.id == "org.libvirt.unix.manage" && subject.isInGroup("libvirt")) {
    return polkit.Result.YES;
  }
});

SASL provides user name and password authentication and data encryption (digest-md5, by default). Since SASL maintains its own user database, the users do not need to exist on the VM Host Server. SASL is required by TCP connections and on top of TLS/SSL connections.

Important: Plain TCP and SASL with digest-md5 encryption

Using digest-md5 encryption on an otherwise not encrypted TCP connection does not provide enough security for production environments. It is recommended to only use it in testing environments.

Tip: SASL authentication on top of TLS/SSL

Access from remote TLS/SSL connections can be indirectly controlled on the client side by restricting access to the certificate's key file. However, this might prove error-prone when dealing with many clients. Using SASL with TLS adds security by additionally controlling access on the server side.

To configure SASL authentication, proceed as follows:

Tip: virsh and SASL authentication

When using SASL authentication, you will be prompted for a user name and password every time you issue a virsh command. Avoid this by using virsh in shell mode.

SASL provides user name and password authentication and data encryption. Since SASL maintains its own user database, the users do not need to exist on the VM Host Server. As with SASL authentication for libvirt, you may use SASL on top of TLS/SSL connections. Refer to Section 12.3.2.4.2, “VNC over TLS/SSL: client configuration” for details on configuring these connections.

To configure SASL authentication for VNC, proceed as follows:

Note: Supported VNC viewers

SASL authentication is currently supported by Virtual Machine Manager and virt-viewer. Both of these viewers also support TLS/SSL connections.

Access to the VNC server may also be controlled by setting a VNC password. You can either set a global password for all VM Guests or set individual passwords for each guest. The latter requires to edit the VM Guest's configuration files.

Note: Always set a global password

If you are using single password authentication, it is good practice to set a global password even if setting passwords for each VM Guest. This will always leave your virtual machines protected with a “fallback” password if you forget to set a per-machine password. The global password will only be used if no other password is set for the machine.

Warning: Security of the VNC protocol

The VNC protocol is not considered to be safe. Although the password is sent encrypted, it might be vulnerable when an attacker can sniff both the encrypted password and the encryption key. Therefore, it is recommended to use VNC with TLS/SSL or tunneled over SSH. virt-viewer, Virtual Machine Manager and Remmina (refer to Section 14.2, “Remmina: the remote desktop client”) support both methods.

TLS (Transport Layer Security) encrypts the communication between two computers by using certificates. The computer starting the connection is always considered the “client”, using a “client certificate”, while the receiving computer is always considered the “server”, using a “server certificate”. This scenario applies, for example, if you manage your VM Host Servers from a central desktop.

If connections are initiated from both computers, each needs to have a client and a server certificate. This is the case, for example, if you migrate a VM Guest from one host to another.

Each x509 certificate has a matching private key file. Only the combination of certificate and private key file can identify itself correctly. To assure that a certificate was issued by the assumed owner, it is signed and issued by a central certificate called certificate authority (CA). Both the client and the server certificates must be issued by the same CA.

Important: User authentication

Using a remote TLS/SSL connection only ensures that two computers are allowed to communicate in a certain direction. Restricting access to certain users can indirectly be achieved on the client side by restricting access to the certificates. For more information, see Section 12.3.2.5, “Restricting access (security considerations)”.

libvirt also supports user authentication on the server with SASL. For more information, see Section 12.3.2.6, “Central user authentication with SASL for TLS sockets”.

The VM Host Server is the machine receiving connections. Therefore, the server certificates need to be installed. The CA certificate needs to be installed, too. When the certificates are in place, TLS support can be turned on for libvirt.

Important: Restarting libvirtd with TLS enabled

If you enable TLS for libvirt, the server certificates need to be in place, otherwise restarting libvirtd will fail. You also need to restart libvirtd in case you change the certificates.

The client is the machine initiating connections. Therefore the client certificates need to be installed. The CA certificate needs to be installed, too.

Currently, VNC communication over TLS is only supported by a few tools. Common VNC viewers such as tightvnc or tigervnc do not support TLS/SSL. The only supported alternative to Virtual Machine Manager and virt-viewer is remmina (refer to Section 14.2, “Remmina: the remote desktop client”).

12.3.2.4.2 VNC over TLS/SSL: client configuration #

  

The only action needed on the client side is to place the x509 client certificates in a location recognized by the client of choice. Unfortunately, Virtual Machine Manager and virt-viewer expect the certificates in a different location. Virtual Machine Manager can either read from a system-wide location applying to all users, or from a per-user location. Remmina (refer to Section 14.2, “Remmina: the remote desktop client”) asks for the location of certificates when initializing the connection to the remote VNC session.

Virtual Machine Manager (virt-manager)

To connect to the remote host, Virtual Machine Manager requires the setup explained in Section 12.3.2.3, “Configuring the client and testing the setup”. To be able to connect via VNC, the client certificates also need to be placed in the following locations:

System-wide location

/etc/pki/CA/cacert.pem

/etc/pki/libvirt-vnc/clientcert.pem

/etc/pki/libvirt-vnc/private/clientkey.pem

Per-user location

/etc/pki/CA/cacert.pem

~/.pki/libvirt-vnc/clientcert.pem

~/.pki/libvirt-vnc/private/clientkey.pem

virt-viewer

virt-viewer only accepts certificates from a system-wide location:

/etc/pki/CA/cacert.pem

/etc/pki/libvirt-vnc/clientcert.pem

/etc/pki/libvirt-vnc/private/clientkey.pem

Important: Restrict access to certificates

Make sure to restrict access to certificates as explained in Section 12.3.2.5, “Restricting access (security considerations)”.

Each x509 certificate consists of two pieces: the public certificate and a private key. A client can only authenticate using both pieces. Therefore, any user that has read access to the client certificate and its private key can access your VM Host Server. On the other hand, an arbitrary machine equipped with the full server certificate can pretend to be the VM Host Server. Since this is probably not desirable, access to at least the private key files needs to be restricted as much as possible. The easiest way to control access to a key file is to use access permissions.

tls_allowed_dn_list = [
   "C=US,L=Provo,O=SUSE Linux Products GmbH,OU=*,CN=venus.example.com,EMAIL=*",
   "C=DE,L=Nuremberg,O=SUSE Linux Products GmbH,OU=Documentation,CN=*"]

> certtool -i --infile /etc/pki/libvirt/clientcert.pem | grep "Subject:"

> sudo systemctl restart libvirtd

A direct user authentication via TLS is not possible—this is handled indirectly on each client via the read permissions for the certificates as explained in Section 12.3.2.5, “Restricting access (security considerations)”. However, if a central, server-based user authentication is needed, libvirt also allows to use SASL (Simple Authentication and Security Layer) on top of TLS for direct user authentication. See Section 12.1.1.3, “User name and password authentication with SASL” for configuration details.