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documentation.suse.com / SUSE Linux Enterprise Server-Dokumentation / Virtualization Guide / Managing virtual machines with libvirt / Connecting and authorizing
Applies to SUSE Linux Enterprise Server 15 SP5

12 Connecting and authorizing

 

Managing several VM Host Servers, each hosting multiple VM Guests, quickly becomes difficult. One benefit of libvirt is the ability to connect to several VM Host Servers at once, providing a single interface to manage all VM Guests and to connect to their graphical console.

To ensure only authorized users can connect, libvirt offers several connection types (via TLS, SSH, Unix sockets, and TCP) that can be combined with different authorization mechanisms (socket, Polkit, SASL and Kerberos).

12.1 Authentication

 

The power to manage VM Guests and to access their graphical console is something that should be restricted to a well-defined circle of persons. To achieve this goal, you can use the following authentication techniques on the VM Host Server:

  • Access control for Unix sockets with permissions and group ownership. This method is available for libvirtd connections only.

  • Access control for Unix sockets with Polkit. This method is available for local libvirtd connections only.

  • User name and password authentication with SASL (Simple Authentication and Security Layer). This method is available for both libvirtd and VNC connections. Using SASL does not require real user accounts on the server, since it uses its own database to store user names and passwords. Connections authenticated with SASL are encrypted.

  • Kerberos authentication. This method, available for libvirtd connections only, is not covered in this manual. Refer to https://libvirt.org/auth.html#ACL_server_kerberos for details.

  • Single password authentication. This method is available for VNC connections only.

Important
Important: Authentication for libvirtd and VNC needs to be configured separately

Access to the VM Guest's management functions (via libvirtd) and to its graphical console always needs to be configured separately. When restricting access to the management tools, these restrictions do not automatically apply to VNC connections!

When accessing VM Guests from remote via TLS/SSL connections, access can be indirectly controlled on each client by restricting read permissions to the certificate's key file to a certain group. See Section 12.3.2.5, “Restricting access (security considerations)” for details.

12.1.1 libvirtd authentication

 

libvirtd authentication is configured in /etc/libvirt/libvirtd.conf. The configuration made here applies to all libvirt tools such as the Virtual Machine Manager or virsh.

libvirt offers two sockets: a read-only socket for monitoring purposes and a read-write socket to be used for management operations. Access to both sockets can be configured independently. By default, both sockets are owned by root.root. Default access permissions on the read-write socket are restricted to the user root (0700) and fully open on the read-only socket (0777).

The following instructions describe how to configure access permissions for the read-write socket. The same instructions also apply to the read-only socket. All configuration steps need to be carried out on the VM Host Server.

Note
Note: Default authentication settings on SUSE Linux Enterprise Server

The default authentication method on SUSE Linux Enterprise Server is access control for Unix sockets. Only the user root may authenticate. When accessing the libvirt tools as a non-root user directly on the VM Host Server, you need to provide the root password through Polkit once. You are then granted access for the current and for future sessions.

Alternatively, you can configure libvirt to allow “system” access to non-privileged users. See Section 12.2.1, ““system” access for non-privileged users” for details.

Recommended authorization methods
 
Local connections
Section 12.1.1.2, “Local access control for Unix sockets with Polkit”
Section 12.1.1.1, “Access control for Unix sockets with permissions and group ownership”
Remote tunnel over SSH
Section 12.1.1.1, “Access control for Unix sockets with permissions and group ownership”
Remote TLS/SSL connection
Section 12.1.1.3, “User name and password authentication with SASL”
none (access controlled on the client side by restricting access to the certificates)

12.1.1.1 Access control for Unix sockets with permissions and group ownership

 

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.

  1. In case it does not exist, create the group that should own the socket: 

    > sudo groupadd libvirt
    Important
    Important: Group needs to exist

    The group must exist before restarting libvirtd. If not, the restart fails.

  2. Add the desired users to the group: 

    > sudo usermod --append --groups libvirt tux

  3. Change the configuration in /etc/libvirt/libvirtd.conf as follows: 

    unix_sock_group = "libvirt"1
unix_sock_rw_perms = "0770"2
auth_unix_rw = "none"3

    1

    Group ownership is set to the group libvirt.

    2

    Sets the access permissions for the socket (srwxrwx---).

    3

    Disables other authentication methods (Polkit or SASL). Access is solely controlled by the socket permissions.

  4. Restart libvirtd: 

    > sudo systemctl start libvirtd

12.1.1.2 Local access control for Unix sockets with Polkit

 

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
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:

  • org.libvirt.unix.monitor: accessing the read-only socket

  • org.libvirt.unix.manage: accessing the read-write socket

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;
  }
});

12.1.1.3 User name and password authentication with SASL

 

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
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
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 may 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:

  1. Change the configuration in /etc/libvirt/libvirtd.conf as follows:

    1. To enable SASL for TCP connections: 

      auth_tcp = "sasl"

    2. To enable SASL for TLS/SSL connections: 

      auth_tls = "sasl"

  2. Restart libvirtd: 

    > sudo systemctl restart libvirtd

  3. The libvirt SASL configuration file is located at /etc/sasl2/libvirtd.conf. Normally, there is no need to change the defaults. However, if using SASL on top of TLS, you may turn off session encryption to avoid additional overhead (TLS connections are already encrypted) by commenting the line setting the mech_list parameter. Only do this for TLS/SASL. For TCP connections, this parameter must be set to digest-md5. 

    #mech_list: digest-md5

  4. By default, no SASL users are configured, so no logins are possible. Use the following commands to manage users:

    Add the user tux
    saslpasswd2 -a libvirt tux
    Delete the user tux
    saslpasswd2 -a libvirt -d tux
    List existing users
    sasldblistusers2 -f /etc/libvirt/passwd.db
Tip
Tip: virsh and SASL authentication

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

12.1.2 VNC authentication

 

Since access to the graphical console of a VM Guest is not controlled by libvirt, but by the specific hypervisor, it is always necessary to additionally configure VNC authentication. The main configuration file is /etc/libvirt/<hypervisor>.conf. This section describes the QEMU/KVM hypervisor, so the target configuration file is /etc/libvirt/qemu.conf.

Note
Note: VNC authentication for Xen

In contrast with KVM, Xen does not yet offer VNC authentication more sophisticated than setting a password on a per-VM basis. See the <graphics type='vnc'... libvirt configuration option below.

Two authentication types are available: SASL and single-password authentication. If you are using SASL for libvirt authentication, it is strongly recommended to use it for VNC authentication as well—it is possible to share the same database.

A third method to restrict access to the VM Guest is to enable the use of TLS encryption on the VNC server. This requires the VNC clients to have access to x509 client certificates. By restricting access to these certificates, access can indirectly be controlled on the client side. Refer to Section 12.3.2.4.2, “VNC over TLS/SSL: client configuration” for details.

12.1.2.1 User name and password authentication with SASL

 

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:

  1. Create a SASL configuration file. It is recommended to use the existing libvirt file. If you have already configured SASL for libvirt and are planning to use the same settings, including the same user name and password database, a simple link is suitable: 

    > sudo ln -s /etc/sasl2/libvirt.conf /etc/sasl2/qemu.conf

    If are setting up SASL for VNC only or you are planning to use a different configuration than for libvirt, copy the existing file to use as a template: 

    > sudo cp /etc/sasl2/libvirt.conf /etc/sasl2/qemu.conf

    Then edit it according to your needs.

  2. Change the configuration in /etc/libvirt/qemu.conf as follows: 

    vnc_listen = "0.0.0.0"
vnc_sasl = 1
sasldb_path: /etc/libvirt/qemu_passwd.db

    The first parameter enables VNC to listen on all public interfaces (rather than to the local host only), and the second parameter enables SASL authentication.

  3. By default, no SASL users are configured, so no logins are possible. Use the following commands to manage users:

    Add the user tux
    > saslpasswd2 -f /etc/libvirt/qemu_passwd.db -a qemu tux
    Delete the user tux
    > saslpasswd2 -f /etc/libvirt/qemu_passwd.db -a qemu -d tux
    List existing users
    > sasldblistusers2 -f /etc/libvirt/qemu_passwd.db

  4. Restart libvirtd: 

    > sudo systemctl restart libvirtd

  5. Restart all VM Guests that have been running before changing the configuration. VM Guests that have not been restarted cannot use SASL authentication for VNC connects.

Note
Note: Supported VNC viewers

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

12.1.2.2 Single password authentication

 

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 editing the VM Guest's configuration files.

Note
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 protects your virtual machines with a “fallback” password if you forget to set a per-machine password. The global password is only used if no other password is set for the machine.

Procedure 12.1: Setting a global VNC password
 

  1. Change the configuration in /etc/libvirt/qemu.conf as follows: 

    vnc_listen = "0.0.0.0"
vnc_password = "PASSWORD"

    The first parameter enables VNC to listen on all public interfaces (rather than to the local host only), and the second parameter sets the password. The maximum length of the password is eight characters.

  2. Restart libvirtd: 

    > sudo systemctl restart libvirtd

  3. Restart all VM Guests that have been running before changing the configuration. VM Guests that have not been restarted cannot use password authentication for VNC connects.

Procedure 12.2: Setting a VM Guest specific VNC password
 

  1. Change the configuration in /etc/libvirt/qemu.conf as follows to enable VNC to listen on all public interfaces (rather than to the local host only). 

    vnc_listen = "0.0.0.0"

  2. Open the VM Guest's XML configuration file in an editor. Replace VM_NAME in the following example with the name of the VM Guest. The editor that is used defaults to $EDITOR. If that variable is not set, vi is used. 

    > virsh edit VM_NAME

  3. Search for the element <graphics> with the attribute type='vnc', for example: 

    <graphics type='vnc' port='-1' autoport='yes'/>

  4. Add the passwd=PASSWORD attribute, save the file and exit the editor. The maximum length of the password is eight characters. 

    <graphics type='vnc' port='-1' autoport='yes' passwd='PASSWORD'/>

  5. Restart libvirtd: 

    > sudo systemctl restart libvirtd

  6. Restart all VM Guests that have been running before changing the configuration. VM Guests that have not been restarted cannot use password authentication for VNC connects.

Warning
Warning: Security of the VNC protocol

The VNC protocol is not considered to be safe. Although the password is sent encrypted, it may 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 Abschnitt 14.2, „Remmina: Remote-Desktop-Client“) support both methods.

12.2 Connecting to a VM Host Server

 

To connect to a hypervisor with libvirt, you need to specify a uniform resource identifier (URI). This URI is needed with virsh and virt-viewer (except when working as root on the VM Host Server) and is optional for the Virtual Machine Manager. Although the latter can be called with a connection parameter (for example, virt-manager -c qemu:///system), it also offers a graphical interface to create connection URIs. See Section 12.2.2, “Managing connections with Virtual Machine Manager” for details. 

HYPERVISOR1+PROTOCOL2://USER@REMOTE3/CONNECTION_TYPE4

1

Specify the hypervisor. SUSE Linux Enterprise Server currently supports the following hypervisors: test (dummy for testing), qemu (KVM), and xen (Xen). This parameter is mandatory.

2

When connecting to a remote host, specify the protocol here. It can be one of: ssh (connection via SSH tunnel), tcp (TCP connection with SASL/Kerberos authentication), tls (TLS/SSL encrypted connection with authentication via x509 certificates).

3

When connecting to a remote host, specify the user name and the remote host name. If no user name is specified, the user name that has called the command ($USER) is used. See below for more information. For TLS connections, the host name needs to be specified exactly as in the x509 certificate.

4

When connecting to the QEMU/KVM hypervisor, two connection types are accepted: system for full access rights, or session for restricted access. Since session access is not supported on SUSE Linux Enterprise Server, this documentation focuses on system access.

Example hypervisor connection URIs
 
test:///default

Connect to the local dummy hypervisor. Useful for testing.

qemu:///system or xen:///system

Connect to the QEMU/Xen hypervisor on the local host having full access (type system).

qemu+ssh://tux@mercury.example.com/system or xen+ssh://tux@mercury.example.com/system

Connect to the QEMU/Xen hypervisor on the remote host mercury.example.com. The connection is established via an SSH tunnel.

qemu+tls://saturn.example.com/system or xen+tls://saturn.example.com/system

Connect to the QEMU/Xen hypervisor on the remote host mercury.example.com. The connection is established using TLS/SSL.

For more details and examples, refer to the libvirt documentation at https://libvirt.org/uri.html.

Note
Note: User names in URIs

A user name needs to be specified when using Unix socket authentication (regardless of whether using the user/password authentication scheme or Polkit). This applies to all SSH and local connections.

There is no need to specify a user name when using SASL authentication (for TCP or TLS connections) or when doing no additional server-side authentication for TLS connections. With SASL, the user name is not evaluated—you are prompted for an SASL user/password combination in any case.

12.2.1 “system” access for non-privileged users

 

As mentioned above, a connection to the QEMU hypervisor can be established using two different protocols: session and system. A “session” connection is spawned with the same privileges as the client program. Such a connection is intended for desktop virtualization, since it is restricted (for example, no USB/PCI device assignments, no virtual network setup, limited remote access to libvirtd).

The “system” connection intended for server virtualization has no functional restrictions but is, by default, only accessible by root. However, with the addition of the DAC (Discretionary Access Control) driver to libvirt, it is now possible to grant non-privileged users “system” access. To grant “system” access to the user tux, proceed as follows:

Procedure 12.3: Granting “system” access to a regular user
 

  1. Enable access via Unix sockets, as described in Section 12.1.1.1, “Access control for Unix sockets with permissions and group ownership”. In that example, access to libvirt is granted to all members of the group libvirt and tux made a member of this group. This ensures that tux can connect using virsh or Virtual Machine Manager.

  2. Edit /etc/libvirt/qemu.conf and change the configuration as follows: 

    user = "tux"
group = "libvirt"
dynamic_ownership = 1

    This ensures that the VM Guests are started by tux and that resources bound to the guest (for example virtual disks) can be accessed and modified by tux.

  3. Make tux a member of the group kvm: 

    > sudo usermod --append --groups kvm tux

    This step is needed to grant access to /dev/kvm, which is required to start VM Guests.

  4. Restart libvirtd: 

    > sudo systemctl restart libvirtd

12.2.2 Managing connections with Virtual Machine Manager

 

The Virtual Machine Manager uses a Connection for every VM Host Server it manages. Each connection contains all VM Guests on the respective host. By default, a connection to the local host is already configured and connected.

All configured connections are displayed in the Virtual Machine Manager main window. Active connections are marked with a small triangle, which you can click to fold or unfold the list of VM Guests for this connection.

Inactive connections are listed gray and are marked with Not Connected. Either double-click or right-click it and choose Connect from the context menu. You can also Delete an existing connection from this menu.

Note
Note: Editing existing connections

It is not possible to edit an existing connection. To change a connection, create a new one with the desired parameters and delete the “old” one.

To add a new connection in the Virtual Machine Manager, proceed as follows:

  1. Choose File › Add Connection

  2. Choose the host's Hypervisor (Xen or QEMU/KVM)

  3. (Optional) To set up a remote connection, choose Connect to remote host. For more information, see Section 12.3, “Configuring remote connections”.

    In case of a remote connection, specify the Hostname of the remote machine in the format USERNAME@REMOTE _HOST.

    Important
    Important: Specifying a user name

    There is no need to specify a user name for TCP and TLS connections: in these cases, it is not evaluated. However, for SSH connections, specifying a user name is necessary when you want to connect as a user other than root.

  4. If you do not want the connection to be automatically started when starting the Virtual Machine Manager, deactivate Autoconnect.

  5. Finish the configuration by clicking Connect.

12.3 Configuring remote connections

 

A major benefit of libvirt is the ability to manage VM Guests on different remote hosts from a central location. This section gives detailed instructions on how to configure server and client to allow remote connections.

12.3.1 Remote tunnel over SSH (qemu+ssh or xen+ssh)

 

Enabling a remote connection that is tunneled over SSH on the VM Host Server only requires the ability to accept SSH connections. Make sure the SSH daemon is started (systemctl status sshd) and that the ports for service SSH are opened in the firewall.

User authentication for SSH connections can be done using traditional file user/group ownership and permissions as described in Section 12.1.1.1, “Access control for Unix sockets with permissions and group ownership”. Connecting as user tux (qemu+ssh://tuxsIVname;/system or xen+ssh://tuxsIVname;/system) works out of the box and does not require additional configuration on the libvirt side.

When connecting via SSH qemu+ssh://USER@SYSTEM or xen+ssh://USER@SYSTEM you need to provide the password for USER. This can be avoided by copying your public key to ~USER/.ssh/authorized_keys on the VM Host Server as explained in Section 22.6, “Public key authentication”. Using gnome-keyring on the machine from which you are connecting adds even more convenience. For more information, see Section 22.9, “Automated public key logins with gnome-keyring”.

12.3.2 Remote TLS/SSL connection with x509 certificate (qemu+tls or xen+tls)

 

Using TCP connections with TLS/SSL encryption and authentication via x509 certificates is much more complicated to set up than SSH, but it is a lot more scalable. Use this method if you need to manage several VM Host Servers with a varying number of administrators.

12.3.2.1 Basic concept

 

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
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”.

12.3.2.2 Configuring the VM Host Server

 

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.

  1. Create the server certificate and export it together with the respective CA certificate.

  2. Create the following directories on the VM Host Server: 

    > sudo mkdir -p /etc/pki/CA/ /etc/pki/libvirt/private/

    Install the certificates as follows: 

    > sudo /etc/pki/CA/cacert.pem
> sudo /etc/pki/libvirt/servercert.pem
> sudo /etc/pki/libvirt/private/serverkey.pem
    Important
    Important: Restrict access to certificates

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

  3. Enable TLS support by enabling the relevant socket and restarting libvirtd: 

    > sudo systemctl stop libvirtd.service
> sudo systemctl enable --now libvirtd-tls.socket
> sudo systemctl start libvirtd.service

  4. By default, libvirt uses the TCP port 16514 for accepting secure TLS connections. Open this port in the firewall.

Important
Important: Restarting libvirtd with TLS enabled

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

12.3.2.3 Configuring the client and testing the setup

 

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

  1. Create the client certificate and export it together with the respective CA certificate.

  2. Create the following directories on the client: 

    > sudo mkdir -p /etc/pki/CA/ /etc/pki/libvirt/private/

    Install the certificates as follows: 

    > sudo /etc/pki/CA/cacert.pem
> sudo /etc/pki/libvirt/clientcert.pem
> sudo /etc/pki/libvirt/private/clientkey.pem
    Important
    Important: Restrict access to certificates

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

  3. Test the client/server setup by issuing the following command. Replace mercury.example.com with the name of your VM Host Server. Specify the same fully qualified host name as used when creating the server certificate. 

    #QEMU/KVM
virsh -c qemu+tls://mercury.example.com/system list --all

#Xen
virsh -c xen+tls://mercury.example.com/system list --all

    If your setup is correct, you can see a list of all VM Guests registered with libvirt on the VM Host Server.

12.3.2.4 Enabling VNC for TLS/SSL connections

 

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 Abschnitt 14.2, „Remmina: Remote-Desktop-Client“).

12.3.2.4.1 VNC over TLS/SSL: VM Host Server configuration
 

To access the graphical console via VNC over TLS/SSL, you need to configure the VM Host Server as follows:

  1. Open ports for the service VNC in your firewall.

  2. Create a directory /etc/pki/libvirt-vnc and link the certificates into this directory as follows: 

    > sudo mkdir -p /etc/pki/libvirt-vnc && cd /etc/pki/libvirt-vnc
> sudo ln -s /etc/pki/CA/cacert.pem ca-cert.pem
> sudo ln -s /etc/pki/libvirt/servercert.pem server-cert.pem
> sudo ln -s /etc/pki/libvirt/private/serverkey.pem server-key.pem

  3. Edit /etc/libvirt/qemu.conf and set the following parameters: 

    vnc_listen = "0.0.0.0"
        vnc_tls = 1
        vnc_tls_x509_verify = 1

  4. Restart the libvirtd: 

    > sudo systemctl restart libvirtd
    Important
    Important: VM Guests need to be restarted

    The VNC TLS setting is only set when starting a VM Guest. Therefore, you need to restart all machines that have been running before making the configuration change.

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. However, 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 Abschnitt 14.2, „Remmina: 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
Important: Restrict access to certificates

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

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 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.

Server certificates

Server certificates need to be readable for QEMU processes. On SUSE Linux Enterprise Server QEMU, processes started from libvirt tools are owned by root, so it is sufficient if the root can read the certificates: 

> chmod 700 /etc/pki/libvirt/private/
> chmod 600 /etc/pki/libvirt/private/serverkey.pem

If you change the ownership for QEMU processes in /etc/libvirt/qemu.conf, you also need to adjust the ownership of the key file.

System-wide client certificates

To control access to a key file that is available system-wide, restrict read access to a certain group, so that only members of that group can read the key file. In the following example, a group libvirt is created, and group ownership of the clientkey.pem file and its parent directory is set to libvirt. Afterward, the access permissions are restricted to owner and group. Finally, the user tux is added to the group libvirt, and thus can access the key file. 

CERTPATH="/etc/pki/libvirt/"
# create group libvirt
groupadd libvirt
# change ownership to user root and group libvirt
chown root.libvirt $CERTPATH/private $CERTPATH/clientkey.pem
# restrict permissions
chmod 750 $CERTPATH/private
chmod 640 $CERTPATH/private/clientkey.pem
# add user tux to group libvirt
usermod --append --groups libvirt tux
Per-user certificates

User-specific client certificates for accessing the graphical console of a VM Guest via VNC need to be placed in the user's home directory in ~/.pki. Contrary to SSH, for example, the VNC viewer using these certificates does not check the access permissions of the private key file. Therefore, it is solely the user's responsibility to make sure the key file is not readable by others.

12.3.2.5.1 Restricting access from the server side
 

By default, every client that is equipped with appropriate client certificates may connect to a VM Host Server accepting TLS connections. Therefore, it is possible to use additional server-side authentication with SASL as described in Section 12.1.1.3, “User name and password authentication with SASL”.

It is also possible to restrict access with a allowlist of DNs (distinguished names), so only clients with a certificate matching a DN from the list can connect.

Add a list of allowed DNs to tls_allowed_dn_list in /etc/libvirt/libvirtd.conf. This list may contain wild cards. Do not specify an empty list, since that would result in refusing all connections. 

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=*"]

Get the distinguished name of a certificate with the following command: 

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

Restart libvirtd after having changed the configuration: 

> sudo systemctl restart libvirtd

12.3.2.6 Central user authentication with SASL for TLS sockets

 

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.

12.3.2.7 Troubleshooting

 
12.3.2.7.1 Virtual Machine Manager/virsh cannot connect to server
 

Check the following in the given order:

Is it a firewall issue (TCP port 16514 needs to be open on the server)?
Is the client certificate (certificate and key) readable by the user that has started Virtual Machine Manager/ virsh?
Has the same full qualified host name as in the server certificate been specified with the connection?
Is TLS enabled on the server (listen_tls = 1)?
Has libvirtd been restarted on the server?
12.3.2.7.2 VNC connection fails
 

Ensure that you can connect to the remote server using Virtual Machine Manager. If so, check whether the virtual machine on the server has been started with TLS support. The virtual machine's name in the following example is sles. 

> ps ax | grep qemu | grep "\-name sles" | awk -F" -vnc " '{ print FS $2 }'

If the output does not begin with a string similar to the following, the machine has not been started with TLS support and must be restarted. 

 -vnc 0.0.0.0:0,tls,x509verify=/etc/pki/libvirt