libvirt: TLS x509 certificate setup
Description: libvirt, virtualization, virtualization API

# TLS x509 certificate setup

Contents

- 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

# 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 the client and server | CA's certificate | n/a
$HOME/.pki/cacert.pem | Installed on the client | CA's certificate | n/a
/etc/pki/libvirt/private/serverkey.pem | Installed on the server | Server's private key | n/a
/etc/pki/libvirt/servercert.pem | Installed on the server | Server's certificate signed by the CA | CommonName (CN) must be the hostname of the server as it is seen by clients. All hostname and IP address variants that might be used to reach the server should be listed in Subject Alt Name fields.
/etc/pki/libvirt/private/clientkey.pem | Installed on the client | Client's private key | n/a
/etc/pki/libvirt/clientcert.pem | Installed on the client | Client's certificate signed by the CA | Distinguished Name (DN) can be checked against an access control list (tls_allowed_dn_list).
$HOME/.pki/libvirt/clientkey.pem | Installed on the client | Client's private key | n/a
$HOME/.pki/libvirt/clientcert.pem | Installed on the client | Client's certificate signed by the CA (see Issuing client certificates) | Distinguished Name (DN) can be checked against an access control list (tls_allowed_dn_list).

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 first. If the required CA certificate cannot be found, then the global default location (/etc/pki/CA/cacert.pem) will be used. Likewise, if either the client certificate or the client key cannot be found, then the global default locations (/etc/pki/libvirt/clientcert.pem, /etc/pki/libvirt/private/clientkey.pem) will be used.

- For the root user, the global default locations will always 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=Libvirt Project
    Issuer: CN=Libvirt Project
    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 containing one or more hostnames and/or IP addresses. Historically the CommonName (CN) field would contain the hostname of the server and would match the hostname used in the URI that clients pass to libvirt. In most TLS implementations the CN field is considered legacy data. The preferential mechanism is to use Subject Alt Name (SAN) extension fields to validate against. In the future use of the CN field for validation may be discontinued entirely, so it is strongly recommended to include the SAN fields.

In the example below, clients will be connecting to the server using a URI of qemu://compute1.libvirt.org/system, so the CN must be "compute1.libvirt.org".

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. The template file will contain a number of fields to define the server as follows:

    organization = Name of your organization
    cn = compute1.libvirt.org
    dns_name = compute1
    dns_name = compute1.libvirt.org
    ip_address = 10.0.0.74
    ip_address = 192.168.1.24
    ip_address = 2001:cafe::74
    ip_address = fe20::24
    tls_www_server
    encryption_key
    signing_key

The 'cn' field should refer to the fully qualified public hostname of the server. For the SAN extension data, there must also be one or more 'dns_name' fields that contain all possible hostnames that can be reasonably used by clients to reach the server, both with and without domain name qualifiers. If clients are likely to connect to the server by IP address, then one or more 'ip_address' fields should also be added.

Use the template file as input to a certtool command to sign the server certificate:

    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=Libvirt Project,CN=compute1.libvirt.org
    Issuer: CN=Libvirt Project
    Signature Algorithm: RSA-SHA
    Validity:
            Not Before: Wed Oct 04 09:09:44 UTC 2017
            Not After: Thu Oct 04 09:09:44 UTC 2018
    Extensions:
            Basic Constraints (critical):
                    Certificate Authority (CA): FALSE
            Subject Alternative Name (not critical):
                    DNSname: compute1
                    DNSname: compute1.libvirt.org
                    IPAddress: 10.0.0.74
                    IPAddress: 192.168.1.24
                    IPAddress: 2001:cafe::74
                    IPAddress: fe20::24

Note the "Issuer" CN is "Libvirt Project" (the CA) and the "Subject" CN is "compute1.libvirt.org" (the server). Notice that the hostname listed in the CN must also be duplicated as a DNSname entry

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:

    C=GB,ST=London,L=London,O=Libvirt Project,CN=name_of_client

The process is the same as for Issuing server certificates 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 = Libvirt Project
       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.