TLS setup for network services

Almost all network services in QEMU have the ability to use TLS for session data encryption, along with x509 certificates for simple client authentication. What follows is a description of how to generate certificates suitable for usage with QEMU, and applies to the VNC server, character devices with the TCP backend, NBD server and client, and migration server and client.

At a high level, QEMU requires certificates and private keys to be provided in PEM format. Aside from the core fields, the certificates should include various extension data sets, including v3 basic constraints data, key purpose, key usage and subject alt name.

The GnuTLS package includes a command called certtool which can be used to easily generate certificates and keys in the required format with expected data present. Alternatively a certificate management service may be used.

At a minimum it is necessary to setup a certificate authority, and issue certificates to each server. If using x509 certificates for authentication, then each client will also need to be issued a certificate.

Assuming that the QEMU network services will only ever be exposed to clients on a private intranet, there is no need to use a commercial certificate authority to create certificates. A self-signed CA is sufficient, and in fact likely to be more secure since it removes the ability of malicious 3rd parties to trick the CA into mis-issuing certs for impersonating your services. The only likely exception where a commercial CA might be desirable is if enabling the VNC websockets server and exposing it directly to remote browser clients. In such a case it might be useful to use a commercial CA to avoid needing to install custom CA certs in the web browsers.

The recommendation is for the server to keep its certificates in either /etc/pki/qemu or for unprivileged users in $HOME/.pki/qemu.

Setup the Certificate Authority

This step only needs to be performed once per organization / organizational unit. First the CA needs a private key. This key must be kept VERY secret and secure. If this key is compromised the entire trust chain of the certificates issued with it is lost.

# certtool --generate-privkey > ca-key.pem

To generate a self-signed certificate requires one core piece of information, the name of the organization. A template file ca.info should be populated with the desired data to avoid having to deal with interactive prompts from certtool:

# cat > ca.info <<EOF
cn = Name of your organization
ca
cert_signing_key
EOF
# certtool --generate-self-signed \
           --load-privkey ca-key.pem
           --template ca.info \
           --outfile ca-cert.pem

The ca keyword in the template sets the v3 basic constraints extension to indicate this certificate is for a CA, while cert_signing_key sets the key usage extension to indicate this will be used for signing other keys. The generated ca-cert.pem file should be copied to all servers and clients wishing to utilize TLS support in the VNC server. The ca-key.pem must not be disclosed/copied anywhere except the host responsible for issuing certificates.

Issuing server certificates

Each server (or host) needs to be issued with a key and certificate. When connecting the certificate is sent to the client which validates it against the CA certificate. The core pieces of information for a server certificate are the hostnames and/or IP addresses that will be used by clients when connecting. The hostname / IP address that the client specifies when connecting will be validated against the hostname(s) and IP address(es) recorded in the server certificate, and if no match is found the client will close the connection.

Thus it is recommended that the server certificate include both the fully qualified and unqualified hostnames. If the server will have permanently assigned IP address(es), and clients are likely to use them when connecting, they may also be included in the certificate. Both IPv4 and IPv6 addresses are supported. Historically certificates only included 1 hostname in the CN field, however, usage of this field for validation is now deprecated. Instead modern TLS clients will validate against the Subject Alt Name extension data, which allows for multiple entries. In the future usage of the CN field may be discontinued entirely, so providing SAN extension data is strongly recommended.

On the host holding the CA, create template files containing the information for each server, and use it to issue server certificates.

# cat > server-hostNNN.info <<EOF
organization = Name  of your organization
cn = hostNNN.foo.example.com
dns_name = hostNNN
dns_name = hostNNN.foo.example.com
ip_address = 10.0.1.87
ip_address = 192.8.0.92
ip_address = 2620:0:cafe::87
ip_address = 2001:24::92
tls_www_server
encryption_key
signing_key
EOF
# certtool --generate-privkey > server-hostNNN-key.pem
# certtool --generate-certificate \
           --load-ca-certificate ca-cert.pem \
           --load-ca-privkey ca-key.pem \
           --load-privkey server-hostNNN-key.pem \
           --template server-hostNNN.info \
           --outfile server-hostNNN-cert.pem

The dns_name and ip_address fields in the template are setting the subject alt name extension data. The tls_www_server keyword is the key purpose extension to indicate this certificate is intended for usage in a web server. Although QEMU network services are not in fact HTTP servers (except for VNC websockets), setting this key purpose is still recommended. The encryption_key and signing_key keyword is the key usage extension to indicate this certificate is intended for usage in the data session.

The server-hostNNN-key.pem and server-hostNNN-cert.pem files should now be securely copied to the server for which they were generated, and renamed to server-key.pem and server-cert.pem when added to the /etc/pki/qemu directory on the target host. The server-key.pem file is security sensitive and should be kept protected with file mode 0600 to prevent disclosure.

Issuing client certificates

The QEMU x509 TLS credential setup defaults to enabling client verification using certificates, providing a simple authentication mechanism. If this default is used, each client also needs to be issued a certificate. The client certificate contains enough metadata to uniquely identify the client with the scope of the certificate authority. The client certificate would typically include fields for organization, state, city, building, etc.

Once again on the host holding the CA, create template files containing the information for each client, and use it to issue client certificates.

# cat > client-hostNNN.info <<EOF
country = GB
state = London
locality = City Of London
organization = Name of your organization
cn = hostNNN.foo.example.com
tls_www_client
encryption_key
signing_key
EOF
# certtool --generate-privkey > client-hostNNN-key.pem
# certtool --generate-certificate \
           --load-ca-certificate ca-cert.pem \
           --load-ca-privkey ca-key.pem \
           --load-privkey client-hostNNN-key.pem \
           --template client-hostNNN.info \
           --outfile client-hostNNN-cert.pem

The subject alt name extension data is not required for clients, so the the dns_name and ip_address fields are not included. The tls_www_client keyword is the key purpose extension to indicate this certificate is intended for usage in a web client. Although QEMU network clients are not in fact HTTP clients, setting this key purpose is still recommended. The encryption_key and signing_key keyword is the key usage extension to indicate this certificate is intended for usage in the data session.

The client-hostNNN-key.pem and client-hostNNN-cert.pem files should now be securely copied to the client for which they were generated, and renamed to client-key.pem and client-cert.pem when added to the /etc/pki/qemu directory on the target host. The client-key.pem file is security sensitive and should be kept protected with file mode 0600 to prevent disclosure.

If a single host is going to be using TLS in both a client and server role, it is possible to create a single certificate to cover both roles. This would be quite common for the migration and NBD services, where a QEMU process will be started by accepting a TLS protected incoming migration, and later itself be migrated out to another host. To generate a single certificate, simply include the template data from both the client and server instructions in one.

# cat > both-hostNNN.info <<EOF
country = GB
state = London
locality = City Of London
organization = Name of your organization
cn = hostNNN.foo.example.com
dns_name = hostNNN
dns_name = hostNNN.foo.example.com
ip_address = 10.0.1.87
ip_address = 192.8.0.92
ip_address = 2620:0:cafe::87
ip_address = 2001:24::92
tls_www_server
tls_www_client
encryption_key
signing_key
EOF
# certtool --generate-privkey > both-hostNNN-key.pem
# certtool --generate-certificate \
           --load-ca-certificate ca-cert.pem \
           --load-ca-privkey ca-key.pem \
           --load-privkey both-hostNNN-key.pem \
           --template both-hostNNN.info \
           --outfile both-hostNNN-cert.pem

When copying the PEM files to the target host, save them twice, once as server-cert.pem and server-key.pem, and again as client-cert.pem and client-key.pem.

TLS x509 credential configuration

QEMU has a standard mechanism for loading x509 credentials that will be used for network services and clients. It requires specifying the tls-creds-x509 class name to the --object command line argument for the system emulators. Each set of credentials loaded should be given a unique string identifier via the id parameter. A single set of TLS credentials can be used for multiple network backends, so VNC, migration, NBD, character devices can all share the same credentials. Note, however, that credentials for use in a client endpoint must be loaded separately from those used in a server endpoint.

When specifying the object, the dir parameters specifies which directory contains the credential files. This directory is expected to contain files with the names mentioned previously, ca-cert.pem, server-key.pem, server-cert.pem, client-key.pem and client-cert.pem as appropriate. It is also possible to include a set of pre-generated Diffie-Hellman (DH) parameters in a file dh-params.pem, which can be created using the certtool --generate-dh-params command. If omitted, QEMU will dynamically generate DH parameters when loading the credentials.

The endpoint parameter indicates whether the credentials will be used for a network client or server, and determines which PEM files are loaded.

The verify parameter determines whether x509 certificate validation should be performed. This defaults to enabled, meaning clients will always validate the server hostname against the certificate subject alt name fields and/or CN field. It also means that servers will request that clients provide a certificate and validate them. Verification should never be turned off for client endpoints, however, it may be turned off for server endpoints if an alternative mechanism is used to authenticate clients. For example, the VNC server can use SASL to authenticate clients instead.

To load server credentials with client certificate validation enabled

qemu-system-x86_64 -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server

while to load client credentials use

qemu-system-x86_64 -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=client

Network services which support TLS will all have a tls-creds parameter which expects the ID of the TLS credentials object. For example with VNC:

qemu-system-x86_64 -vnc 0.0.0.0:0,tls-creds=tls0

TLS Pre-Shared Keys (PSK)

Instead of using certificates, you may also use TLS Pre-Shared Keys (TLS-PSK). This can be simpler to set up than certificates but is less scalable.

Use the GnuTLS psktool program to generate a keys.psk file containing one or more usernames and random keys:

mkdir -m 0700 /tmp/keys
psktool -u rich -p /tmp/keys/keys.psk

TLS-enabled servers such as qemu-nbd can use this directory like so:

qemu-nbd \
  -t -x / \
  --object tls-creds-psk,id=tls0,endpoint=server,dir=/tmp/keys \
  --tls-creds tls0 \
  image.qcow2

When connecting from a qemu-based client you must specify the directory containing keys.psk and an optional username (defaults to “qemu”):

qemu-img info \
  --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=rich,endpoint=client \
  --image-opts \
  file.driver=nbd,file.host=localhost,file.port=10809,file.tls-creds=tls0,file.export=/