xref: /openbmc/qemu/docs/system/tls.rst (revision 2e1cacfb)
1.. _network_005ftls:
2
3TLS setup for network services
4------------------------------
5
6Almost all network services in QEMU have the ability to use TLS for
7session data encryption, along with x509 certificates for simple client
8authentication. What follows is a description of how to generate
9certificates suitable for usage with QEMU, and applies to the VNC
10server, character devices with the TCP backend, NBD server and client,
11and migration server and client.
12
13At a high level, QEMU requires certificates and private keys to be
14provided in PEM format. Aside from the core fields, the certificates
15should include various extension data sets, including v3 basic
16constraints data, key purpose, key usage and subject alt name.
17
18The GnuTLS package includes a command called ``certtool`` which can be
19used to easily generate certificates and keys in the required format
20with expected data present. Alternatively a certificate management
21service may be used.
22
23At a minimum it is necessary to setup a certificate authority, and issue
24certificates to each server. If using x509 certificates for
25authentication, then each client will also need to be issued a
26certificate.
27
28Assuming that the QEMU network services will only ever be exposed to
29clients on a private intranet, there is no need to use a commercial
30certificate authority to create certificates. A self-signed CA is
31sufficient, and in fact likely to be more secure since it removes the
32ability of malicious 3rd parties to trick the CA into mis-issuing certs
33for impersonating your services. The only likely exception where a
34commercial CA might be desirable is if enabling the VNC websockets
35server and exposing it directly to remote browser clients. In such a
36case it might be useful to use a commercial CA to avoid needing to
37install custom CA certs in the web browsers.
38
39The recommendation is for the server to keep its certificates in either
40``/etc/pki/qemu`` or for unprivileged users in ``$HOME/.pki/qemu``.
41
42.. _tls_005fgenerate_005fca:
43
44Setup the Certificate Authority
45~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
46
47This step only needs to be performed once per organization /
48organizational unit. First the CA needs a private key. This key must be
49kept VERY secret and secure. If this key is compromised the entire trust
50chain of the certificates issued with it is lost.
51
52::
53
54   # certtool --generate-privkey > ca-key.pem
55
56To generate a self-signed certificate requires one core piece of
57information, the name of the organization. A template file ``ca.info``
58should be populated with the desired data to avoid having to deal with
59interactive prompts from certtool::
60
61   # cat > ca.info <<EOF
62   cn = Name of your organization
63   ca
64   cert_signing_key
65   EOF
66   # certtool --generate-self-signed \
67              --load-privkey ca-key.pem \
68              --template ca.info \
69              --outfile ca-cert.pem
70
71The ``ca`` keyword in the template sets the v3 basic constraints
72extension to indicate this certificate is for a CA, while
73``cert_signing_key`` sets the key usage extension to indicate this will
74be used for signing other keys. The generated ``ca-cert.pem`` file
75should be copied to all servers and clients wishing to utilize TLS
76support in the VNC server. The ``ca-key.pem`` must not be
77disclosed/copied anywhere except the host responsible for issuing
78certificates.
79
80.. _tls_005fgenerate_005fserver:
81
82Issuing server certificates
83~~~~~~~~~~~~~~~~~~~~~~~~~~~
84
85Each server (or host) needs to be issued with a key and certificate.
86When connecting the certificate is sent to the client which validates it
87against the CA certificate. The core pieces of information for a server
88certificate are the hostnames and/or IP addresses that will be used by
89clients when connecting. The hostname / IP address that the client
90specifies when connecting will be validated against the hostname(s) and
91IP address(es) recorded in the server certificate, and if no match is
92found the client will close the connection.
93
94Thus it is recommended that the server certificate include both the
95fully qualified and unqualified hostnames. If the server will have
96permanently assigned IP address(es), and clients are likely to use them
97when connecting, they may also be included in the certificate. Both IPv4
98and IPv6 addresses are supported. Historically certificates only
99included 1 hostname in the ``CN`` field, however, usage of this field
100for validation is now deprecated. Instead modern TLS clients will
101validate against the Subject Alt Name extension data, which allows for
102multiple entries. In the future usage of the ``CN`` field may be
103discontinued entirely, so providing SAN extension data is strongly
104recommended.
105
106On the host holding the CA, create template files containing the
107information for each server, and use it to issue server certificates.
108
109::
110
111   # cat > server-hostNNN.info <<EOF
112   organization = Name  of your organization
113   cn = hostNNN.foo.example.com
114   dns_name = hostNNN
115   dns_name = hostNNN.foo.example.com
116   ip_address = 10.0.1.87
117   ip_address = 192.8.0.92
118   ip_address = 2620:0:cafe::87
119   ip_address = 2001:24::92
120   tls_www_server
121   encryption_key
122   signing_key
123   EOF
124   # certtool --generate-privkey > server-hostNNN-key.pem
125   # certtool --generate-certificate \
126              --load-ca-certificate ca-cert.pem \
127              --load-ca-privkey ca-key.pem \
128              --load-privkey server-hostNNN-key.pem \
129              --template server-hostNNN.info \
130              --outfile server-hostNNN-cert.pem
131
132The ``dns_name`` and ``ip_address`` fields in the template are setting
133the subject alt name extension data. The ``tls_www_server`` keyword is
134the key purpose extension to indicate this certificate is intended for
135usage in a web server. Although QEMU network services are not in fact
136HTTP servers (except for VNC websockets), setting this key purpose is
137still recommended. The ``encryption_key`` and ``signing_key`` keyword is
138the key usage extension to indicate this certificate is intended for
139usage in the data session.
140
141The ``server-hostNNN-key.pem`` and ``server-hostNNN-cert.pem`` files
142should now be securely copied to the server for which they were
143generated, and renamed to ``server-key.pem`` and ``server-cert.pem``
144when added to the ``/etc/pki/qemu`` directory on the target host. The
145``server-key.pem`` file is security sensitive and should be kept
146protected with file mode 0600 to prevent disclosure.
147
148.. _tls_005fgenerate_005fclient:
149
150Issuing client certificates
151~~~~~~~~~~~~~~~~~~~~~~~~~~~
152
153The QEMU x509 TLS credential setup defaults to enabling client
154verification using certificates, providing a simple authentication
155mechanism. If this default is used, each client also needs to be issued
156a certificate. The client certificate contains enough metadata to
157uniquely identify the client with the scope of the certificate
158authority. The client certificate would typically include fields for
159organization, state, city, building, etc.
160
161Once again on the host holding the CA, create template files containing
162the information for each client, and use it to issue client
163certificates.
164
165::
166
167   # cat > client-hostNNN.info <<EOF
168   country = GB
169   state = London
170   locality = City Of London
171   organization = Name of your organization
172   cn = hostNNN.foo.example.com
173   tls_www_client
174   encryption_key
175   signing_key
176   EOF
177   # certtool --generate-privkey > client-hostNNN-key.pem
178   # certtool --generate-certificate \
179              --load-ca-certificate ca-cert.pem \
180              --load-ca-privkey ca-key.pem \
181              --load-privkey client-hostNNN-key.pem \
182              --template client-hostNNN.info \
183              --outfile client-hostNNN-cert.pem
184
185The subject alt name extension data is not required for clients, so
186the ``dns_name`` and ``ip_address`` fields are not included. The
187``tls_www_client`` keyword is the key purpose extension to indicate this
188certificate is intended for usage in a web client. Although QEMU network
189clients are not in fact HTTP clients, setting this key purpose is still
190recommended. The ``encryption_key`` and ``signing_key`` keyword is the
191key usage extension to indicate this certificate is intended for usage
192in the data session.
193
194The ``client-hostNNN-key.pem`` and ``client-hostNNN-cert.pem`` files
195should now be securely copied to the client for which they were
196generated, and renamed to ``client-key.pem`` and ``client-cert.pem``
197when added to the ``/etc/pki/qemu`` directory on the target host. The
198``client-key.pem`` file is security sensitive and should be kept
199protected with file mode 0600 to prevent disclosure.
200
201If a single host is going to be using TLS in both a client and server
202role, it is possible to create a single certificate to cover both roles.
203This would be quite common for the migration and NBD services, where a
204QEMU process will be started by accepting a TLS protected incoming
205migration, and later itself be migrated out to another host. To generate
206a single certificate, simply include the template data from both the
207client and server instructions in one.
208
209::
210
211   # cat > both-hostNNN.info <<EOF
212   country = GB
213   state = London
214   locality = City Of London
215   organization = Name of your organization
216   cn = hostNNN.foo.example.com
217   dns_name = hostNNN
218   dns_name = hostNNN.foo.example.com
219   ip_address = 10.0.1.87
220   ip_address = 192.8.0.92
221   ip_address = 2620:0:cafe::87
222   ip_address = 2001:24::92
223   tls_www_server
224   tls_www_client
225   encryption_key
226   signing_key
227   EOF
228   # certtool --generate-privkey > both-hostNNN-key.pem
229   # certtool --generate-certificate \
230              --load-ca-certificate ca-cert.pem \
231              --load-ca-privkey ca-key.pem \
232              --load-privkey both-hostNNN-key.pem \
233              --template both-hostNNN.info \
234              --outfile both-hostNNN-cert.pem
235
236When copying the PEM files to the target host, save them twice, once as
237``server-cert.pem`` and ``server-key.pem``, and again as
238``client-cert.pem`` and ``client-key.pem``.
239
240.. _tls_005fcreds_005fsetup:
241
242TLS x509 credential configuration
243~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
244
245QEMU has a standard mechanism for loading x509 credentials that will be
246used for network services and clients. It requires specifying the
247``tls-creds-x509`` class name to the ``--object`` command line argument
248for the system emulators. Each set of credentials loaded should be given
249a unique string identifier via the ``id`` parameter. A single set of TLS
250credentials can be used for multiple network backends, so VNC,
251migration, NBD, character devices can all share the same credentials.
252Note, however, that credentials for use in a client endpoint must be
253loaded separately from those used in a server endpoint.
254
255When specifying the object, the ``dir`` parameters specifies which
256directory contains the credential files. This directory is expected to
257contain files with the names mentioned previously, ``ca-cert.pem``,
258``server-key.pem``, ``server-cert.pem``, ``client-key.pem`` and
259``client-cert.pem`` as appropriate. It is also possible to include a set
260of pre-generated Diffie-Hellman (DH) parameters in a file
261``dh-params.pem``, which can be created using the
262``certtool --generate-dh-params`` command. If omitted, QEMU will
263dynamically generate DH parameters when loading the credentials.
264
265The ``endpoint`` parameter indicates whether the credentials will be
266used for a network client or server, and determines which PEM files are
267loaded.
268
269The ``verify`` parameter determines whether x509 certificate validation
270should be performed. This defaults to enabled, meaning clients will
271always validate the server hostname against the certificate subject alt
272name fields and/or CN field. It also means that servers will request
273that clients provide a certificate and validate them. Verification
274should never be turned off for client endpoints, however, it may be
275turned off for server endpoints if an alternative mechanism is used to
276authenticate clients. For example, the VNC server can use SASL to
277authenticate clients instead.
278
279To load server credentials with client certificate validation enabled
280
281.. parsed-literal::
282
283   |qemu_system| -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server
284
285while to load client credentials use
286
287.. parsed-literal::
288
289   |qemu_system| -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=client
290
291Network services which support TLS will all have a ``tls-creds``
292parameter which expects the ID of the TLS credentials object. For
293example with VNC:
294
295.. parsed-literal::
296
297   |qemu_system| -vnc 0.0.0.0:0,tls-creds=tls0
298
299.. _tls_005fpsk:
300
301TLS Pre-Shared Keys (PSK)
302~~~~~~~~~~~~~~~~~~~~~~~~~
303
304Instead of using certificates, you may also use TLS Pre-Shared Keys
305(TLS-PSK). This can be simpler to set up than certificates but is less
306scalable.
307
308Use the GnuTLS ``psktool`` program to generate a ``keys.psk`` file
309containing one or more usernames and random keys::
310
311   mkdir -m 0700 /tmp/keys
312   psktool -u rich -p /tmp/keys/keys.psk
313
314TLS-enabled servers such as ``qemu-nbd`` can use this directory like so::
315
316   qemu-nbd \
317     -t -x / \
318     --object tls-creds-psk,id=tls0,endpoint=server,dir=/tmp/keys \
319     --tls-creds tls0 \
320     image.qcow2
321
322When connecting from a qemu-based client you must specify the directory
323containing ``keys.psk`` and an optional username (defaults to "qemu")::
324
325   qemu-img info \
326     --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=rich,endpoint=client \
327     --image-opts \
328     file.driver=nbd,file.host=localhost,file.port=10809,file.tls-creds=tls0,file.export=/
329