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 the 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