=head1 NAME

nbdkit-tls - authentication and encryption of NBD connections
(sometimes incorrectly called "SSL")

=head1 SYNOPSIS

 nbdkit [--tls=off|on|require] [--tls-certificates /path/to/certificates]
        [--tls-psk /path/to/pskfile] [--tls-verify-peer]
        PLUGIN [...]

=head1 DESCRIPTION

TLS (authentication and encryption, sometimes incorrectly called
"SSL") is supported if nbdkit was compiled with GnuTLS.  This allows
the server to verify that the client is allowed access, and to encrypt
the contents of the protocol in transit over the network.

TLS can be disabled or enabled by specifying either I<--tls=off> or
I<--tls=on>.  With I<--tls=off>, if a client tries to use TLS to
connect, it will be rejected by the server (in other words, as if the
server doesn't support TLS).

I<--tls=on> means that the client may choose to connect either with or
without TLS.

Because I<--tls=on> is subject to downgrade attacks where a malicious
proxy pretends not to support TLS in order to force either the client
or server to communicate in plaintext, you can also specify
I<--tls=require>, where the server enables TLS B<and> rejects all
non-TLS connection attempts.

=head2 TLS with X.509 certificates

When nbdkit starts up, it loads TLS certificates from some built-in
paths, or from the directory specified by the I<--tls-certificates>
option.

Without I<--tls-certificates>, if nbdkit is started as a non-root user
(note this does not include use of the I<-u> or I<-g> options), nbdkit
looks in each of these paths in turn:

 $HOME/.pki/nbdkit/
 $HOME/.config/pki/nbdkit/

Without I<--tls-certificates>, if nbdkit is started as root, nbkit
looks in:

 $sysconfdir/pki/nbdkit/

(Use C<nbdkit --dump-config> and look at the
C<root_tls_certificates_dir> setting to get the actual directory built
into the binary.)

You can override both directories above by using
I<--tls-certificates /path/to/certificates>.

In this directory, nbdkit expects to find several files:

=over 4

=item F<ca-cert.pem>

The Certificate Authority certificate.

=item F<server-cert.pem>

The server certificate.

=item F<server-key.pem>

The server private key.

=item F<ca-crl.pem>

(Optional) The certificate revocation list.

=back

=head3 Setting up the Certificate Authority

This step only needs to be done once per organization.  It may be that
your organization already has a CA.

 $ certtool --generate-privkey > ca-key.pem
 $ chmod 0600 ca-key.pem

The F<ca-key.pem> file is the CA private key and is I<extremely>
sensitive data.  With possession of this key, anyone can create
certificates pretending to be your organization!

To create the CA certificate file:

 $ 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

=head3 Issuing a server certificate for the nbdkit server

Each nbdkit server (or host) needs a secret key and certificate.

 $ certtool --generate-privkey > server-key.pem
 $ chmod 0600 server-key.pem

The server key file is sensitive.  Setting the mode to C<0600> helps
to prevent other users on the same machine from reading it.

The server DNS name (C<cn> below) must be the fully qualified hostname
— and the only hostname — that the client connects to.

 $ cat > server.info <<EOF
 organization = Name of your organization
 cn = nbd-server.example.com
 tls_www_server
 encryption_key
 signing_key
 EOF
 $ certtool --generate-certificate \
            --load-ca-certificate ca-cert.pem \
            --load-ca-privkey ca-key.pem \
            --load-privkey server-key.pem \
            --template server.info \
            --outfile server-cert.pem

=head3 Issuing and checking client certificates

B<Note:>
You don't need to create client certificates unless you want to check
and limit which clients can connect to nbdkit.  nbdkit B<does not>
check client certificates unless you specify the I<--tls-verify-peer>
option on the command line.

For each client you should generate a private key and a client
certificate:

 $ certtool --generate-privkey > client-key.pem
 $ chmod 0600 client-key.pem

The client key file is sensitive.

The client DNS name (C<cn> below) is the client's name that nbdkit
sees and checks.

 $ cat > client.info <<EOF
 country = US
 state = New York
 locality = New York
 organization = Name of your organization
 cn = client.example.com
 tls_www_client
 encryption_key
 signing_key
 EOF
 $ certtool --generate-certificate \
            --load-ca-certificate ca-cert.pem \
            --load-ca-privkey ca-key.pem \
            --load-privkey client-key.pem \
            --template client.info \
            --outfile client-cert.pem

Client certificates do I<not> need to be present anywhere on the
nbdkit host.  You don't need to copy them into nbdkit's TLS
certificates directory.  The security comes from the fact that the
client must present a client certificate signed by the Certificate
Authority, and nbdkit can check this because it has the F<ca-cert.pem>
file.

To enable checking of client certificates, specify the
I<--tls-verify-peer> option on the command line.  Clients which don't
present a valid certificate (eg. not signed, incorrect signature) are
denied.  Also denied are clients which present a valid certificate
signed by another CA.  Also denied are clients with certificates added
to the certificate revocation list (F<ca-crl.pem>).

=head2 TLS with Pre-Shared Keys (PSK)

As a simpler alternative to TLS certificates, you may used pre-shared
keys to authenticate clients.

Create a PSK file containing one or more C<username:key> pairs.  It is
easiest to use L<psktool(1)> for this:

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

The PSK file contains the hex-encoded random keys in plaintext.  Any
client which can read this file will be able to connect to the server.

Use the nbdkit I<--tls-psk> option to start the server:

 nbdkit --tls=require --tls-psk=/tmp/keys/keys.psk -e / file disk.img

This option overrides X.509 certificate authentication.

Clients must supply one of the usernames in the PSK file and the
corresponding key in order to connect.  An example of connecting using
L<qemu-img(1)> is:

 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=/

=head2 Default TLS behaviour

If nbdkit was compiled without GnuTLS support, then TLS is disabled
and TLS connections will be rejected (as if I<--tls=off> was specified
on the command line).  Also it is impossible to turn on TLS in this
scenario.  You can tell if nbdkit was compiled without GnuTLS support
because C<nbdkit --dump-config> will contain C<tls=no>.

If TLS certificates cannot be loaded either from the built-in path or
from the directory specified by I<--tls-certificates>, then TLS
defaults to disabled.  Turning TLS on will give a warning
(I<--tls=on>) or error (I<--tls=require>) about the missing
certificates.

If TLS certificates can be loaded from the built-in path or from the
I<--tls-certificates> directory, then TLS will by default be enabled
(like I<--tls=on>), but it is not required.  Clients can choose
whether or not to use TLS and whether or not to present certificates.

TLS client certificates are I<not> checked by default unless you
specify I<--tls-verify-peer>.

If the I<--tls-psk> option is used then TLS is enabled (but I<not>
required).  To ensure that all clients are authorized you must use
I<--tls=require>.

Each of these defaults is insecure to some extent (including
I<--tls=on> which could be subject to a downgrade attack), so if you
expect TLS then it is best to specify the I<--tls> option that you
require, and if you want to check client certificates, specify the
I<--tls-verify-peer> option.

=head2 Choice of TLS algorithms

TLS has a bewildering choice of algorithms that can be used.  To
enable you to choose a default set of algorithms, there is a configure
setting C<--with-tls-priority>.  This defaults to C<NORMAL> which, to
quote the GnuTLS documentation:

=over 4

"C<NORMAL> means all C<secure> ciphersuites.  The 256-bit ciphers are
included as a fallback only.  The ciphers are sorted by security
margin."

=back

You could also set the TLS priority so that it can be configured from
a file at runtime:

 ./configure --with-tls-priority=@SYSTEM

means use the policy from F</etc/crypto-policies/config>.

 ./configure --with-tls-priority=@NBDKIT,SYSTEM

means use the policy from
F</etc/crypto-policies/local.d/nbdkit.config> and fall back to
F</etc/crypto-policies/config> if the first file does not exist.

More information can be found in L<gnutls_priority_init(3)>.

=head1 SEE ALSO

L<nbdkit(1)>,
L<gnutls_priority_init(3)>,
L<psktool(1)>,
L<https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md>,
L<https://nbd.sourceforge.io/>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

Pino Toscano

=head1 COPYRIGHT

Copyright (C) 2013-2018 Red Hat Inc.
