nbdkit-tls - authentication and encryption of NBD connections (sometimes incorrectly called "SSL")
nbdkit [--tls=off|on|require] [--tls-certificates /path/to/certificates] [--tls-psk /path/to/pskfile] [--tls-verify-peer] PLUGIN [...]
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 --tls=off or --tls=on. With --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).
--tls=on means that the client may choose to connect either with or without TLS.
Because --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 --tls=require, where the server enables TLS and rejects all non-TLS connection attempts.
When nbdkit starts up, it loads TLS certificates from some built-in paths, or from the directory specified by the --tls-certificates option.
Without --tls-certificates, if nbdkit is started as a non-root user (note this does not include use of the -u or -g options), nbdkit looks in each of these paths in turn:
Without --tls-certificates, if nbdkit is started as root, nbkit looks in:
nbdkit --dump-config and look at the
root_tls_certificates_dir setting to get the actual directory built into the binary.)
You can override both directories above by using --tls-certificates /path/to/certificates.
In this directory, nbdkit expects to find several files:
The Certificate Authority certificate.
The server certificate.
The server private key.
(Optional) The certificate revocation list.
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 ca-key.pem file is the CA private key and is 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
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
0600 helps to prevent other users on the same machine from reading it.
The server DNS name (
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
Note: You don't need to create client certificates unless you want to check and limit which clients can connect to nbdkit. nbdkit does not check client certificates unless you specify the --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 (
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 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 ca-cert.pem file.
To enable checking of client certificates, specify the --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 (ca-crl.pem).
As a simpler alternative to TLS certificates, you may used pre-shared keys to authenticate clients.
Create a PSK file containing one or more
username:key pairs. It is easiest to use 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 --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 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=/
If nbdkit was compiled without GnuTLS support, then TLS is disabled and TLS connections will be rejected (as if --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
nbdkit --dump-config will contain
If TLS certificates cannot be loaded either from the built-in path or from the directory specified by --tls-certificates, then TLS defaults to disabled. Turning TLS on will give a warning (--tls=on) or error (--tls=require) about the missing certificates.
If TLS certificates can be loaded from the built-in path or from the --tls-certificates directory, then TLS will by default be enabled (like --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 not checked by default unless you specify --tls-verify-peer.
If the --tls-psk option is used then TLS is enabled (but not required). To ensure that all clients are authorized you must use --tls=require.
Each of these defaults is insecure to some extent (including --tls=on which could be subject to a downgrade attack), so if you expect TLS then it is best to specify the --tls option that you require, and if you want to check client certificates, specify the --tls-verify-peer option.
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
--with-tls-priority. This defaults to
NORMAL which, to quote the GnuTLS documentation:
NORMAL means all
secure ciphersuites. The 256-bit ciphers are included as a fallback only. The ciphers are sorted by security margin."
You could also set the TLS priority so that it can be configured from a file at runtime:
means use the policy from /etc/crypto-policies/config.
means use the policy from /etc/crypto-policies/local.d/nbdkit.config and fall back to /etc/crypto-policies/config if the first file does not exist.
More information can be found in gnutls_priority_init(3).
nbdkit(1), gnutls_priority_init(3), psktool(1), https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md, https://nbd.sourceforge.io/.
Richard W.M. Jones
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