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Enabling TLS

The NATS server uses modern TLS semantics to encrypt client, route, and monitoring connections. Server configuration revolves around a tls map, which has the following properties:

Property Description
cert_file TLS certificate file.
key_file TLS certificate key file.
ca_file TLS certificate authority file. When not present, default to the system trust store.
cipher_suites When set, only the specified TLS cipher suites will be allowed. Values must match the golang version used to build the server.
curve_preferences List of TLS cipher curves to use in order.
insecure Skip certificate verification. This only applies to outgoing connections, NOT incoming client connections. NOT Recommended
timeout TLS handshake timeout in fractional seconds. Default set to 0.5 seconds.
verify If true, require and verify client certificates. To support use by Browser, this option does not apply to monitoring.
verify_and_map If true, require and verify client certificates and map certificate values for authentication purposes. Does not apply to monitoring either.
verify_cert_and_check_known_urls Only settable in a non client context where verify: true is the default (cluster/gateway). The incoming connections certificate's X509v3 Subject Alternative Name DNS entries will be matched against all urls in the configuration context that contains this tls map. If a match is found, the connection is accepted and rejected otherwise. Meaning for gateways we will match all DNS entries in the certificate against all gateway urls. For cluster we will match against all route urls. As a consequence of this, dynamic cluster growth may require config changes in other cluster where this flag is true. DNS name checking is performed according to rfc6125. Only the full wildcard * is supported for the left most label. This would be one way to keep cluster growth flexible.
pinned_certs List of hex-encoded SHA256 of DER encoded public key fingerprints. When present, during the TLS handshake, the provided certificate's fingerprint is required to be present in the list or the connection is closed. This sequence of commands generates an entry for a provided certificate: `openssl x509 -noout -pubkey -in openssl pkey -pubin -outform DER openssl dgst -sha256`.

The simplest configuration:

tls: {
  cert_file: "./server-cert.pem"
  key_file: "./server-key.pem"
}

Or by using server options:

> nats-server --tls --tlscert=./server-cert.pem --tlskey=./server-key.pem
[21417] 2019/05/16 11:21:19.801539 [INF] Starting nats-server version 2.0.0
[21417] 2019/05/16 11:21:19.801621 [INF] Git commit [not set]
[21417] 2019/05/16 11:21:19.801777 [INF] Listening for client connections on 0.0.0.0:4222
[21417] 2019/05/16 11:21:19.801782 [INF] TLS required for client connections
[21417] 2019/05/16 11:21:19.801785 [INF] Server id is ND6ZZDQQDGKYQGDD6QN2Y26YEGLTH6BMMOJZ2XJB2VASPVII3XD6RFOQ
[21417] 2019/05/16 11:21:19.801787 [INF] Server is ready

Notice that the log indicates that the client connections will be required to use TLS. If you run the server in Debug mode with -D or -DV, the logs will show the cipher suite selection for each connected client:

[22242] 2019/05/16 11:22:20.216322 [DBG] 127.0.0.1:51383 - cid:1 - Client connection created
[22242] 2019/05/16 11:22:20.216539 [DBG] 127.0.0.1:51383 - cid:1 - Starting TLS client connection handshake
[22242] 2019/05/16 11:22:20.367275 [DBG] 127.0.0.1:51383 - cid:1 - TLS handshake complete
[22242] 2019/05/16 11:22:20.367291 [DBG] 127.0.0.1:51383 - cid:1 - TLS version 1.2, cipher suite TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

When a tls section is specified at the root of the configuration, it also affects the monitoring port if https_port option is specified. Other sections such as cluster can specify a tls block.

TLS Timeout

The timeout setting enables you to control the amount of time that a client is allowed to upgrade its connection to tls. If your clients are experiencing disconnects during TLS handshake, you'll want to increase the value, however, if you do be aware that an extended timeout exposes your server to attacks where a client doesn't upgrade to TLS and thus consumes resources. Conversely, if you reduce the TLS timeout too much, you are likely to experience handshake errors.

tls: {
  cert_file: "./server-cert.pem"
  key_file: "./server-key.pem"
  # clients will fail to connect (value is too low)
  timeout: 0.0001
}

Certificate Authorities

The ca_file file should contain one or more Certificate Authorities in PEM format, in a bundle. This is a common format.

When a certificate is issued, it is often accompanied by a copy of the intermediate certificate used to issue it. This is useful for validating that certificate. It is not necessarily a good choice as the only CA suitable for use in verifying other certificates a server may see.

Do consider though that organizations issuing certificates will change the intermediate they use. For instance, a CA might issue intermediates in pairs, with an active and a standby, and reserve the right to switch to the standby without notice. You probably would want to trust both of those for the ca_file directive, to be prepared for such a day, and then after the first CA has been compromised you can remove it. This way the roll from one CA to another will not break your NATS server deployment.

Self Signed Certificates for Testing

Explaining Public key infrastructure, Certificate Authorities (CA) and x509 certificates fall well outside the scope of this document. So does an explanation on how to obtain a properly trusted certificates.

If anybody outside your organization needs to connect, get certs from a public certificate authority. Think carefully about revocation and cycling times, as well as automation, when picking a CA. If arbitrary applications inside your organization need to connect, use a cert from your in-house CA. If only resources inside a specific environment need to connect, that environment might have its own dedicated automatic CA, eg in Kubernetes clusters, so use that.

Only for testing purposes does it makes sense to generate self signed certificates, even your own CA. This is a short guide on how to do just that and what to watch out for.

DO NOT USE these certificates in production!!!

Problems With Self Signed Certificates

Missing in Relevant Trust Stores

As they should, these are not trusted by the system your server or clients are running on.

One option is to specify the CA in every client you are using. In case you make use of verify, verify_and_map or verify_cert_and_check_known_urls you need to specify ca_file in the server. If you are having a more complex setup involving cluster, gateways or leaf nodes, ca_file needs to be present in tls maps used to connect to the server with self signed certificates. While this works for server and libraries from the NATS eco system, you will experience issues when connecting with other tools such as your Browser.

Another option is to configure your system's trust store to include self signed certificate(s). Which trust store needs to be configured depends on what you are testing.

  • This may be your OS for server and certain clients.
  • The runtime environment for other clients like Java, Python or Node.js.
  • Your browser for monitoring endpoints and websockets.

Please check your system's documentation on how to trust a particular self signed certificate.

Missing Subject Alternative Name

Another common problem is failed identity validation. The IP or DNS name to connect to needs to match a Subject Alternative Name (SAN) inside the certificate. Meaning, if a client/browser/server connect via tls to 127.0.0.1, the server needs to present a certificate with a SAN containing the IP 127.0.0.1 or the connection will be closed with a handshake error.

When verify_cert_and_check_known_urls is specified, Subject Alternative Name (SAN) DNS records are necessary. In order to succesfully connect there must be an overlap between the DNS records provided as part of the certificate and the urls configured. If you dynaimcally grow your cluster and use a new certificate, this route or gateway the server connects to will have to be reconfigured to include a url for the new server. Only then can the new server connect. If the DNS record is a wildcard, matching according to rfc6125 will be performed. Using certificates with a wildcard Subject Alternative Name (SAN) and configuration with url(s) that would match are a way to keep the flexibility of dynamic cluster growth without configuration changes in ohter cluster.

Wrong Key Usage

When generating your certificate you need to make sure to include the right purpose for which you want to use the certificate. This is encoded in key usage and extended key usage. The necessary values for key usage depend on the ciphers used. Digital Signature and Key Encipherment are an interoperable choice.

With respect to NATS the relevant values for extended key usage are:

  • TLS WWW server authentication - To authenticate as server for incoming connections. A NATS server will need a certificate containing this.
  • TLS WWW client authentication - To authenticate as client for outgoing connections. Only needed when connecting to a server where verify, verify_and_map or verify_cert_and_check_known_urls are specified. In these cases, a NATS client will need a certificate with this value.
    • Leaf node connections can be configured with verify as well. Then the connecting NATS server will have to present a certificate with this value too. Certificates containing both values are an option.
    • Cluster connections always have verify enabled. Which server acts as client and server comes down to timing and therefore can't be individually configured. Certificates containing both values are a must.
    • Gateway connections always have verify enabled. Unlike cluster outgoing connections can specify a separate cert. Certificates containing both values are an option that reduce configuration.

Note that it's common practice for non-web protocols to use the TLS WWW authentication fields, as a matter of history those have become embedded as generic options.

Creating Self Signed Certificates for Testing

The simplest way to generate a CA as well as client and server certificates is mkcert. This zero config tool generates and installs the CA into your local system trust store(s) and makes providing SAN straight forward. Check it's documentation for installation and your system's trust store. Here is a simple example:

Generate a CA as well as a certificate, valid for server authentication by localhost and the IP ::1(-cert-file and -key-file overwrite default file names). Then start a NATS server using the generated certificate.

mkcert -install
mkcert -cert-file server-cert.pem -key-file server-key.pem localhost ::1
nats-server --tls --tlscert=server-cert.pem --tlskey=server-key.pem -ms 8222

Now you should be able to access the monitoring endpoint https://localhost:8222 with your browser.
https://127.0.0.1:8222 however should result in an error as 127.0.0.1 is not listed as SAN. You will not be able to establish a connection from another computer either. For that to work you have to provide appropriate DNS and/or IP SAN(s)

To generate certificates that work with verify and cluster/gateway/leaf_nodes provide the -client option. It will cause the appropriate key usage for client authentication to be added. This example also adds a SAN email for usage as user name in verify_and_map.

mkcert -client -cert-file client-cert.pem -key-file client-key.pem localhost ::1 email@localhost

Please note:

  • That client refers to connecting process, not necessarily a NATS client.
  • mkcert -client will generate a certificate with key usage suitable for client and server authentication.

Examples in this document make use of the certificates generated so far. To simplify examples using the CA certificate, copy rootCA.pem into the same folder where the certificates were generated. To obtain the CA certificate's location use this command:

mkcert -CAROOT

Once you are done testing, remove the CA from your local system trust store(s).

mkcert -uninstall

Alternatively, you can also use openssl to generate certificates. This tool allows a lot more customization of the generated certificates. It is more complex and does not manage installation into the system trust store(s).

However, for inspecting certificates it is quite handy. To inspect the certificates from the above example execute these commands:

openssl x509 -noout -text -in server-cert.pem
openssl x509 -noout -text -in client-cert.pem