SSL Setup

This page provides instructions on how to enable TLS/SSL authentication and encryption for network communication with and between Flink processes. NOTE: TLS/SSL authentication is not enabled by default.

Internal and External Connectivity

When securing network connections between machines processes through authentication and encryption, Apache Flink differentiates between internal and external connectivity. Internal Connectivity refers to all connections made between Flink processes. These connections run Flink custom protocols. Users never connect directly to internal connectivity endpoints. External / REST Connectivity endpoints refers to all connections made from the outside to Flink processes. This includes the web UI and REST commands to start and control running Flink jobs/applications, including the communication of the Flink CLI with the JobManager / Dispatcher.

For more flexibility, security for internal and external connectivity can be enabled and configured separately.

Internal Connectivity

Internal connectivity includes:

• Control messages: RPC between JobManager / TaskManager / Dispatcher / ResourceManager
• The data plane: The connections between TaskManagers to exchange data during shuffles, broadcasts, redistribution, etc.
• The Blob Service (distribution of libraries and other artifacts).

All internal connections are SSL authenticated and encrypted. The connections use mutual authentication, meaning both server and client side of each connection need to present the certificate to each other. The certificate acts effectively as a shared secret when a dedicated CA is used to exclusively sign an internal cert. The certificate for internal communication is not needed by any other party to interact with Flink, and can be simply added to the container images, or attached to the YARN deployment.

• The easiest way to realize this setup is by generating a dedicated public/private key pair and self-signed certificate for the Flink deployment. The key- and truststore are identical and contains only that key pair / certificate. An example is shown below.

• In an environment where operators are constrained to use firm-wide Internal CA (cannot generate self-signed certificates), the recommendation is to still have a dedicated key pair / certificate for the Flink deployment, signed by that CA. However, the TrustStore must then also contain the CA’s public certificate tho accept the deployment’s certificate during the SSL handshake (requirement in JDK TrustStore implementation).

  NOTE: Because of that, it is critical that you specify the fingerprint of the deployment certificate (security.ssl.internal.cert.fingerprint), when it is not self-signed, to pin that certificate as the only trusted certificate and prevent the TrustStore from trusting all certificates signed by that CA.

Note: Because internal connections are mutually authenticated with shared certificates, Flink can skip hostname verification. This makes container-based setups easier.

External / REST Connectivity

All external connectivity is exposed via an HTTP/REST endpoint, used for example by the web UI and the CLI:

• Communication with the Dispatcher to submit jobs (session clusters)
• Communication with the JobManager to inspect and modify a running job/application

The REST endpoints can be configured to require SSL connections. The server will, however, accept connections from any client by default, meaning the REST endpoint does not authenticate the client.

Simple mutual authentication may be enabled by configuration if authentication of connections to the REST endpoint is required, but we recommend to deploy a “side car proxy”: Bind the REST endpoint to the loopback interface (or the pod-local interface in Kubernetes) and start a REST proxy that authenticates and forwards the requests to Flink. Examples for proxies that Flink users have deployed are Envoy Proxy or NGINX with MOD_AUTH.

The rationale behind delegating authentication to a proxy is that such proxies offer a wide variety of authentication options and thus better integration into existing infrastructures.

Queryable State

Connections to the queryable state endpoints is currently not authenticated or encrypted.

Configuring SSL

SSL can be enabled separately for internal and external connectivity:

• security.ssl.internal.enabled: Enable SSL for all internal connections.
• security.ssl.rest.enabled: Enable SSL for REST / external connections.

Note: For backwards compatibility, the security.ssl.enabled option still exists and enables SSL for both internal and REST endpoints.

For internal connectivity, you can optionally disable security for different connection types separately. When security.ssl.internal.enabled is set to true, you can set the following parameters to false to disable SSL for that particular connection type:

• taskmanager.data.ssl.enabled: Data communication between TaskManagers
• blob.service.ssl.enabled: Transport of BLOBs from JobManager to TaskManager
• akka.ssl.enabled: Akka-based RPC connections between JobManager / TaskManager / ResourceManager

Keystores and Truststores

The SSL configuration requires to configure a keystore and a truststore. The keystore contains the public certificate (public key) and the private key, while the truststore contains the trusted certificates or the trusted authorities. Both stores need to be set up such that the truststore trusts the keystore’s certificate.

Internal Connectivity

Because internal communication is mutually authenticated between server and client side, keystore and truststore typically refer to a dedicated certificate that acts as a shared secret. In such a setup, the certificate can use wild card hostnames or addresses. WHen using self-signed certificates, it is even possible to use the same file as keystore and truststore.

security.ssl.internal.keystore: /path/to/file.keystore
security.ssl.internal.keystore-password: keystore_password
security.ssl.internal.key-password: key_password
security.ssl.internal.truststore: /path/to/file.truststore
security.ssl.internal.truststore-password: truststore_password

When using a certificate that is not self-signed, but signed by a CA, you need to use certificate pinning to allow only a a specific certificate to be trusted when establishing the connectivity.

security.ssl.internal.cert.fingerprint: 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00

REST Endpoints (external connectivity)

For REST endpoints, by default the keystore is used by the server endpoint, and the truststore is used by the REST clients (including the CLI client) to accept the server’s certificate. In the case where the REST keystore has a self-signed certificate, the truststore must trust that certificate directly. If the REST endpoint uses a certificate that is signed through a proper certification hierarchy, the roots of that hierarchy should be in the trust store.

If mutual authentication is enabled, the keystore and the truststore are used by both, the server endpoint and the REST clients as with internal connectivity.

security.ssl.rest.keystore: /path/to/file.keystore
security.ssl.rest.keystore-password: keystore_password
security.ssl.rest.key-password: key_password
security.ssl.rest.truststore: /path/to/file.truststore
security.ssl.rest.truststore-password: truststore_password
security.ssl.rest.authentication-enabled: false

Cipher suites

IMPORTANT The IETF RFC 7525 recommends to use a specific set of cipher suites for strong security. Because these cipher suites were not available on many setups out of the box, Flink’s default value is set to a slightly weaker but more compatible cipher suite. We recommend that SSL setups update to the stronger cipher suites, if possible, by adding the below entry to the Flink configuration:

security.ssl.algorithms: TLS_DHE_RSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,TLS_DHE_RSA_WITH_AES_256_GCM_SHA384,TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

If these cipher suites are not supported on your setup, you will see that Flink processes will not be able to connect to each other.

Complete List of SSL Options

Creating and Deploying Keystores and Truststores

Keys, Certificates, and the Keystores and Truststores can be generated using the keytool utility. You need to have an appropriate Java Keystore and Truststore accessible from each node in the Flink cluster.

• For standalone setups, this means copying the files to each node, or adding them to a shared mounted directory.
• For container based setups, add the keystore and truststore files to the container images.
• For Yarn/Mesos setups, the cluster deployment phase can automatically distribute the keystore and truststore files.

For the externally facing REST endpoint, the common name or subject alternative names in the certificate should match the node’s hostname and IP address.

Example SSL Setup Standalone and Kubernetes

Internal Connectivity

Execute the following keytool commands to create a key pair in a keystore:

keytool -genkeypair -alias flink.internal -keystore internal.keystore -dname "CN=flink.internal" -storepass internal_store_password -keyalg RSA -keysize 4096 -storetype PKCS12

The single key/certificate in the keystore is used the same way by the server and client endpoints (mutual authentication). The key pair acts as the shared secret for internal security, and we can directly use it as keystore and truststore.

security.ssl.internal.enabled: true
security.ssl.internal.keystore: /path/to/flink/conf/internal.keystore
security.ssl.internal.truststore: /path/to/flink/conf/internal.keystore
security.ssl.internal.keystore-password: internal_store_password
security.ssl.internal.truststore-password: internal_store_password
security.ssl.internal.key-password: internal_store_password

REST Endpoint

The REST endpoint may receive connections from external processes, including tools that are not part of Flink (for example curl request to the REST API). Setting up a proper certificate that is signed though a CA hierarchy may make sense for the REST endpoint.

However, as mentioned above, the REST endpoint does not authenticate clients and thus typically needs to be secured via a proxy anyways.

REST Endpoint (simple self signed certificate)

This example shows how to create a simple keystore / truststore pair. The truststore does not contain the primary key and can be shared with other applications. In this example, myhost.company.org / ip:10.0.2.15 is the node (or service) for the JobManager.

keytool -genkeypair -alias flink.rest -keystore rest.keystore -dname "CN=myhost.company.org" -ext "SAN=dns:myhost.company.org,ip:10.0.2.15" -storepass rest_keystore_password -keyalg RSA -keysize 4096 -storetype PKCS12
keytool -exportcert -keystore rest.keystore -alias flink.rest -storepass rest_keystore_password -file flink.cer
keytool -importcert -keystore rest.truststore -alias flink.rest -storepass rest_truststore_password -file flink.cer -noprompt

security.ssl.rest.enabled: true
security.ssl.rest.keystore: /path/to/flink/conf/rest.keystore
security.ssl.rest.truststore: /path/to/flink/conf/rest.truststore
security.ssl.rest.keystore-password: rest_keystore_password
security.ssl.rest.truststore-password: rest_truststore_password
security.ssl.rest.key-password: rest_keystore_password

REST Endpoint (with a self signed CA)

Execute the following keytool commands to create a truststore with a self signed CA.

keytool -genkeypair -alias ca -keystore ca.keystore -dname "CN=Sample CA" -storepass ca_keystore_password -keyalg RSA -keysize 4096 -ext "bc=ca:true" -storetype PKCS12
keytool -exportcert -keystore ca.keystore -alias ca -storepass ca_keystore_password -file ca.cer
keytool -importcert -keystore ca.truststore -alias ca -storepass ca_truststore_password -file ca.cer -noprompt

Now create a keystore for the REST endpoint with a certificate signed by the above CA. Let flink.company.org / ip:10.0.2.15 be the hostname of the JobManager.

keytool -genkeypair -alias flink.rest -keystore rest.signed.keystore -dname "CN=flink.company.org" -ext "SAN=dns:flink.company.org" -storepass rest_keystore_password -keyalg RSA -keysize 4096 -storetype PKCS12
keytool -certreq -alias flink.rest -keystore rest.signed.keystore -storepass rest_keystore_password -file rest.csr
keytool -gencert -alias ca -keystore ca.keystore -storepass ca_keystore_password -ext "SAN=dns:flink.company.org,ip:10.0.2.15" -infile rest.csr -outfile rest.cer
keytool -importcert -keystore rest.signed.keystore -storepass rest_keystore_password -file ca.cer -alias ca -noprompt
keytool -importcert -keystore rest.signed.keystore -storepass rest_keystore_password -file rest.cer -alias flink.rest -noprompt

Now add the following configuration to your flink-conf.yaml:

security.ssl.rest.enabled: true
security.ssl.rest.keystore: /path/to/flink/conf/rest.signed.keystore
security.ssl.rest.truststore: /path/to/flink/conf/ca.truststore
security.ssl.rest.keystore-password: rest_keystore_password
security.ssl.rest.key-password: rest_keystore_password
security.ssl.rest.truststore-password: ca_truststore_password

Tips to query REST Endpoint with curl utility

You can convert the keystore into the PEM format using openssl:

openssl pkcs12 -passin pass:rest_keystore_password -in rest.keystore -out rest.pem -nodes

Then you can query REST Endpoint with curl:

curl --cacert rest.pem flink_url

If mutual SSL is enabled:

curl --cacert rest.pem --cert rest.pem flink_url

Tips for YARN / Mesos Deployment

For YARN and Mesos, you can use the tools of Yarn and Mesos to help:

• Configuring security for internal communication is exactly the same as in the example above.

• To secure the REST endpoint, you need to issue the REST endpoint’s certificate such that it is valid for all hosts that the JobManager may get deployed to. This can be done with a wild card DNS name, or by adding multiple DNS names.

• The easiest way to deploy keystores and truststore is by YARN client’s ship files option (-yt). Copy the keystore and truststore files into a local directory (say deploy-keys/) and start the YARN session as follows: flink run -m yarn-cluster -yt deploy-keys/ flinkapp.jar

• When deployed using YARN, Flink’s web dashboard is accessible through YARN proxy’s Tracking URL. To ensure that the YARN proxy is able to access Flink’s HTTPS URL, you need to configure YARN proxy to accept Flink’s SSL certificates. For that, add the custom CA certificate into Java’s default truststore on the YARN Proxy node.

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