There are two different systems that need to be configured separately to encrypt communication within the cluster: gossip encryption and TLS. TLS is used to secure the RPC calls between agents. Gossip encryption is secured with a symmetric key, since gossip between nodes is done over UDP. In this guide we will configure both.
To enable gossip encryption, you need to use an encryption key when starting the Consul agent. The key can be simple set with the `encrypt` parameter in the agent configuration file. Alternatively, the encryption key can be placed in a separate configuration file with only the `encrypt` field, since the agent can merge multiple configuration files. The key must be 32-bytes, Base64 encoded.
"Encrypt: true" will be included in the output, if encryption is properly configured.
Note: all nodes within a cluster must share the same encryption key in order to send and receive cluster information, including clients and servers. Additionally, if you're using multiple WAN joined datacenters, be sure to use _the same encryption key_ in all datacenters.
Gossip encryption can also be enabled on an existing cluster, but requires several extra steps. The additional configuration of the agent configuration parameters, [`encrypt_verify_incoming`](/docs/agent/options#encrypt_verify_incoming) and [`encrypt_verify_outgoing`](/docs/agent/options#encrypt_verify_outgoing) is necessary.
**Step 2**: Set the [`encrypt`](/docs/agent/options#_encrypt) key, and set `encrypt_verify_incoming` and `encrypt_verify_outgoing` to `false` in the agent configuration file. Then initiate a rolling update of the cluster with these new values. After this step, the agents will be able to decrypt gossip but will not yet be sending encrypted traffic.
A rolling update can be made by restarting the Consul agents (clients and servers) in turn. `consul reload` or `kill -HUP <process_id>` is _not_ sufficient to change the gossip configuration.
**Step 3**: Update the `encrypt_verify_outgoing` setting to `true` and perform another rolling update of the cluster by restarting Consul on each agent. The agents will now be sending encrypted gossip but will still allow incoming unencrypted traffic.
**Step 4**: The previous step, enabling verify outgoing, must be completed on all agents before continuing. Update the `encrypt_verify_incoming` setting to `true` and perform a final rolling update of the cluster.
TLS can be used to verify the authenticity of the servers with [`verify_outgoing`](/docs/agent/options#verify_outgoing) and [`verify_server_hostname`](/docs/agent/options#verify_server_hostname). It can also optionally verify client certificates when using [`verify_incoming`](/docs/agent/options#verify_incoming)
The `verify_outgoing` parameter enables agents to verify the authenticity of Consul servers for outgoing connections. The `verify_server_hostname` parameter requires outgoing connections to perform hostname verification and is critically important to prevent compromised client agents from becoming servers and revealing all state to the attacker. Finally, the `verify_incoming` parameter enables the servers to verify the authenticity of all incoming connections.
Enabling TLS on an existing cluster is supported. This process assumes a starting point of a running cluster with no TLS settings configured, and involves an intermediate step in order to get to full TLS encryption.
**Step 1**: [Generate the necessary keys and certificates](/docs/guides/creating-certificates), then set the `ca_file` or `ca_path`, `cert_file`, and `key_file` settings in the configuration for each agent. Make sure the `verify_outgoing` and `verify_incoming` options are set to `false`. HTTPS for the API can be enabled at this point by setting the [`https`](/docs/agent/options#http_port) port.
Next, perform a rolling restart of each agent in the cluster. After this step, TLS should be enabled everywhere but the agents will not yet be enforcing TLS. Again, `consul reload` or `kill -HUP <process_id>` is _not_ sufficient to update the configuration.
**Step 2**: (Optional, Enterprise-only) If applicable, set the `Use TLS` setting in any network areas to `true`. This can be done either through the [`consul operator area update`](/docs/commands/operator/area) command or the [Operator API](/api/operator/area).
**Step 3**: Change the `verify_incoming`, `verify_outgoing`, and `verify_server_hostname` to `true` then perform another rolling restart of each agent in the cluster.
In this guide we configured both gossip encryption and TLS for RPC. Securing agent communication is a recommended set in setting up a production ready cluster.
