» Agent Communication Encryption

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 complete the RPC encryption section, you must have configured agent certificates.

» Gossip Encryption

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 seperate configuration file with only the encrypt field, since the agent can merge multiple configuration files. The key must be 16-bytes, Base64 encoded.

You can use the Consul CLI command, consul keygen, to generate a cryptographically suitable key.

$ consul keygen
cg8StVXbQJ0gPvMd9o7yrg==

» Enable Gossip Encryption: New Cluster

To enable gossip on a new cluster, we will add the encryption key parameter to the agent configuration file and then pass the file at startup with the -config-dir flag.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw=="
}

$ consul agent -config-dir=/etc/consul.d/
==> Starting Consul agent...
==> Starting Consul agent RPC...
==> Consul agent running!
         Node name: 'Armons-MacBook-Air.local'
        Datacenter: 'dc1'
            Server: false (bootstrap: false)
       Client Addr: 127.0.0.1 (HTTP: 8500, HTTPS: -1, DNS: 8600, RPC: 8400)
      Cluster Addr: 10.1.10.12 (LAN: 8301, WAN: 8302)
    Gossip encrypt: true, RPC-TLS: false, TLS-Incoming: false
...

"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.

» Enable Gossip Encryption: Existing Cluster

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 and encrypt_verify_outgoing is necessary.

Step 1: Generate an encryption key using consul keygen.

$ consul keygen
JY34uTPZyfUE+6tinMYEVw==

Step 2: Set the 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.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "encrypt_verify_incoming": false,
  "encrypt_verify_outgoing": false
}

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.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "encrypt_verify_incoming": false,
  "encrypt_verify_outgoing": true
}

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.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "encrypt_verify_incoming": true,
  "encrypt_verify_outgoing": true
}

All the agents will now be strictly enforcing encrypted gossip. Note, the default behavior of both encrypt_verify_incoming and encrypt_verify_outgoing is true. We have set them in the configuration file as an explicit example.

» TLS Encryption for RPC

Consul supports using TLS to verify the authenticity of servers and clients. To enable TLS, Consul requires that all servers have certificates that are signed by a single Certificate Authority. Clients may optionally authenticate with a client certificate generated by the same CA. Please see this tutorial on creating a CA and signing certificates.

TLS can be used to verify the authenticity of the servers with verify_outgoing and verify_server_hostname. It can also optionally verify client certificates when using verify_incoming

Review the docs for specifics.

In Consul version 0.8.4 and newer, migrating to TLS encrypted traffic on a running cluster is supported.

» Enable TLS: New Cluster

After TLS has been configured on all the agents, you can start the agents and RPC communication will be encrypted.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "verify_incoming": true,
  "verify_outgoing": true,
  "verify_server_hostname": true,
  "ca_file": "consul-agent-ca.pem",
  "cert_file": "consul-server-dc1-0.pem",
  "key_file": "consul-server-dc1-0-key.pem"
}

Note, for clients, the default cert_file and key_file will be named according to their cluster for. For example, consul-client-dc1-0.pem.

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.

» Enable TLS: Existing Cluster

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, 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 port.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "verify_incoming": false,
  "verify_outgoing": false,
  "ca_file": "consul-agent-ca.pem",
  "cert_file": "consul-server-dc1-0.pem",
  "key_file": "consul-server-dc1-0-key.pem"
}

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 command or the Operator API.

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.

{
  "data_dir": "/opt/consul",
  "log_level": "INFO",
  "node_name": "bulldog",
  "server": true,
  "encrypt": "JY34uTPZyfUE+6tinMYEVw==",
  "verify_incoming": true,
  "verify_outgoing": true,
  "verify_server_hostname": true,
  "ca_file": "consul-agent-ca.pem",
  "cert_file": "consul-server-dc1-0.pem",
  "key_file": "consul-server-dc1-0-key.pem"
}

At this point, full TLS encryption for RPC communication is enabled. To disable HTTP connections, which may still be in use by clients for API and CLI communications, update the agent configuration.

» Summary

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.