» Encryption

The Consul agent supports encrypting all of its network traffic. The exact method of encryption is described on the encryption internals page. There are two separate encryption systems, one for gossip traffic and one for RPC.

» Gossip Encryption

Enabling gossip encryption only requires that you set an encryption key when starting the Consul agent. The key can be set via the encrypt parameter: the value of this setting is a configuration file containing the encryption key.

The key must be 16-bytes, Base64 encoded. As a convenience, Consul provides the consul keygen command to generate a cryptographically suitable key:

$ consul keygen

With that key, you can enable encryption on the agent. If encryption is enabled, the output of consul agent will include "Encrypted: true":

$ cat encrypt.json
{"encrypt": "cg8StVXbQJ0gPvMd9o7yrg=="}

$ consul agent -data-dir=/tmp/consul -config-file=encrypt.json
==> WARNING: LAN keyring exists but -encrypt given, using keyring
==> WARNING: WAN keyring exists but -encrypt given, using keyring
==> Starting Consul agent...
==> Starting Consul agent RPC...
==> Consul agent running!
         Node name: 'Armons-MacBook-Air.local'
        Datacenter: 'dc1'
            Server: false (bootstrap: false)
       Client Addr: (HTTP: 8500, HTTPS: -1, DNS: 8600, RPC: 8400)
      Cluster Addr: (LAN: 8301, WAN: 8302)
    Gossip encrypt: true, RPC-TLS: false, TLS-Incoming: false

All nodes within a Consul cluster must share the same encryption key in order to send and receive cluster information.

» Configuring Gossip Encryption on an existing cluster

As of version 0.8.4, Consul supports upshifting to encrypted gossip on a running cluster through the following process.

  1. Generate an encryption key using consul keygen
  2. Set the encrypt key in the agent configuration and set encrypt_verify_incoming and encrypt_verify_outgoing to false, doing 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.
  3. Remove the encrypt_verify_outgoing setting to change it back to true (the default) and perform another rolling update of the cluster. The agents will now be sending encrypted gossip but will still allow incoming unencrypted traffic.
  4. Remove the encrypt_verify_incoming setting to change it back to true (the default) and perform a final rolling update of the cluster. All the agents will now be strictly enforcing encrypted gossip.

» RPC Encryption with TLS

Consul supports using TLS to verify the authenticity of servers and clients. To enable this, Consul requires that all clients and servers have key pairs that are generated by a single Certificate Authority. This can be a private CA, used only internally. The CA then signs keys for each of the agents, as in this tutorial on generating both a CA and signing keys using OpenSSL.

TLS can be used to verify the authenticity of the servers or verify the authenticity of clients. These modes are controlled by the verify_outgoing, verify_server_hostname, and verify_incoming options, respectively.

If verify_outgoing is set, agents verify the authenticity of Consul for outgoing connections. Server nodes must present a certificate signed by a common certificate authority present on all agents, set via the agent's ca_file and ca_path options. All server nodes must have an appropriate key pair set using cert_file and key_file.

If verify_server_hostname is set, then outgoing connections perform hostname verification. All servers must have a certificate valid for server.<datacenter>.<domain> or the client will reject the handshake. This is a new configuration as of 0.5.1, and it is used to prevent a compromised client from being able to restart in server mode and perform a MITM (Man-In-The-Middle) attack. New deployments should set this to true, and generate the proper certificates, but this is defaulted to false to avoid breaking existing deployments.

If verify_incoming is set, the servers verify the authenticity of all incoming connections. All clients must have a valid key pair set using cert_file and key_file. Servers will also disallow any non-TLS connections. To force clients to use TLS, verify_outgoing must also be set.

TLS is used to secure the RPC calls between agents, but gossip between nodes is done over UDP and is secured using a symmetric key. See above for enabling gossip encryption.

» Configuring TLS on an existing cluster

As of version 0.8.4, Consul supports migrating to TLS-encrypted traffic on a running cluster without downtime. This process assumes a starting point with no TLS settings configured, and involves an intermediate step in order to get to full TLS encryption:

  1. Generate the necessary keys/certs and set the ca_file/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.
  2. 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.
  3. (Optional, Enterprise-only) If applicable, set the UseTLS setting in any network areas to true. This can be done either through the consul operator area update command or the Operator API.
  4. Change the verify_incoming and verify_outgoing settings (as well as verify_server_hostname if applicable) to true.
  5. Perform another rolling restart of each agent in the cluster.

At this point, full TLS encryption for RPC communication should be enabled.