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 commmand 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=/tmp/consul -config-file encrypt.json
==> Starting Consul agent...
==> Starting Consul agent RPC...
==> Consul agent running!
         Node name: 'Armons-MacBook-Air.local'
        Datacenter: 'dc1'
    Advertise addr: ''
          RPC addr: ''
         HTTP addr: ''
          DNS addr: ''
         Encrypted: true
            Server: false (bootstrap: false)

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

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. Note: client certificates must have Extended Key Usage enabled for client and server authentication.

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 the certificate authority present on all agents, set via the agent's ca_file option. 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 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.