The Consul agent supports encrypting all of its network traffic. The exact method of this encryption is described on the encryption internals page. There are two seperate 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 by setting the encrypt parameter in a configuration file for the agent. The key must be 16-bytes that are base64 encoded. The easiest method to obtain a cryptographically suitable key is by using consul keygen.

$ consul keygen

With that key, you can enable encryption on the agent. You can verify encryption is enabled because the output 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. For this to work, 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. Here is a tutorial on generating both a CA and signing keys using OpenSSL. Client certificates must have extended key usage enabled for client and server authentication.

There are a number of things to consider when setting up TLS for Consul. Either we can use TLS just to verify the authenticity of the servers, or we can also verify the authenticity of clients. The former can be used to prevent unauthorized access. This behavior is controlled using either the verify_incoming and verify_outgoing options.

If verify_outgoing is set, then agents verify the authenticity of Consuls for outgoing connections. This means server nodes must present a certificate signed by the ca_file that the agent has. This option must be set on all agents, and there must be a ca_file provided to check the certificate against. If this is set, then all server nodes must have an appropriate key pair set using cert_file and key_file.

If verify_incoming is set, then the servers verify the authenticity of all incoming connections. Servers will also disallow any non-TLS connections. If this is set, then all clients must have a valid key pair set using cert_file and key_file. 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.