On the servers they must have a certificate.
On the clients they just have to set verify_outgoing to true to attempt TLS connections for RPCs.
Eventually we may relax these restrictions but right now all of the settings we push down (acl tokens, acl related settings, certificates, gossip key) are sensitive and shouldn’t be transmitted over an unencrypted connection. Our guides and docs should recoommend verify_server_hostname on the clients as well.
Another reason to do this is weird things happen when making an insecure RPC when TLS is not enabled. Basically it tries TLS anyways. We should probably fix that to make it clearer what is going on.
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