According to https://etcd.io/docs/v3.3.12/op-guide/configuration/#listen-peer-urls, we should use an IP address to specify where should etcd listen for peers.

However, in profile::etcd, the base class is called like:

class { '::etcd':
        host               => $::fqdn,
        [...]

Which in turn, ends up being:

$peer_url = "http://${host}:${peer_port}" # Peer TLS is currently broken? in modules/etcd/manifests/init.pp

In my testing setup for a new Toolforge k8s cluster, I saw this error message:

etcd[16341]: error verifying flags, expected IP in URL for binding (http://toolsbeta-arturo-k8s-etcd-1.toolsbeta.eqiad.wmflabs:2380). See 'etcd --help'.

The upstream documentation mentions that it defaults to http://localhost:2380.