export VAULT_CACERT=/usr/local/etc/ssl/ca.pem
export VAULT_ADDR=https://vault0.example.com:8200
First setup the PKI
vault write pki/root/generate/internal common_name=example.com ttl=8760h
vault write pki/roles/example allowed_domains=example.com allow_subdomains=true max_ttl=72h