Terraform init fails with error
x509: certificate signed by unknown authority (possibly because of "x509: invalid signature: parent certificate cannot sign this kind of certificate" while trying to verify candidate authority certificate
Make sure the machine has your root CA certificate in the store when proxy server used with the terraform. Error occurs if machine where terraform is running doesn't have valid certificate for the proxy server.
Internet < proxy < terraform
Work with your Network and Unix/Linux to have the Proxy certificate added to the CA ROOT of the terraform server.
You should be able to test with a command like
curl --proxy "user:firstname.lastname@example.org:1234" "http://httpbin.org/ip"
Where the last part are the url for the terraform provider url.