Terraform and AWS wildcard certificates validation

Context #

When defining an AWS ACM certificate in Terraform, it is common to define the validation for that certificate as well (this is a requirement to attach the certificate to a Load Balancer). The current documentation example doesn't work when the certificate in question contains wildcard domains (e.g. example.com, *.example.com).

Implementation #

The following solution leverages the fact that AWS ACM will generate the same validation CNAME for both *.example.com and example.com by filtering the map of values to exclude any non-wildcard SAN.

resource "aws_route53_zone" "example" {
name = "example.com"
}

resource "aws_acm_certificate" "example" {
domain_name = "example.com"
validation_method = "DNS"

subject_alternative_names = [
"*.example.com",
"*.docs.example.com"
]

lifecycle {
create_before_destroy = true
}
}

resource "aws_route53_record" "example" {
for_each = {
for dvo in aws_acm_certificate.example.domain_validation_options : dvo.domain_name => {
name = dvo.resource_record_name
record = dvo.resource_record_value
type = dvo.resource_record_type
}
# Skips the domain if it doesn't contain a wildcard
if length(regexall("\\*\\..+", dvo.domain_name)) > 0
}

allow_overwrite = true
name = each.value.name
records = [each.value.record]
ttl = 60
type = each.value.type
zone_id = aws_route53_zone.example.zone_id
}

The above solution works in cases where the list of SANs contains wildcard domains (by itself or alongside the parent domain), e.g:

This implementation won't work if you have a SANs list as follows

In this case, the only record validation generated would be for *.help.example.com. If you need a more complete solution check the gist in the reference section.

Notes #

References #

Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer’s view in any way.