Let's Encrypt ACME DNS alias mode

If your DNS provider doesn't support API access, or if you're concerned about security problems from giving the DNS API access to your main domain, then you can use DNS alias mode.

For example, your main domain is example.com, which doesn't have API access, or you don't want to give the API access to acme.sh, since it's important.

And you have another domain: aliasDomainForValidationOnly.com, which has a supported DNS API. This domain is less important, and maybe it's used for validation only.

Ok, let's start.

1. First set domain CNAME:

_acme-challenge.example.com
   =>   _acme-challenge.aliasDomainForValidationOnly.com

or, in standard DNS zone file format, (like ISC BIND or NSD):

_acme-challenge.example.com       IN      CNAME   _acme-challenge.aliasDomainForValidationOnly.com.

2. Issue a cert:

acme.sh --issue  \
  -d  example.com --challenge-alias aliasDomainForValidationOnly.com --dns dns_cf

The Letsencrypt CA server checks the txt record of original domain _acme-challenge.example.com to validate your domain, but you have set the CNAME in step 1, so it goes forward to the aliased domain _acme-challenge.aliasDomainForValidationOnly.com to check.

And acme.sh knows that, so it just added the correct txt record to _acme-challenge.aliasDomainForValidationOnly.com.

So, it's done. you will get a cert for example.com, but you don't need to give the domain control out.

3. Share the same aliased domain:

If you have multiple (sub)domains, you need add CNAME for each (sub)domain, but they can share the same aliased domain. For example, you can add the CNAME like:

_acme-challenge.example.com
  =>   _acme-challenge.aliasDomainForValidationOnly.com

_acme-challenge.www.example.com
  =>   _acme-challenge.aliasDomainForValidationOnly.com

_acme-challenge.sub.example.com
  =>   _acme-challenge.aliasDomainForValidationOnly.com

_acme-challenge.example.net
  =>   _acme-challenge.aliasDomainForValidationOnly.com

_acme-challenge.example.org
  =>   _acme-challenge.aliasDomainForValidationOnly.com

And then issue cert like bellow:

acme.sh --issue  \
  -d  example.com --challenge-alias aliasDomainForValidationOnly.com --dns dns_cf \
  -d  www.example.com \
  -d  sub.example.com \
  -d  example.net \
  -d  example.org

Even with ACME v2 wildcard cert:

acme.sh --issue  \
  -d  example.com --challenge-alias aliasDomainForValidationOnly.com --dns dns_cf \
  -d  www.example.com \
  -d  sub.example.com \
  -d  example.net \
  -d  example.org \
  -d  *.example.com \
  -d  *.example.net \
  -d  *.example.org

4. Specify different aliased domains for each domain.

Yes, you know, acme.sh support to set alias domain for each domain. Even with different dns provider:

You can set CNAME like:

_acme-challenge.example.com
  =>   _acme-challenge.aliasDomainForValidationOnly.com

_acme-challenge.example.net
  => _acme-challenge.aliasDomainForValidationOnly2.com

Then issue cert:

acme.sh --issue  \
  -d  example.com --challenge-alias aliasDomainForValidationOnly.com --dns dns_cf \
  -d  example.net --challenge-alias aliasDomainForValidationOnly2.com

Even with different dns provider:

acme.sh --issue  \
  -d  example.com --challenge-alias aliasDomainForValidationOnly.com --dns dns_cf \
  -d  example.net --challenge-alias aliasDomainForValidationOnly2.com  --dns dns_gd

Let's assume the first domain aliasDomainForValidationOnly.com is hosted at cloudflare, and the second is hosted at godaddy.

5. Last

Do not remove the CNAME like : _acme-challenge.example.com after you issue the cert. It will be reused when acme.sh tries to renew the cert. The left cname record _acme-challenge.example.com doesn't harm your domain at all. Just keep it there.

6. challenge-alias or domain-alias

We have another parameter: --domain-alias, it has the same meaning with --challenge-alias.

But with --domain-alias you don't need to add _acme-challenge. prefix.

For example, if you use --challenge-alias, you must set CNAME like bellow:

CNAME:
_acme-challenge.A.com
   =>   _acme-challenge.B.com

Then issue cert like:

acme.sh --issue -d  a.com  --challenge-alias  b.com  --dns dns_cf

If you use --domain-alias, the CNAME should be like:

CNAME:
_acme-challenge.A.com
   =>    myalias.B.com

Then issue cert like:

acme.sh --issue -d  a.com  --domain-alias  myalias.B.com   --dns dns_cf

Note: Don't use the domain name only for --domain-alias.

acme.sh --issue -d  a.com  --domain-alias B.com   --dns dns_cf

This would require that a TXT record is created at the domain apex i.e. @ TXT "myvalidationcode". Since adding a value at the apex of a domain requires a different syntax for adding the DNS records it cannot be used in this form.

If you really want to create the validation records at the domain apex then depending on the implementation of the dns api you have to use

acme.sh --issue -d  a.com  --domain-alias @.B.com   --dns dns_cf

or

acme.sh --issue -d  a.com  --domain-alias .B.com   --dns dns_cf

from Hacker News https://ift.tt/3z5LWuk