**NOTE on CAA**: Please ensure that your DNS provider answers correctly to CAA record requests. [If your DNS provider answer with an error, Let's Encrypt won't issue a certificate for your domain](https://letsencrypt.org/docs/caa/). Let's Encrypt do not require that you set a CAA record on your domain, just that your DNS provider answers correctly.
**NOTE on IPv6**: If the domain or sub domain you want to issue certificate for has an AAAA record set, Let's Encrypt will favor challenge validation over IPv6. [There is an IPv6 to IPv4 fallback in place but Let's Encrypt can't guarantee it'll work in every possible case](https://github.com/letsencrypt/boulder/issues/2770#issuecomment-340489871), so bottom line is **if you are not sure of both your host and your host's Docker reachability over IPv6, do not advertise an AAAA record** or LE challenge validation might fail.
As described on [basic usage](./Basic-usage.md), the `LETSENCRYPT_HOST` environment variables needs to be declared in each to-be-proxied application containers for which you want to enable SSL and create certificate. It most likely needs to be the same as the `VIRTUAL_HOST` variable and must resolve to your host (which has to be publicly reachable).
The following environment variables are optional and parametrize the way the Let's Encrypt client works.
Specify multiple hosts with a comma delimiter to create multi-domains ([SAN](https://www.digicert.com/subject-alternative-name.htm)) certificates (the first domain in the list will be the base domain).
Let's Encrypt has a limit of [100 domains per certificate](https://letsencrypt.org/fr/docs/rate-limits/), while Buypass limit is [15 domains per certificate](https://www.buypass.com/ssl/products/go-ssl-campaign).
The example above will issue a single domain certificate for all the domains listed in the `LETSENCRYPT_HOST` environment variable. If you need to have a separate certificate for each of the domains, you can add set the `LETSENCRYPT_SINGLE_DOMAIN_CERTS` environment variable to `true`.
Every hour (3600 seconds) the certificates are checked and per default every certificate that have been issued at least [60 days](https://github.com/acmesh-official/acme.sh/blob/f2d350002e7c387fad9777a42cf9befe34996c35/acme.sh#L61) ago is renewed. For Let's Encrypt certificates, that mean they will be renewed 30 days before expiration.
The `LETSENCRYPT_EMAIL` environment variable must be a valid email and will be used by Let's Encrypt to warn you of impeding certificate expiration (should the automated renewal fail) and to recover an account.
The `LETSENCRYPT_KEYSIZE` environment variable determines the type and size of the requested key. Supported values are `2048`, `3072` and `4096` for RSA keys, and `ec-256` or `ec-384` for elliptic curve keys. The default is RSA 4096.
To change the global default set the `DEFAULT_KEY_SIZE` environment variable on the **acme-companion** container to one of the supported values specified above.
The `ACME_OCSP` environment variable, when set to `true` on a proxied application container, will add the [OCSP Must-Staple extension](https://blog.apnic.net/2019/01/15/is-the-web-ready-for-ocsp-must-staple/) to the issued certificate. Please read about OCSP Must-Staple support in Nginx if you intend to use this feature (https://trac.nginx.org/nginx/ticket/812 and https://trac.nginx.org/nginx/ticket/1830)
The `LETSENCRYPT_TEST` environment variable, when set to `true` on a proxied application container, will create a test certificates that don't have the [5 certs/week/domain limits](https://letsencrypt.org/docs/rate-limits/) and are signed by an untrusted intermediate (they won't be trusted by browsers).
If you want to do this globally for all containers, set `ACME_CA_URI` on the **acme-companion** container as described in [Container configuration](./Container-configuration.md).
The `ACME_CA_URI` environment variable is used to set the ACME API endpoint from which the container's certificate(s) will be requested (defaults to ``https://acme-v02.api.letsencrypt.org/directory``).
If the ACME CA provides multiple cert chain, you can use the `ACME_PREFERRED_CHAIN` environment variable to select one. See [`acme.sh --preferred-chain` documentation](https://github.com/acmesh-official/acme.sh/wiki/Preferred-Chain) for more info.
The `LETSENCRYPT_RESTART_CONTAINER` environment variable, when set to `true` on an application container, will restart this container whenever the corresponding cert (`LETSENCRYPT_HOST`) is renewed. This is useful when certificates are directly used inside a container for other purposes than HTTPS (e.g. an FTPS server), to make sure those containers always use an up to date certificate.
The `ACME_PRE_HOOK` and `ACME_POST_HOOK` let you use the [`acme.sh` Pre- and Post-Hooks feature](https://github.com/acmesh-official/acme.sh/wiki/Using-pre-hook-post-hook-renew-hook-reloadcmd) to run commands respectively before and after the container's certificate has been issued. For more information see [Pre- and Post-Hook](./Hooks.md)
The `DEFAULT_EMAIL` variable must be a valid email and, when set on the **acme-companion** container, will be used as a fallback when no email address is provided using proxyed container's `LETSENCRYPT_EMAIL` environment variables. It is highly recommended to set this variable to a valid email address that you own.
The `RENEW_PRIVATE_KEYS` environment variable, when set to `false` on the **acme-companion** container, will set `acme.sh` to reuse previously generated private key instead of generating a new one at renewal for all domains.
Reusing private keys can help if you intend to use [HPKP](https://developer.mozilla.org/en-US/docs/Web/HTTP/Public_Key_Pinning), but please note that HPKP has been deprecated by Google's Chrome and that it is therefore strongly discouraged to use it at all.
