**Caddy's default TLS settings are secure. Only change these settings if you have a good reason and understand the implications.** The most common use of this directive will be to specify an ACME account email address, change the ACME CA endpoint, or to provide your own certificates.
Compatibility note: Due to its sensitive nature as a security protocol, deliberate adjustments to TLS defaults may be made in new minor or patch releases. Old or broken TLS versions, ciphers, features, etc. may be removed at any time. If your deployment is extremely sensitive to changes, you should explicitly specify those values which must remain constant, and be vigilant about upgrades. In almost every case, we recommend using the default settings.
- **ciphers** specifies the list of cipher suite names in descending preference order. It is recommended to not change these unless you know what you're doing. Note that cipher suites are not customizable for TLS 1.3; and not all TLS 1.2 ciphers are enabled by default. The supported names are (in no particular order here):
- **ca** changes the ACME CA endpoint. This is most often used to set [Let's Encrypt's staging endpoint](https://letsencrypt.org/docs/staging-environment/) when testing, or an internal ACME server. (To change this value for the whole Caddyfile, use the `acme_ca` [global option](/docs/caddyfile/options) instead.)
- **dns** enables the [DNS challenge](/docs/automatic-https#dns-challenge) using the specified provider plugin, which must be plugged in from one of the [caddy-dns](https://github.com/caddy-dns) repositories. Each provider plugin may have their own syntax following their name; refer to their docs for details. Maintaining support for each DNS provider is a community effort. [Learn how to enable the DNS challenge for your provider at our wiki.](https://caddy.community/t/how-to-use-dns-provider-modules-in-caddy-2/8148)
- **resolvers** customizes the DNS resolvers used when performing the DNS challenge; these take precedence over system resolvers or any default ones. If set here, the resolvers will propagate to all configured certificate issuers.
- **on_demand** enables [on-demand TLS](/docs/automatic-https#on-demand-tls) for the hostnames given in the site block's address(es). **Security warning:** Doing so in production is insecure unless you also configure the [`on_demand_tls` global option](https://caddyserver.com/docs/caddyfile/options#on-demand-tls) to mitigate abuse.
Multiple `trusted_*` directives may be used to specify multiple CA or leaf certificates. Client certificates which are not listed as one of the leaf certificates or signed by any of the specified CAs will be rejected according to the **mode**.
- **issuer** configures a custom certificate issuer, or a source from which to obtain certificates. Which issuer is used and the options that follow in this segment depend on the issuer modules that are available (see below for the standard issuers; plugins may add others). Some of the other subdirectives such as `ca` and `dns` are actually shortcuts for configuring the `acme` issuer (and this subdirective was added later), so specifying this directive and some of the others is confusing and thus prohibited. This subdirective can be specified multiple times to configure multiple, redundant issuers; if one fails to issue a cert, the next one will be tried.
- **dir** is the URL to the ACME CA's directory. Default: `https://acme-v02.api.letsencrypt.org/directory`
- **test_dir** is an optional fallback directory to use when retrying challenges; if all challenges fail, this endpoint will be used during retries; useful if a CA has a staging endpoint where you want to avoid rate limits on their production endpoint. Default: `https://acme-staging-v02.api.letsencrypt.org/directory`
- **email** is the ACME account contact email address.
- **timeout** is how long to wait before timing out an ACME operation.
- **disable_http_challenge** will disable the HTTP challenge.
- **disable_tlsalpn_challenge** will disable the TLS-ALPN challenge.
- **alt_http_port** is an alternate port on which to serve the HTTP challenge; it has to happen on port 80 so you must forward packets to this alternate port.
- **alt_tlsalpn_port** is an alternate port on which to serve the TLS-ALPN challenge; it has to happen on port 443 so you must forward packets to this alternate port.
- **eab** specifies an External Account Binding which may be required with some ACME CAs.
- **trusted_roots** is one or more root certificates (as PEM filenames) to trust when connecting to the ACME CA server.
- **propagation_timeout** is a [duration value](/docs/conventions#durations) that sets how long to wait for DNS TXT records to propagate. Default 2 minutes.
- **preferred_chains** specifies which certificate chains Caddy should prefer; useful if your CA provides multiple chains. Use one of the following options:
- **smallest** will tell Caddy to prefer chains with the fewest amount of bytes.
- **root_common_name** is a list of one or more common names; Caddy will choose the first chain that has a root that matches with at least one of the specified common names.
- **any_common_name** is a list of one or more common names; Caddy will choose the first chain that has an issuer that matches with at least one of the specified common names.
The functionality of the `zerossl` issuer is the same as the `acme` issuer, except that it will use ZeroSSL's directory by default and it can automatically negotiate EAB credentials (whereas with the `acme` issuer, you have to manually provide EAB credentials and set the directory endpoint).
Use locally-trusted certificates for all hosts on the current site block, rather than public certificates via ACME / Let's Encrypt (useful in dev environments):
Specify an email address for your ACME account (but if only one email is used for all sites, we recommend the `email` [global option](/docs/caddyfile/options) instead):
Enable TLS Client Authentication and require clients to present a valid certificate that is verified against all the provided CA's via `trusted_ca_cert_file`
