Apache Module mod_md

MDBaseServer Directive

Controls if the base server, the one outside all VirtualHosts should be managed by mod_md or not. Default is to not do this, for the very reason that it may have confusing side-effects. It is recommended that you have virtual hosts for all managed domains and do not rely on the global, fallback server configuration.

MDCAChallenges Directive

Sets challenge types and their execution order when proving domain ownership. The names are protocol specific. The current ACME protocol version implemented by Let's Encrypt defines two challenge types that are supported by mod_md. By default, it will try the one on port 443 when available.

MDCertificateAgreement Directive

When you use mod_md to obtain a certificate, you become a customer of the CA (e.g. Let's Encrypt). That means you need to read and agree to their Terms of Service, so that you understand what they offer and what they might exclude or require from you. mod_md cannot, by itself, agree to such a thing.

In case of Let's Encrypt, their current Terms of Service are here. Those terms might (and probably will) change over time. So, the certificate renewal might require you to update this agreement URL.

MDCertificateAgreement https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf
MDomain example.org www.example.org mail.example.org

MDCertificateAuthority Directive

The URL where the CA offers its service.

Let's Encrypt offers, right now, two such URLs. One for the real certificates and one for testing (their staging area, at https://acme-staging.api.letsencrypt.org/directory). In order to have mod_md use this testing service, configure your server like this:

MDCertificateAuthority https://acme-staging.api.letsencrypt.org/directory
MDCertificateAgreement https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf

MDCertificateProtocol Directive

Specifies the protocol to use. Currently, only ACME is supported.

MDDriveMode Directive

In 'auto' mode, mod_md will drive a Managed Domain's properties (e.g. certificate management) whenever necessary. When a MD is not used in any virtual host, the module will do nothing. When a certificate is missing, it will try to get one. When a certificate expires soon (see MDRenewWindow), it will renew it.

In 'manual' mode, it is your duty to do all this. The module will provide the existing certificate to mod_ssl, if available. But it will not contact the CA for signup/renewal. This can be useful in clustered setups where you want just one node to perform the driving.

The third mode 'always' is like 'auto', with the difference that mod_md will not check if the MD is actually used.

MDHttpProxy Directive

Use a http proxy to connect to the MDCertificateAuthority. Define this if your webserver can only reach the internet with a forward proxy.

MDMember Directive

Instead of listing all dns names on the same line, you may use MDMember to add such names to a managed domain.

<MDomainSet example.org>
    MDMember www.example.org
    MDMember mail.example.org
</MDomainSet example.org>

If you use it in the global context, outside a specific MD, you can only specify one value, 'auto' or 'manual' as the default for all other MDs. See MDomain for a description of these special values.

MDMembers Directive

Defines if the ServerName and ServerAlias values of a VirtualHost are automatically added to the members of a Managed Domain or not.

MDMustStaple Directive

Defines if newly requested certificate should have the OCSP Must Staple flag set or not. If a certificate has this flag, the server is required to send a OCSP stapling response to every client. This only works if you configure mod_ssl to generate this (see SSLUseStapling and friends).

MDNotifyCmd Directive

The configured executable is run when Managed Domains have signed up or renewed their certificates. It is given the names of the processed MDs as additional arguments (after the parameters specified here). It should return status code 0 to indicate that it has run successfully.

MDomain Directive

All the names in the list are managed as one Managed Domain (MD). mod_md will request one single certificate that is valid for all these names. This directive uses the global settings (see other MD directives below). If you need specific settings for one MD, use the <MDomainSet>.

There are 2 additional settings that are necessary for a Managed Domain: ServerAdmin and MDCertificateAgreement. The mail address of ServerAdmin is used to register at the CA (Let's Encrypt by default). The CA may use it to notify you about changes in its service or status of your certificates.

The second setting, MDCertificateAgreement, is the URL of the Terms of Service of the CA. When you configure the URL, you confirm that you have read and agree to the terms described in the linked document. Before you do that, the CA will not hand out certificates to you.

ServerAdmin mailto:[email protected]
MDCertificateAgreement https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf
MDomain example.org www.example.org

<VirtualHost *:443>
    ServerName example.org
    DocumentRoot htdocs/root

    SSLEngine on
</VirtualHost>

<VirtualHost *:443>
    ServerName www.example.org
    DocumentRoot htdocs/www

    SSLEngine on
</VirtualHost>

There are two special names that you may use in this directive: 'manual' and 'auto'. This determines if a Managed Domain shall have exactly the name list as is configured ('manual') or offer more convenience. With 'auto' all names of a virtual host are added to a MD. Conventiently, 'auto' is also the default.

MDomain example.org

<VirtualHost *:443>
    ServerName example.org
    ServerAlias www.example.org
    DocumentRoot htdocs/root

    SSLEngine on
</VirtualHost>

MDomain example2.org auto

<VirtualHost *:443>
    ServerName example2.org
    ServerAlias www.example2.org
    ...
</VirtualHost>

In this example, the domain 'www.example.org' is automatically added to the MD 'example.org'. Similarly for 'example2.org' where 'auto' is configured explicitly. Whenever you add more ServerAlias names to this virtual host, they will be added as well to the Managed Domain.

If you prefer to explicitly declare all the domain names, use 'manual' mode. An error will be logged if the names do not match with the expected ones.

<MDomainSet> Directive

This directive allows you to define a Managed Domain (MD) with specific settings, different from the global MD* ones. For example, you can have such an MD use another CA then Let's Encrypt, have its unique renewal duration etc.

<MDomainSet sandbox.example.org>
    MDCertificateAuthority   https://someotherca.com/ACME
    MDCertificateAgreement   https://someotherca.com/terms/v_1.02.pdf
</MDomainSet>

This is a specialized version of MDomain, it should be used only when a fine grained configuration is required. MDomain is the suggested choice for the general use case.

MDPortMap Directive

The ACME protocol provides two methods to verify domain ownership: one that uses port 80 and one for port 443. If your server is not reachable by at least one of the two, ACME will not work for you.

mod_md will look at your server configuration and try to figure out which of those are available. Then it can select the proper ACME challenge to create a certificate for your site.

However if you have some fancy port forwarding in place, your server may be reachable from the Internet on port 443, but the local port that httpd uses is another one. Your server might only listen on ports 5001 and 5002, but be reached on ports 443 and 80. How should mod_md figure that one out?

With MDPortMap you can tell it which 'Internet port' corresponds to which local port.

MDPortMap 80:- 443:5002

This example says that the server is not reachable on port 80 from the outside, but local port 5002 is the one responding to https: requests.

MDPrivateKeys Directive

Defines what kind of private keys are generated for a managed domain and with what parameters. The only supported type right now is 'RSA' and the only parameter it takes is the number of bits used for the key.

The current (2017) recommendation is at least 2048 bits and a smaller number is not accepted here. Higher numbers offer longer security, but are computationally more expensive, e.g. increase the load on your server. That might or might not be an issue for you.

Other key types will be defined in the future.

MDPrivateKeys RSA 3072

Please note that this setting only has an effect on new keys. Any existing private key you have remains unaffected. Also, this only affects private keys generated for certificates. ACME account keys are unaffected by this.

MDRenewWindow Directive

If the validity of the certificate falls below duration, mod_md will get a new signed certificate.

Normally, certificates are valid for around 90 days and mod_md will renew them the earliest 33% of their complete lifetime before they expire (so for 90 days validity, 30 days before it expires). If you think this is not what you need, you can specify either the exact time, as in:

# 21 days before expiry
MDRenewWindow 21d 
# 30 seconds (might be close)
MDRenewWindow 30s
# 10% of the cert lifetime
MDRenewWindow 10%

When in auto drive mode, the module will check every 12 hours at least what the status of the managed domains is and if it needs to do something. On errors, for example when the CA is unreachable, it will initially retry after some seconds. Should that continue to fail, it will back off to a maximum interval of hourly checks.

MDRequireHttps Directive

This is a convenience directive to ease http: to https: migration of your Managed Domains. With:

MDRequireHttps temporary

you announce that you want all traffic via http: URLs to be redirected to the https: ones, for now. This is safe and you can remove this again at any time.

The following has consequences: if you want client to no longer use the http: URLs, configure:

MDRequireHttps permanent

This does two things:

1. All request to the http: resources are redirected to the same url with the https: scheme using the 301 status code. This tells clients that this is intended to be forever and the should update any links they have accordingly.
2. All answers to https: requests will carry the header Strict-Transport-Security with a life time of half a year. This tells the browser that it never (for half a year) shall use http: when talking to this domain name. Browsers will, after having seen this, refuse to contact your unencrypted site. This prevents malicious middleware to downgrade connections and listen/manipulate the traffic. Which is good. But you cannot simply take it back again.

You can achieve the same with mod_alias and some Redirect configuration, basically. If you do it yourself, please make sure to exclude the paths /.well-known/* from your redirection, otherwise mod_md might have trouble signing on new certificates.

If you set this globally, it applies to all managed domains. If you want it for a specific domain only, use:

<MDomainSet xxx.yyy>
  MDRequireHttps temporary
</MDomainSet>

MDStoreDir Directive

Defines where on the local file system the Managed Domain data is stored. This is an absolute path or interpreted relative to the server root. The default will create a directory 'md' in your server root.

If you move this and have already data, be sure to move/copy the data first to the new location, reconfigure and then restart the server. If you reconfigure and restart first, the server will try to get new certificates that it thinks are missing.