Apache Module mod_md

MDBaseServer Directive

Controls if the base server, the one outside all VirtualHosts should be managed by mod_md or not. By default, it will not. 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 (in order of preference) when proving domain ownership. Supported by the module are the challenge methods 'tls-alpn-01', 'dns-01' and 'http-01'. The module will look at the overall configuration of the server to find out which methods can be used.

If the server listens on port 80, for example, the 'http-01' method is available. The prerequisite for 'dns-01' is a configured 'MDChallengeDns01' command. 'tls-alpn-01' is described above in 'https: Challenges'.

This auto selection works for most setups. But since Apache is a very powerful server with many configuration options, the situation is not clear for all possible cases. For example: it may listen on multiple IP addresses where some are reachable on `https:` and some not.

If you configure 'MDCAChallenges' directly, this auto selection is disabled. Instead, the module will use the configured challenge list when talking to the ACME server (a challenge type must be offered by the server as well). This challenges are examined in the order specified.

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.

MDCertificateAuthority Directive

The URL where the CA offers its service.

Let's Encrypt offers, right now, four such URLs. Two for the own legacy version of the ACME protocol, commonly named ACMEv1. And two for the RFC 8555 version, named ACMEv2.

Each version has 2 endpoints, as their is a production endpoint and a "staging" endpoint for testing. The testing endpoint works the same, but will not give you certificates recognized by browsers. However, it also has very relaxed rate limits. This allows testing of the service repeatedly without you blocking yourself.

MDCertificateAuthority https://acme-staging-v02.api.letsencrypt.org/directory

MDCertificateFile Directive

This is used inside a MDomainSet and specifies the file holding the certificate chain for the Managed Domain. The matching key is specified via MDCertificateKeyFile.

<MDomain mydomain.com>
  MDCertificateFile /etc/ssl/my.cert
  MDCertificateKeyFile /etc/ssl/my.key
</MDomain>

This is that equivalent of the mod_ssl SSLCertificateFile directive. It has several uses.

If you want to migrate an existing domain, using static files, to automated Let's Encrypt certificates, for one. You define the MDomainSet, add the files here and remove the SSLCertificateFile from your VirtualHosts.

This will give you the same as before, with maybe less repeating lines in your configuration. Then you can add MDRenewMode 'always' to it and the module will get a new certificate before the one from the file expires. When it has done so, you remove the MDCertificateFile and reload the server.

Another use case is that you renew your Let's Encrypt certificates with another ACME clients, for example the excellent certbot. Then let your MDs point to the files from certbot and have both working together.

MDCertificateKeyFile Directive

This is used inside a MDomainSet and specifies the file holding the private key for the Managed Domain. The matching certificate is specified via MDCertificateFile.

This is that equivalent of the mod_ssl SSLCertificateKeyFile directive.

MDCertificateMonitor Directive

This is part of the 'server-status' HTML user interface and has nothing to do with the core functioning itself. It defines the link offered on that page for easy checking of a certificate monitor. The SHA256 fingerprint of the certificate is appended to the configured url.

Certificate Monitors offer supervision of Certificate Transparency (CT) Logs to track the use of certificates for domains. The least you may see is that Let's Encrypt (or whichever CA you have configured) has entered your certificates into the CTLogs.

Caveat: certificate logs update and monitor's intakes of those updates suffer some delay. This varies between logs and monitors. A brand new certificate will not be known immediately.

MDCertificateProtocol Directive

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

MDCertificateStatus Directive

When enabled, a resources is available in Managed Domains at 'https://domain/.httpd/certificate-status' that returns a JSON document list key properties of the current and of a renewed certificate - when available.

{
  "valid-until": "Thu, 29 Aug 2019 16:06:35 GMT",
  "valid-from": "Fri, 31 May 2019 16:06:35 GMT",
  "serial": "03039C464D454EDE79FCD2CAE859F668F269",
  "sha256-fingerprint": "1ff3bfd2c7c199489ed04df6e29a9b4ea6c015fe8a1b0ce3deb88afc751e352d"
  "renewal" : { ...renewed cert information... }
}

MDChallengeDns01 Directive

Define a program to be called when the `dns-01` challenge needs to be setup/torn down. The program is given the argument `setup` or `teardown` followed by the domain name. For `setup` the challenge content is additionally given.

You do not need to specify this, as long as a 'http:' or 'https:' challenge method is possible. However, Let's Encrypt makes 'dns-01' the only challenge available for wildcard certificates. If you require one of those, you need to configure this.

See the section about wildcard certificates above for more details.

MDContactEmail Directive

The ACME protocol requires you to give a contact url when you sign up. Currently, Let's Encrypt wants an email address (and it will use it to inform you about renewals or changed terms of service). mod_md uses the MDContactEmail directive email in your Apache configuration, so please specify the correct address there. If MDContactEmail is not present, mod_md will use the ServerAdmin directive.

MDDriveMode Directive

This directive exists for backward compatibility as the old name for MDRenewMode.

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.

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

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.

MDMessageCmd Directive

This command gets called when one of the following events happen for a Managed Domain: "renewed", "installed", "expiring", "errored". The command may be invoked for more than these in the future and ignore events it is not prepared to handle.

This is the more flexible companion to MDNotifyCmd.

The program should not block, as the module will wait for it to finish. A return code other than 0 is regarded as an error.

'errored' is no immediate cause for concern since renewal is attempted early enough to allow the internet to come back. This is reported at most once per hour.

'expiring' should be taken serious. It is issued when the MDWarnWindow is reached. By default this is 10% of the certificate lifetime, so for Let's Encrypt this currently means 9 days before it expires. The warning is repeated at most once a day.

'renewed' means that a new certificate has been obtained and is stored in the 'staging' area in the MD store. It will be activated on the next server restart/reload.

'installed' is triggered when a new certificate has been transferred from staging into the domains location in MD store. This happens at server startup/reload. Different to all other invocations, MDMessageCmd is run with root permissions (on *nix systems) and has access to the certificate files (and keys). Certificates needed for other applications or in different formats can be processed on this event.

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 a Managed Domain has signed up or renewed its certificate. It is given the name of the processed MD 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: a contact Email address (via MDContactEmail or 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, should have the value "accepted". By specifying this, you confirm that your accept the Terms of Service of the CA.

MDContactEmail admin@example.org
MDCertificateAgreement accepted
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. Conveniently, '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 is the directive MDomain with the added possibility to add setting just for this MD. In fact, you may also use "<MDomain ..>" as a shortcut.

This allows you to configure an MD that uses another Certificate Authority, have other renewal requirements, etc.

<MDomain sandbox.example.org>
    MDCertificateAuthority   https://someotherca.com/ACME
</MDomain>

A common use case is to configure https: requirements separately for your domains.

<MDomain example.org>
    MDRequireHttps temporary
</MDomain>

MDPortMap Directive

The ACME protocol provides two methods to verify domain ownership via HTTP: one that uses 'http:' urls (port 80) and one for 'https:' urls (port 443). If your server is not reachable by at least one of the two, ACME may only work by configuring your DNS server, see MDChallengeDns01.

On most public facing servers, 'http:' arrives on port 80 and 'https:' on port 443. The module checks the ports your Apache server is listening on and assumes those are available. This means that when your server does not listen on port 80, it assumes that 'http:' requests from the internet will not work.

This is a good guess, but it may be wrong. For example, your Apache might listen to port 80, but your firewall might block it. 'http:' is only available in your intranet. So, the module will falsely assume that Let's Encrypt can use 'http:' challenges with your server. This will then fail, because your firewall will drop those.

MDPortMap http:- https:8433

The above example shows how you can specify that 'http:' requests from the internet will never arrive. In addition it says that 'https:' requests will arrive on local port 8433.

This is necessary if you have 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 8443 and 8000, but be reached on ports 443 and 80 (from the internet).

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.

MDRenewMode Directive

In the default 'auto' mode, the module will do what makes most sense of each Managed Domain. For a domain without any certificates, it will obtain them from the Certificate Authority.

However, if you have defined an MD that is not used by any of Apache's VirtualHosts, it will not bother. And for MDs with static certificate files (see MDCertificateFile), it assumes that you have your own source, and will not renew them either.

You can override this default in either way. If you specify 'always', the module will renew certificates for an MD, regardless if the domains are in use or if there are static files.

For the opposite effect, configure 'manual' and no renewal will be attempted.

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:

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

MDServerStatus Directive

Apaches 'server-status' handler allows you configure a resource to monitor what is going on. This includes now a section listing all Managed Domains with the DNS names, renewal status, lifetimes and main properties.

You can switch that off using this directive.

MDStapleOthers Directive

This setting only takes effect when `MDStapling` is enabled. It controls if `mod_md` should also provide stapling information for certificates that are not directly controlled by it, e.g. renewed via an ACME CA.

MDStapling Directive

mod_md offers an implementation for providing OCSP stapling information. This is an alternative to the one provided by 'mod_ssl'. For backward compatibility, this is disabled by default.

The stapling can be switched on for all certificates on the server or for an individual MDomain. This will replace any stapling configuration in `mod_ssl` for these hosts. When disabled, the 'mod_ssl' stapling will do the work (if it is itself enabled, of course). This allows for a gradual shift over from one implementation to the other.

The stapling of `mod_md` will also work for domains where the certificates are not managed by this module (see MDStapleOthers for how to control this). This allows use of the new stapling without using any ACME certificate management.

MDStaplingKeepResponse Directive

This time window specifies when OCSP response data used in stapling shall be removed from the store again. Response information older than 7 days (default) is deleted on server restart/reload. This keeps the store from growing when certificates are renewed/reconfigured frequently.

MDStaplingRenewWindow Directive

If the validity of the OCSP response used in stapling falls below 'duration', mod_md will obtain a new OCSP response.

The CA issuing a certificate commonly also operates the OCSP responder service and determines how long its signed response about the validity of a certificate are itself valid. The longer a response is valid, the longer it can be cached which mean better overall performance for everyone. The shorter the life time, the more rapidly certificate revocations spread to clients. Also, service reliability is a consideration.

By adjusting the stapling renew window you can control parts of this yourself. If you make the renew time short (e.g. a short time before the current information expires), you gain maximum cache time. But a service outage (down for maintenance, for example) will affect you. If you renew a long time before expiry, updates will be made more frequent, cause more load on the CA server infrastructure and also more coordination between the child processes of your server.

The default is chosen as 33%, which means renewal is started when only a third of the response lifetime is left. For a CA that issues OCSP responses with lifetime of 3 days, this means 2 days of caching and 1 day for renewal attempts. A service outage would have to last full 24 hours to affect your domains.

Setting an absolute renew window, like `2d` (2 days), is also possible.

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.

MDWarnWindow Directive

See MDRenewWindow for a description on how you can specify the time.

The modules checks the remaining lifetime of certificates and invokes MDMessageCmd when there is less than the warn window left. With the default, this mean 9 days for certificates from Let's Encrypt.

It also applies to Managed Domains with static certificate files ( see MDCertificateFile).