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ACME for Apache httpd with mod_md

This post is part of a series of ACME client demonstrations. See also the posts about Certbot standalone HTTP and Certbot with FreeIPA DNS.

mod_md is an ACME client module for Apache httpd. In this post I demonstrate the use mod_md with the FreeIPA ACME service to automatically acquire certificates for managed domains from the FreeIPA CA.

mod_md supports the http-01 and tls-alpn-01 challenges (also dns-01 via external programs). The FreeIPA ACME service does not implement tls-alpn-01 so we will use the HTTP-based challenge. For this httpd needs to be listening on port 80, which is the case in the default Fedora configuration:

[root@f31-0 ~]# grep ^Listen /etc/httpd/conf/httpd.conf
Listen 80

First step was to install the module:

[root@f31-0 ~]# dnf install -y mod_md
  <stuff happens>

Looking at the installed configuration files and their contents, I see the relevant load directives already in place:

[root@f31-0 ~]# rpm -qc mod_md

[root@f31-0 ~]# cat /etc/httpd/conf.modules.d/01-md.conf
LoadModule md_module modules/mod_md.so

I created a minimal VirtualHost configuration:

[root@f31-0 ~]# cat >/etc/httpd/conf.d/acme.conf <<EOF
LogLevel warn md:notice

MDCertificateAuthority https://ipa-ca.ipa.local/acme/directory
MDCertificateAgreement accepted

MDomain f31-0.ipa.local

<VirtualHost *:443>
    ServerName f31-0.ipa.local

    SSLEngine on
    # no certificates specification

Starting httpd and watching the error log, I observed that shortly after startup it only took mod_md about 5 seconds to create an account, submit an order, prove control of the f31-0.ipa.local DNS name and retrieve the issued certificate:

[Wed May 06 15:51:37.371414 2020] [core:notice] [pid 82766:tid
  140661368246592] AH00094: Command line: '/usr/sbin/httpd -D
[Wed May 06 15:51:43.086719 2020] [md:notice] [pid 82778:tid
  140661321930496] AH10059: The Managed Domain f31-0.ipa.local has
  been setup and changes will be activated on next (graceful) server

The notice that we still need to perform a (graceful) restart is important. Indeed a requests from another host still fails with a self-signed certificate warning:

[f31-1:~] ftweedal% curl https://f31-0.ipa.local/
curl: (60) SSL certificate problem: self signed certificate
More details here: https://curl.haxx.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore
could not establish a secure connection to it. To learn more about
this situation and how to fix it, please visit the web page
mentioned above.

After preforming a (graceful) restart of httpd:

[f31-0:~] ftweedal% sudo systemctl reload httpd

Requests now work (never mind the 403 response status):

[f31-1:~] ftweedal% curl --head https://f31-0.ipa.local/
HTTP/1.1 403 Forbidden
Date: Wed, 06 May 2020 06:11:43 GMT
Server: Apache/2.4.43 (Fedora) OpenSSL/1.1.1d mod_auth_gssapi/1.6.1 mod_wsgi/4.6.6 Python/3.7
Last-Modified: Thu, 25 Jul 2019 05:18:03 GMT
ETag: "15bc-58e7a8ccdb8c0"
Accept-Ranges: bytes
Content-Length: 5564
Content-Type: text/html; charset=UTF-8

curl -v output included the following certificate detail:

* Server certificate:
*  subject: CN=f31-0.ipa.local
*  start date: May  6 05:51:41 2020 GMT
*  expire date: Aug  4 05:51:41 2020 GMT
*  subjectAltName: host "f31-0.ipa.local" matched cert's "f31-0.ipa.local"
*  issuer: O=IPA.LOCAL 202004011654; CN=Certificate Authority
*  SSL certificate verify ok.

Observe that it is a short-lived certificate issued by the FreeIPA CA.

The fact that a graceful restart was required suggests that if you are using mod_md in production, you should configure a cron job (or equivalent) to execute that on a regular schedule. The MDRenewWindow directive defines the remaining certificate lifetime at which mod_md will first attempt to renew the certificate. The default value is 33% which for 90 day certificates is 30 days. Therefore with 90 days certificates and the default MDRenewWindow 33%, restarting weekly seems reasonable.

One last curiousity: by default mod_md publishes a “certificate status” resource at .httpd/certificate-status for each managed domain:

[f31-1:~] ftweedal% curl \
  "valid": {
    "until": "Tue, 04 Aug 2020 05:51:41 GMT",
    "from": "Wed, 06 May 2020 05:51:41 GMT"
  "serial": "1E",
  "sha256-fingerprint": "a70d2182f347cf9dddfbd19a14243c5efe24df55fa5728297c667494a28e7d2e"

This can be suppressed by MDCertificateStatus off which is a server-wide setting.

Discussion §

Confession time. The above scenario did not go anywhere near as smoothly as portrayed above. In fact, mod_md was failing immediately after retrieving the directory resource:

[Tue May 05 22:28:32.462108 2020] [md:warn] [pid 68047:tid
140418815502080] (22)Invalid argument: md[f31-0.ipa.local]
while[Contacting ACME server for f31-0.ipa.local at
https://ipa-ca.ipa.local/acme/directory] detail[Unable to understand
ACME server response from <https://ipa-ca.ipa.local/acme/directory>.
Wrong ACME protocol version or link?]

I went to the mod_md source code to investigate. The problem was that mod_md required the ACME revokeCert and keyChange (account key rollover) resources to be defined in the resource document, even though mod_md does not use those capabilities (at this time). The Dogtag ACME responder has not yet implemented key rollover. As a consequence, mod_md refused to interact with it.

What does RFC 8555 have to say about this? §7.1 states:

The server MUST provide “directory” and “newNonce” resources.

But there is no explicit statement about whether other resources are, or are not, required (with the exception of the newAuthz resource other resource which is optional). My conclusion is that mod_md, in checking for resources it doesn’t even use, is too strict. I submitted a pull request to https://github.com/icing/mod_md to relax the check. It was accepted and merged the next day.

Note that mod_md has also been pulled into the httpd codebase, although it does not seem to be as actively maintained there at this point in time. I suppose that the httpd code is periodically updated with the code from the icing respository. Nevertheless I also submitted a pull request to httpd. At time of publication of this post there has been no activity. I have also submitted bugs against the Fedora and RHEL mod_md packages.

In the meantime I built a version of the Fedora package containing my patch. This time mod_md was able to successfully validate the identifier and finalise the order, causing the certificate to be issued. But it was not able to retrieve the certificate; mod_md does not handle the absense of the Location header in the response to the finalise request. This header was required in an earlier (pre-RFC) draft of the ACME protocol, but it is not required any more. Boulder (the ACME server implementation used by Let’s Encrypt) does set it so mod_md works fine with Boulder. But the Dogtag ACME service did not set it and mod_md fails at this point, putting the client-side order data into an unrecoverable state.

The quick fix was to update the Dogtag ACME service to include the Location header. I also reported the issue in the upstream repository.

That’s it for this demo. For my next FreeIPA ACME demo I’m going to attempt DNS-based identifier validation challenges with Certbot and FreeIPA’s integrated DNS.

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