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posts about ansible.

this year, let's encrypt added two great features:

  1. they enabled the acme v2 protocol, and allow to obtain wildcard certificates through this.
  2. they improved their certificate transparency support by including signed certificate timestamp (sct) records in the certificates. chrome will, for example, require scts from april 2018 on.

i've already tried out both wildcard certificates and scts, and so far they work flawlessly! i've been using the acme v2 support in the letsencrypt module of ansible 2.5 (with a bugfix), into which i invested quite some work.

four days ago, arch linux switched to openssl 1.1.0. openssl 1.1.0 was originally released at the end of last august, but since it has some breaking api changes, it's only slowly creeping into new linux distributions.

this also means that i can finally test my let's encrypt library, let's encrypt ansible role and ocspbot against openssl 1.1.0. the let's encrypt code worked out of the box (i've already incorporated a change somewhen earlier, even without being able to properly test it), but ocspbot needed a bit more work. there's a command line syntax change between 1.0.x and 1.1.0 when specifying http headers to ocsp calls; the old syntax was -header name value, the new one is -header name=value. so i had to add a version detection (i.e. parsing the output of openssl version) to use the correct syntax depending on the used version. but now it works with both openssl 1.0.x and 1.1.0!

using openssl 1.1.0 on my server also allowed me to use x25519, using daniel j. bernstein's curve25519 in edwards form, for secret key negotation (i.e. ephemeral diffie-hellman). using it in nginx is pretty easy:

ssl_ecdh_curve X25519:secp521r1:secp384r1;

this uses x25519 as the default curve/key exchange, followed by the fallsbacks using ecdhe with a 521-bit nist curve and then a 384-bit nist curve as a third fallback. (btw, note the uppercase x in x25519 — if you use the lowercase variant, nginx won't load the config.) the third curve is the only one supported by almost every browser; only a few support the 521-bit one, and right now only chrome supports x25519.

introduction

classically, revokation of certificates was accomplished with certificate revokation lists (crls). the idea was that browsers regularly download crls from the certificate authorities (cas) and check whether certificates they see are on the list. this doesn't scale well, though. nowadays, there are many cas trusted by browsers in their default configuration, and crls tend to get huge.

a better solution is the online certificate status protocol (ocsp): a browser, when encountering a new certificate, asks the ocsp server of the browser (the url for it is contained in the certificate) whether the certificate is still valid. this has several downsides as well: first, ocsp servers are not always reliable. if a browser cannot connect to it (or doesn't get a reply), what should it do? deny access to the site? besides that, there's another large downside: privacy. the ocsp server knows which page you are visiting, because the browser tells it by asking whether its certificate is valid.

ocsp stapling was invented to improve upon this: the idea is that the webserver itself asks the ocsp server for the status of its certificate, and delivers the answer together with the certificate to the connecting browser. as the ocsp response is signed by the cas certificate, the browser can verify that the response is valid. also, the expiration time of ocsp responses is much less than the one for certificates, so if a certificate is revoked, existing ocsp responses will only be valid for a couple of more days.

this is pretty good already, except that a malicious webserver could simply not send the ocsp response with its certificate. if a browser cannot contact the ocsp server itself, it has no way to know whether the certificate is revoked or not. to overcome this, ocsp must-staple was invented. this is a flag in the certificate itself which says that the certificate is only valid with a valid and good ocsp response. so if a browser encounters a certificate with this flag, and the webserver isn't ocsp stapling, the browser knows that something is fishy.

unfortunately, there are some downsides. first, most the most common webservers for linux, apache and nginx, while having ocsp stapling support, do in some situations send replies without ocsp stapling. if the certificate has the ocsp must-staple flag set, these answers result in error pages shown in browsers. and that's something you really want to avoid, that visitors of your page thing there's something bad happening.

fortunately, at least for nginx, you can specify a file containing an ocsp response directly with the ssl_stapling_file directive. unfortunately, you have to make sure you always have a good and valid ocsp response at that place, and reload nginx in case the response is updated. other programs allow to specify an ocsp response in a similar way, such as exim with the tls_ocsp_file directive, and thus have the same problem. to solve this problem, i've started creating ocsp bot:

ocsp bot

ocsp bot is a python script which should be called frequently (as in: once per hour or so), which checks a set of x.509 certificates to obtain up-to-date ocsp responses. in case the current ocsp responses will expire soon, or aren't there, it will try to get a new response. it will only copy the new response to the correct place if it is valid and good. calling it frequently will ensure that in case of problems getting a new response, it will retry every hour (or so) until a good and valid response could have been obtained. so a user response is only necessary if the process fails several times in a row.

ocsp bot will signal with its exit code whether responses have been updated, allowing to reload/restart the corresponding service to use the new response.

you can install ocsp bot with pip install ocsp from pypi.

integration with ansible

i'm using ansible to configure my server. to copy certificates and obtain ocsp responses, i'm using a custom role.

the ansible tasks for the role are as follows. ocsp bot is installed in /var/www/ocsp:

- name: Create OCSP log folder
  file: dest=/var/www/ocsp/logs state=directory
- name: Create OCSP response folder
  file: dest=/var/www/ocsp/responses state=directory
- name: Install pyyaml
  pip: name=pyyaml
- name: Install OCSP response utility
  copy: src=ocspbot.py dest=/var/www/ocsp/ocspbot.py mode=0755
  # ocspbot.py is ocspbot/__main__.py from https://github.com/felixfontein/ocspbot/
- name: Install OCSP bash script
  template: src=ocspbot.sh.j2 dest=/var/www/ocsp/ocspbot.sh mode=0755
- name: Install OCSP response utility configurations
  template: src=ocspbot.yaml.j2 dest=/var/www/ocsp/ocspbot-{{ item.key }}.yaml
  with_dict: "{{ certificates }}"
- name: Install OCSP response cronjob
  cron: name="Update OCSP responses" hour=* minute=0 job=/var/www/ocsp/ocspbot.sh state=present

the variable certificates is defined as follows:

certificates:
  nginx:
    domains:
    - example.com
    - example.net
    reload:
    - nginx
    key_owner: root
    key_group: root
    key_mode: "0400"
  mailserver:
    domains:
    - mail.example.com
    reload:
    - dovecot
    - exim
    key_owner: root
    key_group: exim
    key_mode: "0440"

The template for ocspbot.sh:

#!/bin/bash
RC=0
{% for name, data in certificates|dictsort %}

# Renew OCSP responses for {{ name }}
/var/www/ocsp/ocspbot.py /home/ocsp/ocspbot-{{ name }}.yaml
RESULT=$?
if [ $RESULT -gt 0 ]; then
{%   for service in data.reload %}
    systemctl reload {{ service }}
{%   endfor %}
elif [ $RESULT -lt 0 ]; then
    RC=1
fi
{% endfor %}

exit $RC

the template for the configuration yaml files:

make_backups: True

minimum_validity: 3d
minimum_validity_percentage: 42.8

ocsp_folder: /var/www/ocsp/responses
output_log: /var/www/ocsp/logs/{{ item.key }}-{year}{month}{day}-{hour}{minute}{second}.log

domains:
{% for domain in item.value.domains|sort %}
  {{ domain }}:
    cert: /var/www/certs/{{ domain }}.pem
    chain: /var/www/certs/{{ domain }}-chain.pem
    rootchain: /var/www/certs/{{ domain }}-rootchain.pem
    ocsp: {{ domain }}.ocsp-resp
{% endfor %}

the certificates are copied with the following ansible tasks:

- name: copy private keys
  copy: src=keys/{{ item.1 }}.key dest=/var/www/keys/{{ item.1 }}.key owner={{ item.0.value.key_owner }} group={{ item.0.value.key_group }} mode={{ item.0.value.key_mode }}
  with_dependent:
  - "certificates"
  - "item.0.value.domains"
  notify: update OCSP responses
- name: copy certificates
  copy: src=keys/{{ item.1 }}{{ item.2 }} dest=/var/www/certs/{{ item.1 }}{{ item.2 }} owner=root group=root mode=0444
  with_dependent:
  - "certificates"
  - "item.0.value.domains"
  - '["-rootchain.pem", "-fullchain.pem", "-chain.pem", ".pem"]'
  notify: update OCSP responses

(here, the dependent loop lookup plugin is used.)

the handler update OCSP responses is defined as follows:

- name: update OCSP responses
  command: /var/www/ocsp/ocspbot.sh
  register: result
  failed_when: result.rc != 0
  notify:
  - reload nginx
  - reload exim
  - reload dovecot

i'm using this setup for some weeks now, and it seems to work fine. so far, i'm not using ocsp must-staple certificates (except for some test subdomains). if everything seems to be fine for some time, i'll switch to ocsp must-staple certificates.

three days ago, let’s encrypt started their public beta. for those of you who don’t know: let’s encrypt is a certificate authority issuing free certificates for protecting https connections.

this is awesome!

for one, this allows me to get some “real” certificates (as opposed to my self-signed ones) without paying a larger sum of money per year (i’m using quite many subdomains of fontein.de and two other domains, which results in quite some sum even when using cheap resellers of resellers of resellers).

then, their goal is to automate the whole process as much as possible. so instead of a lof of manual work (mostly filling out forms, handling payment of fees, reacting to emails or domain challenge requests, etc.) it should be possible to run one command, maybe even as a cronjob, to get a (renewed) certificate for a domain or a set of domains.

on thursday, when the beta officially started, i tried out the official client. as mentioned already by lots of others, it has some serious downside: it is a huge python program which needs to be run as root. (not necessarily on the webserver, though, even though in that case you cannot automate stuff anymore.) but there were already alternatives: a static website telling you what to do and doing some calculations in javascript, or a tiny python client. (both are by daniel roesler.)

that’s already much better, but still not what i want, as this is hard to automate when you don’t want to run that on the webserver itself. i’m prefering something which can run somewhere else, and can be integrated in an orchestration tool like ansible. well, so i took daniel roesler’s code (including a python 3 patch by collin anderson) and converted it into a more modular tool, which allows to split up the process so that with some more scripting, it can easily be used to do the process from remote. you can find the result on github. i also created an ansible role which allows to simply generate keys, certificate signing requests and get complete certificates from let’s encrypt with ansible; that project can also be found on github. i’m using it in production for my personal webserver: as a result you can now look at spielwiese without having to accept my self-signed certificate! maybe also others will find this useful.