Automate ACME certificates for Marathon apps served by marathon-lb
There is one big requirement for deploying marathon-acme: there must
be shared persistent storage between marathon-acme and all
marathon-lb instances. This will be used to store the certificates.
marathon-acmewatches Marathon for changes to app definitions.- It collects the values of all
MARATHON_ACME_{n}_DOMAINlabels on apps. This will form the set of domains to fetch certificates for. - It generates, verifies and stores certificates for any new domains using the configured ACME certificate authority.
- It tells
marathon-lbto reload using themarathon-lbHTTP API. - It issues new certificates for soon-to-expire certificates once a day.
marathon-acme is written in Python using
Twisted. The certificate issuing
functionality is possible thanks to the
txacme library.
The ACME provider that most people are likely to use is Let's
Encrypt. Before using marathon-acme
with Let's Encrypt, make sure you are aware of their rate
limits.
The entire certificate-issuing workflow is shown below:
marathon-acme is available as a pip-installable Python package on
PyPI. However, most
users will probably want to use the Docker image available from Docker
Hub.
> $ docker run --rm praekeltfoundation/marathon-acme --help
usage: marathon-acme [-h] [-a ACME] [-e EMAIL] [-m MARATHON[,MARATHON,...]]
[-l LB[,LB,...]] [-g GROUP] [--allow-multiple-certs]
[--listen LISTEN] [--sse-timeout SSE_TIMEOUT]
[--log-level {debug,info,warn,error,critical}]
storage-dir
Automatically manage ACME certificates for Marathon apps
positional arguments:
storage-dir Path to directory for storing certificates
optional arguments:
-h, --help show this help message and exit
-a ACME, --acme ACME The address for the ACME Directory Resource (default:
https://acme-v01.api.letsencrypt.org/directory)
-e EMAIL, --email EMAIL
An email address to register with the ACME service
(optional)
-m MARATHON[,MARATHON,...], --marathon MARATHON[,MARATHON,...]
The addresses for the Marathon HTTP API (default:
http://marathon.mesos:8080)
-l LB[,LB,...], --lb LB[,LB,...]
The addresses for the marathon-lb HTTP API (default:
http://marathon-lb.marathon.mesos:9090)
-g GROUP, --group GROUP
The marathon-lb group to issue certificates for
(default: external)
--allow-multiple-certs
Allow multiple certificates for a single app port.
This allows multiple domains for an app, but is not
recommended.
--listen LISTEN The address for the port to listen on (default: :8000)
--sse-timeout SSE_TIMEOUT
Amount of time in seconds to wait for some event data
to be received from Marathon. Set to 0 to disable.
(default: 60)
--log-level {debug,info,warn,error,critical}
The minimum severity level to log messages at
(default: info)
--version show program's version number and exit
marathon-acme should be deployed as a Marathon app.
{
"id": "/marathon-acme",
"cpus": 0.01,
"mem": 128.0,
"args": [
"--email", "[email protected]",
"--marathon", "http://marathon1:8080,http://marathon2:8080,http://marathon3:8080",
"--lb", "http://lb1:9090,http://lb2:9090",
"/var/lib/marathon-acme"
],
"labels": {
"HAPROXY_GROUP": "external",
"HAPROXY_0_VHOST": "marathon-acme.example.com",
"HAPROXY_0_BACKEND_WEIGHT": "1",
"HAPROXY_0_PATH": "/.well-known/acme-challenge/",
"HAPROXY_0_HTTP_FRONTEND_ACL_WITH_PATH": " acl host_{cleanedUpHostname} hdr(host) -i {hostname}\n acl path_{backend} path_beg {path}\n redirect prefix http://{hostname} code 302 if !host_{cleanedUpHostname} path_{backend}\n use_backend {backend} if host_{cleanedUpHostname} path_{backend}\n"
},
"container": {
"type": "DOCKER",
"docker": {
"image": "praekeltfoundation/marathon-acme",
"network": "BRIDGE",
"portMappings": [
{ "containerPort": 8000, "hostPort": 0 }
],
"parameters": [
{
"value": "my-volume-driver",
"key": "volume-driver"
},
{
"value": "marathon-acme-certs:/var/lib/marathon-acme",
"key": "volume"
}
],
}
}
}The above should mostly be standard across different deployments. The volume parameters will depend on your particular networked storage solution.
"labels": {
"HAPROXY_GROUP": "external",
"HAPROXY_0_VHOST": "marathon-acme.example.com",
"HAPROXY_0_BACKEND_WEIGHT": "1",
"HAPROXY_0_PATH": "/.well-known/acme-challenge/",
"HAPROXY_0_HTTP_FRONTEND_ACL_WITH_PATH": " acl host_{cleanedUpHostname} hdr(host) -i {hostname}\n acl path_{backend} path_beg {path}\n redirect prefix http://{hostname} code 302 if !host_{cleanedUpHostname} path_{backend}\n use_backend {backend} if host_{cleanedUpHostname} path_{backend}\n"
}Several special marathon-lb labels are needed in order to forward
all HTTP requests whose path begins with
/.well-known/acme-challenge/ to marathon-acme, in order to serve
ACME HTTP
challenge
responses.
external
marathon-lb instances are assigned a group. Only Marathon apps with
a HAPROXY_GROUP label that matches their group are routed with that
instance. "external" is the common name for publicly-facing load
balancers.
marathon-acme.example.com
marathon-acme needs its own domain to respond to ACME challenge
requests on. This domain must resolve to your marathon-lb
instance(s).
1
We want this rule in HAProxy's config file to come before any others so
that requests are routed to marathon-acme before we do the (usually)
domain-based routing for the other Marathon apps. The default weight is
0, so we set to 1 so that the rule comes first.
/.well-known/acme-challenge/
This is the beginning of the HTTP path to ACME validation challenges.
acl host_{cleanedUpHostname} hdr(host) -i {hostname}
acl path_{backend} path_beg {path}
redirect prefix http://{hostname} code 302 if !host_{cleanedUpHostname} path_{backend}
use_backend {backend} if host_{cleanedUpHostname} path_{backend}
This is where it gets complicated... It’s possible to edit the templates
used for generating the HAProxy on a per-app basis using labels. This is
necessary because by default marathon-lb will route based on domain
first, but we don’t want to do that. You can see the standard template
here.
Here, we add an extra redirect rule. This redirects all requests
matching the ACME challenge path to marathon-acme, except those
requests already headed for marathon-acme. The Let's Encrypt server
will follow redirects.
It is possible to have marathon-acme serve ACME challenge requests
over HTTPS, although this is usually not necessary. In this case, a
certificate needs to be issued for marathon-acme and the HTTP
redirect label needs to be modified:
"labels": {
...,
"MARATHON_ACME_0_DOMAIN": "marathon-acme.example.com",
"HAPROXY_0_HTTP_FRONTEND_ACL_WITH_PATH": " acl host_{cleanedUpHostname} hdr(host) -i {hostname}\n acl path_{backend} path_beg {path}\n redirect prefix https://{hostname} code 302 if path_{backend}\n"
}Note that using the HAPROXY_0_REDIRECT_TO_HTTPS label for
marathon-acme will break things. This label is difficult for us to
use because of the way marathon-lb's templating works.
marathon-acme.example.com
Here we set up marathon-acme to fetch a certificate for itself.
acl host_{cleanedUpHostname} hdr(host) -i {hostname}
acl path_{backend} path_beg {path}
redirect prefix https://{hostname} code 302 if path_{backend}
We redirect to the HTTPS address (https://{hostname}) for all
domains (including marathon-acme's) for requests to the ACME
challenge path. The use_backend directive can now be removed since
the backend is never used over HTTP as all requests are redirected.
Note that this label can only be set after marathon-acme has
fetched the first certificate for its own domain. In other words, set
the MARATHON_ACME_0_DOMAIN first and make sure it has taken effect
before setting this one.
Docker images are available from Docker Hub. There are two different streams of Docker images available:
:latest/:<version>: Tracks the latest released version ofmarathon-acmeon PyPI. The Dockerfile for these is in the praekeltfoundation/docker-marathon-acme repo.:develop: Tracks thedevelopbranch of this repo and is built using the Dockerfile in this repo.
For more details on the Docker images, see the praekeltfoundation/docker-marathon-acme repo.
The marathon-acme container defaults to the
/var/lib/marathon-acme directory to store certificates and the ACME
client private key. This is the path inside the container that should be
mounted as a shared volume.
The container also defaults to listening on port 8000 on all interfaces.
You can override these values by providing arguments to the Docker container.
marathon-acme creates the following directory/file structure:
/var/lib/marathon-acme/client.key: The ACME client private keydefault.pem: A self-signed wildcard cert for HAProxy to fallback tocerts/- www.example.com.pem: An issued ACME certificate for a domain
unmanaged-certs/: A directory for certs thatmarathon-acmedoesn't manage
marathon-acme does nothing with the unmanaged-certs/ directory
after creating it. HAProxy fails if any path in its certificate config
doesn't exist, so it reduces setup friction to have a standard place to
put unmanaged certificates.
marathon-acme requires marathon-lb 1.4.0 or later in order to be
able to trigger HAProxy reloads.
As mentioned earlier, marathon-lb must share persistent storage with
marathon-acme. BYONS: bring your own networked storage.
The only real configuration needed for marathon-lb is to add the
path to marathon-acme's certificate storage directory as a source of
certificates. HAProxy supports loading certificates from a directory.
You should set marathon-lb's --ssl-certs CLI option to the
certificate directory path as well as the fallback certificate (if
HAProxy cannot find any certificates in the paths it is given it will
fail to start).
--ssl-certs <storage-dir>/certs,<storage-dir>/default.pem
marathon-acme uses a single marathon-lb-like label to assign
domains to app ports: MARATHON_ACME_{n}_DOMAIN, where {n} is the
port index. The value of the label is a set of comma- and/or
whitespace-separated domain names, although by default only the first
domain name will be considered.
Currently, marathon-acme can only issue certificates with a single
domain. This means multiple certificates need to be issued for apps with
multiple configured domains.
A limitation was added that limits apps to a single domain. This limit
can be removed by passing the --allow-multiple-certs command-line
option, although this is not recommended as it makes it possible for a
large number of certificates to be issued for a single app, potentially
exhausting the Let's Encrypt rate limit.
The app or its port must must be in the same HAPROXY_GROUP as
marathon-acme was configured with at start-up.
We decided not to reuse the HAPROXY_{n}_VHOST label so as to limit
the number of domains that certificates are issued for.
The library used for ACME certificate management, txacme, is
currently quite limited in its functionality. The two biggest
limitations are:
- There is no Subject Alternative Name (SAN) support yet (#37). Each certificate will correspond to exactly one domain name. This limitation makes it easier to hit Let's Encrypt's rate limits.
- There is no support for removing certificates from
txacme's certificate store (#77). Oncemarathon-acmeissues a certificate for an app it will try to renew that certificate forever unless it is manually deleted from the certificate store.
For a more complete list of issues, see the issues page for this repo.
One common problem is that marathon-lb is misconfigured and ACME
challenge requests are unable to reach marathon-acme. You can test
challenge request routing to marathon-acme using the challenge ping
endpoint.
It should be possible to reach the /.well-known/acme-challenge/ping
path from all domains served by marathon-lb:
> $ curl cake-service.example.com/.well-known/acme-challenge/ping
{"message": "pong"}
> $ curl soda-service.example.com/.well-known/acme-challenge/ping
{"message": "pong"}
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