acme-dns

A simplified DNS server with a RESTful HTTP API to provide a simple way to automate ACME DNS challenges.

Why?

Many DNS servers do not provide an API to enable automation for the ACME DNS challenges. Those which do, give the keys way too much power. Leaving the keys laying around your random boxes is too often a requirement to have a meaningful process automation.

Acme-dns provides a simple API exclusively for TXT record updates and should be used with ACME magic "_acme-challenge" - subdomain CNAME records. This way, in the unfortunate exposure of API keys, the effects are limited to the subdomain TXT record in question.

So basically it boils down to accessibility and security.

For longer explanation of the underlying issue and other proposed solutions, see a blog post on the topic from EFF deeplinks blog: https://www.eff.org/deeplinks/2018/02/technical-deep-dive-securing-automation-acme-dns-challenge-validation

Features

• Simplified DNS server, serving your ACME DNS challenges (TXT)
• Custom records (have your required A, AAAA, NS, etc. records served)
• HTTP API automatically acquires and uses Let's Encrypt TLS certificate
• Limit /update API endpoint access to specific CIDR mask(s), defined in the /register request
• Supports SQLite & PostgreSQL as DB backends
• Rolling update of two TXT records to be able to answer to challenges for certificates that have both names: yourdomain.tld and *.yourdomain.tld, as both of the challenges point to the same subdomain.
• Simple deployment (it's Go after all)

Usage

A client application for acme-dns with support for Certbot authentication hooks is available at: https://github.com/acme-dns/acme-dns-client.

Using acme-dns is a three-step process (provided you already have the self-hosted server set up):

• Get credentials and unique subdomain (simple POST request to eg. https://auth.acme-dns.io/register)
• Create a (ACME magic) CNAME record to your existing zone, pointing to the subdomain you got from the registration. (eg. _acme-challenge.domainiwantcertfor.tld. CNAME a097455b-52cc-4569-90c8-7a4b97c6eba8.auth.example.org )
• Use your credentials to POST new DNS challenge values to an acme-dns server for the CA to validate from.
• Crontab and forget.

API

Register endpoint

The method returns a new unique subdomain and credentials needed to update your record. Fulldomain is where you can point your own _acme-challenge subdomain CNAME record to. With the credentials, you can update the TXT response in the service to match the challenge token, later referred as ___validation_token_received_from_the_ca___, given out by the Certificate Authority.

Optional:: You can POST JSON data to limit the /update requests to predefined source networks using CIDR notation.

POST /register

OPTIONAL Example input

{
    "allowfrom": [
        "192.168.100.1/24",
        "1.2.3.4/32",
        "2002:c0a8:2a00::0/40"
    ]
}

Status: 201 Created

{
    "allowfrom": [
        "192.168.100.1/24",
        "1.2.3.4/32",
        "2002:c0a8:2a00::0/40"
    ],
    "fulldomain": "8e5700ea-a4bf-41c7-8a77-e990661dcc6a.auth.acme-dns.io",
    "password": "htB9mR9DYgcu9bX_afHF62erXaH2TS7bg9KW3F7Z",
    "subdomain": "8e5700ea-a4bf-41c7-8a77-e990661dcc6a",
    "username": "c36f50e8-4632-44f0-83fe-e070fef28a10"
}

Update endpoint

The method allows you to update the TXT answer contents of your unique subdomain. Usually carried automatically by automated ACME client.

POST /update

Required headers

Header name	Description	Example
X-Api-User	UUIDv4 username received from registration	X-Api-User: c36f50e8-4632-44f0-83fe-e070fef28a10
X-Api-Key	Password received from registration	X-Api-Key: htB9mR9DYgcu9bX_afHF62erXaH2TS7bg9KW3F7Z

Example input

{
    "subdomain": "8e5700ea-a4bf-41c7-8a77-e990661dcc6a",
    "txt": "___validation_token_received_from_the_ca___"
}

Response

Status: 200 OK

{
    "txt": "___validation_token_received_from_the_ca___"
}

Health check endpoint

The method can be used to check readiness and/or liveness of the server. It will return status code 200 on success or won't be reachable.

GET /health

Self-hosted

You are encouraged to run your own acme-dns instance, because you are effectively authorizing the acme-dns server to act on your behalf in providing the answer to the challenging CA, making the instance able to request (and get issued) a TLS certificate for the domain that has CNAME pointing to it.

See the INSTALL section for information on how to do this.

Installation

1. Install Go 1.13 or newer.

2. Build acme-dns:

git clone https://github.com/joohoi/acme-dns
cd acme-dns
export GOPATH=/tmp/acme-dns
go build

3. Move the built acme-dns binary to a directory in your $PATH, for example: sudo mv acme-dns /usr/local/bin

4. Edit config.cfg to suit your needs (see configuration). acme-dns will read the configuration file from /etc/acme-dns/config.cfg or ./config.cfg, or a location specified with the -c flag.

5. If your system has systemd, you can optionally install acme-dns as a service so that it will start on boot and be tracked by systemd. This also allows us to add the CAP_NET_BIND_SERVICE capability so that acme-dns can be run by a user other than root.

   1. Make sure that you have moved the configuration file to /etc/acme-dns/config.cfg so that acme-dns can access it globally.

   2. Move the acme-dns executable from ~/go/bin/acme-dns to /usr/local/bin/acme-dns (Any location will work, just be sure to change acme-dns.service to match).

   3. Create a minimal acme-dns user: sudo adduser --system --gecos "acme-dns Service" --disabled-password --group --home /var/lib/acme-dns acme-dns.

   4. Move the systemd service unit from acme-dns.service to /etc/systemd/system/acme-dns.service.

   5. Reload systemd units: sudo systemctl daemon-reload.

   6. Enable acme-dns on boot: sudo systemctl enable acme-dns.service.

   7. Run acme-dns: sudo systemctl start acme-dns.service.

6. If you did not install the systemd service, run acme-dns. Please note that acme-dns needs to open a privileged port (53, domain), so it needs to be run with elevated privileges.

Using Docker

1. Pull the latest acme-dns Docker image: docker pull joohoi/acme-dns.

2. Create directories: config for the configuration file, and data for the sqlite3 database.

3. Copy configuration template to config/config.cfg.

4. Modify the config.cfg to suit your needs.

5. Run Docker, this example expects that you have port = "80" in your config.cfg:

docker run --rm --name acmedns                 \
 -p 53:53                                      \
 -p 53:53/udp                                  \
 -p 80:80                                      \
 -v /path/to/your/config:/etc/acme-dns:ro      \
 -v /path/to/your/data:/var/lib/acme-dns       \
 -d joohoi/acme-dns

Docker Compose

1. Create directories: config for the configuration file, and data for the sqlite3 database.

2. Copy configuration template to config/config.cfg.

3. Copy docker-compose.yml from the project, or create your own.

4. Edit the config/config.cfg and docker-compose.yml to suit your needs, and run docker-compose up -d.

DNS Records

Note: In this documentation:

• auth.example.org is the hostname of the acme-dns server
• acme-dns will serve *.auth.example.org records
• 198.51.100.1 is the public IP address of the system running acme-dns

These values should be changed based on your environment.

You will need to add some DNS records on your domain's regular DNS server:

• NS record for auth.example.org pointing to auth.example.org (this means, that auth.example.org is responsible for any *.auth.example.org records)
• A record for auth.example.org pointing to 198.51.100.1
• If using IPv6, an AAAA record pointing to the IPv6 address.
• Each domain you will be authenticating will need a _acme-challenge CNAME subdomain added. The client you use will explain how to do this.

Testing It Out

You may want to test that acme-dns is working before using it for real queries.

1. Confirm that DNS lookups for the acme-dns subdomain works as expected: dig auth.example.org.

2. Call the /register API endpoint to register a test domain:

$ curl -X POST https://auth.example.org/register
{"username":"eabcdb41-d89f-4580-826f-3e62e9755ef2","password":"pbAXVjlIOE01xbut7YnAbkhMQIkcwoHO0ek2j4Q0","fulldomain":"d420c923-bbd7-4056-ab64-c3ca54c9b3cf.auth.example.org","subdomain":"d420c923-bbd7-4056-ab64-c3ca54c9b3cf","allowfrom":[]}

3. Call the /update API endpoint to set a test TXT record. Pass the username, password and subdomain received from the register call performed above:

$ curl -X POST \
  -H "X-Api-User: eabcdb41-d89f-4580-826f-3e62e9755ef2" \
  -H "X-Api-Key: pbAXVjlIOE01xbut7YnAbkhMQIkcwoHO0ek2j4Q0" \
  -d '{"subdomain": "d420c923-bbd7-4056-ab64-c3ca54c9b3cf", "txt": "___validation_token_received_from_the_ca___"}' \
  https://auth.example.org/update

Note: The txt field must be exactly 43 characters long, otherwise acme-dns will reject it

4. Perform a DNS lookup to the test subdomain to confirm the updated TXT record is being served:

$ dig -t txt @auth.example.org d420c923-bbd7-4056-ab64-c3ca54c9b3cf.auth.example.org

Configuration

[general]
# DNS interface. Note that systemd-resolved may reserve port 53 on 127.0.0.53
# In this case acme-dns will error out and you will need to define the listening interface
# for example: listen = "127.0.0.1:53"
listen = "127.0.0.1:53"
# protocol, "both", "both4", "both6", "udp", "udp4", "udp6" or "tcp", "tcp4", "tcp6"
protocol = "both"
# domain name to serve the requests off of
domain = "auth.example.org"
# zone name server
nsname = "auth.example.org"
# admin email address, where @ is substituted with .
nsadmin = "admin.example.org"
# predefined records served in addition to the TXT
records = [
    # domain pointing to the public IP of your acme-dns server 
    "auth.example.org. A 198.51.100.1",
    # specify that auth.example.org will resolve any *.auth.example.org records
    "auth.example.org. NS auth.example.org.",
]
# debug messages from CORS etc
debug = false

[database]
# Database engine to use, sqlite3 or postgres
engine = "sqlite3"
# Connection string, filename for sqlite3 and postgres://$username:$password@$host/$db_name for postgres
# Please note that the default Docker image uses path /var/lib/acme-dns/acme-dns.db for sqlite3
connection = "/var/lib/acme-dns/acme-dns.db"
# connection = "postgres://user:password@localhost/acmedns_db"

[api]
# listen ip eg. 127.0.0.1
ip = "0.0.0.0"
# disable registration endpoint
disable_registration = false
# listen port, eg. 443 for default HTTPS
port = "443"
# possible values: "letsencrypt", "letsencryptstaging", "cert", "none"
tls = "letsencryptstaging"
# only used if tls = "cert"
tls_cert_privkey = "/etc/tls/example.org/privkey.pem"
tls_cert_fullchain = "/etc/tls/example.org/fullchain.pem"
# only used if tls = "letsencrypt"
acme_cache_dir = "api-certs"
# optional e-mail address to which Let's Encrypt will send expiration notices for the API's cert
notification_email = ""
# CORS AllowOrigins, wildcards can be used
corsorigins = [
    "*"
]
# use HTTP header to get the client ip
use_header = false
# header name to pull the ip address / list of ip addresses from
header_name = "X-Forwarded-For"

[logconfig]
# logging level: "error", "warning", "info" or "debug"
loglevel = "debug"
# possible values: stdout, TODO file & integrations
logtype = "stdout"
# file path for logfile TODO
# logfile = "./acme-dns.log"
# format, either "json" or "text"
logformat = "text"

HTTPS API

The RESTful acme-dns API can be exposed over HTTPS in two ways:

1. Using tls = "letsencrypt" and letting acme-dns issue its own certificate automatically with Let's Encrypt.
2. Using tls = "cert" and providing your own HTTPS certificate chain and private key with tls_cert_fullchain and tls_cert_privkey.

Where possible the first option is recommended. This is the easiest and safest way to have acme-dns expose its API over HTTPS.

Warning: If you choose to use tls = "cert" you must take care that the certificate does not expire! If it does and the ACME client you use to issue the certificate depends on the ACME DNS API to update TXT records you will be stuck in a position where the API certificate has expired but it can't be renewed because the ACME client will refuse to connect to the ACME DNS API it needs to use for the renewal.

Clients

• acme.sh: https://github.com/Neilpang/acme.sh
• Certify The Web: https://github.com/webprofusion/certify
• cert-manager: https://github.com/jetstack/cert-manager
• Lego: https://github.com/xenolf/lego
• Posh-ACME: https://github.com/rmbolger/Posh-ACME
• Sewer: https://github.com/komuw/sewer
• Traefik: https://github.com/containous/traefik
• Windows ACME Simple (WACS): https://www.win-acme.com

Authentication hooks

• acme-dns-client with Certbot authentication hook: https://github.com/acme-dns/acme-dns-client
• Certbot authentication hook in Python: https://github.com/joohoi/acme-dns-certbot-joohoi
• Certbot authentication hook in Go: https://github.com/koesie10/acme-dns-certbot-hook

Libraries

• Generic client library in Python (PyPI): https://github.com/joohoi/pyacmedns
• Generic client library in Go: https://github.com/cpu/goacmedns

Changelog

• v0.8
  • NOTE: configuration option: "api_domain" deprecated!
  • New
    • Automatic HTTP API certificate provisioning using DNS challenges making acme-dns able to acquire certificates even with HTTP api not being accessible from public internet.
    • Configuration value for "tls": "letsencryptstaging". Setting it will help you to debug possible issues with HTTP API certificate acquiring process. This is the new default value.
  • Changed
    • Fixed: EDNS0 support
    • Migrated from autocert to certmagic for HTTP API certificate handling
• v0.7.2
  • Changed
    • Fixed: Regression error of not being able to answer to incoming random-case requests.
    • Fixed: SOA record added to a correct header field in NXDOMAIN responses.
• v0.7.1
  • Changed
    • Fixed: SOA record correctly added to the TCP DNS server when using both, UDP and TCP servers.
• v0.7
  • New
    • Added an endpoint to perform health checks
  • Changed
    • A new protocol selection for DNS server "both", that binds both - UDP and TCP ports.
    • Refactored DNS server internals.
    • Handle some aspects of DNS spec better.
• v0.6
  • New
    • Command line flag -c to specify location of config file.
    • Proper refusal of dynamic update requests.
    • Release signing
  • Changed
    • Better error messages for goroutines
• v0.5
  • New
    • Configurable certificate cache directory
  • Changed
    • Process wide umask to ensure created files are only readable by the user running acme-dns
    • Replaced package that handles UUIDs because of a flaw in the original package
    • Updated dependencies
    • Better error messages
• v0.4 Clear error messages for bad TXT record content, proper handling of static CNAME records, fixed IP address parsing from the request, added option to disable registration endpoint in the configuration.
• v0.3.2 Dockerfile was fixed for users using autocert feature
• v0.3.1 Added goreleaser for distributing binary builds of the releases
• v0.3 Changed autocert to use HTTP-01 challenges, as TLS-SNI is disabled by Let's Encrypt
• v0.2 Now powered by httprouter, support wildcard certificates, Docker images
• v0.1 Initial release

TODO

• Logging to a file
• DNSSEC
• Want to see something implemented, make a feature request!

Contributing

acme-dns is open for contributions. If you have an idea for improvement, please open an new issue or feel free to write a PR!

License

acme-dns is released under the MIT License.