ACME

This plugin allows Kong to apply certificates from Let’s Encrypt or any other ACMEv2 service and serve them dynamically. Renewal is handled with a configurable threshold time.

Storage configuration considerations

config.storage_config is a table for all possible storage types. By default, it is:

    "storage_config": {
      "kong": {},
      "shm": {
        "shm_name": "kong"
      },
      "redis": {
        "auth": null,
        "port": 6379,
        "database": 0,
        "host": "127.0.0.1",
        "ssl": false,
        "ssl_verify": false,
        "ssl_server_name": null,
        "namespace": "",
      },
       "consul": {
          "host": "127.0.0.1",
          "port": 8500,
          "token": null,
          "kv_path": "acme",
          "timeout": 2000,
          "https": false
      },
      "vault": {
          "host": "127.0.0.1",
          "port": 8200,
          "token": null,
          "kv_path": "acme",
          "timeout": 2000,
          "https": false,
          "tls_verify": true,
          "tls_server_name": null,
          "auth_method": "token",
          "auth_path": null,
          "auth_role": null,
          "jwt_path": null,
      },
    }

The consul.token, redis.auth, and vault.token fields are referenceable, which means they can be securely stored as secrets in a vault. References must follow a specific format.

To configure a storage type other than kong, refer to lua-resty-acme.

Workflow

A http-01 challenge workflow between the Kong Gateway and the ACME server is described below:

1. The client sends a proxy or Admin API request that triggers certificate generation for mydomain.com.
2. The Kong Gateway sends a request to the ACME server to start the validation process.
3. The ACME server returns a challenge response detail to the Kong Gateway.
4. mydomain.com is publicly resolvable to the Kong Gateway that serves the challenge response.
5. The ACME server checks if the previous challenge has a response at mydomain.com.
6. The Kong Gateway checks the challenge status and if passed, downloads the certificate from the ACME server.
7. The Kong Gateway uses the new certificate to serve TLS requests.

Using the plugin

Configure Kong

• Kong needs to listen on port 80 or proxy a load balancer that listens for port 80.
• lua_ssl_trusted_certificate needs to be set in kong.conf to ensure the plugin can properly verify the Let’s Encrypt API. The CA-bundle file is usually /etc/ssl/certs/ca-certificates.crt for Ubuntu/Debian and /etc/ssl/certs/ca-bundle.crt for CentOS/Fedora/RHEL. Starting with Kong v2.2, users can set this config to system to auto pick CA-bundle from OS.

Configure plugin

Here’s a sample declarative configuration with redis as storage:

_format_version: "3.0"
# this section is not necessary if there's already a route that matches
# /.well-known/acme-challenge path with http protocol
services:
  - name: acme-dummy
    url: http://127.0.0.1:65535
    routes:
      - name: acme-dummy
        protocols:
          - http
        paths:
          - /.well-known/acme-challenge
plugins:
  - name: acme
    config:
      account_email: example@myexample.com
      account_key:
        key_id: "1234"
        key_set: "example-key-set"
      domains:
        - "*.example.com"
        - "example.com"
      tos_accepted: true
      storage: redis
      storage_config:
        redis:
          host: redis.service
          port: 6379

Enable the plugin

For each the domain that needs a certificate, make sure DOMAIN/.well-known/acme-challenge is mapped to a route in Kong. You can check this by sending curl KONG_IP/.well-known/acme-challenge/x -H "host:DOMAIN" and getting the response Not found. You can also use the Admin API to verify the setup. If not, add a route and a dummy service to catch this route.

# add a dummy service if needed
curl http://localhost:8001/services \
  -d name=acme-dummy \
  -d url=http://127.0.0.1:65535
# add a dummy route if needed
curl http://localhost:8001/routes \
  -d name=acme-dummy \
  -d paths[]=/.well-known/acme-challenge \
  -d service.name=acme-dummy
# add the plugin
curl http://localhost:8001/plugins \
  -d name=acme \
  -d config.account_email=yourname@yourdomain.com \
  -d config.tos_accepted=true \
  -d config.domains[]=my.secret.domains.com

Note by setting tos_accepted to true implies that you have read and accepted terms of service.

This plugin can only be configured as a global plugin. The plugin terminates /.well-known/acme-challenge/ path for matching domains. To create certificates and terminate challenges only for certain domains, refer to the Parameters section.

Trigger certificate creation

Assume Kong proxy is accessible via http://mydomain.com and https://mydomain.com.

# Trigger asynchronous creation from proxy requests
# The following request returns immediately with Kong's default certificate
# Wait up to 1 minute for the background process to finish
curl https://mydomain.com -k
# OR create from Admin API synchronously
# User can also use this endpoint to force "renew" a certificate
curl http://localhost:8001/acme -d host=mydomain.com
# Furthermore, it's possible to run a sanity test on your Kong setup
# before creating any certificate
curl http://localhost:8001/acme -d host=mydomain.com -d test_http_challenge_flow=true
curl https://mydomain.com
# Now gives you a valid Let's Encrypt certicate

Renew certificates

The plugin runs daily checks and automatically renews all certificates that will expire in less than the configured renew_threshold_days value. If the renewal of an individual certificate throws an error, the plugin will continue renewing the other certificates. It will try renewing all certificates, including those that previously failed, once per day. Note the renewal configuration is stored in the configured storage backend. If the storage is cleared or modified outside of Kong, renewal might not complete properly.

It’s also possible to actively trigger the renewal. The following request schedules a renewal in the background and returns immediately.

curl http://localhost:8001/acme -XPATCH

Monitoring and debugging

The ACME plugin exposes several endpoints through Admin API that can be used for debugging and monitoring certificate creation and renewal.

Apply certificate

Endpoint

Applies or renews the certificate and returns the result.

Request body

Update certificate

Apply or renew the certificate and return the result. Unlike POST, PATCH runs the process in the background.

Endpoint

Request body

Get ACME certificates

List the certificates being created by the ACME plugin. You can use this endpoint to monitor certificate existence and expiry.

Endpoint

Get certificate by host

List certificates with a specific host.

Endpoint

Example response for listing certificates:

{
  "data": [
    {
      "not_after": "2022-09-21 23:59:59",
      "pubkey_type": "id-ecPublicKey",
      "digest": "A9:49:55:06:A7:B6:1D:2B:13:47:C5:58:5B:AC:DA:43:B5:25:E0:86",
      "issuer_cn": "ZeroSSL ECC Domain Secure Site CA",
      "valid": true,
      "host": "subdomain1.domain.com",
      "not_before": "2022-06-23 00:00:00",
      "serial_number": "93:B8:E9:D5:C6:36:ED:B4:A8:B3:FD:C5:9E:A8:08:88"
    },
    {
      "not_after": "2022-09-21 23:59:59",
      "pubkey_type": "id-ecPublicKey",
      "digest": "26:12:A2:C4:6A:F5:A5:90:9D:03:15:CB:FE:A7:BF:32:1C:42:49:CE",
      "issuer_cn": "ZeroSSL ECC Domain Secure Site CA",
      "valid": true,
      "host": "subdomain2.domain.com",
      "not_before": "2022-06-23 00:00:00",
      "serial_number": "F1:15:74:E3:E1:DD:21:72:48:C0:4F:06:25:1B:71:F7"
    }
  ]
}

Hybrid mode

"shm" storage type is not available in Hybrid Mode.

Due to current the limitations of Hybrid Mode, "kong" storage only supports certificate generation from the Admin API but not the proxy side.

"kong" storage in Hybrid Mode works in following flow:

1. The client sends an Admin API request that triggers certificate generation for mydomain.com.
2. The Kong Control Plane requests the ACME server to start the validation process.
3. The ACME server returns a challenge response detail to the Kong Control Plane.
4. The Kong Control Plane propagates the challenge response detail to the Kong Data Plane.
5. mydomain.com is publicly resolvable to the Kong Data Plane that serves the challenge response.
6. The ACME server checks if the previous challenge has a response at mydomain.com.
7. The Kong Control Plane checks the challenge status and if passed, downloads the certificate from the ACME server.
8. The Kong Control Plane propagates the new certificates to the Kong Data Plane.
9. The Kong Data Plane uses the new certificate to serve TLS requests.

All external storage types work as usual in Hybrid Mode. Note both the Control Plane and Data Planes need to connect to the same external storage cluster. It’s also a good idea to setup replicas to avoid connecting to same node directly for external storage.

External storage in Hybrid Mode works in following flow:

1. The client send a proxy or Admin API request that triggers certificate generation for mydomain.com.
2. The Kong Control Plane or Data Plane requests the ACME server to start the validation process.
3. The ACME server returns a challenge response detail to the Kong Gateway.
4. The Kong Control Plane or Data Plane stores the challenge response detail in external storage.
5. mydomain.com is publicly resolvable to the Kong Data Plane that reads and serves the challenge response from external storage.
6. The ACME server checks if the previous challenge has a response at mydomain.com.
7. The Kong Control Plane or Data Plane checks the challenge status and if passed, downloads the certificate from the ACME server.
8. The Kong Control Plane or Data Plane stores the new certificates in external storage.
9. The Kong Data Plane reads from external storage and uses the new certificate to serve TLS requests.

Local testing and development

Run ngrok

ngrok exposes a local URL to the internet. Download ngrok and install.

ngrok is only needed for local testing or development, it’s not a requirement for the plugin itself.

Run ngrok with:

./ngrok http localhost:8000
# Shows something like
# ...
# Forwarding                    http://e2e034a5.ngrok.io -> http://localhost:8000
# Forwarding                    https://e2e034a5.ngrok.io -> http://localhost:8000
# ...
# Substitute "e2e034a5.ngrok.io" with the host shows in your ngrok output
export NGROK_HOST=e2e034a5.ngrok.io

Leave the process running.

Configure route and service

curl http://localhost:8001/services \
  -d name=acme-test \
  -d url=http://mockbin.org
curl http://localhost:8001/routes \
  -d service.name=acme-test \
  -d hosts=$NGROK_HOST \
  -d paths=/mock

Enable plugin

curl localhost:8001/plugins \
  -d name=acme \
  -d config.account_email=test@test.com \
  -d config.tos_accepted=true \
  -d config.domains[]=$NGROK_HOST

Trigger certificate creation

curl https://$NGROK_HOST:8443 --resolve $NGROK_HOST:8443:127.0.0.1 -vk
# Wait for several seconds

Check new certificate

echo q |openssl s_client -connect localhost -port 8443 -servername $NGROK_HOST 2>/dev/null |openssl x509 -text -noout

Notes

• In database mode, the plugin creates an SNI and Certificate entity in Kong to serve the certificate. If SNI or Certificate for the current request is already set in the database, they will be overwritten.
• In DB-less mode, the plugin takes over certificate handling. If the SNI or Certificate entity is already defined in Kong, they will be overridden by the response.
• The plugin only supports http-01 challenge, meaning a user will need a public IP and set up a resolvable DNS. Kong also needs to accept proxy traffic from port 80. Also, note that a wildcard or star (*) certificate is not supported. Each domain must have its own certificate.