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About

acme-proxy is a standalone ACME server built on step-ca that operates in registration authority (RA) mode. It accepts certificate orders and validates certificate requests using the ACME protocol (RFC 8555), but does NOT sign certificates or store private keys.

Documentation

Checkout our documentation site for detailed examples on user guide, installation instructions, configuration etc.

How It Works

acme-proxy runs as an ACME server inside your trusted network, acting as an intermediary between your internal infrastructure and an external certificate authority service (such as Sectigo). When a client successfully completes an ACME challenge, acme-proxy forwards the certificate signing request to an external certificate authority (CA) that supports External Account Binding (EAB). The external CA signs the certificate and returns it to the client through acme-proxy.

Certificate Request Flow:

  1. Your internal server (behind a firewall perimeter) requests a certificate from acme-proxy using standard ACME clients like certbot, acme.sh or cert-manager.io if you're using Kubernetes.
  2. acme-proxy presents cryptographic challenges to verify domain ownership
  3. Once validation succeeds, acme-proxy forwards the certificate signing request to your external CA using External Account Binding (EAB)
  4. The external CA signs the certificate
  5. acme-proxy retrieves the certificate bundle and returns it to your server

sequence diagram

Note: LetsEncrypt does not support EAB. However, commercial CAs such as Sectigo and ZeroSSL do.

Use Cases

This architecture addresses typical enterprise constraints that prevent direct certificate issuance from LetsEncrypt:

HTTP-01 Challenge Limitations:

  • Security policies prohibit exposing port 80 to the public internet

DNS-01 Challenge Limitations:

  • Legacy DNS infrastructure lacks REST API support or ACME client integration
  • Security policies restrict distribution of API tokens or TSIG keys for large DNS zones

For more information on security considerations when using DNS-01 challenge:

Quick Start

curl -fsSL https://raw.githubusercontent.com/esnet/acme-proxy/main/install.sh | sudo sh

Installing binary to /opt/acme-proxy...
Creating acme-proxy service user...
Setting ownership of installation directory...
Installing systemd service...
Reloading systemd daemon...
Enabling acme-proxy service...
Created symlink /etc/systemd/system/multi-user.target.wants/acme-proxy.service → /etc/systemd/system/acme-proxy.service.

Installation complete!

Next steps:
  1. Edit /opt/acme-proxy/ca.json and configure:
     - dnsNames: Your ACME proxy hostname
     - ca_url: Your upstream ACME CA URL
     - account_email: Your account email
     - eab_kid: External Account Binding Key ID
     - eab_hmac_key: External Account Binding HMAC key

  2. Start the service:
     sudo systemctl start acme-proxy

  3. Check status:
     sudo systemctl status acme-proxy
     sudo journalctl -u acme-proxy -f

The script installs acme-proxy as a systemd service with sensible defaults, all of which can be overridden with environment variables:

# Default values (all overridable)
INSTALL_DIR="${INSTALL_DIR:-/opt/acme-proxy}"
DB_DIR="${DB_DIR:-${INSTALL_DIR}/db}"
CONFIG_FILE="${CONFIG_FILE:-${INSTALL_DIR}/ca.json}"
SERVICE_USER="${SERVICE_USER:-acme-proxy}"
SERVICE_GROUP="${SERVICE_GROUP:-acme-proxy}"

Configure

Review and update configuration options in ca.json before starting the acme-proxy server.

vim ca.json

Checkout our official docs for full set of configuration options. For quick start the most relevant config bits are:

  "dnsNames": ["acmeproxy.example.com"],
  ...
  "authority": {
    "type": "externalcas",
    "config": {
      "ca_url": "",
      "account_email": "",
      "eab_kid": "",
      "eab_hmac_key": "",
      "metrics": {
        "enabled": true,
        "port": 9234,
        "dataSource": "/opt/acme-proxy/db/metrics"
      }
    },
  ...
    "commonName": "acmeproxy.example.com"
  }

Most commercial certificate authorities (such as Sectigo) support certificate issuance over external account binding. You will need to get EAB credentials i.e HMAC Key and Key ID associated with your account. To get signed certs from InCommon use https://acme.sectigo.com/v2/InCommonRSAOV as shown below

  "ca_url": "https://acme.sectigo.com/v2/InCommonRSAOV"
  "account_email": "certadmin@example.com",
  "eab_kid": "",
  "eab_hmac_key": "",

Starting acme-proxy

After configuring ca.json file simply start the systemd service

sudo systemctl start acme-proxy

Upon starting acme-proxy it automatically obtains a SSL certificate for itself as part of bootstrapping. This certificate and it's private key are stored in memory and are automatically rotated using the EAB credentials provided in ca.json

$ sudo systemctl status acme-proxy

badger 2025/07/15 22:12:24 INFO: All 1 tables opened in 0s
badger 2025/07/15 22:12:24 INFO: Replaying file id: 0 at offset: 105133
badger 2025/07/15 22:12:24 INFO: Replay took: 5.99µs
2025/07/15 22:12:25 Building new tls configuration using step-ca x509 Signer Interface
2025/07/15 22:12:25 Initializing ACME client...
2025/07/15 22:12:25 [INFO] acme: Registering account for admin@example.com
2025/07/15 22:12:26 ACME client initialized successfully
2025/07/15 22:12:26 Processing certificate request for domains: [proxy.example.com]
2025/07/15 22:12:26 Starting certificate request processing for domains: [proxy.example.com]
2025/07/15 22:12:26 [INFO] [proxy.example.com] acme: Obtaining bundled SAN certificate given a CSR
2025/07/15 22:12:27 [INFO] [proxy.example.com] AuthURL: https://acme.sectigo.com/v2/InCommonRSAOV/authz/sx4qvINAdWw2IjplmyH6kg
2025/07/15 22:12:27 [INFO] [proxy.example.com] acme: authorization already valid; skipping challenge
2025/07/15 22:12:27 [INFO] [proxy.example.com] acme: Validations succeeded; requesting certificates
2025/07/15 22:12:27 [INFO] Wait for certificate [timeout: 30s, interval: 500ms]
2025/07/15 22:12:33 [INFO] [proxy.example.com] Server responded with a certificate.
2025/07/15 22:12:33 Successfully obtained certificate from InCommon for domains: [proxy.example.com]
2025/07/15 22:12:33 Starting Smallstep CA/0000000-dev (linux/amd64)
2025/07/15 22:12:33 Documentation: https://u.step.sm/docs/ca
2025/07/15 22:12:33 Community Discord: https://u.step.sm/discord
2025/07/15 22:12:33 Config file: ca.json
2025/07/15 22:12:33 The primary server URL is https://acmeproxy.example.com:443
2025/07/15 22:12:33 Root certificates are available at https://acmeproxy.example.com:443/roots.pem
2025/07/15 22:12:33 X.509 Root Fingerprint: a6cf64dbb4c8d5fd19ce48896068db03b533a8d1336c6256a87d00cbb3def3ea
2025/07/15 22:12:33 Serving HTTPS on proxy.example.com:443 ...

Obtaining a certificate

While the example below uses acme.sh as the ACME client in standalone mode, we've also tested using certbot and cert-manager on Kubernetes with equal success. For more examples see user guide.

$ ./acme.sh --issue \
    --server https://acmeproxy.example.com/acme/acme/directory \
    --domain myserver.example.com \
    --standalone \
    --listen-v6

[Tue 15 Jul 22:41:01 CDT 2025] Using CA: https://acmeproxy.example.com/acme/acme/directory
[Tue 15 Jul 22:41:01 CDT 2025] Standalone mode.
[Tue 15 Jul 22:41:01 CDT 2025] Creating domain key
[Tue 15 Jul 22:41:01 CDT 2025] The domain key is here: /root/.acme.sh/myserver.example.com_ecc/myserver.example.com.key
[Tue 15 Jul 22:41:01 CDT 2025] Single domain='myserver.example.com'
[Tue 15 Jul 22:41:02 CDT 2025] Getting webroot for domain='myserver.example.com'
[Tue 15 Jul 22:41:02 CDT 2025] Verifying: myserver.example.com
[Tue 15 Jul 22:41:02 CDT 2025] Standalone mode server
[Tue 15 Jul 22:41:04 CDT 2025] Success
[Tue 15 Jul 22:41:04 CDT 2025] Verification finished, beginning signing.
[Tue 15 Jul 22:41:04 CDT 2025] Let's finalize the order.
[Tue 15 Jul 22:41:04 CDT 2025] Le_OrderFinalize='https://acmeproxy.example.com/acme/acme/order/ugickkyMzE0hoHZhBLGuGqG3ab1N0hwz/finalize'
[Tue 15 Jul 22:41:12 CDT 2025] Downloading cert.
[Tue 15 Jul 22:41:12 CDT 2025] Le_LinkCert='https://acmeproxy.example.com/acme/acme/certificate/b3A7d7rZA78ijaKwcR0n5xtxf8PAeE1v'
[Tue 15 Jul 22:41:13 CDT 2025] Cert success.
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
[Tue 15 Jul 22:41:13 CDT 2025] Your cert is in: /root/.acme.sh/myserver.example.com_ecc/myserver.example.com.cer
[Tue 15 Jul 22:41:13 CDT 2025] Your cert key is in: /root/.acme.sh/myserver.example.com_ecc/myserver.example.com.key
[Tue 15 Jul 22:41:13 CDT 2025] The intermediate CA cert is in: /root/.acme.sh/myserver.example.com_ecc/ca.cer
[Tue 15 Jul 22:41:13 CDT 2025] And the full-chain cert is in: /root/.acme.sh/myserver.example.com_ecc/fullchain.cer

Verify

Let's decode the certificate just to be sure ;-)

$ openssl x509 -in myserver.example.com.cer -noout -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            13:1c:e0:69:50:13:f6:04:68:f2:a7:9f:48:d5:4c:87
        Signature Algorithm: ecdsa-with-SHA256
        Issuer: C = US, O = Internet2, CN = InCommon ECC Server CA 2
        Validity
            Not Before: Jul 16 00:00:00 2025 GMT
            Not After : Jul 16 23:59:59 2026 GMT
        Subject: C = US, ST = California, O = Energy Sciences Network, CN = myserver.example.com
        Subject Public Key Info:

We have our certificate signed by our certificate authority i.e InCommon 🎉

Benefits

Using ACME with commercial CAs in enterprise environments provides several advantages:

Trusted Certificates:

  • Certificates are signed by publicly trusted CAs are already in system trust stores
  • Eliminates the operational burden of distributing and maintaining custom root certificates across endpoints, servers, and client devices

Automation and Self-Service:

  • Leverage standard ACME clients (Certbot, acme.sh, cert-manager.io) for certificate issuance, automatic renewals.
  • Enable self-service certificate requests for development teams

About

Solve ACME http-01 challenge without opening port 80 to the internet, obtain certs from an external certificate authority.

software.es.net/acme-proxy/

Topics

golang proxy certificates acme acme-server rfc8555 step-ca

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Apache-2.0 license
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Release v0.5.0 Latest
Apr 9, 2026
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