Let's Encrypt client and ACME library written in Go

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Let's Encrypt client and ACME library written in Go

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This is a work in progress. Please do NOT run this on a production server and please report any bugs you find!


lego supports both binary installs and install from source.

To get the binary just download the latest release for your OS/Arch from the release page and put the binary somewhere convenient. lego does not assume anything about the location you run it from.

To install from source, just run

go get -u


  • Register with CA
  • Obtain certificates
  • Renew certificates
  • Revoke certificates
  • Robust implementation of all ACME challenges
    • HTTP (http-01)
    • TLS with Server Name Indication (tls-sni-01)
    • DNS (dns-01)
  • SAN certificate support
  • Comes with multiple optional DNS providers
  • Custom challenge solvers
  • Certificate bundling
  • OCSP helper function

Please keep in mind that CLI switches and APIs are still subject to change.

When using the standard --path option, all certificates and account configurations are saved to a folder .lego in the current working directory.


The CLI does not require root permissions but needs to bind to port 80 and 443 for certain challenges. To run the CLI without sudo, you have four options:

  • Use setcap 'cap_net_bind_service=+ep' /path/to/program
  • Pass the --http or/and the --tls option and specify a custom port to bind to. In this case you have to forward port 80/443 to these custom ports (see Port Usage).
  • Pass the --webroot option and specify the path to your webroot folder. In this case the challenge will be written in a file in .well-known/acme-challenge/ inside your webroot.
  • Pass the --dns option and specify a DNS provider.

Port Usage

By default lego assumes it is able to bind to ports 80 and 443 to solve challenges. If this is not possible in your environment, you can use the --http and --tls options to instruct lego to listen on that interface:port for any incoming challenges.

If you are using this option, make sure you proxy all of the following traffic to these ports.

HTTP Port:

  • All plaintext HTTP requests to port 80 which begin with a request path of /.well-known/acme-challenge/ for the HTTP challenge.

TLS Port:

  • All TLS handshakes on port 443 for the TLS-SNI challenge.

This traffic redirection is only needed as long as lego solves challenges. As soon as you have received your certificates you can deactivate the forwarding.


   lego - Let's Encrypt client written in Go

   lego [global options] command [command options] [arguments...]


   run      Register an account, then create and install a certificate
   revoke   Revoke a certificate
   renew    Renew a certificate
   dnshelp  Shows additional help for the --dns global option
   help, h  Shows a list of commands or help for one command

   --domains, -d [--domains option --domains option]            Add domains to the process
   --server, -s ""    CA hostname (and optionally :port). The server certificate must be trusted in order to avoid further modifications to the client.
   --email, -m                              Email used for registration and recovery contact.
   --accept-tos, -a                         By setting this flag to true you indicate that you accept the current Let's Encrypt terms of service.
   --key-type, -k "rsa2048"                     Key type to use for private keys. Supported: rsa2048, rsa4096, rsa8192, ec256, ec384
   --path "${CWD}/.lego"    Directory to use for storing the data
   --exclude, -x [--exclude option --exclude option]            Explicitly disallow solvers by name from being used. Solvers: "http-01", "tls-sni-01".
   --webroot                                Set the webroot folder to use for HTTP based challenges to write directly in a file in .well-known/acme-challenge
   --http                               Set the port and interface to use for HTTP based challenges to listen on. Supported: interface:port or :port
   --tls                                Set the port and interface to use for TLS based challenges to listen on. Supported: interface:port or :port
   --dns                                Solve a DNS challenge using the specified provider. Disables all other challenges. Run 'lego dnshelp' for help on usage.
   --help, -h                               show help
   --version, -v                            print the version
CLI Example

Assumes the lego binary has permission to bind to ports 80 and 443. You can get a pre-built binary from the releases page. If your environment does not allow you to bind to these ports, please read Port Usage.

Obtain a certificate:

$ lego --email="" --domains="" run

(Find your certificate in the .lego folder of current working directory.)

To renew the certificate:

$ lego --email="" --domains="" renew

Obtain a certificate using the DNS challenge and AWS Route 53:

$ AWS_REGION=us-east-1 AWS_ACCESS_KEY_ID=my_id AWS_SECRET_ACCESS_KEY=my_key lego --email="" --domains="" --dns="route53" run

Note that --dns=foo implies --exclude=http-01 and --exclude=tls-sni-01. lego will not attempt other challenges if you've told it to use DNS instead.

lego defaults to communicating with the production Let's Encrypt ACME server. If you'd like to test something without issuing real certificates, consider using the staging endpoint instead:

$ lego --server= …

DNS Challenge API Details

AWS Route 53

The following AWS IAM policy document describes the permissions required for lego to complete the DNS challenge. Replace <INSERT_YOUR_HOSTED_ZONE_ID_HERE> with the Route 53 zone ID of the domain you are authorizing.

    "Version": "2012-10-17",
    "Statement": [
            "Effect": "Allow",
            "Action": [
            "Resource": [
            "Effect": "Allow",
            "Action": [
            "Resource": [

ACME Library Usage

A valid, but bare-bones example use of the acme package:

// You'll need a user or account type that implements acme.User
type MyUser struct {
    Email        string
    Registration *acme.RegistrationResource
    key          crypto.PrivateKey
func (u MyUser) GetEmail() string {
    return u.Email
func (u MyUser) GetRegistration() *acme.RegistrationResource {
    return u.Registration
func (u MyUser) GetPrivateKey() crypto.PrivateKey {
    return u.key

// Create a user. New accounts need an email and private key to start.
const rsaKeySize = 2048
privateKey, err := rsa.GenerateKey(rand.Reader, rsaKeySize)
if err != nil {
myUser := MyUser{
    Email: "",
    key: privateKey,

// A client facilitates communication with the CA server. This CA URL is
// configured for a local dev instance of Boulder running in Docker in a VM.
client, err := acme.NewClient("", &myUser, acme.RSA2048)
if err != nil {

// We specify an http port of 5002 and an tls port of 5001 on all interfaces
// because we aren't running as root and can't bind a listener to port 80 and 443 
// (used later when we attempt to pass challenges). Keep in mind that we still
// need to proxy challenge traffic to port 5002 and 5001.

// New users will need to register
reg, err := client.Register()
if err != nil {
myUser.Registration = reg


// The client has a URL to the current Let's Encrypt Subscriber
// Agreement. The user will need to agree to it.
err = client.AgreeToTOS()
if err != nil {

// The acme library takes care of completing the challenges to obtain the certificate(s).
// The domains must resolve to this machine or you have to use the DNS challenge.
bundle := false
certificates, failures := client.ObtainCertificate([]string{""}, bundle, nil)
if len(failures) > 0 {

// Each certificate comes back with the cert bytes, the bytes of the client's
// private key, and a certificate URL. SAVE THESE TO DISK.
fmt.Printf("%#v\n", certificates)

// ... all done.