Automatic HTTPS for any Go program: fully-managed TLS certificate issuance and renewal
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mholt Calling ObtainCert() should be no-op if certificate already obtained
This does not affect RenewCert(), which should obviously happen if a
certificate already exists.

Fixes mholt/caddy#2400
Latest commit 01ffe8b Jan 18, 2019
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appveyor.yml ci: Fix gometalinter command Dec 10, 2018
cache.go Don't initialize a default cache until one is needed Dec 12, 2018
cache_test.go storage: Replace TryLock and Wait with Lock; simplify FileStorage Dec 19, 2018
certificates.go storage: Replace TryLock and Wait with Lock; simplify FileStorage Dec 19, 2018
certificates_test.go Move some tests over Dec 10, 2018
certmagic.go client: Make cert obtain timeout configurable Jan 11, 2019
certmagic_test.go Move some tests over Dec 10, 2018
client.go client: Make cert obtain timeout configurable Jan 11, 2019
config.go Calling ObtainCert() should be no-op if certificate already obtained Jan 18, 2019
config_test.go storage: Replace TryLock and Wait with Lock; simplify FileStorage Dec 19, 2018
crypto.go Make HTTPS() use production-ready TLS and HTTP configurations (issue #9) Dec 17, 2018
crypto_test.go Move some tests over Dec 10, 2018
doc_test.go Add CI scripts Dec 10, 2018
filestorage.go exported KeyBuilder.Safe to use with external pkgs (#13) Jan 7, 2019
handshake.go Use fmt.Errorf; customize HTTPTimeout and UserAgent Dec 11, 2018
handshake_test.go Move some tests over Dec 10, 2018
httphandler.go Move some tests over Dec 10, 2018
httphandler_test.go Move some tests over Dec 10, 2018
maintain.go Print which cache is doing maintenance in log entries Dec 12, 2018
ocsp.go Export methods to build storage keys and prefixes Dec 11, 2018
solvers.go exported KeyBuilder.Safe to use with external pkgs (#13) Jan 7, 2019
storage.go exported KeyBuilder.Safe to use with external pkgs (#13) Jan 7, 2019
storage_test.go Export methods to build storage keys and prefixes Dec 11, 2018
user.go FileStorage: Fix List(); modify Storage interface (fixes #4) Dec 12, 2018
user_test.go storage: Replace TryLock and Wait with Lock; simplify FileStorage Dec 19, 2018


Easy and Powerful TLS Automation

The same library used by the Caddy Web Server

Caddy's automagic TLS features, now for your own Go programs, in one powerful and easy-to-use library!

CertMagic is the most mature, robust, and capable ACME client integration for Go.

With CertMagic, you can add one line to your Go application to serve securely over TLS, without ever having to touch certificates.

Instead of:

// plaintext HTTP, gross 🤢
http.ListenAndServe(":80", mux)

Use CertMagic:

// encrypted HTTPS with HTTP->HTTPS redirects - yay! 🔒😍
certmagic.HTTPS([]string{""}, mux)

That line of code will serve your HTTP router mux over HTTPS, complete with HTTP->HTTPS redirects. It obtains and renews the TLS certificates. It staples OCSP responses for greater privacy and security. As long as your domain name points to your server, CertMagic will keep its connections secure.

Compared to other ACME client libraries for Go, only CertMagic supports the full suite of ACME features, and no other library matches CertMagic's maturity and reliability.

CertMagic - Automatic HTTPS using Let's Encrypt

Sponsored by Relica - Cross-platform local and cloud file backup:

Relica - Cross-platform file backup to the cloud, local disks, or other computers



  • Fully automated certificate management including issuance and renewal
  • One-liner, fully managed HTTPS servers
  • Full control over almost every aspect of the system
  • HTTP->HTTPS redirects (for HTTP applications)
  • Solves all 3 ACME challenges: HTTP, TLS-ALPN, and DNS
  • Over 50 DNS providers work out-of-the-box (powered by lego!)
  • Pluggable storage implementations (default: file system)
  • Wildcard certificates (requires DNS challenge)
  • OCSP stapling for each qualifying certificate (done right)
  • Distributed solving of all challenges (works behind load balancers)
  • Supports "on-demand" issuance of certificates (during TLS handshakes!)
    • Custom decision functions
    • Hostname whitelist
    • Ask an external URL
    • Rate limiting
  • Optional event hooks to observe internal behaviors
  • Works with any certificate authority (CA) compliant with the ACME specification
  • Certificate revocation (please, only if private key is compromised)
  • Must-Staple (optional; not default)
  • Cross-platform support! Mac, Windows, Linux, BSD, Android...
  • Scales well to thousands of names/certificates per instance
  • Use in conjunction with your own certificates


  1. Public DNS name(s) you control
  2. Server reachable from public Internet
    • Or use the DNS challenge to waive this requirement
  3. Control over port 80 (HTTP) and/or 443 (HTTPS)
    • Or they can be forwarded to other ports you control
    • Or use the DNS challenge to waive this requirement
    • (This is a requirement of the ACME protocol, not a library limitation)
  4. Persistent storage
    • Typically the local file system (default)
    • Other integrations available/possible

Before using this library, your domain names MUST be pointed (A/AAAA records) at your server (unless you use the DNS challenge)!


$ go get -u


Package Overview

Certificate authority

This library uses Let's Encrypt by default, but you can use any certificate authority that conforms to the ACME specification. Known/common CAs are provided as consts in the package, for example LetsEncryptStagingCA and LetsEncryptProductionCA.

The Config type

The certmagic.Config struct is how you can wield the power of this fully armed and operational battle station. However, an empty config is not a valid one! In time, you will learn to use the force of certmagic.New(certmagic.Config{...}) as I have.


For every field in the Config struct, there is a corresponding package-level variable you can set as a default value. These defaults will be used when you call any of the high-level convenience functions like HTTPS() or Listen() or anywhere else a default Config is used. They are also used for any Config fields that are zero-valued when you call New().

You can set these values easily, for example: certmagic.Email = ... sets the email address to use for everything unless you explicitly override it in a Config.

Providing an email address

Although not strictly required, this is highly recommended best practice. It allows you to receive expiration emails if your certificates are expiring for some reason, and also allows the CA's engineers to potentially get in touch with you if something is wrong. I recommend setting certmagic.Email or always setting the Email field of the Config struct.

Development and Testing

Note that Let's Encrypt imposes strict rate limits at its production endpoint, so using it while developing your application may lock you out for a few days if you aren't careful!

While developing your application and testing it, use their staging endpoint which has much higher rate limits. Even then, don't hammer it: but it's much safer for when you're testing. When deploying, though, use their production CA because their staging CA doesn't issue trusted certificates.

To use staging, set certmagic.CA = certmagic.LetsEncryptStagingCA or set CA of every Config struct.


There are many ways to use this library. We'll start with the highest-level (simplest) and work down (more control).

First, we'll follow best practices and do the following:

// read and agree to your CA's legal documents
certmagic.Agreed = true

// provide an email address
certmagic.Email = ""

// use the staging endpoint while we're developing
certmagic.CA = certmagic.LetsEncryptStagingCA

For fully-functional program examples, check out this Twitter thread (or read it unrolled into a single post).

Serving HTTP handlers with HTTPS

err := certmagic.HTTPS([]string{"", ""}, mux)
if err != nil {
	return err

This starts HTTP and HTTPS listeners and redirects HTTP to HTTPS!

Starting a TLS listener

ln, err := certmagic.Listen([]string{""})
if err != nil {
	return err

Getting a tls.Config

tlsConfig, err := certmagic.TLS([]string{""})
if err != nil {
	return err

Advanced use

For more control, you'll make and use a Config like so:

magic := certmagic.New(certmagic.Config{
	CA:     certmagic.LetsEncryptStagingCA,
	Email:  "",
	Agreed: true,
	// plus any other customization you want

// this obtains certificates or renews them if necessary
err := magic.Manage([]string{"", ""})
if err != nil {
	return err

// to use its certificates and solve the TLS-ALPN challenge,
// you can get a TLS config to use in a TLS listener!
tlsConfig := magic.TLSConfig()

// if you already have a TLS config you don't want to replace,
// we can simply set its GetCertificate field and append the
// TLS-ALPN challenge protocol to the NextProtos
myTLSConfig.GetCertificate = magic.GetCertificate
myTLSConfig.NextProtos = append(myTLSConfig.NextProtos, tlsalpn01.ACMETLS1Protocol}

// the HTTP challenge has to be handled by your HTTP server;
// if you don't have one, you should have disabled it earlier
// when you made the certmagic.Config
httpMux = magic.HTTPChallengeHandler(httpMux)

Great! This example grants you much more flexibility for advanced programs. However, the vast majority of you will only use the high-level functions described earlier, especially since you can still customize them by setting the package-level defaults.

If you want to use the default configuration but you still need a certmagic.Config, you can call certmagic.Manage() directly to get one:

magic, err := certmagic.Manage([]string{""})
if err != nil {
	return err

And then it's the same as above, as if you had made the Config yourself.

Wildcard certificates

At time of writing (December 2018), Let's Encrypt only issues wildcard certificates with the DNS challenge.

Behind a load balancer (or in a cluster)

CertMagic runs effectively behind load balancers and/or in cluster/fleet environments. In other words, you can have 10 or 1,000 servers all serving the same domain names, all sharing certificates and OCSP staples.

To do so, simply ensure that each instance is using the same Storage. That is the sole criteria for determining whether an instance is part of a cluster.

The default Storage is implemented using the file system, so mounting the same shared folder is sufficient (see Storage for more on that)! If you need an alternate Storage implementation, feel free to use one, provided that all the instances use the same one. :)

See Storage and the associated godoc for more information!

The ACME Challenges

This section describes how to solve the ACME challenges. Challenges are how you demonstrate to the certificate authority some control over your domain name, thus authorizing them to grant you a certificate for that name. The great innovation of ACME is that verification by CAs can now be automated, rather than having to click links in emails (who ever thought that was a good idea??).

If you're using the high-level convenience functions like HTTPS(), Listen(), or TLS(), the HTTP and/or TLS-ALPN challenges are solved for you because they also start listeners. However, if you're making a Config and you start your own server manually, you'll need to be sure the ACME challenges can be solved so certificates can be renewed.

The HTTP and TLS-ALPN challenges are the defaults because they don't require configuration from you, but they require that your server is accessible from external IPs on low ports. If that is not possible in your situation, you can enable the DNS challenge, which will disable the HTTP and TLS-ALPN challenges and use the DNS challenge exclusively.

Technically, only one challenge needs to be enabled for things to work, but using multiple is good for reliability in case a challenge is discontinued by the CA. This happened to the TLS-SNI challenge in early 2018—many popular ACME clients such as Traefik and Autocert broke, resulting in downtime for some sites, until new releases were made and patches deployed, because they used only one challenge; Caddy, however—this library's forerunner—was unaffected because it also used the HTTP challenge. If multiple challenges are enabled, they are chosen randomly to help prevent false reliance on a single challenge type.

HTTP Challenge

Per the ACME spec, the HTTP challenge requires port 80, or at least packet forwarding from port 80. It works by serving a specific HTTP response that only the genuine server would have to a normal HTTP request at a special endpoint.

If you are running an HTTP server, solving this challenge is very easy: just wrap your handler in HTTPChallengeHandler or call SolveHTTPChallenge() inside your own ServeHTTP() method.

For example, if you're using the standard library:

mux := http.NewServeMux()
mux.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
	fmt.Fprintf(w, "Lookit my cool website over HTTPS!")

http.ListenAndServe(":80", magic.HTTPChallengeHandler(mux))

If wrapping your handler is not a good solution, try this inside your ServeHTTP() instead:

magic := certmagic.NewDefault()

func ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if magic.HandleHTTPChallenge(w, r) {
		return // challenge handled; nothing else to do

If you are not running an HTTP server, you should disable the HTTP challenge or run an HTTP server whose sole job it is to solve the HTTP challenge.

TLS-ALPN Challenge

Per the ACME spec, the TLS-ALPN challenge requires port 443, or at least packet forwarding from port 443. It works by providing a special certificate using a standard TLS extension, Application Layer Protocol Negotiation (ALPN), having a special value. This is the most convenient challenge type because it usually requires no extra configuration and uses the standard TLS port which is where the certificates are used, also.

This challenge is easy to solve: just use the provided tls.Config when you make your TLS listener:

// use this to configure a TLS listener
tlsConfig := magic.TLSConfig()

Or make two simple changes to an existing tls.Config:

myTLSConfig.GetCertificate = magic.GetCertificate
myTLSConfig.NextProtos = append(myTLSConfig.NextProtos, tlsalpn01.ACMETLS1Protocol}

Then just make sure your TLS listener is listening on port 443:

ln, err := tls.Listen("tcp", ":443", myTLSConfig)

DNS Challenge

The DNS challenge is perhaps the most useful challenge because it allows you to obtain certificates without your server needing to be publicly accessible on the Internet, and it's the only challenge by which Let's Encrypt will issue wildcard certificates.

This challenge works by setting a special record in the domain's zone. To do this automatically, your DNS provider needs to offer an API by which changes can be made to domain names, and the changes need to take effect immediately for best results. CertMagic supports all of lego's DNS provider implementations! All of them clean up the temporary record after the challenge completes.

To enable it, just set the DNSProvider field on a certmagic.Config struct, or set the default certmagic.DNSProvider variable. For example, if my domains' DNS was served by DNSimple (they're great, by the way) and I set my DNSimple API credentials in environment variables:

import ""

provider, err := dnsimple.NewProvider()
if err != nil {
	return err

certmagic.DNSProvider = provider

Now the DNS challenge will be used by default, and I can obtain certificates for wildcard domains. See the godoc documentation for the provider you're using to learn how to configure it. Most can be configured by env variables or by passing in a config struct. If you pass a config struct instead of using env variables, you will probably need to set some other defaults (that's just how lego works, currently):

PropagationTimeout: dns01.DefaultPollingInterval,
PollingInterval:    dns01.DefaultPollingInterval,
TTL:                dns01.DefaultTTL,

Enabling the DNS challenge disables the other challenges for that certmagic.Config instance.

On-Demand TLS

Normally, certificates are obtained and renewed before a listener starts serving, and then those certificates are maintained throughout the lifetime of the program. In other words, the certificate names are static. But sometimes you don't know all the names ahead of time. This is where On-Demand TLS shines.

Originally invented for use in Caddy (which was the first program to use such technology), On-Demand TLS makes it possible and easy to serve certificates for arbitrary names during the lifetime of the server. When a TLS handshake is received, CertMagic will read the Server Name Indication (SNI) value and either load and present that certificate in the ServerHello, or if one does not exist, it will obtain it from a CA right then-and-there.

Of course, this has some obvious security implications. You don't want to DoS a CA or allow arbitrary clients to fill your storage with spammy TLS handshakes. That's why, in order to enable On-Demand issuance, you'll need to set some limits or some policy to allow getting a certificate.

CertMagic provides several ways to enforce decision policies for On-Demand TLS, in descending order of priority:

  • A generic function that you write which will decide whether to allow the certificate request
  • A name whitelist
  • The ability to make an HTTP request to a URL for permission
  • Rate limiting

The simplest way to enable On-Demand issuance is to set the OnDemand field of a Config (or the default package-level value):

certmagic.OnDemand = &certmagic.OnDemandConfig{MaxObtain: 5}

This allows only 5 certificates to be requested and is the simplest way to enable On-Demand TLS, but is the least recommended. It prevents abuse, but only in the least helpful way.

The godoc describes how to use the other policies, all of which are much more recommended! :)

If OnDemand is set and Manage() is called, then the names given to Manage() will be whitelisted rather than obtained right away.


CertMagic relies on storage to store certificates and other TLS assets (OCSP staple cache, coordinating locks, etc). Persistent storage is a requirement when using CertMagic: ephemeral storage will likely lead to rate limiting on the CA-side as CertMagic will always have to get new certificates.

By default, CertMagic stores assets on the local file system in $HOME/.local/share/certmagic (and honors $XDG_DATA_HOME if set). CertMagic will create the directory if it does not exist. If writes are denied, things will not be happy, so make sure CertMagic can write to it!

The notion of a "cluster" or "fleet" of instances that may be serving the same site and sharing certificates, etc, is tied to storage. Simply, any instances that use the same storage facilities are considered part of the cluster. So if you deploy 100 instances of CertMagic behind a load balancer, they are all part of the same cluster if they share the same storage configuration. Sharing storage could be mounting a shared folder, or implementing some other distributed storage system such as a database server or KV store.

The easiest way to change the storage being used is to set certmagic.DefaultStorage to a value that satisfies the Storage interface. Keep in mind that a valid Storage must be able to implement some operations atomically in order to provide locking and synchronization.

If you write a Storage implementation, let us know and we'll add it to the project so people can find it!


All of the certificates in use are de-duplicated and cached in memory for optimal performance at handshake-time. This cache must be backed by persistent storage as described above.

Most applications will not need to interact with certificate caches directly. Usually, the closest you will come is to set the package-wide certmagic.DefaultStorage variable (before attempting to create any Configs). However, if your use case requires using different storage facilities for different Configs (that's highly unlikely and NOT recommended! Even Caddy doesn't get that crazy), you will need to call certmagic.NewCache() and pass in the storage you want to use, then get new Config structs with certmagic.NewWithCache() and pass in the cache.

Again, if you're needing to do this, you've probably over-complicated your application design.


Can I use some of my own certificates while using CertMagic?

Yes, just call the relevant method on the Config to add your own certificate to the cache:

Keep in mind that unmanaged certificates are (obviously) not renewed for you, so you'll have to replace them when you do. However, OCSP stapling is performed even for unmanaged certificates that qualify.

Does CertMagic obtain SAN certificates?

Technically all certificates these days are SAN certificates because CommonName is deprecated. But if you're asking whether CertMagic issues and manages certificates with multiple SANs, the answer is no. But it does support serving them, if you provide your own.

How can I listen on ports 80 and 443? Do I have to run as root?

On Linux, you can use setcap to grant your binary the permission to bind low ports:

$ sudo setcap cap_net_bind_service=+ep /path/to/your/binary

and then you will not need to run with root privileges.


We welcome your contributions! Please see our contributing guidelines for instructions.

Project History

CertMagic is the core of Caddy's advanced TLS automation code, extracted into a library. The underlying ACME client implementation is lego, which was originally developed for use in Caddy even before Let's Encrypt entered public beta in 2015.

In the years since then, Caddy's TLS automation techniques have been widely adopted, tried and tested in production, and served millions of sites and secured trillions of connections.

Now, CertMagic is the actual library used by Caddy. It's incredibly powerful and feature-rich, but also easy to use for simple Go programs: one line of code can enable fully-automated HTTPS applications with HTTP->HTTPS redirects.

Caddy is known for its robust HTTPS+ACME features. When ACME certificate authorities have had outages, in some cases Caddy was the only major client that didn't experience any downtime. Caddy can weather OCSP outages lasting days, or CA outages lasting weeks, without taking your sites offline.

Caddy was also the first to sport "on-demand" issuance technology, which obtains certificates during the first TLS handshake for an allowed SNI name.

Consequently, CertMagic brings all these (and more) features and capabilities right into your own Go programs.

You can watch a 2016 dotGo talk by the author of this library about using ACME to automate certificate management in Go programs:

Matthew Holt speaking at dotGo 2016 about ACME in Go

Credits and License

CertMagic is a project by Matthew Holt, who is the author; and various contributors, who are credited in the commit history of either CertMagic or Caddy.

CertMagic is licensed under Apache 2.0, an open source license. For convenience, its main points are summarized as follows (but this is no replacement for the actual license text):

  • The author owns the copyright to this code
  • Use, distribute, and modify the software freely
  • Private and internal use is allowed
  • License text and copyright notices must stay intact and be included with distributions
  • Any and all changes to the code must be documented