Go TLS configuration module

This module eases configuration of TLS for clients and servers written in Go.

Features:

• The Config struct has YAML and JSON struct tags.
• It can be used for both clients and servers.
• A Manager can generate a corresponding tls.Config.
• On-disk certificates can be automatically reloaded without downtime.

The main idea behind it is that you can integrate it in your code once and then automatically gain support for any new certificate management features in the future.

YAML configuration examples

Note that depending on the application, the configuration can also be in JSON or another format.

The examples below group all TLS options under a tls key, but this name depends on the name used in the program.

Servers

Load certificate from file:

tls:
  cert_file: path/to/cert.pem
  key_file: path/to/key.pem

With automatic reloading:

tls:
  cert_file: path/to/cert.pem
  key_file: path/to/key.pem
  watch_certs: true
  watch_certs_poll_interval: 5s

Same with a custom CA and client certificate support:

tls:
  ca_file: path/to/ca.pem
  cert_file: path/to/cert.pem
  key_file: path/to/key.pem
  watch_certs: true
  watch_certs_poll_interval: 5s
  require_client_cert: true

Clients

Clients that just want to use OS provides CAs can leave the configuration empty.

To use custom CA certs to verify the connection:

tls:
  ca_file: path/to/ca.pem

To allow both custom CA certs and system CA certs to verify the connection:

tls:
  ca_file: path/to/ca.pem
  add_system_ca_pool: true

To use a client certificate:

tls:
  ca_file: path/to/ca.pem
  cert_file: path/to/cert.pem
  key_file: path/to/key.pem

INSECURE - to disable certificate validation:

tls:
  insecure_skip_verify: true

General options

The ca_file, cert_file and key_file options have corresponding ca, cert and key options that allow you to inline the PEM certificate into the config file:

tls:
  cert: |
    -----BEGIN CERTIFICATE-----
    ...

Note that there is no way to do automatic reloading this way.

The insecure_key_log_file option can be set to a log file path for the use of tools like Wireshark to decode encrypted traffic in development. See this Mozilla page for more information.

Usage in Go code

You can use the Config struct in you existing configuration structures:

import "github.com/PowerDNS/go-tlsconfig"

type MyConfig struct {
	Server Server `yaml:"server"`
	// ...
}

type Server struct {
	TLS tlsconfig.Config `yaml:"tls"`
}

After you have loaded your configration, you can the use a Manager to generate a tls.Config (error handling omitted):

manager, err := tlsconfig.NewManager(ctx, config.Server.TLS, tlsconfig.Options{
	IsServer: true,
})

tlsConfig, err := manager.TLSConfig()

hs := http.Server{
	TLSConfig: tlsConfig,
}
err = hs.ListenAndServeTLS("", "") // Certificates are handled by the TLSConfig

Example of how to use a custom TLS config with an HTTP client:

manager, err := tlsconfig.NewManager(ctx, config.Client.TLS, tlsconfig.Options{
	IsClient: true,
})

tlsConfig, err := manager.TLSConfig()

transport := &http.Transport{
	TLSClientConfig: tlsConfig,
    ForceAttemptHTTP2: true, // not attempted by default when TLSClientConfig is set
}
client := &http.Client{Transport: transport}
resp, err := client.Get("https://some.example/")

Or you can use this convenience method for a more opinionated HTTP client with various timeouts set, but do read the source code for this method so that you understand the implications:

manager, err := tlsconfig.NewManager(ctx, config.Client.TLS, tlsconfig.Options{
	IsClient: true,
})

client, err := manager.HTTPClient()

resp, err := client.Get("https://some.example/")

The testca/testca_test.go file contains an example that uses client certificates.

The Manager performs certificate reloads in the background. To keep track of what it is doing and see error messages, you can provide a logr.Logger interface in Options.Logr. The genericr module makes it easy to use custom logging logic here.

Stability

At this point we do not guarantee any API and config file format stability between versions. If you want to use it in your own project, pin it at a speciifc version. Once we are confident that our approach is sane, we will release a 1.0.0 version that will have these guarantees.