Common tls (#474)

* Add common TLS config routines

These routines can be used to load TLS configs based upon the
args in the Corefile.

* Add common routine for Corefile arg handling

Add the NewTLSConfigFromArgs routine so that you can just pass
in the Corefile args and get a tls.Config. This ensures the
parameters are handled consistently across middleware.

* Change to varargs style params

Change to use args ...string instead of []string. Add
documentation of what each call means.

middleware/tls/tls.go

@@ -0,0 +1,104 @@
+package tls
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"io/ioutil"
+)
+
+// NewTLSConfigFromArgs returns a TLS config based upon the passed
+// in list of arguments. Typically these come straight from the
+// Corefile.
+// no args
+//  - creates a Config with no cert and using system CAs
+//  - use for a client that talks to a server with a public signed cert (CA installed in system)
+//  - the client will not be authenticated by the server since there is no cert
+// one arg: the path to CA PEM file
+//  - creates a Config with no cert using a specific CA
+//  - use for a client that talks to a server with a private signed cert (CA not installed in system)
+//  - the client will not be authenticated by the server since there is no cert
+// two args: path to cert PEM file, the path to private key PEM file
+//  - creates a Config with a cert, using system CAs to validate the other end
+//  - use for:
+//    - a server; or,
+//    - a client that talks to a server with a public cert and needs certificate-based authentication
+//  - the other end will authenticate this end via the provided cert
+//  - the cert of the other end will be verified via system CAs
+// three args: path to cert PEM file, path to client private key PEM file, path to CA PEM file
+//  - creates a Config with the cert, using specified CA to validate the other end
+//  - use for:
+//    - a server; or,
+//    - a client that talks to a server with a privately signed cert and needs certificate-based
+//      authentication
+//  - the other end will authenticate this end via the provided cert
+//  - this end will verify the other end's cert using the specified CA
+func NewTLSConfigFromArgs(args ...string) (*tls.Config, error) {
+	var err error
+	var c *tls.Config
+	switch len(args) {
+	case 0:
+		// No client cert, use system CA
+		c, err = NewTLSClientConfig("")
+	case 1:
+		// No client cert, use specified CA
+		c, err = NewTLSClientConfig(args[0])
+	case 2:
+		// Client cert, use system CA
+		c, err = NewTLSConfig(args[0], args[1], "")
+	case 3:
+		// Client cert, use specified CA
+		c, err = NewTLSConfig(args[0], args[1], args[2])
+	default:
+		err = fmt.Errorf("Maximum of three arguments allowed for TLS config, found %d", len(args))
+	}
+	if err != nil {
+		return nil, err
+	}
+	return c, nil
+}
+
+// NewTLSConfig returns a TLS config that includes a certificate
+// Use for server TLS config or when using a client certificate
+// If caPath is empty, system CAs will be used
+func NewTLSConfig(certPath, keyPath, caPath string) (*tls.Config, error) {
+	cert, err := tls.LoadX509KeyPair(certPath, keyPath)
+	if err != nil {
+		return nil, fmt.Errorf("Could not load TLS cert: %s", err)
+	}
+
+	roots, err := loadRoots(caPath)
+	if err != nil {
+		return nil, err
+	}
+
+	return &tls.Config{Certificates: []tls.Certificate{cert}, RootCAs: roots}, nil
+}
+
+// NewTLSClientConfig returns a TLS config for a client connection
+// If caPath is empty, system CAs will be used
+func NewTLSClientConfig(caPath string) (*tls.Config, error) {
+	roots, err := loadRoots(caPath)
+	if err != nil {
+		return nil, err
+	}
+
+	return &tls.Config{RootCAs: roots}, nil
+}
+
+func loadRoots(caPath string) (*x509.CertPool, error) {
+	if caPath == "" {
+		return nil, nil
+	}
+
+	roots := x509.NewCertPool()
+	pem, err := ioutil.ReadFile(caPath)
+	if err != nil {
+		return nil, fmt.Errorf("Error reading %s: %s", caPath, err)
+	}
+	ok := roots.AppendCertsFromPEM(pem)
+	if !ok {
+		return nil, fmt.Errorf("Could not read root certs: %s", err)
+	}
+	return roots, nil
+}