Add support for additional AWS authentication sources

AWS client tools commonly support passing credentials via
`AWS_ACCESS_KEY_ID` + `AWS_SECRET_ACCESS_KEY`, but supporting only this is
insufficient. For example, access key IDs provided by STS require passing in
`AWS_SECURITY_TOKEN` as a third value, and EC2 instances are often provided
dynamic credentials at runtime via the EC2 metadata service.

This changeset makes `lego` attempt to find credentials in the same way that
the `aws` CLI tool attempts to find credentials. The result is even less
auth code than before because `goamz` provides all this with `aws.GetAuth()`.

acme/dns_challenge_route53.go

@@ -15,30 +15,30 @@ type DNSProviderRoute53 struct {
 }
 
 // NewDNSProviderRoute53 returns a DNSProviderRoute53 instance with a configured route53 client.
-// Authentication is either done using the passed credentials or - when empty -
+// Authentication is either done using the passed credentials or - when empty - falling back to
-// using the environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY.
+// the customary AWS credential mechanisms, including the file refernced by $AWS_CREDENTIAL_FILE
+// (defaulting to $HOME/.aws/credentials) optionally scoped to $AWS_PROFILE, credentials
+// supplied by the environment variables AWS_ACCESS_KEY_ID + AWS_SECRET_ACCESS_KEY [ + AWS_SECURITY_TOKEN ],
+// and finally credentials available via the EC2 instance metadata service.
 func NewDNSProviderRoute53(awsAccessKey, awsSecretKey, awsRegionName string) (*DNSProviderRoute53, error) {
 	region, ok := aws.Regions[awsRegionName]
 	if !ok {
 		return nil, fmt.Errorf("Invalid AWS region name %s", awsRegionName)
 	}
 
-	var auth aws.Auth
+	// use aws.GetAuth, which tries really hard to find credentails:
-	// First try passed in credentials
+	//   - uses awsAccessKey and awsSecretKey, if provided
-	if awsAccessKey != "" && awsSecretKey != "" {
+	//   - uses AWS_PROFILE / AWS_CREDENTIAL_FILE, if provided
-		auth = aws.Auth{awsAccessKey, awsSecretKey, ""}
+	//   - uses AWS_ACCESS_KEY_ID + AWS_SECRET_ACCESS_KEY and optionally AWS_SECURITY_TOKEN, if provided
+	//   - uses EC2 instance metadata credentials (http://169.254.169.254/latest/meta-data/…), if available
+	//  ...and otherwise returns an error
+	if auth, err := aws.GetAuth(awsAccessKey, awsSecretKey); err != nil {
+		return nil, err
 	} else {
-		// try getting credentials from environment
-		envAuth, err := aws.EnvAuth()
-		if err != nil {
-			return nil, fmt.Errorf("AWS credentials missing")
-		}
-		auth = envAuth
-	}
-
 		client := route53.New(auth, region)
 		return &DNSProviderRoute53{client: client}, nil
 	}
+}
 
 // Present creates a TXT record using the specified parameters
 func (r *DNSProviderRoute53) Present(domain, token, keyAuth string) error {

acme/dns_challenge_route53_test.go

@@ -13,6 +13,8 @@ import (
 var (
 	route53Secret     string
 	route53Key        string
+	awsCredentialFile string
+	homeDir           string
 	testServer        *testutil.HTTPServer
 )
 
@@ -69,6 +71,8 @@ var serverResponseMap = testutil.ResponseMap{
 func init() {
 	route53Key = os.Getenv("AWS_ACCESS_KEY_ID")
 	route53Secret = os.Getenv("AWS_SECRET_ACCESS_KEY")
+	awsCredentialFile = os.Getenv("AWS_CREDENTIAL_FILE")
+	homeDir = os.Getenv("HOME")
 	testServer = testutil.NewHTTPServer()
 	testServer.Start()
 }
@@ -76,6 +80,8 @@ func init() {
 func restoreRoute53Env() {
 	os.Setenv("AWS_ACCESS_KEY_ID", route53Key)
 	os.Setenv("AWS_SECRET_ACCESS_KEY", route53Secret)
+	os.Setenv("AWS_CREDENTIAL_FILE", awsCredentialFile)
+	os.Setenv("HOME", homeDir)
 }
 
 func makeRoute53TestServer() *testutil.HTTPServer {
@@ -108,8 +114,10 @@ func TestNewDNSProviderRoute53ValidEnv(t *testing.T) {
 func TestNewDNSProviderRoute53MissingAuthErr(t *testing.T) {
 	os.Setenv("AWS_ACCESS_KEY_ID", "")
 	os.Setenv("AWS_SECRET_ACCESS_KEY", "")
+	os.Setenv("AWS_CREDENTIAL_FILE", "") // in case test machine has this variable set
+	os.Setenv("HOME", "/")               // in case test machine has ~/.aws/credentials
 	_, err := NewDNSProviderRoute53("", "", "us-east-1")
-	assert.EqualError(t, err, "AWS credentials missing")
+	assert.EqualError(t, err, "No valid AWS authentication found")
 	restoreRoute53Env()
 }