provider/aws: Refactor AWS Authentication chain

builtin/providers/aws/config.go

@@ -3,14 +3,19 @@ package aws
 import (
 	"fmt"
 	"log"
+	"net/http"
+	"os"
 	"strings"
+	"time"
 
 	"github.com/hashicorp/go-cleanhttp"
 	"github.com/hashicorp/go-multierror"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"
-	"github.com/aws/aws-sdk-go/aws/credentials"
+	awsCredentials "github.com/aws/aws-sdk-go/aws/credentials"
+	"github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds"
+	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/autoscaling"
 	"github.com/aws/aws-sdk-go/service/cloudformation"
@@ -104,9 +109,14 @@ func (c *Config) Client() (interface{}, error) {
 		client.region = c.Region
 
 		log.Println("[INFO] Building AWS auth structure")
-		// We fetched all credential sources in Provider. If they are
-		// available, they'll already be in c. See Provider definition.
-		creds := credentials.NewStaticCredentials(c.AccessKey, c.SecretKey, c.Token)
+		creds := getCreds(c.AccessKey, c.SecretKey, c.Token)
+		// Call Get to check for credential provider. If nothing found, we'll get an
+		// error, and we can present it nicely to the user
+		_, err = creds.Get()
+		if err != nil {
+			errs = append(errs, fmt.Errorf("Error loading credentials for AWS Provider: %s", err))
+			return nil, &multierror.Error{Errors: errs}
+		}
 		awsConfig := &aws.Config{
 			Credentials: creds,
 			Region:      aws.String(c.Region),
@@ -118,7 +128,7 @@ func (c *Config) Client() (interface{}, error) {
 		sess := session.New(awsConfig)
 		client.iamconn = iam.New(sess)
 
-		err := c.ValidateCredentials(client.iamconn)
+		err = c.ValidateCredentials(client.iamconn)
 		if err != nil {
 			errs = append(errs, err)
 		}
@@ -316,3 +326,53 @@ func (c *Config) ValidateAccountId(iamconn *iam.IAM) error {
 
 	return nil
 }
+
+// This function is responsible for reading credentials from the
+// environment in the case that they're not explicitly specified
+// in the Terraform configuration.
+func getCreds(key, secret, token string) *awsCredentials.Credentials {
+	// build a chain provider, lazy-evaulated by aws-sdk
+	providers := []awsCredentials.Provider{
+		&awsCredentials.StaticProvider{Value: awsCredentials.Value{
+			AccessKeyID:     key,
+			SecretAccessKey: secret,
+			SessionToken:    token,
+		}},
+		&awsCredentials.EnvProvider{},
+		&awsCredentials.SharedCredentialsProvider{},
+	}
+
+	// We only look in the EC2 metadata API if we can connect
+	// to the metadata service within a reasonable amount of time
+	metadataURL := os.Getenv("AWS_METADATA_URL")
+	if metadataURL == "" {
+		metadataURL = "http://169.254.169.254:80/latest"
+	}
+	c := http.Client{
+		Timeout: 100 * time.Millisecond,
+	}
+
+	r, err := c.Get(metadataURL)
+	// Flag to determine if we should add the EC2Meta data provider. Default false
+	var useIAM bool
+	if err == nil {
+		// AWS will add a "Server: EC2ws" header value for the metadata request. We
+		// check the headers for this value to ensure something else didn't just
+		// happent to be listening on that IP:Port
+		if r.Header["Server"] != nil && strings.Contains(r.Header["Server"][0], "EC2") {
+			useIAM = true
+		}
+	}
+
+	if useIAM {
+		log.Printf("[DEBUG] EC2 Metadata service found, adding EC2 Role Credential Provider")
+		providers = append(providers, &ec2rolecreds.EC2RoleProvider{
+			Client: ec2metadata.New(session.New(&aws.Config{
+				Endpoint: aws.String(metadataURL),
+			})),
+		})
+	} else {
+		log.Printf("[DEBUG] EC2 Metadata service not found, not adding EC2 Role Credential Provider")
+	}
+	return awsCredentials.NewChainCredentials(providers)
+}