Etcd DNS discovery (#2521)

* etcd: Add discovery_srv option

physical/etcd.go

@@ -3,7 +3,10 @@ package physical
 import (
 	"context"
 	"errors"
+	"fmt"
+	"net/url"
 	"os"
+	"strings"
 
 	"github.com/coreos/etcd/client"
 	"github.com/coreos/go-semver/semver"
@@ -13,6 +16,7 @@ import (
 var (
 	EtcdSyncConfigError          = errors.New("client setup failed: unable to parse etcd sync field in config")
 	EtcdSyncClusterError         = errors.New("client setup failed: unable to sync etcd cluster")
+	EtcdMultipleBootstrapError   = errors.New("client setup failed: multiple discovery or bootstrap flags specified, use either \"address\" or \"discovery_srv\"")
 	EtcdAddressError             = errors.New("client setup failed: address must be valid URL (ex. 'scheme://host:port')")
 	EtcdSemaphoreKeysEmptyError  = errors.New("lock queue is empty")
 	EtcdLockHeldError            = errors.New("lock already held")
@@ -95,3 +99,47 @@ func getEtcdAPIVersion(c client.Client) (string, error) {
 
 	return "3", nil
 }
+
+// Retrieves the config option in order of priority:
+//  1. The named environment variable if it exist
+//  2. The key in the config map
+func getEtcdOption(conf map[string]string, confKey, envVar string) (string, bool) {
+	confVal, inConf := conf[confKey]
+	envVal, inEnv := os.LookupEnv(envVar)
+	if inEnv {
+		return envVal, true
+	}
+	return confVal, inConf
+}
+
+func getEtcdEndpoints(conf map[string]string) ([]string, error) {
+	address, staticBootstrap := getEtcdOption(conf, "address", "ETCD_ADDR")
+	domain, useSrv := getEtcdOption(conf, "discovery_srv", "ETCD_DISCOVERY_SRV")
+	if useSrv && staticBootstrap {
+		return nil, EtcdMultipleBootstrapError
+	}
+
+	if staticBootstrap {
+		endpoints := strings.Split(address, Etcd2MachineDelimiter)
+		// Verify that the machines are valid URLs
+		for _, e := range endpoints {
+			u, urlErr := url.Parse(e)
+			if urlErr != nil || u.Scheme == "" {
+				return nil, EtcdAddressError
+			}
+		}
+		return endpoints, nil
+	}
+
+	if useSrv {
+		discoverer := client.NewSRVDiscover()
+		endpoints, err := discoverer.Discover(domain)
+		if err != nil {
+			return nil, fmt.Errorf("failed to discover etcd endpoints through SRV discovery: %v", err)
+		}
+		return endpoints, nil
+	}
+
+	// Set a default endpoints list if no option was set
+	return []string{"http://127.0.0.1:2379"}, nil
+}

physical/etcd2.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"encoding/base64"
 	"fmt"
-	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -118,23 +117,9 @@ func newEtcd2Backend(conf map[string]string, logger log.Logger) (Backend, error)
 }
 
 func newEtcdV2Client(conf map[string]string) (client.Client, error) {
-	// Set a default machines list and check for an overriding address value.
-	machines := "http://127.0.0.1:2379"
-	if address, ok := conf["address"]; ok {
-		machines = address
-	}
-	machinesEnv := os.Getenv("ETCD_ADDR")
-	if machinesEnv != "" {
-		machines = machinesEnv
-	}
-	machinesParsed := strings.Split(machines, Etcd2MachineDelimiter)
-
-	// Verify that the machines are valid URLs
-	for _, machine := range machinesParsed {
-		u, urlErr := url.Parse(machine)
-		if urlErr != nil || u.Scheme == "" {
-			return nil, EtcdAddressError
-		}
+	endpoints, err := getEtcdEndpoints(conf)
+	if err != nil {
+		return nil, err
 	}
 
 	// Create a new client from the supplied address and attempt to sync with the
@@ -160,7 +145,7 @@ func newEtcdV2Client(conf map[string]string) (client.Client, error) {
 	}
 
 	cfg := client.Config{
-		Endpoints: machinesParsed,
+		Endpoints: endpoints,
 		Transport: cTransport,
 	}
 

physical/etcd3.go

@@ -48,10 +48,9 @@ func newEtcd3Backend(conf map[string]string, logger log.Logger) (Backend, error)
 		path = "/" + path
 	}
 
-	// Set a default machines list and check for an overriding address value.
-	endpoints := []string{"http://127.0.0.1:2379"}
-	if address, ok := conf["address"]; ok {
-		endpoints = strings.Split(address, ",")
+	endpoints, err := getEtcdEndpoints(conf)
+	if err != nil {
+		return nil, err
 	}
 
 	cfg := clientv3.Config{

website/source/docs/configuration/storage/etcd.html.md

@@ -36,6 +36,10 @@ storage "etcd" {
   Etcd instances as a comma-separated list. This can also be provided via the
   environment variable `ETCD_ADDR`.
 
+- `discovery_srv` `(string: "example.com")` - Specifies the domain name to
+  query for SRV records describing cluster endpoints. This can also be provided
+  via the environment variable `ETCD_DISCOVERY_SRV`.
+
 - `etcd_api` `(string: "<varies>")` – Specifies the version of the API to
   communicate with. By default, this is derived automatically. If the cluster
   version is 3.1+ and there has been no data written using the v2 API, the
@@ -89,6 +93,18 @@ discussed in more detail in the [HA concepts page](/docs/concepts/ha.html).
 
 ## `etcd` Examples
 
+### DNS Discovery of cluster members
+
+This example configures vault to discover the Etcd cluster members via SRV
+records as outlined in the
+[DNS Discovery protocol documentation][dns discovery].
+
+```hcl
+storage "etcd" {
+  discovery_srv = "example.com"
+}
+```
+
 ### Custom Authentication
 
 This example shows connecting to the Etcd cluster using a username and password.
@@ -122,3 +138,4 @@ storage "etcd" {
 ```
 
 [etcd]: https://coreos.com/etcd "Etcd by CoreOS"
+[dns discovery]: https://coreos.com/etcd/docs/latest/op-guide/clustering.html#dns-discovery "Etcd cluster DNS Discovery"