Fix elasticsearch url parsing

use regex to check if host given contains the protocol scheme. If scheme is
missing, prepend before attempting to parse the complete URL

CHANGELOG.asciidoc

@@ -38,6 +38,7 @@ https://github.com/elastic/beats/compare/v5.1.1...master[Check the HEAD diff]
 *Affecting all Beats*
 
 - Add `_id`, `_type`, `_index` and `_score` fields in the generated index pattern. {pull}3282[3282]
+- Fix potential elasticsearch output URL parsing error if protocol scheme is missing. {pull}3671[3671]
 
 *Filebeat*
 - Always use absolute path for event and registry. {pull}3328[3328]

libbeat/outputs/elasticsearch/url.go

@@ -4,9 +4,12 @@ import (
 	"fmt"
 	"net"
 	"net/url"
+	"regexp"
 	"strings"
 )
 
+var hasScheme = regexp.MustCompile(`^([a-z][a-z0-9+\-.]*)://`)
+
 // Creates the url based on the url configuration.
 // Adds missing parts with defaults (scheme, host, port)
 func MakeURL(defaultScheme string, defaultPath string, rawURL string) (string, error) {
@@ -15,6 +18,10 @@ func MakeURL(defaultScheme string, defaultPath string, rawURL string) (string, e
 		defaultScheme = "http"
 	}
 
+	if !hasScheme.MatchString(rawURL) {
+		rawURL = fmt.Sprintf("%v://%v", defaultScheme, rawURL)
+	}
+
 	addr, err := url.Parse(rawURL)
 	if err != nil {
 		return "", err
@@ -24,22 +31,6 @@ func MakeURL(defaultScheme string, defaultPath string, rawURL string) (string, e
 	host := addr.Host
 	port := "9200"
 
-	// sanitize parse errors if url does not contain scheme
-	// if parse url looks funny, prepend schema and try again:
-	if addr.Scheme == "" || (addr.Host == "" && addr.Path == "" && addr.Opaque != "") {
-		rawURL = fmt.Sprintf("%v://%v", defaultScheme, rawURL)
-		if tmpAddr, err := url.Parse(rawURL); err == nil {
-			addr = tmpAddr
-			scheme = addr.Scheme
-			host = addr.Host
-		} else {
-			// If url doesn't have a scheme, host is written into path. For example: 192.168.3.7
-			scheme = defaultScheme
-			host = addr.Path
-			addr.Path = ""
-		}
-	}
-
 	if host == "" {
 		host = "localhost"
 	} else {

libbeat/outputs/elasticsearch/url_test.go

@@ -79,6 +79,8 @@ func TestGetUrl(t *testing.T) {
 		"http://localhost/":    "http://localhost:9200/",
 
 		// no schema + hostname
+		"33f3600fd5c1bb599af557c36a4efb08.host":       "http://33f3600fd5c1bb599af557c36a4efb08.host:9200",
+		"33f3600fd5c1bb599af557c36a4efb08.host:12345": "http://33f3600fd5c1bb599af557c36a4efb08.host:12345",
 		"localhost":        "http://localhost:9200",
 		"localhost:80":     "http://localhost:80",
 		"localhost:80/":    "http://localhost:80/",