diff --git a/shared/definition.go b/shared/definition.go
index 5d70918..64bfd0d 100644
--- a/shared/definition.go
+++ b/shared/definition.go
@@ -32,7 +32,7 @@ type DefinitionImage struct {
 // A DefinitionSource specifies the download type and location
 type DefinitionSource struct {
 	Downloader string   `yaml:"downloader"`
-	URL        string   `yaml:"url"`
+	URL        string   `yaml:"url,omitempty"`
 	Keys       []string `yaml:"keys,omitempty"`
 	Keyserver  string   `yaml:"keyserver,omitempty"`
 	Variant    string   `yaml:"variant,omitempty"`
@@ -122,10 +122,6 @@ func ValidateDefinition(def Definition) error {
 		return fmt.Errorf("source.downloader must be one of %v", validDownloaders)
 	}
 
-	if strings.TrimSpace(def.Source.URL) == "" {
-		return errors.New("source.url may not be empty")
-	}
-
 	validManagers := []string{
 		"apk",
 		"apt",
diff --git a/shared/definition_test.go b/shared/definition_test.go
index 32b1886..ded182f 100644
--- a/shared/definition_test.go
+++ b/shared/definition_test.go
@@ -65,6 +65,23 @@ func TestValidateDefinition(t *testing.T) {
 			"",
 			false,
 		},
+		{
+			"valid Defintion without source.url",
+			Definition{
+				Image: DefinitionImage{
+					Distribution: "ubuntu",
+					Release:      "artful",
+				},
+				Source: DefinitionSource{
+					Downloader: "debootstrap",
+				},
+				Packages: DefinitionPackages{
+					Manager: "apt",
+				},
+			},
+			"",
+			false,
+		},
 		{
 			"empty image.distribution",
 			Definition{},
@@ -85,20 +102,6 @@ func TestValidateDefinition(t *testing.T) {
 			"source.downloader must be one of .+",
 			true,
 		},
-		{
-			"empty source.url",
-			Definition{
-				Image: DefinitionImage{
-					Distribution: "ubuntu",
-					Release:      "artful",
-				},
-				Source: DefinitionSource{
-					Downloader: "debootstrap",
-				},
-			},
-			"source.url may not be empty",
-			true,
-		},
 		{
 			"invalid package.manager",
 			Definition{