diff --git a/config/validate/rules.go b/config/validate/rules.go
index 459877d..416e6ec 100644
--- a/config/validate/rules.go
+++ b/config/validate/rules.go
@@ -18,6 +18,7 @@ package validate
 
 import (
 	"fmt"
+	"net/url"
 	"path"
 	"reflect"
 	"strings"
@@ -29,11 +30,24 @@ type rule func(config node, report *Report)
 
 // Rules contains all of the validation rules.
 var Rules []rule = []rule{
+	checkDiscoveryUrl,
 	checkStructure,
 	checkValidity,
 	checkWriteFiles,
 }
 
+// checkDiscoveryUrl verifies that the string is a valid url.
+func checkDiscoveryUrl(cfg node, report *Report) {
+	c := cfg.Child("coreos").Child("etcd").Child("discovery")
+	if !c.IsValid() {
+		return
+	}
+
+	if _, err := url.ParseRequestURI(c.String()); err != nil {
+		report.Warning(c.line, "discovery URL is not valid")
+	}
+}
+
 // checkStructure compares the provided config to the empty config.CloudConfig
 // structure. Each node is checked to make sure that it exists in the known
 // structure and that its type is compatible.
diff --git a/config/validate/rules_test.go b/config/validate/rules_test.go
index 55f4387..c767dff 100644
--- a/config/validate/rules_test.go
+++ b/config/validate/rules_test.go
@@ -21,6 +21,39 @@ import (
 	"testing"
 )
 
+func TestCheckDiscoveryUrl(t *testing.T) {
+	tests := []struct {
+		config string
+
+		entries []Entry
+	}{
+		{},
+		{
+			config: "coreos:\n  etcd:\n    discovery: https://discovery.etcd.io/00000000000000000000000000000000",
+		},
+		{
+			config: "coreos:\n  etcd:\n    discovery: http://custom.domain/mytoken",
+		},
+		{
+			config:  "coreos:\n  etcd:\n    discovery: disco",
+			entries: []Entry{{entryWarning, "discovery URL is not valid", 3}},
+		},
+	}
+
+	for i, tt := range tests {
+		r := Report{}
+		n, err := parseCloudConfig([]byte(tt.config), &r)
+		if err != nil {
+			panic(err)
+		}
+		checkDiscoveryUrl(n, &r)
+
+		if e := r.Entries(); !reflect.DeepEqual(tt.entries, e) {
+			t.Errorf("bad report (%d, %q): want %#v, got %#v", i, tt.config, tt.entries, e)
+		}
+	}
+}
+
 func TestCheckStructure(t *testing.T) {
 	tests := []struct {
 		config string