feat(etc-hosts) add support for manage_etc_hosts: localhost

This feature is based on https://github.com/number5/cloud-init/blob/master/doc/examples/cloud-config.txt#L447:L482

Documentation/cloud-config.md

@@ -260,3 +260,16 @@ Provide a list of objects with the following attributes:
 - **content**: Data to write at the provided `path`
 - **permissions**: String representing file permissions in octal notation (i.e. '0644')
 - **owner**: User and group that should own the file written to disk. This is equivalent to the `<user>:<group>` argument to `chown <user>:<group> <path>`.
+
+### manage_etc_hosts
+
+Have coreos-cloudinit manage your /etc/hosts file for local name resolution.
+The only supported value is "localhost" which will cause your system's hostname
+to resolve to "127.0.0.1".  This is helpful when the host does not have DNS
+infrastructure in place to resolve its own hostname, for example, when using Vagrant.
+
+```
+#cloud-config
+
+manage_etc_hosts: localhost
+```

initialize/config.go

@@ -17,9 +17,10 @@ type CloudConfig struct {
 		Units []system.Unit
 		OEM   OEMRelease
 	}
-	WriteFiles []system.File `yaml:"write_files"`
-	Hostname   string
-	Users      []system.User
+	WriteFiles     []system.File `yaml:"write_files"`
+	Hostname       string
+	Users          []system.User
+	ManageEtcHosts string `yaml:"manage_etc_hosts"`
 }
 
 func NewCloudConfig(contents string) (*CloudConfig, error) {
@@ -154,7 +155,7 @@ func Apply(cfg CloudConfig, env *Environment) error {
 				commands["systemd-networkd.service"] = "restart"
 			} else {
 				if unit.Command != "" {
-				    commands[unit.Name] = unit.Command
+					commands[unit.Name] = unit.Command
 				}
 			}
 		}
@@ -169,5 +170,15 @@ func Apply(cfg CloudConfig, env *Environment) error {
 		}
 	}
 
+	if cfg.ManageEtcHosts != "" {
+
+		if err := WriteEtcHosts(cfg.ManageEtcHosts, env.Root()); err != nil {
+			log.Fatalf("Failed to write /etc/hosts to filesystem: %v", err)
+		}
+
+		log.Printf("Wrote /etc/hosts file to filesystem")
+
+	}
+
 	return nil
 }

initialize/manage_etc_hosts.go

@@ -0,0 +1,44 @@
+package initialize
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path"
+
+	"github.com/coreos/coreos-cloudinit/system"
+)
+
+const DefaultIpv4Address = "127.0.0.1"
+
+func generateEtcHosts(option string) (out string, err error) {
+	if option != "localhost" {
+		return "", errors.New("Invalid option to manage_etc_hosts")
+	}
+
+	// use the operating system hostname
+	hostname, err := os.Hostname()
+	if err != nil {
+		return "", err
+	}
+
+	return fmt.Sprintf("%s %s", DefaultIpv4Address, hostname), nil
+
+}
+
+// Write an /etc/hosts file
+func WriteEtcHosts(option string, root string) error {
+
+	etcHosts, err := generateEtcHosts(option)
+	if err != nil {
+		return err
+	}
+
+	file := system.File{
+		Path:               path.Join(root, "etc", "hosts"),
+		RawFilePermissions: "0644",
+		Content:            etcHosts,
+	}
+
+	return system.WriteFile(&file)
+}

initialize/manage_etc_hosts_test.go

@@ -0,0 +1,76 @@
+package initialize
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"testing"
+)
+
+func TestCloudConfigManageEtcHosts(t *testing.T) {
+	contents := `
+manage_etc_hosts: localhost
+`
+	cfg, err := NewCloudConfig(contents)
+	if err != nil {
+		t.Fatalf("Encountered unexpected error: %v", err)
+	}
+
+	manageEtcHosts := cfg.ManageEtcHosts
+
+	if manageEtcHosts != "localhost" {
+		t.Errorf("ManageEtcHosts value is %q, expected 'localhost'", manageEtcHosts)
+	}
+}
+
+func TestManageEtcHostsInvalidValue(t *testing.T) {
+	dir, err := ioutil.TempDir(os.TempDir(), "coreos-cloudinit-")
+	if err != nil {
+		t.Fatalf("Unable to create tempdir: %v", err)
+	}
+	defer rmdir(dir)
+
+	if err := WriteEtcHosts("invalid", dir); err == nil {
+		t.Fatalf("WriteEtcHosts succeeded with invalid value: %v", err)
+	}
+}
+
+func TestEtcHostsWrittenToDisk(t *testing.T) {
+	dir, err := ioutil.TempDir(os.TempDir(), "coreos-cloudinit-")
+	if err != nil {
+		t.Fatalf("Unable to create tempdir: %v", err)
+	}
+	defer rmdir(dir)
+
+	if err := WriteEtcHosts("localhost", dir); err != nil {
+		t.Fatalf("WriteEtcHosts failed: %v", err)
+	}
+
+	fullPath := path.Join(dir, "etc", "hosts")
+
+	fi, err := os.Stat(fullPath)
+	if err != nil {
+		t.Fatalf("Unable to stat file: %v", err)
+	}
+
+	if fi.Mode() != os.FileMode(0644) {
+		t.Errorf("File has incorrect mode: %v", fi.Mode())
+	}
+
+	contents, err := ioutil.ReadFile(fullPath)
+	if err != nil {
+		t.Fatalf("Unable to read expected file: %v", err)
+	}
+
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Fatalf("Unable to read OS hostname: %v", err)
+	}
+
+	expect := fmt.Sprintf("%s %s", DefaultIpv4Address, hostname)
+
+	if string(contents) != expect {
+		t.Fatalf("File has incorrect contents")
+	}
+}