Append extra_hosts from all tasks

.changelog/11074.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+docker: Append `extra_hosts` values from all tasks to `/etc/hosts`.
+```

drivers/docker/driver.go

@@ -962,7 +962,7 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 	// that comes from the pause container
 	if task.NetworkIsolation != nil && driverConfig.NetworkMode == "" {
 		etcHostMount, err := hostnames.GenerateEtcHostsMount(
-			task.AllocDir, task.NetworkIsolation, driverConfig.ExtraHosts)
+			task.Name, task.AllocDir, task.NetworkIsolation, driverConfig.ExtraHosts)
 		if err != nil {
 			return c, fmt.Errorf("failed to build mount for /etc/hosts: %v", err)
 		}

drivers/shared/hostnames/mount.go

@@ -11,12 +11,27 @@ import (
 	"github.com/hashicorp/nomad/plugins/drivers"
 )
 
+const baseHostsFile = `# this file was generated by Nomad
+127.0.0.1 localhost
+::1 localhost
+::1 ip6-localhost ip6-loopback
+fe00::0 ip6-localnet
+ff00::0 ip6-mcastprefix
+ff02::1 ip6-allnodes
+ff02::2 ip6-allrouters
+ff02::3 ip6-allhosts
+
+# this entry is the IP address and hostname of the allocation
+# shared with tasks in the task group's network
+%s %s
+`
+
 // GenerateEtcHostsMount writes a /etc/hosts file using the network spec's
 // hosts configuration, and returns a mount config so that task drivers can
 // bind-mount it into the resulting task's filesystem. The extraHosts
 // parameter is expected to be the same format as the extra_hosts field from
 // the Docker or containerd drivers: []string{"<hostname>:<ip address>"}
-func GenerateEtcHostsMount(taskDir string, conf *drivers.NetworkIsolationSpec, extraHosts []string) (*drivers.MountConfig, error) {
+func GenerateEtcHostsMount(taskName string, taskDir string, conf *drivers.NetworkIsolationSpec, extraHosts []string) (*drivers.MountConfig, error) {
 	if conf == nil || conf.Mode != drivers.NetIsolationModeGroup {
 		return nil, nil
 	}
@@ -26,23 +41,26 @@ func GenerateEtcHostsMount(taskDir string, conf *drivers.NetworkIsolationSpec, e
 	}
 
 	var content strings.Builder
-	fmt.Fprintf(&content, `# this file was generated by Nomad
-127.0.0.1 localhost
-::1 localhost
-::1 ip6-localhost ip6-loopback
-fe00::0 ip6-localnet
-ff00::0 ip6-mcastprefix
-ff02::1 ip6-allnodes
-ff02::2 ip6-allrouters
-ff02::3 ip6-allhosts
 
-# this entry is the IP address and hostname of the allocation
-# shared with tasks in the task group's network
-%s %s
-`, hostsCfg.Address, hostsCfg.Hostname)
+	path := filepath.Join(taskDir, "hosts")
+	_, err := os.Stat(path)
+	if err == nil {
+		// hosts file already exists so it was probably created by another task.
+		// Use it as the starting point.
+		hosts, err := ioutil.ReadFile(path)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read hosts file %s: %v", path, err)
+		}
+		content.Write(hosts)
+	} else if os.IsNotExist(err) {
+		// hosts file doesn't exist, so create a new one from the base content.
+		fmt.Fprintf(&content, baseHostsFile, hostsCfg.Address, hostsCfg.Hostname)
+	} else {
+		return nil, fmt.Errorf("failed to read hosts file info: %v", err)
+	}
 
 	if len(extraHosts) > 0 {
-		content.WriteString("\n# these entries are extra hosts added by the task config")
+		content.WriteString(fmt.Sprintf("\n# these entries are extra hosts added by the task %q", taskName))
 		for _, hostLine := range extraHosts {
 			hostsEntry := strings.SplitN(hostLine, ":", 2)
 			if len(hostsEntry) != 2 {
@@ -56,15 +74,9 @@ ff02::3 ip6-allhosts
 		content.WriteString("\n")
 	}
 
-	path := filepath.Join(taskDir, "hosts")
-
-	// tasks within an alloc should be able to share and modify the file, so
-	// only write to it if it doesn't exist
-	if _, err := os.Stat(path); os.IsNotExist(err) {
-		err := ioutil.WriteFile(path, []byte(content.String()), 0644)
-		if err != nil {
-			return nil, err
-		}
+	err = ioutil.WriteFile(path, []byte(content.String()), 0644)
+	if err != nil {
+		return nil, err
 	}
 
 	// Note that we're not setting readonly. The file is in the task dir

drivers/shared/hostnames/mount_unix_test.go

@@ -15,10 +15,14 @@ import (
 
 func TestGenerateEtcHostsMount(t *testing.T) {
 
+	type task struct {
+		spec       *drivers.NetworkIsolationSpec
+		extraHosts []string
+	}
+
 	testCases := []struct {
 		name        string
-		spec        *drivers.NetworkIsolationSpec
-		extraHosts  []string
+		tasks       map[string]task
 		expected    []string
 		expectedErr string
 	}{
@@ -27,15 +31,23 @@ func TestGenerateEtcHostsMount(t *testing.T) {
 		},
 		{
 			name: "no-hosts-config",
-			spec: &drivers.NetworkIsolationSpec{Mode: drivers.NetIsolationModeGroup},
+			tasks: map[string]task{
+				"test": {
+					spec: &drivers.NetworkIsolationSpec{Mode: drivers.NetIsolationModeGroup},
+				},
+			},
 		},
 		{
 			name: "base-case",
-			spec: &drivers.NetworkIsolationSpec{
-				Mode: drivers.NetIsolationModeGroup,
-				HostsConfig: &drivers.HostsConfig{
-					Address:  "192.168.1.1",
-					Hostname: "xyzzy",
+			tasks: map[string]task{
+				"test": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
 				},
 			},
 			expected: []string{
@@ -44,45 +56,93 @@ func TestGenerateEtcHostsMount(t *testing.T) {
 		},
 		{
 			name: "with-valid-extra-hosts",
-			spec: &drivers.NetworkIsolationSpec{
-				Mode: drivers.NetIsolationModeGroup,
-				HostsConfig: &drivers.HostsConfig{
-					Address:  "192.168.1.1",
-					Hostname: "xyzzy",
+			tasks: map[string]task{
+				"test": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
+					extraHosts: []string{
+						"apple:192.168.1.2",
+						"banana:2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+					},
 				},
 			},
-			extraHosts: []string{
-				"apple:192.168.1.2",
-				"banana:2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+			expected: []string{
+				"192.168.1.1 xyzzy",
+				"192.168.1.2 apple",
+				"2001:0db8:85a3:0000:0000:8a2e:0370:7334 banana",
+			},
+		},
+		{
+			name: "multiple-tasks-with-valid-extra-hosts",
+			tasks: map[string]task{
+				"test-1": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
+					extraHosts: []string{
+						"apple:192.168.1.2",
+						"banana:2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+					},
+				},
+				"test-2": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
+					extraHosts: []string{
+						"cherry:192.168.1.3",
+					},
+				},
 			},
 			expected: []string{
 				"192.168.1.1 xyzzy",
 				"192.168.1.2 apple",
 				"2001:0db8:85a3:0000:0000:8a2e:0370:7334 banana",
+				"192.168.1.3 cherry",
 			},
 		},
 		{
 			name: "invalid-extra-hosts-syntax",
-			spec: &drivers.NetworkIsolationSpec{
-				Mode: drivers.NetIsolationModeGroup,
-				HostsConfig: &drivers.HostsConfig{
-					Address:  "192.168.1.1",
-					Hostname: "xyzzy",
+			tasks: map[string]task{
+				"test": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
+					extraHosts: []string{"apple192.168.1.2"},
 				},
 			},
-			extraHosts:  []string{"apple192.168.1.2"},
 			expectedErr: "invalid hosts entry \"apple192.168.1.2\"",
 		},
 		{
 			name: "invalid-extra-hosts-bad-ip",
-			spec: &drivers.NetworkIsolationSpec{
-				Mode: drivers.NetIsolationModeGroup,
-				HostsConfig: &drivers.HostsConfig{
-					Address:  "192.168.1.1",
-					Hostname: "xyzzy",
+			tasks: map[string]task{
+				"test": {
+					spec: &drivers.NetworkIsolationSpec{
+						Mode: drivers.NetIsolationModeGroup,
+						HostsConfig: &drivers.HostsConfig{
+							Address:  "192.168.1.1",
+							Hostname: "xyzzy",
+						},
+					},
+					extraHosts: []string{"apple:192.168.1.256"},
 				},
 			},
-			extraHosts:  []string{"apple:192.168.1.256"},
 			expectedErr: "invalid IP address \"apple:192.168.1.256\"",
 		},
 	}
@@ -97,7 +157,10 @@ func TestGenerateEtcHostsMount(t *testing.T) {
 			require.NoError(err)
 			dest := filepath.Join(taskDir, "hosts")
 
-			got, err := GenerateEtcHostsMount(taskDir, tc.spec, tc.extraHosts)
+			var got *drivers.MountConfig
+			for taskName, taskConfig := range tc.tasks {
+				got, err = GenerateEtcHostsMount(taskName, taskDir, taskConfig.spec, taskConfig.extraHosts)
+			}
 
 			if tc.expectedErr != "" {
 				require.EqualError(err, tc.expectedErr)

website/content/docs/upgrade/upgrade-specific.mdx

@@ -44,6 +44,11 @@ from the task directory to `/etc/hosts` within the task.  In Nomad 1.1.3 the
 source for the bind mount was moved to the allocation directory so that it is
 shared between all tasks in an allocation.
 
+Please note that this change may prevent [`extra_hosts`] values to be correctly
+propagated to each task when there are multiple tasks within the same group,
+such as when using Consul Connect. Use a newer version of Nomad if this
+affects you.
+
 ## Nomad 1.1.0
 
 #### Enterprise licenses
@@ -1183,3 +1188,4 @@ deleted and then Nomad 0.3.0 can be launched.
 [cap_add_exec]: /docs/drivers/exec#cap_add
 [cap_drop_exec]: /docs/drivers/exec#cap_drop
 [`log_file`]: /docs/configuration#log_file
+[`extra_hosts`]: /docs/drivers/docker#extra_hosts