docker: generate /etc/hosts file for bridge network mode

When `network.mode = "bridge"`, we create a pause container in Docker with no
networking so that we have a process to hold the network namespace we create
in Nomad. The default `/etc/hosts` file of that pause container is then used
for all the Docker tasks that share that network namespace. Some applications
rely on this file being populated.

This changeset generates a `/etc/hosts` file and bind-mounts it to the
container when Nomad owns the network, so that the container's hostname has an
IP in the file as expected.

client/allocrunner/network_hook.go

@@ -9,6 +9,8 @@ import (
 	"github.com/hashicorp/nomad/plugins/drivers"
 )
 
+const dockerNetSpecLabelKey = "docker_sandbox_container_id"
+
 type networkIsolationSetter interface {
 	SetNetworkIsolation(*drivers.NetworkIsolationSpec)
 }
@@ -106,7 +108,10 @@ func (h *networkHook) Prerun() error {
 		if err != nil {
 			return fmt.Errorf("failed to configure networking for alloc: %v", err)
 		}
-
+		h.spec.HostsConfig = &drivers.HostsConfig{
+			Address:  status.Address,
+			Hostname: spec.Labels[dockerNetSpecLabelKey][:12], // TODO: make safe
+		}
 		h.networkStatusSetter.SetNetworkStatus(status)
 	}
 	return nil

drivers/docker/driver.go

@@ -24,6 +24,7 @@ import (
 	"github.com/hashicorp/nomad/drivers/docker/docklog"
 	"github.com/hashicorp/nomad/drivers/shared/capabilities"
 	"github.com/hashicorp/nomad/drivers/shared/eventer"
+	"github.com/hashicorp/nomad/drivers/shared/hostnames"
 	"github.com/hashicorp/nomad/drivers/shared/resolvconf"
 	nstructs "github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/plugins/base"
@@ -954,6 +955,30 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 		hostConfig.Mounts = append(hostConfig.Mounts, *hm)
 	}
 
+	// Setup /etc/hosts
+	// If the task's network_mode is unset our hostname and IP will come from
+	// the Nomad-owned network (if in use), so we need to generate an
+	// /etc/hosts file that matches the network rather than the default one
+	// that comes from the pause container
+	if task.NetworkIsolation != nil && driverConfig.NetworkMode == "" {
+		etcHostMount, err := hostnames.GenerateEtcHostsMount(
+			task.TaskDir().Dir, task.NetworkIsolation, driverConfig.ExtraHosts)
+		if err != nil {
+			return c, fmt.Errorf("failed to build mount for /etc/hosts: %v", err)
+		}
+		if etcHostMount != nil {
+			hostConfig.Mounts = append(hostConfig.Mounts, docker.HostMount{
+				Target:   etcHostMount.TaskPath,
+				Source:   etcHostMount.HostPath,
+				Type:     "bind",
+				ReadOnly: etcHostMount.Readonly,
+				BindOptions: &docker.BindOptions{
+					Propagation: etcHostMount.PropagationMode,
+				},
+			})
+		}
+	}
+
 	// Setup DNS
 	// If task DNS options are configured Nomad will manage the resolv.conf file
 	// Docker driver dns options are not compatible with task dns options

drivers/shared/hostnames/mount.go

@@ -0,0 +1,64 @@
+package hostnames
+
+import (
+	"fmt"
+	"io/ioutil"
+	"path/filepath"
+
+	"github.com/hashicorp/nomad/plugins/drivers"
+)
+
+// 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.
+func GenerateEtcHostsMount(taskDir string, conf *drivers.NetworkIsolationSpec, extraHosts []string) (*drivers.MountConfig, error) {
+	if conf == nil || conf.Mode != drivers.NetIsolationModeGroup {
+		return nil, nil
+	}
+	hostsCfg := conf.HostsConfig
+	if hostsCfg == nil || hostsCfg.Address == "" || hostsCfg.Hostname == "" {
+		return nil, nil
+	}
+
+	content := fmt.Sprintf(`# 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)
+
+	if len(extraHosts) > 0 {
+		content += "\n# these entries are extra hosts added by the task config"
+		for _, hostLine := range extraHosts {
+			content += "\n" + hostLine
+		}
+		content += "\n"
+	}
+
+	path := filepath.Join(taskDir, "hosts")
+	err := ioutil.WriteFile(path, []byte(content), 0755)
+	if err != nil {
+		return nil, err
+	}
+
+	// Note that we're not setting readonly. The file is in the task dir
+	// anyways, so this lets the task overwrite its own hosts file if the
+	// application knows better than Nomad here. Task drivers may override
+	// this behavior.
+	mount := &drivers.MountConfig{
+		TaskPath:        "/etc/hosts",
+		HostPath:        path,
+		Readonly:        false,
+		PropagationMode: "private",
+	}
+
+	return mount, nil
+}

plugins/drivers/driver.go

@@ -200,9 +200,15 @@ var (
 )
 
 type NetworkIsolationSpec struct {
-	Mode   NetIsolationMode
-	Path   string
-	Labels map[string]string
+	Mode        NetIsolationMode
+	Path        string
+	Labels      map[string]string
+	HostsConfig *HostsConfig
+}
+
+type HostsConfig struct {
+	Hostname string
+	Address  string
 }
 
 // MountConfigSupport is an enum that defaults to "all" for backwards