From a66034bb3ae058630f3cf4d6cb5770bee599409b Mon Sep 17 00:00:00 2001
From: Tim Gross <tgross@hashicorp.com>
Date: Wed, 30 Jun 2021 11:10:04 -0400
Subject: [PATCH] docker: move host path for hosts file mount to alloc dir
 (#10823)

In Nomad 1.1.1 we generate a hosts file based on the Nomad-owned network
namespace, rather than using the default hosts file from the pause
container. This hosts file should be shared between tasks in the same
allocation so that tasks can update the file and have the results propagated
between tasks.
---
 .changelog/10823.txt                              |  3 +++
 drivers/docker/driver.go                          |  2 +-
 drivers/shared/hostnames/mount.go                 | 12 +++++++++---
 website/content/docs/upgrade/upgrade-specific.mdx | 10 ++++++++++
 4 files changed, 23 insertions(+), 4 deletions(-)
 create mode 100644 .changelog/10823.txt

diff --git a/.changelog/10823.txt b/.changelog/10823.txt
new file mode 100644
index 00000000000..76535fae27e
--- /dev/null
+++ b/.changelog/10823.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+docker: Moved the generated `/etc/hosts` file's mount source to the allocation directory so that it can be shared between tasks of an allocation.
+```
diff --git a/drivers/docker/driver.go b/drivers/docker/driver.go
index 1f567ba5bb5..9682234607b 100644
--- a/drivers/docker/driver.go
+++ b/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.TaskDir().Dir, task.NetworkIsolation, driverConfig.ExtraHosts)
+			task.AllocDir, task.NetworkIsolation, driverConfig.ExtraHosts)
 		if err != nil {
 			return c, fmt.Errorf("failed to build mount for /etc/hosts: %v", err)
 		}
diff --git a/drivers/shared/hostnames/mount.go b/drivers/shared/hostnames/mount.go
index 32e23240c14..6a468097549 100644
--- a/drivers/shared/hostnames/mount.go
+++ b/drivers/shared/hostnames/mount.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -56,9 +57,14 @@ ff02::3 ip6-allhosts
 	}
 
 	path := filepath.Join(taskDir, "hosts")
-	err := ioutil.WriteFile(path, []byte(content.String()), 0644)
-	if err != nil {
-		return nil, err
+
+	// 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
+		}
 	}
 
 	// Note that we're not setting readonly. The file is in the task dir
diff --git a/website/content/docs/upgrade/upgrade-specific.mdx b/website/content/docs/upgrade/upgrade-specific.mdx
index 2a8afa2110e..09dedcf8599 100644
--- a/website/content/docs/upgrade/upgrade-specific.mdx
+++ b/website/content/docs/upgrade/upgrade-specific.mdx
@@ -13,6 +13,16 @@ upgrade. However, specific versions of Nomad may have more details provided for
 their upgrades as a result of new features or changed behavior. This page is
 used to document those details separately from the standard upgrade flow.
 
+## Nomad 1.1.3
+
+#### Docker Driver
+
+Starting in Nomad 1.1.2, task groups with `network.mode = "bridge"` generated
+a hosts file in Docker containers. This generated hosts file was bind-mounted
+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.
+
 ## Nomad 1.1.0
 
 #### Enterprise licenses