hashicorp/nomad · error

failed to build mount for resolv.conf: %v

Error message

failed to build mount for resolv.conf: %v

What it means

When a task block defines a dns block, Nomad generates a resolv.conf bind mount via resolvconf.GenerateDNSMount instead of passing Docker dns options. If the generator fails (invalid DNS configuration or filesystem problems in the task dir), the error is wrapped with this message and container creation stops.

Source

Thrown at drivers/docker/driver.go:1317

			hostConfig.Mounts = append(hostConfig.Mounts, mount.Mount{
				Target:   etcHostMount.TaskPath,
				Source:   etcHostMount.HostPath,
				Type:     "bind",
				ReadOnly: etcHostMount.Readonly,
				BindOptions: &mount.BindOptions{
					Propagation: mount.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
	if task.DNS != nil {
		dnsMount, err := resolvconf.GenerateDNSMount(task.TaskDir().Dir, task.DNS)
		if err != nil {
			return c, fmt.Errorf("failed to build mount for resolv.conf: %v", err)
		}
		hostConfig.Mounts = append(hostConfig.Mounts, mount.Mount{
			Target:   dnsMount.TaskPath,
			Source:   dnsMount.HostPath,
			Type:     "bind",
			ReadOnly: dnsMount.Readonly,
			BindOptions: &mount.BindOptions{
				Propagation: mount.Propagation(dnsMount.PropagationMode),
			},
		})
	} else {
		if len(driverConfig.DNSSearchDomains) > 0 {
			hostConfig.DNSSearch = driverConfig.DNSSearchDomains
		}
		if len(driverConfig.DNSOptions) > 0 {
			hostConfig.DNSOptions = driverConfig.DNSOptions
		}
		// set DNS servers

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Fix the task's dns block: supply valid servers/searches/options values
2. Check the task directory is writable and has free space
3. Read the wrapped %v inner error to pinpoint the exact DNS field or file failure
4. Remove the dns block to fall back to Docker's default DNS handling

Example fix

// before
task "web" {
  driver = "docker"
  dns {
    servers = [""]
  }
}
// after
task "web" {
  driver = "docker"
  dns {
    servers = ["1.1.1.1", "8.8.8.8"]
  }
}

for _, s := range task.DNS.Servers {
  if net.ParseIP(s) == nil && s != "" {
    return fmt.Errorf("invalid dns server: %q", s)
  }
}

err := client.StartTask(task); if err != nil && strings.Contains(err.Error(), "failed to build mount for resolv.conf") { logRootCause(err); fixDNSBlock() }

When it happens

Trigger: StartTask -> createContainerConfig with task.DNS != nil where GenerateDNSMount(task.TaskDir().Dir, task.DNS) errors, typically from malformed DNS config (empty servers, bad option format) or inability to prepare the file in the task dir.

Common situations: dns blocks with servers/searches/options entries containing invalid characters; read-only or full task directories; upgrades where stale alloc state blocks writing resolv.conf.

Related errors

• failed to build mount for /etc/hosts: %v
• invalid source, must be "" for tmpfs
• invalid mount type, must be "bind", "volume", "tmpfs": %q
• failed to reattach to docker logger process: %v
• failed to launch docker logger plugin: %v

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/45065c4a571be243. Report an issue: GitHub.