203 lines
6 KiB
Plaintext
203 lines
6 KiB
Plaintext
---
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layout: docs
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page_title: 'Drivers: Exec'
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description: The Exec task driver is used to run binaries using OS isolation primitives.
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---
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# Isolated Fork/Exec Driver
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Name: `exec`
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The `exec` driver is used to simply execute a particular command for a task.
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However, unlike [`raw_exec`](/docs/drivers/raw_exec) it uses the underlying isolation
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primitives of the operating system to limit the task's access to resources. While
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simple, since the `exec` driver can invoke any command, it can be used to call
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scripts or other wrappers which provide higher level features.
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## Task Configuration
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```hcl
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task "webservice" {
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driver = "exec"
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config {
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command = "my-binary"
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args = ["-flag", "1"]
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}
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}
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```
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The `exec` driver supports the following configuration in the job spec:
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- `command` - The command to execute. Must be provided. If executing a binary
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that exists on the host, the path must be absolute and within the task's
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[chroot](#chroot). If executing a binary that is downloaded from
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an [`artifact`](/docs/job-specification/artifact), the path can be
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relative from the allocations's root directory.
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- `args` - (Optional) A list of arguments to the `command`. References
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to environment variables or any [interpretable Nomad
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variables](/docs/runtime/interpolation) will be interpreted before
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launching the task.
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- `pid_mode` - (Optional) Set to `"private"` to enable PID namespace isolation for
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this task, or `"host"` to disable isolation. If left unset, the behavior is
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determined from the [`default_pid_mode`][default_pid_mode] in plugin configuration.
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!> **Warning:** If set to `"host"`, other processes running as the same user will
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be able to access sensitive process information like environment variables.
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- `ipc_mode` - (Optional) Set to `"private"` to enable IPC namespace isolation for
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this task, or `"host"` to disable isolation. If left unset, the behavior is
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determined from the [`default_ipc_mode`][default_ipc_mode] in plugin configuration.
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!> **Warning:** If set to `"host"`, other processes running as the same user will be
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able to make use of IPC features, like sending unexpected POSIX signals.
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## Examples
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To run a binary present on the Node:
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```hcl
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task "example" {
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driver = "exec"
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config {
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# When running a binary that exists on the host, the path must be absolute.
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command = "/bin/sleep"
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args = ["1"]
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}
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}
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```
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To execute a binary downloaded from an
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[`artifact`](/docs/job-specification/artifact):
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```hcl
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task "example" {
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driver = "exec"
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config {
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command = "name-of-my-binary"
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}
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artifact {
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source = "https://internal.file.server/name-of-my-binary"
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options {
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checksum = "sha256:abd123445ds4555555555"
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}
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}
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}
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```
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## Capabilities
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The `exec` driver implements the following [capabilities](/docs/internals/plugins/task-drivers#capabilities-capabilities-error).
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| Feature | Implementation |
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| -------------------- | -------------- |
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| `nomad alloc signal` | true |
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| `nomad alloc exec` | true |
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| filesystem isolation | chroot |
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| network isolation | host, group |
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| volume mounting | all |
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## Client Requirements
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The `exec` driver can only be run when on Linux and running Nomad as root.
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`exec` is limited to this configuration because currently isolation of resources
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is only guaranteed on Linux. Further, the host must have cgroups mounted properly
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in order for the driver to work.
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If you are receiving the error:
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```
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* Constraint "missing drivers" filtered <> nodes
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```
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and using the exec driver, check to ensure that you are running Nomad as root.
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This also applies for running Nomad in -dev mode.
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## Plugin Options
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- `default_pid_mode` `(string: optional)` - Defaults to `"private"`. Set to
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`"private"` to enable PID namespace isolation for tasks by default, or `"host"` to
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disable isolation.
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!> **Warning:** If set to `"host"`, other processes running as the same user will
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be able to access sensitive process information like environment variables.
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- `default_ipc_mode` `(string: optional)` - Defaults to `"private"`. Set to
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`"private"` to enable IPC namespace isolation for tasks by default,
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or `"host"` to disable isolation.
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!> **Warning:** If set to `"host"`, other processes running as the same user will be
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able to make use of IPC features, like sending unexpected POSIX signals.
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- `no_pivot_root` `(bool: optional)` - Defaults to `false`. When `true`, the driver uses `chroot`
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for file system isolation without `pivot_root`. This is useful for systems
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where the root is on a ramdisk.
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## Client Attributes
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The `exec` driver will set the following client attributes:
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- `driver.exec` - This will be set to "1", indicating the driver is available.
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## Resource Isolation
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The resource isolation provided varies by the operating system of
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the client and the configuration.
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On Linux, Nomad will use cgroups, and a chroot to isolate the resources of a
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process and as such the Nomad agent must be run as root. Some Linux
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distributions do not boot with all required cgroups enabled by default. You
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can see which cgroups are enabled by reading `/proc/cgroups`, and verifying
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that all the following cgroups are enabled:
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```
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$ awk '{print $1 " " $4}' /proc/cgroups
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#subsys_name enabled
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cpuset 1
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cpu 1
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cpuacct 1
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blkio 1
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memory 1
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devices 1
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freezer 1
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net_cls 1
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perf_event 1
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net_prio 1
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hugetlb 1
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pids 1
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```
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### Chroot
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The chroot is populated with data in the following directories from the host
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machine:
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```
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[
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"/bin",
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"/etc",
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"/lib",
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"/lib32",
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"/lib64",
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"/run/resolvconf",
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"/sbin",
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"/usr",
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]
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```
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The task's chroot is populated by linking or copying the data from the host into
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the chroot. Note that this can take considerable disk space. Since Nomad v0.5.3,
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the client manages garbage collection locally which mitigates any issue this may
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create.
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This list is configurable through the agent client
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[configuration file](/docs/configuration/client#chroot_env).
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[default_pid_mode]: /docs/drivers/exec#default_pid_mode
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[default_ipc_mode]: /docs/drivers/exec#default_ipc_mode
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