# Custom bootstrap actions If you define the [`HeadNode`](HeadNode-v3.md) / [`CustomActions`](HeadNode-v3.md#HeadNode-v3-CustomActions) / [`OnNodeStart`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeStart) configuration settings, AWS ParallelCluster runs arbitrary code immediately after the node starts. If you define the [`HeadNode`](HeadNode-v3.md) / [`CustomActions`](HeadNode-v3.md#HeadNode-v3-CustomActions) / [`OnNodeConfigured`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeConfigured) configuration settings, AWS ParallelCluster runs the code after the node configuration is correctly completed. Starting with AWS ParallelCluster version 3.4.0, the code can be run after the head node update, if you define the [`HeadNode`](HeadNode-v3.md) / [`CustomActions`](HeadNode-v3.md#HeadNode-v3-CustomActions) / [`OnNodeUpdated`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeUpdated) configuration settings. In most cases, this code is stored in Amazon Simple Storage Service (Amazon S3) and accessed through an HTTPS connection. The code is run as `root` and can be in any script language that's supported by the cluster OS. Often the code is in *Bash* or *Python*. **Note** Starting with AWS ParallelCluster version 3.7.0, the cluster [`Imds`](Imds-cluster-v3.md#Imds-cluster-v3.title) / [`ImdsSupport`](Imds-cluster-v3.md#yaml-cluster-Imds-ImdsSupport) default setting is `v2.0`. When you create a new cluster to upgrade to version 3.7.0 and later versions, either update your custom bootstrap action scripts to be compatible with IMDSv2 or set [`Imds`](Imds-cluster-v3.md#Imds-cluster-v3.title) / [`ImdsSupport`](Imds-cluster-v3.md#yaml-cluster-Imds-ImdsSupport) to `v1.0` in your cluster configuration file. **Warning** It is your responsibility to configure the custom scripts and arguments as described in the [Shared responsibility model](https://aws.amazon.com/compliance/shared-responsibility-model/). Verify that your custom bootstrap scripts and arguments are from sources that you trust to have full access to your cluster nodes. **Warning** AWS ParallelCluster doesn't support the use of internal variables that are provided through the `/etc/parallelcluster/cfnconfig` file. This file might be removed as part of a future release. `OnNodeStart` actions are called before any node deployment bootstrap action is started, such as configuring NAT, Amazon Elastic Block Store (Amazon EBS) or the scheduler. `OnNodeStart` bootstrap actions may include modifying storage, adding extra users, and adding packages. **Note** If you configure [`DirectoryService`](DirectoryService-v3.md) and a [`HeadNode`](HeadNode-v3.md) / [`CustomActions`](HeadNode-v3.md#HeadNode-v3-CustomActions) / [`OnNodeStart`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeStart) script for your cluster, AWS ParallelCluster configures `DirectoryService` and restarts the `sssd`, before it runs the `OnNodeStart` script. `OnNodeConfigured` actions are called after the node bootstrap processes are complete. `OnNodeConfigured` actions serve the last actions to occur before an instance is considered fully configured and complete. Some `OnNodeConfigured` actions include changing scheduler settings, modifying storage, and modifying packages. You can pass arguments to scripts by specifying them during configuration. `OnNodeUpdated` actions are called after the head node update is completed and the scheduler and shared storage are aligned with the latest cluster configuration changes. When `OnNodeStart` or `OnNodeConfigured` custom actions succeed, success is indicated with exit code zero (0). Any other exit code indicates the instance bootstrap failed. When `OnNodeUpdated` custom actions succeed, success is signaled with exit code zero (0). Any other exit code indicates the update failed. **Note** If you configure [`OnNodeUpdated`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeUpdated), you must manually restore the `OnNodeUpdated` actions to the previous state on update failures. If an `OnNodeUpdated` custom action fails, the update rolls back to the previous state. However, the `OnNodeUpdated` action is only run at update time and not at stack rollback time. You can specify different scripts for the head node and for each queue, in the [`HeadNode`](HeadNode-v3.md) / [`CustomActions`](HeadNode-v3.md#HeadNode-v3-CustomActions) and i [`Scheduling`](Scheduling-v3.md) / [`SlurmQueues`](Scheduling-v3.md#Scheduling-v3-SlurmQueues) / [`CustomActions`](Scheduling-v3.md#Scheduling-v3-SlurmQueues-CustomActions) configuration sections. [`OnNodeUpdated`](HeadNode-v3.md#yaml-HeadNode-CustomActions-OnNodeUpdated) can only be configured in the `HeadNode` section. **Note** Before AWS ParallelCluster version 3.0, it was not possible to specify different scripts for head and compute nodes. Please refer to [Moving from AWS ParallelCluster 2.x to 3.x](moving-from-v2-to-v3.md). **Topics** + [Configuration settings to define actions and arguments](custom-bootstrap-actions-config-v3.md) + [Arguments](custom-bootstrap-actions-args-v3.md) + [Example cluster with custom bootstrap actions](custom-bootstrap-actions-example-cluster-v3.md) + [Example of how to update a custom bootstrap script for IMDSv2](custom-bootstrap-actions-example-imdsv2-v3.md) + [Example of how to update a configuration for IMDSv1](custom-bootstrap-actions-example-imdsv1-v3.md)