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How can I connect to a Google Compute Engine VM instance via SSH?

Secure Shell (SSH) is a fundamental protocol for securely connecting to your Google Compute Engine (GCE) virtual machine (VM) instances. This guide provides a comprehensive overview of the various methods you can use to establish an SSH connection, including the Google Cloud Console, Google Cloud CLI (gcloud), third-party SSH clients, and Identity-Aware Proxy (IAP) for enhanced security.

Prerequisites

Before you can connect to your VM instance via SSH, ensure that you have the following:

• A Google Cloud Platform (GCP) project with the Compute Engine API enabled.
• A running Compute Engine VM instance.
• Sufficient IAM (Identity and Access Management) permissions to connect to the instance. Typically, the roles/compute.osLogin or roles/compute.instanceAdmin role will grant necessary access.
• If using the Google Cloud CLI (gcloud), ensure it is installed and configured correctly. You can download it from the official Google Cloud SDK documentation. You'll also need to authenticate using gcloud auth login and set the project with gcloud config set project [YOUR_PROJECT_ID].

Methods for Connecting via SSH

1. Using the Google Cloud Console

The simplest method for connecting to a VM instance is through the Google Cloud Console. This option is readily accessible and requires no additional setup on your local machine.

1. Navigate to the Compute Engine VM instances page in the Google Cloud Console.
2. Locate the VM instance you want to connect to.
3. In the "Connect" column, click the "SSH" button. A browser-based SSH client will open in a new window.
4. Select the preferred method from the dropdown menu: "Open in browser window", "Open in a browser window with custom command", or "Connect using gcloud command"
   
   • Open in browser window: This is the easiest method. A secure SSH connection to the VM's operating system prompt is opened directly in the browser.
   • Open in a browser window with custom command: Allows you to specify a command that will run after SSH is established (e.g. tmux attach).
   • Connect using gcloud command: Displays the gcloud compute ssh command to execute on your local machine, rather than launching a browser window. Useful when you prefer working with your local terminal.
   
   

The Google Cloud Console automatically manages SSH key generation and deployment, making this a seamless experience. When using "Open in browser window", if no existing user account and key is setup, Compute Engine can generate a temporary SSH key-pair. If requested, accept by selecting "Create and continue".

2. Using the Google Cloud CLI (gcloud)

The Google Cloud CLI (gcloud) provides a command-line interface for managing your Google Cloud resources. It's a powerful tool for automating tasks and connecting to VMs from your terminal.

1. Open your terminal or command prompt.
2. Use the following command to connect to your VM instance, replacing [INSTANCE_NAME] with the actual name of your VM instance and [ZONE] with the zone where the instance resides:
   
   

   gcloud compute ssh [INSTANCE_NAME] --zone=[ZONE]

   
   For example:
   

   gcloud compute ssh my-vm-instance --zone=us-central1-a

The gcloud compute ssh command automatically handles SSH key generation and management if necessary. The first time you run this command, it might prompt you to create a new SSH key pair and upload the public key to your project's metadata. Answer y (yes) to proceed if prompted.

You can also specify a username using the --project and --dry-run options to test whether it will work properly (for troubleshooting or debugging when you can't connect).

gcloud compute ssh username@[INSTANCE_NAME] --zone=[ZONE] --dry-run

3. Using Third-Party SSH Clients (PuTTY, OpenSSH, etc.)

You can also use third-party SSH clients, such as PuTTY (Windows), OpenSSH (Linux/macOS), or others, to connect to your VM instance. This requires manually managing SSH keys.

1. Generate an SSH Key Pair: If you don't already have an SSH key pair, generate one using ssh-keygen (Linux/macOS) or PuTTYgen (Windows). It's recommended to use a key size of at least 2048 bits (4096 is generally considered more secure):
   
   

   ssh-keygen -t rsa -f ~/.ssh/my-vm-key -b 4096

   
   This creates a private key (~/.ssh/my-vm-key) and a public key (~/.ssh/my-vm-key.pub). Keep the private key secure and never share it with anyone.
2. Add the Public Key to Instance Metadata or Project Metadata:
   
   • Instance Metadata: Adds the key specifically for that VM instance. Navigate to the instance details page in the Cloud Console, click "Edit", and under "SSH Keys", add your public key. Paste the contents of the .pub file (e.g., ~/.ssh/my-vm-key.pub).
   • Project Metadata: Adds the key to all instances within the project. Navigate to the "Metadata" page in the Compute Engine section of the Cloud Console and add an SSH key.
   
   
   The format of the SSH key in the metadata must be:
   

   ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQDx...

   where "ssh-rsa" defines keytype followed by encoded value, you also specify username in this stage. The Cloud Console can generally parse this information.
   
   
3. Connect using your SSH client:

   Using OpenSSH (Linux/macOS):

   ssh -i ~/.ssh/my-vm-key [USERNAME]@[EXTERNAL_IP_ADDRESS]

   Replace [USERNAME] with the username you want to connect as and [EXTERNAL_IP_ADDRESS] with the VM instance's external IP address (found on the VM instance details page).
   
   The username for the ssh session, i.e. username@[IP_ADDRESS], is usually created the first time you successfully create an SSH tunnel to the VM instance via a method like clicking 'SSH' in the cloud console UI, creating the initial OS account. It could also have been previously established with custom provisioning, which is why setting via metadata becomes relevant for manually generating keys.
   
   

   Using PuTTY (Windows):

   • Open PuTTY.
   • Enter the external IP address of your VM instance in the "Host Name (or IP address)" field.
   • Under "Connection > SSH > Auth", browse to your private key file (.ppk, you'll need to convert the ~/.ssh/my-vm-key format into PPK with puttygen).
   • Click "Open" to start the SSH session.
   • Login with the defined username if applicable, otherwise default values may be in place or it is required on account of the linux distribution configured to mandate user information when initializing a VM user session..
   
   

4. Using Identity-Aware Proxy (IAP)

Identity-Aware Proxy (IAP) allows you to securely connect to your VM instances without exposing them directly to the internet. This enhances security by requiring authentication and authorization before allowing access. IAP effectively acts as a gatekeeper. This is particularly relevant for VMs without external IP addresses.

This method assumes that IAP has been enabled at the project or firewall level and a suitable firewall rule that enables traffic on port 22 and 3389 for accessing GCE from IPs associated with Google. Ingress control rules would control inbound ports on your VMs based on firewall configs for subnets. If the proper connectivity rule is established between GCP network IP space and any associated subnet-based IP space (example, 10.128.0.0/9). This approach leverages tunnels which provide the SSH protocol, enabling network packets associated with IAP TCP forwarding for Cloud Shell, gcloud and SSH/browser based utilities via ports. Traffic is filtered to users only that are granted the correct Identity-Aware Proxy roles via the Access/IAM Google UI or from the IAM service accounts used for CLI/SDK interaction/execution in order to SSH into cloud resources such as compute engine instances without going over the public internet. The most specific Cloud IAM principle that provides access over IAP for tunneling connections, in general terms, is:
roles/iap.tunnelResourceAccessor for connecting a local browser window based IAP. The IAP setting typically is under the Compute > VM instance "Overview". To use with CLI, gcloud is usually deployed locally on end point devices and this requires authentication and tunneling that must be setup using gcloud config set tunnel/use_port_forwarding true. If you change it you have to re authenticate (gcloud auth login).

1. Enable IAP: Ensure that IAP is enabled for your project and properly configured in the Compute Engine. Typically enabling involves associating a service account which becomes associated as "secured-tunnel users". This enables access and authenticates any user attempt. Access is often applied under the VPC firewall for instance IAP: secure shell and HTTP connection between user agents associated with IAP and GCE VMs.
2. Install the IAP plugin:
   
   In a Cloud shell prompt, download Google cloud SDK's authenticated tool. It may already be on GCE for most versions of the VM instances if Google accounts exist, and it will automatically fetch keys and register itself, enabling command shell functionality over TCP.
   sudo apt install google-cloud-sdk-appengine google-cloud-sdk-appengine-java google-cloud-sdk-appengine-python google-cloud-sdk-bigtable-emulator google-cloud-sdk-cbt google-cloud-sdk-cloud-datastore-emulator google-cloud-sdk-cloud-run-proxy google-cloud-sdk-components google-cloud-sdk-datalab google-cloud-sdk-datastore-emulator google-cloud-sdk-e2e-test google-cloud-sdk-gcd-emulator google-cloud-sdk-gen-repo-info google-cloud-sdk-minikube google-cloud-sdk-nomulus google-cloud-sdk-ops-agent-policy-migration google-cloud-sdk-package-go google-cloud-sdk-skaffold
3. After IAP is activated and Cloud SDK shell tool is connected with the service account as the IAP principal, running the command

   gcloud compute ssh [INSTANCE_NAME] --zone=[ZONE]

   will be authenticated according the security level deployed for TCP forward and authorized tunneling configured as the underlying security rules in IAM for granting and access permissions across multiple scopes. When enabled, connections between GCP and remote desktops that leverage secure tunnels created between the web or shell tool (webbrowser interface and ssh) for granting authorized authenticated traffic and security policies in cloud identity are established and can traverse internal instances over VPC using Cloud identity based proxies or private routes rather than publicly published routes for any remote destination endpoints

Troubleshooting SSH Connection Issues

If you encounter problems connecting to your VM instance via SSH, consider the following troubleshooting steps:

• Firewall Rules: Ensure that the firewall rules allow incoming traffic on port 22 (for standard SSH). If IAP is enabled, verify that your firewall configuration allows traffic from the IAP proxy IPs. For IAP itself you need firewall rules for ingress (incoming from a security group). The traffic is allowed for TCP forward by default between networks (ingress rules on the local web host (or GCE running local instances)) is required for access into that private cloud (VPC/VLAN for example on VPC 0 which by default are not able to traverse external networks by design) for authenticated tunnels used for establishing authorized users can traverse web interfaces. It can restrict network traffic that would originate through web and API sessions via user devices and only grant authorized instances connected to web interface ports and not permit full scale web accessible connectivity for internal networks
• SSH Key Issues:
  
  • Verify that the public key is correctly added to the instance metadata or project metadata, making sure there aren't any trailing spaces, incomplete ssh keys, extra tabs or special characters when generating an SSH key. The key in Google is usually 400+ ASCII characters or more.
  • Ensure that the private key is being used correctly with your SSH client (e.g., the correct path is specified).
  • For permission-based keys using gcloud over ssh using web connections/TCP with SDK ensure you've done steps to ensure it works, ensure correct network traffic policies and ingress routes have been defined in associated network's network configurations or the access token, key pairs in local client are out of synch for a previously granted session/account's private keys in memory used to authenticate against current configurations
    
  
  
• Instance Status: Confirm that the VM instance is running and has a valid network configuration. It also ensures VMs can obtain DHCP network configurations or access public network paths with egress policies. Without it they will be unable to create SSH web-interface based tunnels without TCP or ports over secure access policies defined that permits external interface (web based) sessions over user or service credentials when they login into Google console or command shell environments associated with a project. IAP is enabled over such contexts so GCE cannot make outgoing DNS/http/ssh related traffic (because the ingress/egress policy has become isolated), so instances cannot reach any network interface over GCP interfaces with SSH related TCP sessions over port access because secure policies from private IPs can not obtain outbound network connectivity unless explicitly assigned using roles or permissions to access/use these interfaces for remote traffic between user end point.
• Networking Issues:
  
  • Check the network configuration to ensure the VM instance has an external IP address (if you're connecting directly without IAP). Without egress or assigned access (and with egress configured as 'isolated') a public route outside of your network that requires IAP for traffic and access is disabled due security compliance policies by Google to isolate environments, traffic requires an established access. Also if your VM cannot get a public network and it remains "internal only", make sure you change and reserve new or temporary addresses.
    
  • Verify that the VM instance is not blocking incoming SSH connections using its internal firewall (iptables or Windows Firewall). Check subnetworks to see ingress policies allow appropriate user or web connection access via Google shell-access web or CLI connections via service based protocols like port access of 4222.
    
  • Permissions via IAM It can also restrict based on Google authentication in web shell command interface session for traffic and it can also restrict external endpoint web sessions from authenticating due to a conflict that requires key rotation if IAM based principals no longer trust Google endpoint authentications.
    
  
  
• OS Login Issues Ensure your IAM config (user) contains a configured value as configured for Cloud SDK to provide SSH traffic access over tunneling:
  
• Make sure it uses correct private/public pair, or regenerate or register a new session when key configuration rotates and user is unaware that they are attempting to use authentication credentials that were registered with a older identity key pair configured that the local web endpoint authentication shell no longer trusts
  

By understanding these methods and potential issues, you should be able to successfully connect to your Google Compute Engine VM instances via SSH. Regularly review and update your security practices to protect your cloud infrastructure.

Compute Engine VM SSH Connection Security 

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