Securing Containerized Applications with SSH Tunneling

As cloud engineers and architects embrace containerization, ensuring secure communication becomes paramount. Data transmission and access control are critical aspects of security that need to be considered. SSH tunneling is a technique that can help achieve secure communication between different components of an application or solution. SSH tunneling creates an encrypted channel over an existing SSH connection, allowing secure data transmission between a local machine (SSH client) and a remote server (SSH server).

In this article, we will show how to set up SSH tunneling between containers running in the cloud that need to communicate with downstream resources via an SSH server hosted in a cloud VM.

 

SSH Tunneling

Before diving into the implementation, let’s have a quick refresher about SSH tunneling. Also known as SSH port forwarding, SSH tunneling allows secure communication between two endpoints by creating an encrypted tunnel over an existing SSH connection. It enables data to be transmitted securely between a local machine (SSH client) and a remote server (SSH server) through an intermediary channel. Here is an overview of different scenarios where SSH tunneling can be used:

 

1. Secure Remote Access to Internal Services: An organization has internal services (e.g., databases, internal web applications) that are not exposed to the public internet for security reasons. Using SSH tunneling, employees can securely connect to these internal services from remote locations without exposing the services to the internet.

2. Bypassing Firewall Restrictions: Developers need to access specific resources that are behind a corporate firewall, but the firewall restricts direct access. By setting up an SSH tunnel, developers can securely forward traffic through the firewall, allowing them to access the restricted resources.

3. Protecting Sensitive Data in Transit: An application needs to send sensitive data between different components or services, and there's a risk of data interception. SSH tunneling can be used to encrypt the data as it travels between the components, ensuring that it remains secure in transit.

4. Accessing a Remote Database Securely: A developer needs to access a remote database server for maintenance or development purposes, but direct access is not permitted due to security policies. The developer can set up an SSH tunnel to securely connect to the remote database server without exposing it to the public internet.

5. Securely Using Insecure Protocols: An application uses an insecure protocol (e.g., FTP, HTTP) to communicate between different services. By wrapping the insecure protocol within an SSH tunnel, the communication can be secured, protecting the data from being intercepted.

6. Remote Debugging: A developer needs to debug an application running on a remote server, but direct access to the debugging port is restricted. SSH tunneling can be used to forward the debugging port from the remote server to the local machine, allowing the developer to securely debug the application.

7. Protecting IoT Device Communication: IoT devices need to communicate with a central server, but the communication is vulnerable to interception or tampering. By establishing an SSH tunnel between the IoT devices and the central server, the communication can be encrypted and secured, protecting the data in transit.

8. Secure File Transfer: Files need to be transferred securely between different systems or locations. SSH tunneling can be used to securely transfer files over the encrypted tunnel, ensuring that the data remains confidential and integrity is maintained.

9. Accessing Remote Services: A user needs to access services or resources hosted on a remote server securely. By setting up an SSH tunnel, the user can securely access the remote services as if they were running locally, protecting the data in transit.

10. Protecting Web Traffic: Web traffic needs to be secured when accessing websites or web applications over untrusted networks. SSH tunneling can be used to create a secure connection to a remote server, encrypting the web traffic and protecting it from eavesdropping or interception.

 

Scenario

For this article, we will implement the following scenario:

 

 

Architecture Components

• myInfraVNet: Virtual network where the downstream resources are deployed.
• nginxVM: A virtual machine running Nginx, a web server or reverse proxy, within myInfraVNet. It is assigned a private IP address, so that it is not directly accessible from the internet.
• nginxVM/NSG: Network Security Group associated with the nginxVM, controlling inbound and outbound traffic.
• myAppVNet: Virtual network where the container apps are deployed.
• Container Apps Environment: This environment hosts two containerized applications:
  • mycontainerapp: A simple containerized Python application that fetches content from the NGINX server running on the VM and renders this content along with other content.
  • sshclientcontainerapp: Another containerized application, used to establish secure SSH tunnels to other resources.
• Container Registry: Stores container images that can be deployed to the container apps.
• VNet Peering: Allows resources in myAppVNet and myInfraVNet to communicate with each other. It essentially bridges the two VNets, enabling low-latency, high-bandwidth interconnectivity.
• SSH Tunnel: The sshclientcontainerapp in the myAppVNet establishes an SSH tunnel to the nginxVM in the myInfraVNet to enable secure communication between the containerized app and the VM.
• Network Security Group (NSG): The nginxVM/NSG ensures that only allowed traffic can reach the nginxVM. It's crucial to configure this NSG correctly to allow SSH traffic from the sshclientcontainerapp and restrict unwanted access.

Scripting the Scenario

Based on the scenario described above, we will now script the implementation of the architecture. The script will create the necessary resources, configure the SSH tunnel, and deploy the containerized applications.

 

Prerequisites

Before running the script, ensure that you have the following prerequisites:

• Azure CLI installed on your local machine.
• Docker installed on your local machine.
• A valid Azure subscription.
• A basic understanding of Azure Container Apps, Azure Container Registry, and Azure Virtual Networks.

Parameters

Let's start by defining the parameters that will be used in the script. These parameters include the resource group name, location, virtual network names, subnet names, VM name, VM image, VM size, SSH key, admin username, admin password, container apps environment name, container registry name, container app image names, SSH client container image name, SSH port, and NGINX port. A random string is generated and appended to the resource group name, container apps environment name, and container registry name to ensure uniqueness.

 

 

random=$(echo $RANDOM | tr '[0-9]' '[a-z]')
echo "Random:" $random
export RESOURCE_GROUP=rg-ssh-$(echo $random)
echo "RESOURCE_GROUP:" $RESOURCE_GROUP
export LOCATION="australiaeast"
export INFRA_VNET_NAME="myInfraVNet"
export APP_VNET_NAME="myAppVNet"
export INFRA_SUBNET_NAME="mySubnet"
export APP_SUBNET_NAME="acaSubnet"
export VM_NAME="nginxVM"
export VM_IMAGE="Ubuntu2204"
export VM_SIZE="Standard_DS1_v2"
export VM_KEY=mykey$(echo $random)
export ADMIN_USERNAME="azureuser"
export ADMIN_PASSWORD="Password123$"  # Replace with your actual password
export CONTAINER_APPS_ENV=sshacae$(echo $random)
export REGISTRY_NAME=sshacr$(echo $random)
export REGISTRY_SKU="Basic"
export CONTAINER_APP_IMAGE="mycontainerapp:latest"
export SSH_CLIENT_CONTAINER_IMAGE="sshclientcontainer:latest"
export CONTAINER_APP_NAME="mycontainerapp"
export SSH_CLIENT_CONTAINER_APP_NAME="sshclientcontainerapp"
export SSH_PORT=22
export NGINX_PORT=80

 

 

Create Resource Group

Create a resource group using the az group create command. The resource group name and location are passed as parameters.

 

 

az group create --name $RESOURCE_GROUP --location $LOCATION  

 

 

Create Virtual Networks and Subnets

Create two virtual networks, myInfraVNet and myAppVNet, using the az network vnet create command. The address prefixes and subnet prefixes are specified for each virtual network. The az network vnet subnet update command is used to delegate the Microsoft.App/environments to the myAppVNet subnet.

 

 

az network vnet create --resource-group $RESOURCE_GROUP --name $INFRA_VNET_NAME --address-prefix 10.0.0.0/16 --subnet-name $INFRA_SUBNET_NAME --subnet-prefix 10.0.0.0/24  
az network vnet create --resource-group $RESOURCE_GROUP --name $APP_VNET_NAME --address-prefix 10.1.0.0/16 --subnet-name $APP_SUBNET_NAME --subnet-prefix 10.1.0.0/24  
az network vnet subnet update --resource-group $RESOURCE_GROUP --vnet-name $APP_VNET_NAME --name $APP_SUBNET_NAME --delegations Microsoft.App/environments

 

 

Create VNET Peering

Create a VNET peering between myInfraVNet and myAppVNet using the az network vnet peering create command. Two peering connections are created, one from myInfraVNet to myAppVNet and the other from myAppVNet to myInfraVNet.

 

 

az network vnet peering create --name VNet1ToVNet2 --resource-group $RESOURCE_GROUP --vnet-name $INFRA_VNET_NAME --remote-vnet $APP_VNET_NAME --allow-vnet-access  
az network vnet peering create --name VNet2ToVNet1 --resource-group $RESOURCE_GROUP --vnet-name $APP_VNET_NAME --remote-vnet $INFRA_VNET_NAME --allow-vnet-access  

 

 

Create Network Security Group and Rules

Create a network security group (NSG) for the nginxVM using the az network nsg create command. Two NSG rules are created to allow SSH traffic on port 22 and HTTP traffic on port 80.

 

 

az network nsg create --resource-group $RESOURCE_GROUP --name ${VM_NAME}NSG  
az network nsg rule create --resource-group $RESOURCE_GROUP --nsg-name ${VM_NAME}NSG --name AllowSSH --protocol Tcp --direction Inbound --priority 1000 --source-address-prefixes '*' --source-port-ranges '*' --destination-address-prefixes '*' --destination-port-ranges $SSH_PORT --access Allow  
az network nsg rule create --resource-group $RESOURCE_GROUP --nsg-name ${VM_NAME}NSG --name AllowHTTP --protocol Tcp --direction Inbound --priority 1001 --source-address-prefixes '*' --source-port-ranges '*' --destination-address-prefixes '*' --destination-port-ranges $NGINX_PORT --access Allow  

 

 

Create Network Interface

Create a network interface for the nginxVM using the az network nic create command. The NIC is associated with the myInfraVNet and mySubnet and the NSG created earlier.

 

 

az network nic create --resource-group $RESOURCE_GROUP --name ${VM_NAME}NIC --vnet-name $INFRA_VNET_NAME --subnet $INFRA_SUBNET_NAME --network-security-group ${VM_NAME}NSG 

 

 

Create VM

Create a virtual machine using the az vm create command. The VM is created with the specified image, size, admin username, and password. The NIC created earlier is associated with the VM. Ensure that you have provided a value for the password in the ADMIN_PASSWORD variable.

 

 

az vm create --resource-group $RESOURCE_GROUP --name $VM_NAME --image $VM_IMAGE --size $VM_SIZE --admin-username $ADMIN_USERNAME --admin-password $ADMIN_PASSWORD --nics ${VM_NAME}NIC  
export VM_PRIVATE_IP=$(az vm show -d -g $RESOURCE_GROUP -n $VM_NAME --query privateIps -o tsv)  
echo "VM Private IP: $VM_PRIVATE_IP"

 

 

Generate SSH Key Pair and Add the Public Key to the VM

Generate an SSH key pair using the ssh-keygen command. The public key is added to the VM using the az vm user update command.

 

 

# Generate an SSH key pair  
ssh-keygen -t rsa -b 4096 -f $VM_KEY -N ""  
# Add the public key to the VM  
az vm user update --resource-group $RESOURCE_GROUP --name $VM_NAME --username $ADMIN_USERNAME --ssh-key-value "$(cat $VM_KEY.pub)"  
# Print success message  
echo "SSH key pair generated and public key added to VM $VM_NAME" 

 

 

Install NGINX and SSH Server on the VM

Install NGINX and SSH server on the VM using the az vm run-command invoke command. This command runs a shell script on the VM to update the package repository, install NGINX, start the NGINX service, install the SSH server, and start the SSH service.

 

 

az vm run-command invoke --command-id RunShellScript --name $VM_NAME --resource-group $RESOURCE_GROUP --scripts "sudo apt-get update && sudo apt-get install -y nginx && sudo systemctl start nginx && sudo apt-get install -y openssh-server && sudo systemctl start ssh"

 

 

Create Azure Container Registry

Create an Azure Container Registry using the az acr create command to store the container images that will be deployed to the container apps.

 

 

az acr create --resource-group $RESOURCE_GROUP --name $REGISTRY_NAME --sku $REGISTRY_SKU --location $LOCATION --admin-enabled true  

 

 

Login to Azure Container Registry

Login to the Azure Container Registry using the az acr login command.

 

 

az acr login --name $REGISTRY_NAME 

 

 

Create Dockerfile for mycontainerapp

Create a Dockerfile for the mycontainerapp. The Dockerfile specifies the base image, working directory, copy files, install packages, expose port, define environment variable, and run the application.

 

 

echo "
# Use an official Python runtime as a parent image  
FROM python:3.8-slim  
# Set the working directory in the container  
WORKDIR /app  
# Copy the current directory contents into the container at /app  
COPY . /app  
# Install any needed packages specified in requirements.txt  
RUN pip install --no-cache-dir -r requirements.txt  
# Make port 80 available to the world outside this container  
EXPOSE 80  
# Define environment variable  
ENV NAME World  
# Run app.py when the container launches  
CMD [\"python\", \"app.py\"]
" > Dockerfile.mycontainerapp

 

 

 

Create Dockerfile for sshclientcontainer

Create a Dockerfile for the sshclientcontainer. The Dockerfile specifies the base image, install SSH client, copy SSH key, set working directory, copy files, expose port, and run the SSH client.

 

 

echo "
# Use an official Ubuntu as a parent image  
FROM ubuntu:20.04  
# Install SSH client  
RUN apt-get update && apt-get install -y openssh-client && apt-get install -y curl  
# Copy SSH key  
COPY ${VM_KEY} /root/.ssh/${VM_KEY}  
RUN chmod 600 /root/.ssh/${VM_KEY}  
# Set the working directory in the container  
WORKDIR /app  
# Copy the current directory contents into the container at /app  
COPY . /app  
# Make port 80 available to the world outside this container  
EXPOSE 80  
# Run the SSH client when the container launches  
CMD [\"bash\", \"-c\", \"ssh -i /root/.ssh/${VM_KEY} -o StrictHostKeyChecking=no -L 0.0.0.0:80:localhost:80 ${ADMIN_USERNAME}@${VM_PRIVATE_IP} -N\"]
" > Dockerfile.sshclientcontainer 

 

 

Create an App for mycontainerapp

Create a simple app that can be hosted on mycontainerapp. The app.py file contains a simple Flask application that fetches content from the NGINX server running on the VM and renders it along with other content.

 

 

echo "
import requests
from flask import Flask, render_template_string

app = Flask(__name__)

@app.route('/')
def hello_world():
    response = requests.get('http://sshclientcontainerapp:80')
    html_content = \"\"\"
    <!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Response Page</title>
    </head>
    <body>
        <header>
            <h1>Welcome to My Container App</h1>
        </header>
        <main>
            <div>Response Content - The following response has been received from the NGINX server running on the VM via SSH tunnel</div>
            <hr/>
            <div>{}</div>
        </main>
        <footer>
            <hr/>
            <p>&copy; 2024 My Flask App</p>
        </footer>
    </body>
    </html>
    \"\"\".format(response.text)
    return render_template_string(html_content)

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=80)
" > app.py

echo "
Flask==2.0.0
Werkzeug==2.2.2
requests==2.25.1
" > requirements.txt

 

 

Build and Push Docker Images

Build the Docker images for sshclientcontainer and mycontainerapp using the docker build command. The images are tagged with the Azure Container Registry name and pushed to the registry using the docker push command.

 

 

# Build the Docker image for sshclientcontainer  
docker build -t $REGISTRY_NAME.azurecr.io/$SSH_CLIENT_CONTAINER_IMAGE -f Dockerfile.sshclientcontainer .  
   
# Push the Docker image for sshclientcontainer to Azure Container Registry  
docker push $REGISTRY_NAME.azurecr.io/$SSH_CLIENT_CONTAINER_IMAGE  
   
# Build the Docker image for mycontainerapp  
docker build -t $REGISTRY_NAME.azurecr.io/$CONTAINER_APP_IMAGE -f Dockerfile.mycontainerapp .  
   
# Push the Docker image for mycontainerapp to Azure Container Registry  
docker push $REGISTRY_NAME.azurecr.io/$CONTAINER_APP_IMAGE  

 

 

Create Azure Container Apps Environment

Create an Azure Container Apps environment using the az containerapp env create command. The environment is associated with the virtual network and subnet created earlier.

 

 

# Get the subnet ID for the infrastructure subnet
export INFRA_SUBNET_ID=$(az network vnet subnet show --resource-group $RESOURCE_GROUP --vnet-name $APP_VNET_NAME --name $APP_SUBNET_NAME --query id --output tsv)
echo $INFRA_SUBNET_ID

# Create the Azure Container Apps environment
az containerapp env create --name $CONTAINER_APPS_ENV --resource-group $RESOURCE_GROUP --location $LOCATION --infrastructure-subnet-resource-id $INFRA_SUBNET_ID

 

 

Deploy Container Apps

Deploy the container apps to the Azure Container Apps environment using the az containerapp create command. The container images are pulled from the Azure Container Registry, and the container apps are configured to use the SSH tunnel for secure communication.

 

Deploy sshclientcontainerapp

 

 

az acr login --name $REGISTRY_NAME

# Deploy sshclientcontainerapp
az containerapp create --name $SSH_CLIENT_CONTAINER_APP_NAME --resource-group $RESOURCE_GROUP --environment $CONTAINER_APPS_ENV --image $REGISTRY_NAME.azurecr.io/$SSH_CLIENT_CONTAINER_IMAGE --target-port 80 --ingress 'external' --registry-server $REGISTRY_NAME.azurecr.io

 

 

Deploy mycontainerapp

 

 

az acr login --name $REGISTRY_NAME

az containerapp create --name $CONTAINER_APP_NAME --resource-group $RESOURCE_GROUP --environment $CONTAINER_APPS_ENV --image $REGISTRY_NAME.azurecr.io/$CONTAINER_APP_IMAGE --target-port 80 --ingress 'external' --registry-server $REGISTRY_NAME.azurecr.io

 

 

Testing the Deployment

After deploying the container apps, you can test the deployment by accessing the public URL of the mycontainerapp. The app should fetch content from the NGINX server running on the VM through the SSH tunnel and render it along with other content.

 

1. Retrieve the public URL of the mycontainerapp:

   MY_CONTAINER_APP_URL=$(az containerapp show --name $CONTAINER_APP_NAME --resource-group $RESOURCE_GROUP --query 'properties.configuration.ingress.fqdn' -o tsv)  
   echo "mycontainerapp URL: http://$MY_CONTAINER_APP_URL"  

2. Open the URL in your web browser by copying and pasting the URL printed in the previous step.

You should see a webpage that includes the response content from the NGINX server running on the VM via the SSH tunnel.

 

 

Clean Up

After testing the deployment, you can clean up the resources by deleting the resource group. This will remove all the resources created in the script.

 

 

az group delete --name $RESOURCE_GROUP --yes --no-wait

 

 

Conclusion

In this article, we demonstrated how to secure containerized applications using SSH tunneling. We covered the steps to set up the necessary infrastructure, create and deploy containerized applications, and establish an SSH tunnel for secure communication between the container apps and a VM hosting an NGINX server.

By following these steps, you can ensure that your containerized applications communicate securely with downstream resources, enhancing the overall security of your cloud-native architecture.

For more information on securing containerized applications, refer to the Azure Container Apps documentation.

If you have any questions or need further assistance, feel free to consult the Azure documentation or reach out to Microsoft support.

 

References

• Azure Container Apps Documentation
• Azure Virtual Network Documentation
• Azure Container Registry Documentation
• Azure CLI Documentation
• SSH Tunneling Documentation