docs: Connect NGINX Agent to NGINX One using squid proxy (#1025)

* feat: new doc for agent squid

Author: JTorreG

content/includes/installation/add-ports-agent-selinux.md

@@ -1,7 +1,7 @@
 ---
 docs:
 files:
-  - content/nginx-one/agent/configure-instance-reporting/configure-selinux.md
+  - content/nginx-one/agent/configure-instances/configure-selinux.md
   - content/nim/system-configuration/configure-selinux.md
   - content/nms/nginx-agent/install-nginx-agent.md
 ---

content/includes/installation/enable-agent-selinux.md

@@ -1,7 +1,7 @@
 ---
 docs:
 files:
-   - content/nginx-one/agent/configure-instance-reporting/configure-selinux.md
+   - content/nginx-one/agent/configure-instances/configure-selinux.md
    - content/nim/system-configuration/configure-selinux.md
    - content/nms/nginx-agent/install-nginx-agent.md
 ---

content/nginx-one/agent/configure-instance-reporting/_index.md

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+---
+title: "Configure instances"
+weight: 400
+url: /nginx-one/agent/configure-instances/
+---

content/nginx-one/agent/configure-instances/_index.md

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+---
+title: Prepare - Set up an explicit forward proxy
+toc: true
+weight: 250
+nd-docs: DOCS-000
+---
+
+NGINX Agent can be configured to connect to NGINX One using an explicit forward
+proxy. This is useful in environments where direct internet access is restricted or monitored.
+
+## Before you start
+
+Ensure you have the following:
+
+- An explicit forward proxy server installed and configured in your network.
+- [NGINX Agent is installed]({{< ref "nginx-one/agent/install-upgrade/" >}})
+- Access to the [NGINX One console]({{< ref "/nginx-one/getting-started.md#before-you-begin" >}}).
+
+
+## NGINX Agent configuration for proxy usage
+
+1. Open a secure connection to your instance using SSH and log in.
+1. Open the NGINX Agent configuration file (/etc/nginx-agent/nginx-agent.conf) with a text editor. To edit this file you need superuser privileges.
+1. Add or modify the `proxy` section to include the proxy URL and timeout settings:
+
+   ```conf
+   server:
+      host: agent.connect.nginx.com
+      port: 443
+      proxy:
+         url: "http://proxy.example.com:3128"
+   ```
+
+1. Restart NGINX Agent to apply the changes:
+
+    ```sh
+    sudo systemctl restart nginx-agent
+    ```
+
+### In a containerized environment
+
+To configure NGINX Agent in a containerized environment:
+
+1. Run the NGINX Agent container with the environment variables set as follows:
+
+   ```sh
+   sudo docker run \
+      --add-host "myproxy.example.com:host-gateway" \
+      --env=NGINX_AGENT_COMMAND_SERVER_PORT=443 \
+      --env=NGINX_AGENT_COMMAND_SERVER_HOST=agent.connect.nginx.com \
+      --env=NGINX_AGENT_COMMAND_AUTH_TOKEN="<your-data-plane-key-here>" \
+      --env=NGINX_AGENT_COMMAND_TLS_SKIP_VERIFY=false \
+      --env=NGINX_AGENT_COMMAND_SERVER_PROXY_URL=http://myproxy.example.com:3128 \
+      --restart=always \
+      --runtime=runc \
+      -d private-registry.nginx.com/nginx-plus/agentv3:latest
+   ```
+
+
+## NGINX Agent proxy authentication
+
+If your forward proxy requires authentication, you can specify the username and password in the `proxy` section of the `agent.conf` file:
+
+1. Open a secure connection to your instance using SSH and log in.
+1. Add or modify the `proxy` section of the NGINX Agent configuration file (/etc/nginx-agent/nginx-agent.conf) to include the authentication details:
+
+   ```conf
+   proxy:
+      url: "http://proxy.example.com:3128"
+      auth_method: "basic"
+      username: "user"
+      password: "pass"
+   ```
+
+1. Restart NGINX Agent to apply the changes:
+
+    ```sh
+    sudo systemctl restart nginx-agent
+    ```
+
+### In a containerized environment
+
+To set proxy authentication in a containerized environment:
+
+1. Run the NGINX Agent container with the environment variables set as follows:
+
+
+   ```sh
+   sudo docker run \
+      --add-host "myproxy.example.com:host-gateway" \
+      --env=NGINX_AGENT_COMMAND_SERVER_PORT=443 \
+      --env=NGINX_AGENT_COMMAND_SERVER_HOST=agent.connect.nginx.com \
+      --env=NGINX_AGENT_COMMAND_AUTH_TOKEN="<your-data-plane-key-here>" \
+      --env=NGINX_AGENT_COMMAND_TLS_SKIP_VERIFY=false \
+      --env NGINX_AGENT_COMMAND_SERVER_PROXY_URL=http://proxy.example.com:3128
+      --env NGINX_AGENT_COMMAND_SERVER_PROXY_AUTH_METHOD=basic
+      --env NGINX_AGENT_COMMAND_SERVER_PROXY_USERNAME="user"
+      --env NGINX_AGENT_COMMAND_SERVER_PROXY_PASSWORD="pass"
+      --restart=always \
+      --runtime=runc \
+      -d private-registry.nginx.com/nginx-plus/agentv3:latest
+   ```
+
+## Validate connectivity between the components
+
+To test the connectivity between NGINX Agent, your proxy, and NGINX One Console, you can use the `curl` command with the proxy settings.
+
+1. Open a secure connection to your instance using SSH and log in.
+1. Run the following `curl` command to test the connection:
+   ```sh
+   curl -x http://proxy.example.com:3128 -U your_user:your_password https://agent.connect.nginx.com/api/v1/agents
+   ```
+
+   - Replace `proxy.example.com:3128` with your proxy address and port.
+   - Replace `your_user` and `your_password` with the credentials you set up for proxy in the previous steps.
+
+To test the configuration from a containerized environment, run the following command from within the container:
+
+   ```sh
+   curl -x http://host.docker.internal:3128 -U your_user:your_password https://agent.connect.nginx.com/api/v1/agents
+   ```
+
+   - Replace `your_user` and `your_password` with the credentials you set up for proxy in the previous steps.
+