# Network filtering with Squid with isolated VPCs using Private Service Connect

This blueprint shows how to deploy a filtering HTTP proxy to restrict Internet access. Here we show one way to do this using isolated VPCs and Private Service Connect:

- The `app` subnet hosts the consumer VMs that will have their Internet access tightly controlled by a non-caching filtering forward proxy.
- The `proxy` subnet hosts a Cloud NAT instance and a [Squid](http://www.squid-cache.org/) server.
- The `psc` subnet is reserved for the Private Service Connect.

The reason for using Privat Service Connect in this setup is to have a common proxy setup between all environments without having to share a VPC between projects. This allows us to enforce the `compute.vmExternalIpAccess` [organization policy](https://cloud.google.com/resource-manager/docs/organization-policy/org-policy-constraints), which prevents the service projects from having external IPs, thus forcing all outbound Internet connections through the proxy.

To allow Internet connectivity to the proxy subnet, a Cloud NAT instance is configured to allow usage from [that subnet only](https://cloud.google.com/nat/docs/using-nat#specify_subnet_ranges_for_nat). All other subnets are not allowed to use the Cloud NAT instance.

To simplify the usage of the proxy, a Cloud DNS private zone is created in each consumer VPC and the IP address of the proxy is exposed with the FQDN `proxy.internal`. In addition, system-wide `http_proxy` and `https_proxy` environment variables and an APT configuration are rolled out via a [startup script](startup.sh).
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## Variables

| name | description | type | required | default |
|---|---|:---:|:---:|:---:|
| [prefix](variables.tf#L44) | Prefix used for resources that need unique names. | <code>string</code> | ✓ |  |
| [project_id](variables.tf#L66) | Project id used for all resources. | <code>string</code> | ✓ |  |
| [allowed_domains](variables.tf#L17) | List of domains allowed by the squid proxy. | <code>list&#40;string&#41;</code> |  | <code title="&#91;&#10;  &#34;.google.com&#34;,&#10;  &#34;.github.com&#34;,&#10;  &#34;.fastlydns.net&#34;,&#10;  &#34;.debian.org&#34;&#10;&#93;">&#91;&#8230;&#93;</code> |
| [cidrs](variables.tf#L28) | CIDR ranges for subnets. | <code>map&#40;string&#41;</code> |  | <code title="&#123;&#10;  app   &#61; &#34;10.0.0.0&#47;24&#34;&#10;  proxy &#61; &#34;10.0.2.0&#47;28&#34;&#10;  psc   &#61; &#34;10.0.3.0&#47;28&#34;&#10;&#125;">&#123;&#8230;&#125;</code> |
| [nat_logging](variables.tf#L38) | Enables Cloud NAT logging if not null, value is one of 'ERRORS_ONLY', 'TRANSLATIONS_ONLY', 'ALL'. | <code>string</code> |  | <code>&#34;ERRORS_ONLY&#34;</code> |
| [project_create](variables.tf#L49) | Set to non null if project needs to be created. | <code title="object&#40;&#123;&#10;  billing_account &#61; string&#10;  parent          &#61; string&#10;&#125;&#41;">object&#40;&#123;&#8230;&#125;&#41;</code> |  | <code>null</code> |
| [region](variables.tf#L71) | Default region for resources. | <code>string</code> |  | <code>&#34;europe-west1&#34;</code> |

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