# Network filtering with Squid
This blueprint shows how to deploy a filtering HTTP proxy to restrict Internet access. Here we show one way to do this using a VPC with two subnets:
- The `apps` subnet hosts the 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 VPC is a Shared VPC and all the service projects will be located under a folder enforcing the `compute.vmExternalIpAccess` [organization policy](https://cloud.google.com/resource-manager/docs/organization-policy/org-policy-constraints). This 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/set-up-manage-network-address-translation#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 and the IP address of the proxy is exposed with the FQDN `proxy.internal`.
You can optionally deploy the Squid server as [Managed Instance Group](https://cloud.google.com/compute/docs/instance-groups) by setting the `mig` option to `true`. This option defaults to `false` which results in a standalone VM.
![High-level diagram](squid.png "High-level diagram")
## Variables
| name | description | type | required | default |
|---|---|:---:|:---:|:---:|
| [billing_account](variables.tf#L26) | Billing account id used as default for new projects. | string
| ✓ | |
| [prefix](variables.tf#L52) | Prefix used for resource names. | string
| ✓ | |
| [root_node](variables.tf#L67) | Root node for the new hierarchy, either 'organizations/org_id' or 'folders/folder_id'. | string
| ✓ | |
| [allowed_domains](variables.tf#L17) | List of domains allowed by the squid proxy. | list(string)
| | […]
|
| [cidrs](variables.tf#L31) | CIDR ranges for subnets. | map(string)
| | {…}
|
| [mig](variables.tf#L40) | Enables the creation of an autoscaling managed instance group of squid instances. | bool
| | false
|
| [nat_logging](variables.tf#L46) | Enables Cloud NAT logging if not null, value is one of 'ERRORS_ONLY', 'TRANSLATIONS_ONLY', 'ALL'. | string
| | "ERRORS_ONLY"
|
| [region](variables.tf#L61) | Default region for resources. | string
| | "europe-west1"
|
## Outputs
| name | description | sensitive |
|---|---|:---:|
| [squid-address](outputs.tf#L17) | IP address of the Squid proxy. | |
## Test
```hcl
module "test1" {
source = "./fabric/blueprints/networking/filtering-proxy"
billing_account = "123456-123456-123456"
mig = true
prefix = "fabric"
root_node = "folders/123456789"
}
# tftest modules=14 resources=36
```
```hcl
module "test2" {
source = "./fabric/blueprints/networking/filtering-proxy"
billing_account = "123456-123456-123456"
mig = false
prefix = "fabric"
root_node = "folders/123456789"
}
# tftest modules=12 resources=30
```