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# Filter by User Agent

> **Warning**
>
> User agents can easily be spoofed by attackers. To prevent potential attacks, we recommend not to filter out traffic using user agents.

If you'd still like to run a test based on a specific user agent, disable this feature after the test.

**TO define a user agent you want to filter out:**

1. Open the **PX_CUSTOM.vcl** fie.
2. Add the required IP as a regex expression using req.http.User-Agent.

**EXAMPLE**

```javascript
sub px_custom_filter_by_user_agent {
if (req.http.User-Agent ~ {"<User_agents_to_filter>"}) {
set req.http.X-PX-filter-by:ua = "1";
    }
}
```

To learn more about custom subroutines, see [Customized subroutines](/applications/fastly-customized-subroutines).

**TO enable the feature:**

1. Open the **PX_CONFIG.vcl** file.
2. Set the px_filter_by_user_agent_enabled value to "true" (enabled). The default is "false" (disabled).

```javascript
table px_configs {
  "px_filter_by_user_agent_enabled": "true",
}
```

> **Test**
>
> 1. Send a request that includes the filtered user agent.
> 2. Openthe console and search for your request. If you can't find the request, the feature is configured correctly.