# Port Forwarding

Port forwarding lets developers securely access processes on their Coder
workspace from a local machine. A common use case is testing web applications in
a browser.

There are three ways to forward ports in Coder:

- The `coder port-forward` command
- Dashboard
- SSH

The `coder port-forward` command is generally more performant.

## The `coder port-forward` command

This command can be used to forward TCP or UDP ports from the remote workspace
so they can be accessed locally. Both the TCP and UDP command line flags
(`--tcp` and `--udp`) can be given once or multiple times.

The supported syntax variations for the `--tcp` and `--udp` flag are:

- Single port with optional remote port: `local_port[:remote_port]`
- Comma separation `local_port1,local_port2`
- Port ranges `start_port-end_port`
- Any combination of the above

### Examples

Forward the remote TCP port `8080` to local port `8000`:

```console
coder port-forward myworkspace --tcp 8000:8080
```

Forward the remote TCP port `3000` and all ports from `9990` to `9999` to their
respective local ports.

```console
coder port-forward myworkspace --tcp 3000,9990-9999
```

For more examples, see `coder port-forward --help`.

## Dashboard

> To enable port forwarding via the dashboard, Coder must be configured with a
> [wildcard access URL](../admin/configure.md#wildcard-access-url). If an access
> URL is not specified, Coder will create
> [a publicly accessible URL](../admin/configure.md#tunnel) to reverse proxy the
> deployment, and port forwarding will work. There is a known limitation where
> if the port forwarding URL length is greater than 63 characters, port
> forwarding will not work.

### From an arbitrary port

One way to port forward in the dashboard is to use the "Port forward" button to
specify an arbitrary port. Coder will also detect if processes are running, and
will list them below the port picklist to click an open the running processes in
the browser.

![Port forwarding in the UI](../images/port-forward-dashboard.png)

### From an coder_app resource

Another way to port forward is to configure a `coder_app` resource in the
workspace's template. This approach shows a visual application icon in the
dashboard. See the following `coder_app` example for a Node React app and note
the `subdomain` and `share` settings:

```hcl
# node app
resource "coder_app" "node-react-app" {
  agent_id  = coder_agent.dev.id
  slug      = "node-react-app"
  icon      = "https://upload.wikimedia.org/wikipedia/commons/a/a7/React-icon.svg"
  url       = "http://localhost:3000"
  subdomain = true
  share     = "authenticated"

  healthcheck {
    url       = "http://localhost:3000/healthz"
    interval  = 10
    threshold = 30
  }

}
```

Valid `share` values include `owner` - private to the user, `authenticated` -
accessible by any user authenticated to the Coder deployment, and `public` -
accessible by users outside of the Coder deployment.

![Port forwarding from an app in the UI](../images/coderapp-port-forward.png)

### Cross-origin resource sharing (CORS)

When forwarding via the dashboard, Coder automatically sets headers that allow
requests between separately forwarded applications belonging to the same user.

When forwarding through other methods the application itself will need to set
its own CORS headers if they are being forwarded through different origins since
Coder does not intercept these cases. See below for the required headers.

#### Authentication

Since ports forwarded through the dashboard are private, cross-origin requests
must include credentials (set `credentials: "include"` if using `fetch`) or the
requests cannot be authenticated and you will see an error resembling the
following:

> Access to fetch at
> 'https://coder.example.com/api/v2/applications/auth-redirect' from origin
> 'https://8000--dev--user--apps.coder.example.com' has been blocked by CORS
> policy: No 'Access-Control-Allow-Origin' header is present on the requested
> resource. If an opaque response serves your needs, set the request's mode to
> 'no-cors' to fetch the resource with CORS disabled.

#### Headers

Below is a list of the cross-origin headers Coder sets with example values:

```
access-control-allow-credentials: true
access-control-allow-methods: PUT
access-control-allow-headers: X-Custom-Header
access-control-allow-origin: https://8000--dev--user--apps.coder.example.com
vary: Origin
vary: Access-Control-Request-Method
vary: Access-Control-Request-Headers
```

The allowed origin will be set to the origin provided by the browser if the
users are identical. Credentials are allowed and the allowed methods and headers
will echo whatever the request sends.

#### Configuration

These cross-origin headers are not configurable by administrative settings.

If applications set any of the above headers they will be stripped from the
response except for `Vary` headers that are set to a value other than the ones
listed above.

In other words, CORS behavior through the dashboard is not currently
configurable by either admins or users.

#### Allowed by default

<table class="tg">
<thead>
  <tr>
    <th class="tg-0pky" rowspan="2"></th>
    <th class="tg-0pky" rowspan="3"></th>
    <th class="tg-0pky">From</th>
    <th class="tg-0pky" colspan="3">Alice</th>
    <th class="tg-0pky">Bob</th>
  </tr>
  <tr>
    <th class="tg-0pky" rowspan="2"></th>
    <th class="tg-0pky">Workspace 1</th>
    <th class="tg-0pky" colspan="2">Workspace 2</th>
    <th class="tg-0pky">Workspace 3</th>
  </tr>
  <tr>
    <th class="tg-0pky">To</th>
    <th class="tg-0pky">App A</th>
    <th class="tg-0pky">App B</th>
    <th class="tg-0pky">App C</th>
    <th class="tg-0pky">App D</th>
  </tr>
</thead>
<tbody>
  <tr>
    <td class="tg-0pky" rowspan="3">Alice</td>
    <td class="tg-0pky" rowspan="2">Workspace 1</td>
    <td class="tg-0pky">App A</td>
    <td class="tg-0pky">✅</td>
    <td class="tg-0pky">✅<span style="font-weight:400;font-style:normal">*</span></td>
    <td class="tg-0pky">✅<span style="font-weight:400;font-style:normal">*</span></td>
    <td class="tg-0pky">❌</td>
  </tr>
  <tr>
    <td class="tg-0pky">App B</td>
    <td class="tg-0pky">✅*</td>
    <td class="tg-0pky">✅</td>
    <td class="tg-0pky">✅<span style="font-weight:400;font-style:normal">*</span></td>
    <td class="tg-0pky">❌</td>
  </tr>
  <tr>
    <td class="tg-0pky">Workspace 2</td>
    <td class="tg-0pky">App C</td>
    <td class="tg-0pky">✅<span style="font-weight:400;font-style:normal">*</span></td>
    <td class="tg-0pky">✅<span style="font-weight:400;font-style:normal">*</span></td>
    <td class="tg-0pky">✅</td>
    <td class="tg-0pky">❌</td>
  </tr>
  <tr>
    <td class="tg-0pky">Bob</td>
    <td class="tg-0pky">Workspace 3</td>
    <td class="tg-0pky">App D</td>
    <td class="tg-0pky">❌</td>
    <td class="tg-0pky">❌</td>
    <td class="tg-0pky">❌</td>
    <td class="tg-0pky">✅</td>
  </tr>
</tbody>
</table>

> '\*' means `credentials: "include"` is required

## SSH

First, [configure SSH](../ides.md#ssh-configuration) on your local machine.
Then, use `ssh` to forward like so:

```console
ssh -L 8080:localhost:8000 coder.myworkspace
```

You can read more on SSH port forwarding
[here](https://www.ssh.com/academy/ssh/tunneling/example).
