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Port Forwarding

Port Forwarding

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 than:

  1. The Dashboard which proxies traffic through the Coder control plane versus peer-to-peer which is possible with the Coder CLI
  2. sshd which does double encryption of traffic with both Wireguard and SSH

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:

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.

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. If an access URL is not specified, Coder will create a publicly accessible URL to reverse proxy the deployment, and port forwarding will work.

There is a DNS limitation where each segment of hostnames must not exceed 63 characters. If your app name, agent name, workspace name and username exceed 63 characters in the hostname, port forwarding via the dashboard 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 apps inside the workspace are listening on ports, and list them below the port input (this is only supported on Windows and Linux workspace agents).

Port forwarding in the UI

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:

# 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

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

FromAliceBob
Workspace 1Workspace 2Workspace 3
ToApp AApp BApp CApp D
AliceWorkspace 1App A**
App B✅**
Workspace 2App C**
BobWorkspace 3App D

'*' means credentials: "include" is required

SSH

First, configure SSH on your local machine. Then, use ssh to forward like so:

ssh -L 8080:localhost:8000 coder.myworkspace

You can read more on SSH port forwarding here.

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