Port forwarding on demand.

The problem

An agent started a dev server on port 3000 inside instance api and you want it in your browser, or psql on your host against that instance’s Postgres. Publishing ports with -p has to happen at create time, collides as soon as a second instance wants the same port, and can’t reach a server listening on the container’s 127.0.0.1.

devsandbox port forwards a port on demand, like VS Code’s Ports tab: it is on localhost until you stop it.

Try it

With an instance api running, from a host terminal (no config needed; runs until Ctrl-C):

devsandbox port api 3000                        # http://localhost:3000
devsandbox port api 8080:3000                   # host :8080 -> api's :3000
devsandbox port api --service postgres 5432     # a service api uses
devsandbox port my-server/web 3000              # an instance on another machine

• --service: forward a service’s port instead of the instance’s.
• --address: the host address to bind (default 127.0.0.1).
• --machine: a global service on a machine (--machine my-server --service redis 6379); the port still opens here.

From the dashboard: p on an instance or service, type the port; live forwards are on the Ports tab (3). The host daemon keeps them, so they last after you close the dashboard and come back on the same host port, until d stops them; devsandbox port forwards only while it runs. To forward a port whenever the instance runs, add forwardPorts = [3000] to the sandbox (forwardPorts).

How it works

• Each connection is tunnelled through the in-container helper, so a server on the container’s 127.0.0.1 is reachable and nothing is published.
• --service goes through a running instance that uses the service (services).
• A bare port taken on the host falls back to the next free one; the printed line says which.
• On a machine, the same tunnel runs over one ssh per forward, reconnecting if it drops; its own forwardPorts aren’t copied here.
• Unix hosts, TCP only.

Source: site/src/content/examples/port-forwarding.mdxEdit this page on GitHub(opens in a new tab)