---
title: Single-Host Network
description: How execd's reverse proxy gives every sandbox access to all HTTP/WebSocket ports through a single exposed host port.
---

# Single-Host Network

Detailed routing for a single-host deployment: how execd’s proxy gives every sandbox access to HTTP and WebSocket ports through one exposed host port.

![Single-host sandbox routing](../public/images/single_host_network.png)

## Single-host routing model
- Every sandbox container starts `execd` listening on container port `44772`. `execd` bundles a lightweight reverse proxy that intercepts requests with the `/proxy/{port}` prefix and forwards them to `127.0.0.1:{port}` inside the same container.
- The Docker runtime binds only the host side of the execd proxy port (labeled `opensandbox.io/embedding-proxy-port`). Callers use `get_endpoint(..., port=X)` to receive `{public_host}:{host_proxy_port}/proxy/{X}`, and execd transparently routes the request back to the sandbox service on port `X`.
- Because the proxy preserves `Upgrade`, `Connection`, and other HTTP headers, HTTP, Server-Sent Events, and WebSocket traffic share the same mapped host port without additional configuration.
- With this setup, a single host port per sandbox suffices to reach **all** container ports. You can safely run many sandboxes on one machine without worrying about overlapping host port allocations.
- When the caller lives inside the same Docker network (e.g., another container or Kubernetes pod), use `get_endpoint(..., resolve_internal=True)` to bypass the host mapping and return the sandbox IP (e.g., `172.17.0.3:5900`) instead.
- The diagram above shows the routing path: host traffic hits the proxy port, execd rewrites the request towards the target container port, and upstream services remain isolated within the sandbox.

## Network modes

### Host network mode (single-host constraints)
- Containers share the host network stack (`network_mode=host`) so sandbox ports are directly accessible on the host.
- Because each sandbox binds its ports on the host, this mode practically limits you to one sandbox instance per host unless you reserve dedicated ports per sandbox.
- `get_endpoint(..., port=X)` returns `{public_host}:{X}` with no `/proxy/` prefix, so the caller needs to know the exact host port and the host must manage firewall rules for each sandbox port.

### Bridge network mode (default for single-host deployments)
- Docker places sandboxes on an isolated bridge network, preventing container ports from being reachable without explicit mapping.
- For single-host scaling, OpenSandbox maps only execd’s proxy port (`44772`) and, optionally, port `8080`. Any other container port stays private and is reached via the proxy.
- The reverse proxy label (`opensandbox.io/embedding-proxy-port`) identifies a host port that fronts `execd`. `get_endpoint(..., port=X)` returns `{public_host}:{host_proxy_port}/proxy/{X}`, so all internal ports can share the same host binding.
- Port `8080` may also receive a direct host binding (`opensandbox.io/http-port`), providing a conventional HTTP endpoint without the proxy path when required.
- This bridge setup lets a single machine host many sandboxes without port conflicts, because the same host proxy port can multiplex requests for HTTP, SSE, WebSocket, VNC, etc.

## Operational notes
- If execd’s proxy port (`44772`) or the optional `8080` host mapping is missing, `get_endpoint` responds with HTTP 500 and a message stating which mapping was unavailable.
- Always keep the `/proxy/{port}` prefix (including any additional path or query string) when embedding URLs in browser-based clients or SDKs so that execd can correctly dispatch the request.
- This proxy-based approach means additional ports never need to be published on the host, simplifying firewall management and improving security.