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Showing content from https://developers.cloudflare.com/workers/runtime-apis/tcp-sockets/ below:

TCP sockets · Cloudflare Workers docs

The Workers runtime provides the connect() API for creating outbound TCP connections ↗ from Workers.

Many application-layer protocols are built on top of the Transmission Control Protocol (TCP). These application-layer protocols, including SSH, MQTT, SMTP, FTP, IRC, and most database wire protocols including MySQL, PostgreSQL, MongoDB, require an underlying TCP socket API in order to work.

Note

Connecting to a PostgreSQL database? You should use Hyperdrive, which provides the connect() API with built-in connection pooling and query caching.

Note

TCP Workers outbound connections are sourced from a prefix that is not part of list of IP ranges ↗.

The connect() function returns a TCP socket, with both a readable and writable stream of data. This allows you to read and write data on an ongoing basis, as long as the connection remains open.

connect() is provided as a Runtime API, and is accessed by importing the connect function from cloudflare:sockets. This process is similar to how one imports built-in modules in Node.js. Refer to the following codeblock for an example of creating a TCP socket, writing to it, and returning the readable side of the socket as a response:

import { connect } from 'cloudflare:sockets';
export default {
  async fetch(req): Promise<Response> {
    const gopherAddr = { hostname: "gopher.floodgap.com", port: 70 };
    const url = new URL(req.url);
    try {
      const socket = connect(gopherAddr);
      const writer = socket.writable.getWriter()
      const encoder = new TextEncoder();
      const encoded = encoder.encode(url.pathname + "\r\n");
      await writer.write(encoded);
      await writer.close();
      return new Response(socket.readable, { headers: { "Content-Type": "text/plain" } });
    } catch (error) {
      return new Response("Socket connection failed: " + error, { status: 500 });
    }
  }
} satisfies ExportedHandler;
Opportunistic TLS (StartTLS)

Many TCP-based systems, including databases and email servers, require that clients use opportunistic TLS (otherwise known as StartTLS ↗) when connecting. In this pattern, the client first creates an insecure TCP socket, without TLS, and then upgrades it to a secure TCP socket, that uses TLS. The connect() API simplifies this by providing a method, startTls(), which returns a new Socket instance that uses TLS:

import { connect } from "cloudflare:sockets"
const address = {
  hostname: "example-postgres-db.com",
  port: 5432
};
const socket = connect(address, { secureTransport: "starttls" });
const secureSocket = socket.startTls();

To handle errors when creating a new TCP socket, reading from a socket, or writing to a socket, wrap these calls inside try/catch ↗ statement blocks. The following example opens a connection to Google.com, initiates a HTTP request, and returns the response. If this fails and throws an exception, it returns a 500 response:

import { connect } from 'cloudflare:sockets';
const connectionUrl = { hostname: "google.com", port: 80 };
export interface Env { }
export default {
  async fetch(req, env, ctx): Promise<Response> {
    try {
      const socket = connect(connectionUrl);
      const writer = socket.writable.getWriter();
      const encoder = new TextEncoder();
      const encoded = encoder.encode("GET / HTTP/1.0\r\n\r\n");
      await writer.write(encoded);
      await writer.close();
      return new Response(socket.readable, { headers: { "Content-Type": "text/plain" } });
    } catch (error) {
      return new Response(`Socket connection failed: ${error}`, { status: 500 });
    }
  }
} satisfies ExportedHandler<Env>;

You can close a TCP connection by calling close() on the socket. This will close both the readable and writable sides of the socket.

import { connect } from "cloudflare:sockets"
const socket = connect({ hostname: "my-url.com", port: 70 });
const reader = socket.readable.getReader();
socket.close();
// After close() is called, you can no longer read from the readable side of the socket
const reader = socket.readable.getReader(); // This fails

Review descriptions of common error messages you may see when working with TCP Sockets, what the error messages mean, and how to solve them.

proxy request failed, cannot connect to the specified address

Your socket is connecting to an address that was disallowed. Examples of a disallowed address include Cloudflare IPs, localhost, and private network IPs.

If you need to connect to addresses on port 80 or 443 to make HTTP requests, use fetch.

Your socket is connecting back to the Worker that initiated the outbound connection. In other words, the Worker is connecting back to itself. This is currently not supported.

Connections to port 25 are prohibited

Your socket is connecting to an address on port 25. This is usually the port used for SMTP mail servers. Workers cannot create outbound connections on port 25. Consider using Cloudflare Email Workers instead.

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