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TCP sockets

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.

​​ connect()

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: Request) {
const gopherAddr = { hostname: "", 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);
return new Response(socket.readable, { headers: { "Content-Type": "text/plain" } });
} catch (error) {
return new Response("Socket connection failed: " + error, { status: 500 });

​​ SocketAddress

  • hostname string

    • The hostname to connect to. Example:
  • port number

    • The port number to connect to. Example: 5432.

​​ SocketOptions

  • secureTransport “off” | “on” | “starttls” — Defaults to off

  • allowHalfOpen boolean — Defaults to false

    • Defines whether the writable side of the TCP socket will automatically close on end-of-file (EOF). When set to false, the writable side of the TCP socket will automatically close on EOF. When set to true, the writable side of the TCP socket will remain open on EOF.
    • This option is similar to that offered by the Node.js net module and allows interoperability with code which utilizes it.

​​ SocketInfo

  • remoteAddress string | null

    • The address of the remote peer the socket is connected to. May not always be set.
  • localAddress string | null

    • The address of the local network endpoint for this socket. May not always be set.

​​ Socket

  • readable : ReadableStream

    • Returns the readable side of the TCP socket.
  • writable : WritableStream

    • Returns the writable side of the TCP socket.
    • The WritableStream returned only accepts chunks of Uint8Array or its views.
  • opened Promise<SocketInfo>

    • This promise is resolved when the socket connection is established and is rejected if the socket encounters an error.
  • closed Promise<void>

    • This promise is resolved when the socket is closed and is rejected if the socket encounters an error.
  • close() Promise<void>

    • Closes the TCP socket. Both the readable and writable streams are forcibly closed.
  • startTls() : Socket

    • Upgrades an insecure socket to a secure one that uses TLS, returning a new Socket. Note that in order to call startTls(), you must set secureTransport to starttls when initially calling connect() to create the socket.

​​ 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: "",
port: 5432
const socket = connect(address, { secureTransport: "starttls" });
const secureSocket = socket.startTls();
  • startTls() can only be called if secureTransport is set to starttls when creating the initial TCP socket.
  • Once startTls() is called, the initial socket is closed and can no longer be read from or written to. In the example above, anytime after startTls() is called, you would use the newly created secureSocket. Any existing readers and writers based off the original socket will no longer work. You must create new readers and writers from the newly created secureSocket.
  • startTls() should only be called once on an existing socket.

​​ Handle errors

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

import { connect } from 'cloudflare:sockets';
const connectionUrl = { hostname: "", port: 80 };
export interface Env { }
export default {
async fetch(req: Request, env: Env, ctx: ExecutionContext): 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);
return new Response(socket.readable, { headers: { "Content-Type": "text/plain" } });
} catch (error) {
return new Response(`Socket connection failed: ${error}`, { status: 500 });

​​ Close TCP connections

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: "", port: 70 });
const reader = socket.readable.getReader();
// After close() is called, you can no longer read from the readable side of the socket
const reader = socket.readable.getReader(); // This fails

​​ Considerations

​​ Troubleshooting

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.

​​ TCP Loop detected

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.