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## TCP echo server
This is an example of a simple server which accepts connections on port 8080,
and returns to the client anything it sends.
```ts
const listener = Deno.listen({ port: 8080 });
console.log("listening on 0.0.0.0:8080");
for await (const conn of listener) {
Deno.copy(conn, conn);
}
```
When this program is started, it throws PermissionDenied error.
```shell
$ deno run https://deno.land/std/examples/echo_server.ts
error: Uncaught PermissionDenied: network access to "0.0.0.0:8080", run again with the --allow-net flag
► $deno$/dispatch_json.ts:40:11
at DenoError ($deno$/errors.ts:20:5)
...
```
For security reasons, Deno does not allow programs to access the network without
explicit permission. To allow accessing the network, use a command-line flag:
```shell
$ deno run --allow-net https://deno.land/std/examples/echo_server.ts
```
To test it, try sending data to it with netcat:
```shell
$ nc localhost 8080
hello world
hello world
```
Like the `cat.ts` example, the `copy()` function here also does not make
unnecessary memory copies. It receives a packet from the kernel and sends back,
without further complexity.
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