Question

I need to send several commands over telnet to a server. If I try to send them without a time delay between every command, the server freaks out:

var net = require('net');
var conn = net.createConnection(8888, 'localhost');
conn.on('connect', function() {
    conn.write(command_1);
    conn.write(command_2);  
    conn.write(command_3);
    //...
    conn.write(command_n);
})

I guess the server needs some time to respond to command n before I send it command n+1. One way is to write something to the log and fake a “wait”:

var net = require('net');
var conn = net.createConnection(8888, 'localhost');
conn.on('connect', function() {
    console.log('connected to server');
    console.log('I'm about to send command #1');
    conn.write(command_1);
    console.log('I'm about to send command #2');
    conn.write(command_2);  
    console.log('I'm about to send command #3');
    conn.write(command_3);
    //...
    console.log('I'm about to send command #n');
    conn.write(command_n);
})

It might also be the fact that conn.write() is asynchronous, and putting one command after another doesn’t guranty the correct order??

Anyway, what is the correct pattern to assure correct order and enough time between two consecutive commands, for the server to respond?

Answer 1

First things first: if this is truly a telnet server, then you should do something with the telnet handshaking (where terminal options are negotiated between the peers, this is the binary data you can see when opening the socket).

If you don’t want to get into that (it will depend on your needs), you can ignore the negotiation and go straight to business, but you will have to read this data and ignore it yourself.

Now, in your code, you’re sending the data as soon as the server accepts the connection. This may be the cause of your troubles. You’re not supposed to “wait” for the response, the response will get to you asynchronously thanks to nodejs 🙂 So you just need to send the commands as soon as you get the “right” response from the server (this is actually useful, because you can see if there were any errors, etc).

I’ve tried this code (based on yours) against a device I’ve got at hand that has a telnet server. It will do a login and then a logout. See how the events are dispatched according to the sever’s response:

var net = require('net');
var conn = net.createConnection(23, '1.1.1.1');
var commands = [ "logout\n" ];
var i = 0;
conn.setEncoding('ascii');
conn.on('connect', function() {
    conn.on('login', function () {
        conn.write('myUsername\n');
    });
    conn.on('password', function () {
        conn.write('myPassword\n');
    });
    conn.on('prompt', function () {
        conn.write(commands[i]);
        i++;
    });
    conn.on('data', function(data) {
        console.log("got: " + data + "\n");
        if (data.indexOf("login") != -1) {
            conn.emit('login');
        }
        if (data.indexOf("password") != -1) {
            conn.emit('password');
        }
        if (data.indexOf(">#") != -1) {
            conn.emit('prompt');
        }
    });
});

See how the commands are in an array, where you can iteratively send them (the prompt event will trigger the next command). So the right response from the server is the next prompt. When the server sends (in this case) the string ># another command is sent.

Hope it helps 🙂