Testing remote host connectivity using telnet

telnet used in this fashion is just establishing a somewhat raw TCP connection. nc or netcat is a better tool for command line testing of IP network connectivity.

In the first instance, you connected to a listening sshd on port 22, and got the first part of an SSH handshake--the server version string.

In the second instance, you connected to port 23 where telnetd traditionally listens. However, as telnet is less secure than ssh, it is typically not enabled by default on modern systems. You are unable to connect because there is no telnetd listening on <remote_host_ipaddr>.

You are not establishing a remote shell session when using telnet like this, which is why you cannot execute shell commands on the remote system, such as ping or date. You are establishing a network connection which allows you to hand-type what would normally be sent over the network for the relevant protocol. With SSH, you are pretty much stuck after the initial version exchange, as the exchanged data is encrypted past this point.

A more typical use would be to test a server running a protocol which is text-based, such as HTTP, IMAP, or SMTP, e.g.

$ telnet <remote_host_ipaddr> 80
> GET /index.html HTTP/1.0
>
...
$

With telnet you can check if a port is opened and listening. But not really run commands on the remote host. telnet is majorly insecure and all modern systems have it disabled. That is also the reason why your connection to port 23 fails: the system is not supposed to accept tenet connections and the port is closed. Connection refused means that the connection has been blocked by a firewall or closed port.

For a more dedicated way of testing you may use our test tool dda-serverspec (https://github.com/DomainDrivenArchitecture/dda-serverspec-crate) for such tasks. You may define your expectation

{:netcat [{:host "mywebserver.com" :port "443"}
          {:host "telnet mywebserver.com" :port "80"}
          {:host "telnet mywebserver.com" :port "8443"}]}

and test these expectation either against localhost or against remote hosts (connect by ssh). For remote tests you've to define a targets:

{:existing [{:node-name "test-vm1"
             :node-ip "35.157.19.218"}
            {:node-name "test-vm2"
             :node-ip "18.194.113.138"}]
 :provisioning-user {:login "ubuntu"}}

You may run the test with java -jar dda-serverspec.jar --targets targets.edn serverspec.edn

Under the hood we're using netcat with some special settings for timeout or minimizing traffic ...