Automating Telnet with PowerShell

Question

How can I write a PowerShell script to automate this set of commands?

• Telnet to a machine,
• execute few commands,
• analyze at the output in the telnet window,
• based on that output, send few more commands

Answer 1

Rather than try to automate a telnet executable, just create the socket and issue the commands, read them back, and make decisions based on that. Here is an oversimplified example connecting to my local web server:

function test() {
  $msg = [System.Text.Encoding]::ASCII.GetBytes("GET / HTTP/1.0`r`nHost: localhost`r`n`r`n")
  $c = New-Object System.Net.Sockets.TcpClient("localhost", 80)
  $str = $c.GetStream()
  $str.Write($msg, 0, $msg.Length)
  $buf = New-Object System.Byte[] 4096
  $count = $str.Read($buf, 0, 4096)
  [System.Text.Encoding]::ASCII.GetString($buf, 0, $count)
  $str.Close()
  $c.Close()
}

Obviously you would need to change it from port 80, and pass a username/password instead of a web request header... but this should be enough to get you started.

Answer 2

Ok this isn't the most elegant solution, and it does rely on shudder VBscript but here it goes...

Create a VBScript to actually expedite the telnet session, this is an example

set oShell = CreateObject("WScript.Shell")
oShell.run("Telnet")
WScript.Sleep 1000
oShell.SendKeys("Open 127.0.0.1 23")
WScript.Sleep 1000
oShell.SendKeys("{Enter}")
WScript.Sleep 1000
oShell.SendKeys("n")
WScript.Sleep 1000
oShell.SendKeys("{Enter}")
WScript.Sleep 1000
oShell.SendKeys"MyName"
WScript.Sleep 1000
oShell.SendKeys("{Enter}")
WScript.Sleep 1000
oShell.SendKeys("MyPassword")
WScript.Sleep 1000
oShell.SendKeys("{Enter}")
WScript.Sleep 1000
oShell.SendKeys("MyCommand")
WScript.Sleep 1000
oShell.SendKeys("{Enter}")
WScript.Sleep 1000

Then use Powershell to invoke that script and pass it the commands you want executing, in the example below these commands are stored in a file called CommandList.txt

function Connect-MyTelnet{
Param(
 [string] $IPAddress,
 [string] $Port,
 [string] $UserName,
 [string] $Password,
 [string] $cmdlistPath
)
    ## - Setting default values:
    if($port -eq $null){ $Port = "23"; };
    if($cmdlistPath -eq $null) { $CmdlistPath = 'c:\temp\cmdlist.txt'; };

    ## create vbscript file: MyTelnetSession.vbs
    ## - For Microsoft Telnet:
    $MyVBScript = @"
                   set oShell = CreateObject("WScript.Shell")`r`n
                   oShell.run("Telnet")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("Open $IPAddress $Port")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("{Enter}")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("n")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("{Enter}")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("$UserName")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("{Enter}")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("$Password")`r`n
                   WScript.Sleep 1000`r`n
                   oShell.SendKeys("{Enter}")`r`n
                   WScript.Sleep 1000`r`n
                 "@;

    ## - Get file with telnet commands:
    [array] $Cmdlist = Get-Content $cmdlistPath;

    ## loop through and build each telnet command line:
    foreach($cmd in $cmdlist)
    {
        ## - Build VBscript lines:
        $MyVBScript += 'oShell.SendKeys("'+$cmd+'")'+"`r`n";
        $MyVBScript += "WScript.Sleep 1000`r`n";
        $MyVBScript += 'oShell.SendKeys("{Enter}")'+"`r`n";
        $MyVBScript += 'WScript.Sleep 1000'+"`r`n";
    }

    ## - Close Telnet Session:
        $MyVBScript += 'oShell.SendKeys("  QUIT")'+"`r`n";
        $MyVBScript += "WScript.Sleep 1000`r`n";
        $MyVBScript += 'oShell.SendKeys("{Enter}")'+"`r`n";
        $MyVBScript += 'WScript.Sleep 1000'+"`r`n";

    ## - Save and execute generated VBscript:
    $MYVBScript | Out-File -FilePath c:\temp\MyTelnet.vbs -Encoding ASCII;
    & c:\temp\MyTelnet.vbs
}; Set-Alias ct Connect-MyTelnet;

And that should do what you are asking...

Note: Not my solution, found from this blog post and I have made use of it once or twice.