How to error handle Invoke-Command over SSH

Question

How to error handle Invoke-Command over SSH

Nanne14 20

I'm currently working with a script that utilizes the Invoke-Command over SSH as shown below. While it works well, I'm looking to implement exception handling, specifically for scenarios where the remote server is not reachable.

Invoke-Command -HostName '192.168.100.1' -ScriptBlock { Write-Output "Hi" }

When the remote server is not reachable, I receive the following error message: "ssh: connect to host 192.168.100.1 port 22: Connection timed out" The challenge is that the script halts, and I have to manually abort it.

So as the script just stops I do not know how to handle such an error as following code just does not get executed. Is there any recommended approach or fix to implement proper exception handling in such cases?

2 answers

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Answer 1

Rich Matheisen 47,901

After the Invoke-Command, what follows next in the script? Are you checking the output from the Invoke-Command?

If you want to get the output, including any errors, try redirecting the error stream (a.k.a., STDERR) to the success stream (a.k.a., STDOUT)

Invoke-Command -HostName '192.168.100.1' -ScriptBlock { Write-Output "Hi" } 2>&1

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_redirection?view=powershell-5.1

Nanne14 20 Reputation points

2023-11-30T16:12:54.2133333+00:00

yes my idea was to save the output in a variable and then check in an if statement for the error message.
$result = Invoke-Command -HostName '192.168.100.1' -ScriptBlock { Write-Output "Hi" } 2>&1

But even with "2>&1" the script just does not continue.

Answer 2

Rich Matheisen 47,901

Your Invoke-Command is never run! Replace "-HostName" with "-ComputerName".

Invoke-Command : A parameter cannot be found that matches parameter name 'HostName'.
At C:\Projects\Exercism\work.ps1:1 char:26
+ $result = Invoke-Command -HostName '192.168.100.1' -ScriptBlock { Wri ...
+                          ~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [Invoke-Command], ParameterBindingException
    + FullyQualifiedErrorId : NamedParameterNotFound,Microsoft.PowerShell.Commands.InvokeCommandCommand

If the machine at the IP address isn't accepting connections the Invoke-Command will either return after the attempt at connecting times out (a function of your client machine's network) or, if the address isn't reachable, it will fail very quickly.

If the Invoke-Command fails, the $result variable will hold an object with the type "ErrorRecord" that will provide the details.

Nanne14 20 Reputation points

2023-12-01T10:29:26.53+00:00

The error you recieve is becuase you probably using powershell version < 6.0. I installed PoweShell 7 and require the SSH transport. For SSH you need to use -HostName instad of -ComputerName
MotoX80 36,291 Reputation points

2023-12-01T16:36:34.2366667+00:00
Looks like a bug to me. I tried it with 7.3.9 and 7.4.0. I tried it with invoke-command and new-pssession. I get this and then it just hangs.

ssh: connect to host 192.168.100.1 port 22: Connection timed out

I have to ctrl+c to break out of the hang.

I added "-ConnectingTimeout 5" and I get:

New-PSSession: [192.168.100.1] SSH connection attempt failed after time out: 0 seconds.

I found this on github, but it's not your exact case.

https://github.com/PowerShell/PowerShell/issues/15174

Rich, have you ever reported an issue on github? That would appear to be the place to report this problem.

Nanne14, You could try running a Test-NetConnection first to see if the ip/port responds.

That bad news to that is that, just for fun, I tried to connect to my laptop on 80 (because I knew it would respond with something) and it still hung on me.
Rich Matheisen 47,901 Reputation points

2023-12-01T16:55:52.26+00:00

Yes, I've reported several problems there. Most were documentation problems, but a few were bugs.

Just to note: if the "ConnectingTimeout" isn't used, the timeout is infinite. That parameter was added to version 7.2.0 Preview 5. It should be present in subsequent versions, too.

The New-PSSession also has a -ConnectingTimeout. That cmdlet also has a -SSHConnection parameter. It would make more sense to use a PSSession than an executable to establish the connection.
Rich Matheisen 47,901 Reputation points

2023-12-01T16:59:29.49+00:00

@Nanne14 Since PS6 and PS7 open-source products, I always assume that code is written to work with any version of PS. Stating which version of PS you're using avoids any confusion and the need to make assumptions.
MotoX80 36,291 Reputation points

2023-12-01T17:24:45.6566667+00:00

PS 7.4 on Win10.

Using ConnectingTimeout times out immediately to a bogus host IP. Without it, the connection times out and then the PS window hangs until I ctrl+c.

@Nanne14 if you specify ConnectingTimeout and then any value, but to a "good" host, can you successfully connect?
Nanne14 20 Reputation points

2023-12-01T17:41:36.2566667+00:00

@MotoX80 yes with a higher Number. Its specified in miliseconds with 600 it always fails but with 1000 or higher it works.
MotoX80 36,291 Reputation points

2023-12-01T17:51:45.45+00:00

Ahh.

Does that then solve your hang problem?
Nanne14 20 Reputation points

2023-12-01T18:26:54.1966667+00:00

Sorry to be specific: I refer to Invoke-Command, When using Enter-PSSession I have the same error as you when trying to connect to a "good" Host.
Nanne14 20 Reputation points

2023-12-01T18:30:32.57+00:00

But For my script that fixes the hang but I was not able to save the error message.
When i prepend $result = nothing gets saved there, the Timout error message only gets printed out.
MotoX80 36,291 Reputation points

2023-12-01T18:35:45.1233333+00:00

Sure. PS 7.4 on Win 10. Invoke-command to a bogus address times out after 10 seconds, and doesn't hang using -ConnectingTimeout 10000.
Rich Matheisen 47,901 Reputation points

2023-12-01T19:55:00.86+00:00
The Invoke-Command takes all "Common Parameters". Try adding "ErrorAction STOP" to the cmdlet. Then wrap the Invoke-Command in a Try/Catch block:

Try{ Invoke-Command . . . -ErrorAction STOP } Catch{ $_ }

I don't know what you might want to do in case of an error, so the Catch block just displays the ErrorRecord and the script continues.

I'm not sure what the Invoke-Commands action might be when dealing with remote machines. If Invoke-Command is working with multiple remote machines (a pretty common thing) you really don't want the failure of a single connection to terminate the Invoke-Command -- that would also terminate any of the other connections (not a "good thing").

Is there a reason you're using SSH to access the remote machines? Does remoting to those machines work with PS remoting default connectivity?
Nanne14 20 Reputation points

2023-12-04T12:50:02.14+00:00

@Rich Matheisen @MotoX80 Thanks you so much for your help. I was able to implement everything that I wanted in my script.

I am using SSH because we have a lot of enviornments (a set of VMs) that are sitting behind VyOS Gateways. (The reson is that behind these Gateways these Enviornments can have the same IP-Configuration. We want them all the have the same for developing and testing purposes.)

To address a maschine behind a Gateway we use the Gateways IP and a Port.

In my case '192.168.1.100:54823'.

(In my question above I omitted the port, just for simplicity)

In the Gateways configuration we have configured that when the Gateway gets contacted on Port 54823, the traffic gets forwarded to a specific maschine on Port 22. (Same for a lot of other things like RDP, HTTPS etc.)

I did not find any way to contact the behind Gateway maschines by using PS remoting default connectivity WinRM, bcause of that I use SSH.

With SSH and -HostName Parameter, I can pass this IP/Port combination, but with the ComputerName Parameter I am not, as I directrly recieve the error message "Enter-PSSession: Invalid URI: The hostname could not be parsed"

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How to error handle Invoke-Command over SSH

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