How does invoke-command transfer commands to remote computer

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How does invoke-command transfer commands to remote computer

Eds89 141

Hi, I have a number of machines that were set with a static IP, Subnet, Gateway and DNS settings. For some reason, we have found that the previous gateway provided by DHCP before setting static settings, has remained as an alternate gateway on the network interface for these machines. This means that on occassion, we are unable to connect to these machines remotely, as Windows decides to use the alternate gateway which is on a totally different subnet to the interface IP. What I'd like to do, is run a script I've written to remotely delete the default routes, and recreate them using the correct gateway IP. My query is, when running Invoke-Command, is everything from the ScriptBlock transfered to the remote machine at the start of the command, or does the parent script execute each part of the script block one command at a time? Reason for asking, is that if it sends the entire script block, it should be unaffected by network drops right? However, if the parent script runs one command at a time, then I would assume as soon as the default routes are deleted, the following commands to create new routes would fail, as the machine will have lost connectivity due to no longer having any default routes? Is there a way I can have it send all the commands in the script block first (assuming this is not the current behaviour), before executing them? Not sure if that's what the "-AsJob" switch does? Cheers James

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Answer 1

Rich Matheisen 48,116

You can use the Invoke-Command with the -InDisconnectedSession parameter if you expect to be disconnected after sending the scriptblock to the remote machine.

Instead of getting the output of the scriptblock you'll get a PSSession object to use to reconnect to the remote machine and retrieve the results.

Be sure to build in a long enough delay to allow the scriptblock to complete before trying to reconnect.

Eds89 141 Reputation points

2023-04-24T07:55:11.4466667+00:00
Thanks for this, that's useful info. I suppose my follow up is two explicit queries;

Should I expect my remote session to be disconnected when running commands to delete default routes and before I create new ones?

Given this switch exists, is it correct to assume that the script block is indeed executed one line at a time remotely, and not sent to the remote machine as one block and executed all at once on the remote machine? Cheers Eds
Eds89 141 Reputation points

2023-04-24T12:24:08.9233333+00:00
Having a bit of trouble rolling this into my existing script; I've gone back to a basic test, and have a script that does this:

$ComputerName = "Computer01" $WindowsCredential = Get-Credential Invoke-Command -ComputerName $ComputerName -Credential $WindowsCredential -SessionName "TestSession" -InDisconnectedSession -ErrorAction Stop -ScriptBlock { Get-Service } Start-Sleep 20 Receive-PSSession -ComputerName $ComputerName -Name "TestSession"

The disconnected session gets created successfully, however the problem is, when doing the "Receive-PSSession", I get an error that states:

Receive-PSSession: The remote session query failed for Computer01 with the following error message: The user name or password is incorrect.

EDIT: Weirdly, if I just do a Get-PSSession -ComputerName Server01, I get an incorrect username or password error. If I just do a Get-PSSession, it lists the session on that server. Weirder still, if I just do Get-PSSession | Receive-PSSession, I get the results back. I can't wrap my head around what is occurring here?

EDIT 2: Ok so at the moment I am using the below to get the results, although still doesn't make sense why I can't just use the ComputerName and Name switches on Receive-PSSession?

$UpdateRemoteSession = Get-PSSession | Where-Object {($_.ComputerName -eq "Server01") -and ($_.Name -eq "TestSession")} | Receive-PSSession

Now I have the results, I want to store them in a variable for querying and manipulating. When doing the below to store them, it also outputs the results to the console? How can I suppress console output, as I want to cherry pick what parts of it to display, not the whole lot

$UpdateRemoteResults = $UpdateRemoteSession | Receive-PSSession
Rich Matheisen 48,116 Reputation points

2023-04-24T14:59:13.9766667+00:00

I wouldn't expect your session to be disconnected because the routing has been changed on the remote machine (that would happen if you changed the IP address, though). But, depending on network software, if a packet is sent to an IP address on one network and the answer comes back on a different one (because the routes on the remote machine have changed), your session may time out.

The Invoke-Command returns the PSSession (instead of the scriptblock's results) when you use -InDisconnectedSession switch, so you should save that and pipe the variable into the Receive-Session.

Have a look at the Invoke-Command help (https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/invoke-command?view=powershell-5.1) and the parameter InDisconnectedSession.
Rich Matheisen 48,116 Reputation points

2023-04-24T15:27:50.2866667+00:00

For the data being echoed to the command line, see the OutputBufferingMode value in the Invoke-Command's -SessionOption parameter.

There's mention of this in the -InDisconnectedState parameters description.

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/invoke-command?view=powershell-5.1
Eds89 141 Reputation points

2023-04-25T08:12:09.89+00:00

Got it, thank you.

Think I now have everything working as expected. Thanks for your help!

Cheers Eds

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