#Requires -Version 5.1
<#
    JbTecWiz Support Centre -- generated fix script

    Fault : VPN errors 809, 789 and 691 -- the tunnel will not come up
    Fix   : Allow IPsec through the NAT in front of the client
    Source: https://jbtecwiz.com/support/win-net-vpn-809

    Run as     : Elevated PowerShell on the client
    Expect     : 15 minutes
    Risk       : medium
    Reversible : yes

    WHEN THIS IS THE RIGHT FIX
    Error 809 or 800, and the same profile works from another network.

    HOW TO UNDO IT
    Remove-ItemProperty -Path
    'HKLM:\SYSTEM\CurrentControlSet\Services\PolicyAgent' -Name
    AssumeUDPEncapsulationContextOnSendRule, then restart.

    This script walks the fix one step at a time and asks before each
    one. Steps with no command are things you do yourself -- it prints
    those and waits. Run with -DryRun to print without executing.

    --------------------------------------------------------------------
    NO WARRANTY - USE AT YOUR OWN RISK

    This script is provided by JbTecWiz as-is and with no warranty of any
    kind, express or implied. You run it entirely at your own risk.

    JbTecWiz accepts no liability for any loss or damage arising from its
    use, including but not limited to data loss, downtime, or configuration
    changes that turn out to be wrong for your system.

    You are responsible for reading this script before running it, for
    satisfying yourself that it suits the machine in front of you, and for
    having a working backup first. Some steps cannot be undone.
    --------------------------------------------------------------------
#>

[CmdletBinding()]
param(
    # Print every step and command without running anything.
    [switch]$DryRun,
    # Do not ask before each step. Read the script first if you use this.
    [switch]$Unattended
)

$ErrorActionPreference = 'Stop'
$script:Failed = 0

# - THIS SCRIPT WILL NOT RUN UNTIL YOU EDIT IT -
# The commands below contain values only you can supply. Search the
# script for each one, replace it, then delete this block.
$script:MustEdit = @(
    'example.com'   # a placeholder domain
)
if ($script:MustEdit.Count) {
    Write-Host ""
    Write-Host '  This script needs editing before it can run.' -ForegroundColor Yellow
    Write-Host '  Replace each of these with a real value:' -ForegroundColor Yellow
    $script:MustEdit | ForEach-Object { Write-Host ("    " + $_) -ForegroundColor Yellow }
    Write-Host ""
    Write-Host '  Then delete the $script:MustEdit block near the top.'
    Write-Host ""
    exit 2
}

function Write-Rule { param([string]$Text)
    Write-Host ''
    Write-Host ('-' * 70) -ForegroundColor DarkGray
    if ($Text) { Write-Host $Text -ForegroundColor Cyan }
}

function Show-Prose { param([string]$Text, [string]$Colour = "Gray")
    if (-not $Text) { return }
    $words = $Text -split "\s+"; $line = "  "
    foreach ($w in $words) {
        if (($line.Length + $w.Length) -gt 74) { Write-Host $line -ForegroundColor $Colour; $line = "  " }
        $line += "$w "
    }
    if ($line.Trim()) { Write-Host $line -ForegroundColor $Colour }
}

function Invoke-Step {
    param(
        [int]$Number, [string]$Do, [string]$Why,
        [scriptblock]$Command, [switch]$Manual, [string]$Shell = "powershell"
    )
    Write-Rule "  Step $Number of 4"
    Show-Prose $Do "White"
    if ($Why) { Write-Host ""; Show-Prose $Why "DarkGray" }

    if ($Manual) {
        Write-Host ''
        Write-Host '  -> Do this yourself, then press Enter to carry on.' -ForegroundColor Yellow
        if (-not $Unattended -and -not $DryRun) { [void](Read-Host) }
        return
    }

    Write-Host ''
    foreach ($l in ($Command.ToString().Trim() -split "`n")) {
        Write-Host ("    " + $l.Trim()) -ForegroundColor Green
    }
    Write-Host ''

    if ($DryRun) { Write-Host "  (dry run -- not executed)" -ForegroundColor DarkGray; return }
    if (-not $Unattended) {
        $a = Read-Host "  Run this step? [Y]es / [S]kip / [Q]uit"
        if ($a -match "^[Qq]") { Write-Host "  Stopped at your request."; exit 0 }
        if ($a -match "^[Ss]") { Write-Host "  Skipped." -ForegroundColor DarkGray; return }
    }
    try { & $Command }
    catch {
        $script:Failed++
        Write-Host ("  Step $Number failed: " + $_.Exception.Message) -ForegroundColor Red
        Show-Prose "The rest of the fix may depend on this. Read the write-up before carrying on." "Red"
        if (-not $Unattended) {
            $c = Read-Host "  Carry on anyway? [y/N]"
            if ($c -notmatch "^[Yy]") { exit 1 }
        }
    }
}

Write-Rule
Write-Host '  VPN errors 809, 789 and 691 -- the tunnel will not come up' -ForegroundColor White
Write-Host '  Allow IPsec through the NAT in front of the client' -ForegroundColor Cyan
Write-Host ''
Write-Host '  Risk: medium   Reversible   15 minutes'
Write-Host ''
Show-Prose 'No warranty. Use at your own risk - JbTecWiz accepts no liability. Read it before you run it, and have a backup.' 'DarkYellow'
Write-Rule

if (-not $Unattended -and -not $DryRun) {
    $go = Read-Host '  Ready? [y/N]'
    if ($go -notmatch "^[Yy]") { Write-Host "  Nothing was changed."; exit 0 }
}

Invoke-Step -Number 1 -Do 'Check whether the server is even reachable on the right port. L2TP is UDP 500 and 4500, SSTP is TCP 443, IKEv2 is UDP 500 and 4500.' -Command {
    Test-NetConnection vpn.example.com -Port 443 -InformationLevel Detailed
}

Invoke-Step -Number 2 -Do 'Set the NAT traversal value so the client will build a tunnel through a NAT device.' -Why 'By default Windows will not build an IPsec tunnel when both ends sit behind NAT. A value of 2 permits it, which is the standard fix for a home router in front of a client and a firewall in front of the server.' -Command {
    New-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\PolicyAgent' -Name AssumeUDPEncapsulationContextOnSendRule -PropertyType DWord -Value 2 -Force
}

Invoke-Step -Number 3 -Do 'Restart the machine. The policy agent reads this at boot.' -Manual

Invoke-Step -Number 4 -Do 'If it still fails on that network only, the router is dropping UDP 500 -- switch the profile to SSTP over TCP 443, which passes through anything.' -Command {
    Set-VpnConnection -Name 'Work VPN' -TunnelType Sstp -Force
}

Write-Rule "  Confirm it worked"
Show-Prose 'The tunnel connects and the assigned address appears.' 'White'
Write-Host ''
if (-not $DryRun) {
    Get-VpnConnection -Name 'Work VPN' | Format-List Name,ConnectionStatus,TunnelType
    ipconfig | findstr /i "PPP"
}

Write-Rule
if ($script:Failed -gt 0) {
    Write-Host ("  Finished with " + $script:Failed + " failed step(s).") -ForegroundColor Yellow
    Show-Prose 'Read the full write-up at https://jbtecwiz.com/support/win-net-vpn-809' 'Yellow'
} else {
    Write-Host '  Finished.' -ForegroundColor Green
}
Write-Host ''
Show-Prose 'To undo: Remove-ItemProperty -Path ''HKLM:\SYSTEM\CurrentControlSet\Services\PolicyAgent'' -Name AssumeUDPEncapsulationContextOnSendRule, then restart.' 'DarkGray'
Write-Rule