Manage DNS Failover

dnsFailover.ps1

param(
    [Parameter(Mandatory=$false)]
    [string]$dnsName,
    
    [Parameter(Mandatory=$false)]
    [string]$dnsServer,
    
    [Parameter(Mandatory=$false)]
    [string]$lookupZone,
    
    [ValidateSet("check", "failover")]
    [string]$Ops = "check",  # Default operation mode
    
    [System.TimeSpan]$TTL = [System.TimeSpan]::FromMinutes(15)  # Sets TTL to 15 minutes
    [int]$waitTimeInit = 300,  # Default init wait time in seconds before 1st check
    [int]$waitTime = 60,  # Default wait time in seconds for each check
    [int]$maxWaitTime = 1800  # Max wait time to verify the failover (double the default TTL 15 mins)
)

function LogMessage($message) {
    Write-Output "$(Get-Date) - $message"
}


function Get-ActiveHost {
    try {
        $ping = Test-Connection -ComputerName $dnsName -Count 1 -ErrorAction Stop
        # Resolve the hostname from the IP address
        $hostEntry = [System.Net.Dns]::GetHostEntry($ping.Address)
        $activeHost = $hostEntry.HostName
        #LogMessage "Active host : $activeHost"
        return $activeHost
    } catch {
    LogMessage "Unable to ping $dnsName or resolve hostname. Error : $_"
        return $null
    }
}


function GetCurrentCnameData {
    try {
        $dnsRecord = Get-DnsServerResourceRecord -Name $dnsName -RRType CName -ZoneName $lookupZone -ComputerName $dnsServer -ErrorAction Stop
        return $dnsRecord.RecordData.HostNameAlias
    } catch {
        LogMessage "Failed to get DNS record for $dnsName. Error : $_"
        return $null
    }
}

function Update-Cname($currentData) {
    $newData = switch ($currentData) {
        {$currentData -match "Prod"} {$currentData -replace "Prod", "DR"}
        {$currentData -match "DR"} {$currentData -replace "DR", "Prod"}
        default {
            LogMessage "No matching 'Prod' or 'DR' found in $currentData."
            return $false
        }
    }

    if ($newData) {
        try {
            Remove-DnsServerResourceRecord -ZoneName $lookupZone -Name $dnsName -RRType CName -ComputerName $dnsServer -Force
            Add-DnsServerResourceRecordCName -Name $dnsName -HostNameAlias $newData -ZoneName $lookupZone -ComputerName $dnsServer -TimeToLive $TTL
            LogMessage "DNS CNAME updated to $newData"
            return $true
        } catch {
            LogMessage "Failed to update DNS CNAME. Error : $_"
            return $false
        }
    }
}

function Verify-Failover($initialHost) {
    $elapsedTime = 0
    Start-Sleep -Seconds $waitTimeInit
    do {
        Start-Sleep -Seconds $waitTime
        $elapsedTime += $waitTime
        $newHost = Get-ActiveHost
        if ($newHost -ne $initialHost) {
            LogMessage "DNS failover succeeded, new host is $newHost"
            return
        }
    } while ($elapsedTime -lt $maxWaitTime)
    LogMessage "DNS failover failed or is still pending propagation after $maxWaitTime seconds."
}

# Main Execution Block
switch ($Ops) {
    "check" {
        Get-ActiveHost
    }
    "failover" {
        $initialHost = Get-ActiveHost
        if ($initialHost) {
            $currentData = GetCurrentCnameData
            if ($currentData) {
                if (Update-Cname $currentData) {
                    Verify-Failover $initialHost
                }
            }
        }
    }
}