On monitoring HTTP status of multiple service instances of load balanced websites

I had a some time today, and this is what I came up with for a v0.1 (Alpha) Advanced custom sensor in PowerShell

Basically, supply it the host name, an optional CSV of IP addresses, a specific page to get, and whether it should use SSL. It will check each IP address, setting the host name as an HTTP header, and get the HTTP status for each address. Intended to work when a host name has multiple IP addresses defined to allow for DNS Round Robin HA/LB, as well as traditional load balancing where you need to identify specific back-end IP addresses that DNS does not resolve to for that host name.

www.paesller.com for example resolves from a CNAME into 8 IP addresses (today). This sensor will show the HTTP status code of each host.

 ##########################################################################
 # PRTG Custom Advanced EXE/XML Sensor v0.2
 #
 # Get HTTP Status of all A/CNAME DNS records, or specified list of IPs, given a specific host name
 #
 # Parameters: -h <hostname> [-ips <csv of IPs to check>] [-page <path/file to get>] [-ssl (Use httpS)]
 # Example: -h my.website.com -ips 10.10.10.1,10.10.10.2,10.10.10.3 -page testpage.htm -ssl
 #
 # TODO: How to identify error when cant connect to port (currently NULL value)
 # TODO: Include other details from IWR
 # TODO: Make hostname required?
 # TODO: Set warning and/or errors 
 # 
 ##########################################################################
 
 param (
   [string]$h = "www.paessler.com",
   [string]$ips = $null,
   [string]$page = $null,
   [switch]$ssl = $false
 )
 $ipa =@()
if ($ips) {
   # Split CSV parameter into array of strings
   $ipa = $ips.Split(",")
}
else {
   # Get all A and CNAME records for host $h into array of strings
   $d=Resolve-DnsName -Name $h -DnsOnly -TcpOnly -Type A
    foreach($i in $d) {
       if ($i.Address) {
          $ipa += $i.Address
        }
    }
   $d=Resolve-DnsName -Name $h -DnsOnly -TcpOnly -Type CNAME
    foreach($i in $d) {
       if ($i.Address) {
          $ipa += $i.Address
        }
    }
}

Write-Host "<?xml version=`"1.0`" encoding="UTF-8"?>"
Write-Host "<prtg>"
Write-Host " <Text>$($h)</Text>"

# For each IP address found or specified, get HTTP status code for using host header
foreach($i in $ipa) {
   # If -ssl parameter set use HTTPS, otherwise use HTTP as default
   if ($ssl) { $req="https://$($i)/$($page)"}
     else {$req="http://$($i)/$($page)"}
  # Get HTTP status
  [int]$rc=$null
  try { $r = Invoke-WebRequest $($req) -headers @{host=$($h)} -UseBasicParsing -UseDefaultCredentials
        # Do best we can to ensure status value meets type constraint
          $z=[int]::TryParse($r.StatusCode , [ref]$rc )
        } catch {
          $z=[int]::TryParse($_.Exception.Response.StatusCode.Value__ , [ref]$rc )
        }
   # Create result record
   Write-Host " <result>"
   Write-Host "  <channel>$($req)</channel>"
   Write-Host "  <value>$($r.StatusCode)</value>"
   Write-Host "  <float>0</float>"
   Write-Host " </result>"
}
Write-Host "</prtg>"