Friday, June 22, 2007

Testing SMTP verbs and sending an alert based on the SMTP reply code

I had an interesting problem with a SMTP gateway this week because one of its internal processes had failed the server was processing mail down to the TO verb in the SMTP Protocol exchange and then failing with a specific Status code. Usually you could detect this type of fault by looking at growing queue sizes or delayed delivery times but I wanted a solution that would go out and proactively check the SMTP verbs on a mail server so I could make sure that I would be alerted if there was any issue with anything going on in the SMTP stack.

A while ago I posted a SMTP test script for Powershell which simulated a telnet test against port 25 using Powershell. Using this as a base I streamlined the code to stop outputting information to the commandline and added some logic to parse the first 3 characters from the exchange SMTP response which will relate to the SMTP status code. If everything is going well with your SMTP box you should get a 250 status code see for a full list of status codes. If you don’t get a 250 (or 200) then there’s a possible problem in your SMTP stack the script then creates and sends a SMTP message. The way I used this was to do a direct delivery to my cell phone email account which then gets sent to me as an SMS. You really want to bypass going through your normal gateway for alerting eg if this script is checking that gateway. For the first helo verb I set the script to detect both 220 and 250 response codes this was because I was having an issue where certain firewalls would affect this response code.

Alerting is great but I didn’t want the script to constantly alert me for the same issue once it was detected so there are routines in the script to ensure that it doesn’t send any more the 1 alert per hour. It does this by creating a file in the temp directory and writing the date the last alert was sent. When an error is detected and an alert needs to be sent this file is checked and if the timespan between the last alert being sent and the current date is greater than 60 minutes then an alert is sent if not the script just exits without sending an alert.

Because the script is meant to be self contained I used hardcoded variables which need to be set

$remoteHost = "host.targetdomain.com"
$domain = "targetdomain.com"
$sendingdomain = "youdomain.com"
$AlertFilePath="c:\temp\msMailAlert.txt"
$SmtpServerforAlertDelivery = "smsgateway.com"
$AlertFrom = mailserverwarning@youdomain.com
$AerttTo = "0419999999@youcelphone.com"

To run the script I used a schedule task which is a little tricky under powershell in the run line of the scheduled task you need a line such as

powershell -nologo -noprofile -command "c:\utils\smtpverbmailalert.ps1"

I’ve put a downloadable copy of the script here the script itself looks like.

$remoteHost = "host.targetdomain.com"
$domain = "targetdomain.com"
$sendingdomain = "youdomain.com"
$Global:SendAlert = 0
$Global:SentAlert = 0
$AlertFilePath="c:\temp\msMailAlert.txt"
$SmtpServerforAlertDelivery = "smsgateway.com"
$AlertFrom = "mailserverwarning@youdomain.com"
$AerttTo = "0419999999@youcelphone.com"

function readResponse($verb) {

while($stream.DataAvailable)
{
$read = $stream.Read($buffer, 0, 1024)
$rstring = $encoding.GetString($buffer, 0, $read)
switch ($verb){
"HELO" {if ($rstring.substring(0,3) -ne "220"){
if ($rstring.substring(0,3) -ne "250"){
SendAlert("HELO verb Result " + $rstring.substring(0,3))}}
}
"FROM" {if ($rstring.substring(0,3) -ne "250"){SendAlert("From verb Result " +
$rstring.substring(0,3))} }
"TO" {if ($rstring.substring(0,3) -ne "250"){SendAlert("To verb ResultStatus " +
$rstring.substring(0,3))} }
}
}
}

function SendAlert($Problem) {

# Create and write to a file
if (! [System.Io.File]::Exists($AlertFilePath))
{ $alertFile=[System.Io.File]::Createtext($AlertFilePath)
$cdate = get-date
$alertFile.WriteLine($cdate.ToString())
$alertFile.Close()
$Global:SendAlert = 1
}
else{
$alertFile = [System.Io.File]::OpenText($AlertFilePath)
$alertTime = $alertFile.ReadLine()
$alertFile.Close()
$LastAlertTimeSpan = New-TimeSpan -start ($alertTime) -end $(Get-Date)
if ($LastAlertTimeSpan.TotalMinutes -gt 60){
[System.Io.File]::Delete($AlertFilePath)
if (! [System.Io.File]::Exists($AlertFilePath))
{ $alertFile=[System.Io.File]::Createtext($AlertFilePath)
$cdate = get-date
$alertFile.WriteLine($cdate.ToString())
$alertFile.Close()
$Global:SendAlert = 1
}
}
else{
$Global:SendAlert = 0
}

}

if ($Global:SendAlert -eq 1 -band $Global:SentAlert -eq 0) {

$Title = "Mail Server Failed " + $Problem
$Body = $Problem
$SmtpClient = new-object system.net.mail.smtpClient
$SmtpClient.host = $SmtpServerforAlertDelivery
$SmtpClient.Send($AlertFrom,$AerttTo,$title,$Body)
$Global:SentAlert = 1

}

}

$port = 25
$socket = new-object System.Net.Sockets.TcpClient($remoteHost, $port)
if($socket -eq $null) { return; }
$stream = $socket.GetStream()
$writer = new-object System.IO.StreamWriter($stream)
$buffer = new-object System.Byte[] 1024
$encoding = new-object System.Text.AsciiEncoding
readResponse($stream)
$command = "HELO "+ $domain
$writer.WriteLine($command)
$writer.Flush()
start-sleep -m 500
readResponse($stream,"HELO")
$command = "MAIL FROM: <smtpcheck@" + $sendingdomain + ">"
$writer.WriteLine($command)
$writer.Flush()
start-sleep -m 500
readResponse($stream,"FROM")
$command = "RCPT TO: <postmaster@" + $domain + ">"
$writer.WriteLine($command)
$writer.Flush()
start-sleep -m 500
readResponse($stream,"TO")
$command = "QUIT"
$writer.WriteLine($command)
$writer.Flush()
start-sleep -m 500
readResponse($stream,"QUIT")
## Close the streams
$writer.Close()
$stream.Close()

1 comment:

DaveShack said...

How awesome you are, Glen.