#!/usr/bin/bash


#############################################################################################################
# check-postfix.sh                                                                                          #
#############################################################################################################
#                                                                                                           #
#  This script is just something I cooked up for work. I setup a server with posftix configured as an`      #
#  email/smtp relay so that some of our old equipment can still function without having to try and          #
#  configure our commercial Email Server to accept unauthed requests to send email.                         #
#                                                                                                           #
#  This requres a few packages to be installed:                                                             #
#  - postfix (of course)                                                                                    #
#  - telnet                                                                                                 #
#  - sendmail                                                                                               #
#                                                                                                           #
#  The issue is if the postfix service dies and fails to come back up after a restart via this script,      #
#  you will be unable to send any emails via a smtp client (as they will try to contact the service         #
#  on the server itself). So the only workaround I was able to find is to manually send the email via       #
#  telnet using a coproc. You could use a smtp library with python or perl, but I like to keep these        #
#  scripts using a single language wherever possible.                                                       #
#                                                                                                           #
#  Since telnet is just a basic tcp client we are able to interact with the smpt port on a mailserver       #
#  (port 25). So setting it up as coproc allows us to run a concurrent process and pipe commands into       #
#  the file-descriptor to send the commands to the mailserver. Which allows us to manually send an email    #
#  to an authoritative email server (provided they don't have your IP blocklisted or are filtering          #
#  traffic on port 25).                                                                                     #
#                                                                                                           #
#############################################################################################################

# BEGIN: Variables
## Duration to wait to see if service has come back up
DURATION=10
## Wait duration to send next part of message
WAIT=1

## Test flag
TEST=0

## Email addresses
NOTIFY_EMAIL=recipient@domain.com
MAIL_FROM=username@domain.net
FROM_DOMAIN=` echo $FROM_EMAIL | cut -d @ -f 2 `
DAMAIN=` echo $NOTIFY_EMAIL | cut -d @ -f 2 `

# END: Variables


# BEGIN: Helper Functions

send_email () {
	local MESSAGE="${1:?"send_email: No message was passed through"}"
	sendmail $NOTIFY_EMAIL <<< "$MESSAGE"

}


: "
The only option is to manually telnet to the smtp port on the authoritative mailserver for the target domain. As otherwise unless a mail-host is configured as an email relay, you will be unable to send an email to a user outside of the mailservers domain (without authentication). 
"
do_emergency_email () {
	local MESSAGE="${1:-"The Postfix service has failed to come up on `hostname` (`hostname -i`) after a service restart. Please ssh into server to troubleshoot the issues."}"
	## Getting a mailserver IP for manual message
	MAILSERVER=`dig $DOMAIN mx | grep -Eo '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' | head -n 1`

	## Setting up coprocess to send commands to telnet session coproc TELNET { telnet $MAILSERVER 25; }

	## Commands to send email manually
	local -a commands=(
		"ehlo $FROM_DOMAIN\n"
		"mail from: <$MAIL_FROM>\n"
		"rcpt to: <$NOTIFY_EMAIL>\n"
		"data\n"
		"Subject: Postfix Service Failure\nFrom: $MAIL_FROM\nTo: $NOTIFY_EMAIL\n"
		"$MESSAGE\n\n.\n"
		"quit\n"
	)

	## Iterate through commands and send them to coprocesses
	### We need to wait before each command as the remote mailserver will not catch everything otherwise.
	for i in ${!commands[@]}; do
		COMMAND="${commands[$i]}"
		echo -e "$COMMAND" >&${TELNET[1]}
		sleep $WAIT
	done
}

# END: Helper Functions


# BEGIN: Test Check 

if [[ $TEST -eq 1 ]]; then
	MESSAGE_1="
This is a test message to verify that postfix can send an email
"
	send_email "$MESSAGE_1"

	MESSAGE_2="This is a test email to make sure the postfix crash workaround email works"

	do_emergency_email "$MESSAGE_2"
	exit
fi

# END: Test Check


# BEGIN: Work

if [[ `systemctl is-active postfix`  != 'active' ]]; then
	systemctl restart postfix

	sleep $DURATION

	SERVICE_STATUS=` systemctl is-active postfix `

	if [[ "$SERVICE_STATUS" == 'active' ]]; then
		MESSAGE="
SUBJECT: Postfix Service Failure
FROM: root
TO: $NOTIFY_EMAIL

The postfix service on `hostname` (`hostname -i`) was found to be not running.

The service has been restarted, and after waiting $DURATION seconds it was found to be $SERVICE_STATUS.
"
		send_email "$MESSAGE"
	else
		do_emergency_email
	fi
fi

# END: Work