how to setup real-time email-notification for crit

a few weeks ago, i wrote a short article about the advantages of using for all your logging needs. syslog is the standard logging solution for *nix platforms and integrates into virtually all application servers, network devices, and programming languages.

it is often important for system administrators to get real time notification of critical events. unfortunately, it isn't immediately obvious how to do this in the syslog framework. in this article i show you step-by-step how to do this.

as usual, all code and configurations have been tested on debian etch but should be useful for other *nix flavors with subtle modifications.

the syslog advantage

one of the big advantages of syslog is the separation between the log request and the logging action. for example, a shell script might contain a log request like:

logger -p local0.crit "my pants are on fire"

this statement logs the message "my pants are on fire" at a critical level to a facility local0.

without changing your script, you can configure syslog perform some or all of these actions:

1. write this to a local logfile
2. log this to the console
3. write this to a remote logging server
4. send an email / sms in real time

let's focus on number 4. real-time notification is a good choice when your pants are on fire.

named-pipes

later versions of syslog have support for writing to named-pipes. a is a special type of file that implements a simple stream, allowing processes to talk to each other. we'll exploit named-pipes to implement real-time messaging between syslog and our mailer.

in our example, we'll take all critical messages written to the local0 facility and (in addition to logging) send them to the mail recipient, [e-mail].

configuring syslog to write to a named-pipe

first, create a named-pipe for critical messages, for example:

# mkdir /etc/syslog.pipes
# mknod /etc/syslog.pipes/criticalMessages p
# chmod 600 /etc/syslog.pipes/criticalMessages

next, configure syslog to log all critical messages written to the local0 facility to this pipe. add the following statement to your syslog.conf file.

local0.crit   |/etc/syslog.pipes/criticalMessages

sending out messages

the final step is to mail out any messages that are written to the pipe. you can do this with a simple shell script. i've included an example below, let's call it /usr/bin/syslogMailer:

#!/bin/bash

# syslogMailer: a script to read stdin and turn each line into an alert
# email typically this is used to read a named-pipe written to by syslog
#
#   example usage: syslogMailer < /etc/syslog.pipes/criticalMessages
#

alertRecipient="[e-mail]"      # the mail recipient for alerts
TMOUT=1                                   # don't wait > 1 second for input

# process each line of input and produce an alert email
while read line
do
   # remove any repeated messages
   echo ${line} | grep "message repeated" > /dev/null 2>&1
   if test $? -eq 1
   then
      # send the alert
      echo "${line}" | mailx -s "critical error on syslog" ${alertRecipient}
   fi
done

daemon vs cron?

you'll notice that i've included the following line in the script:

TMOUT=1                                 # don't wait > 1 second for input

this line specifies a one second timeout for the bash builtin, read. the script therefore runs to completion after processing one batch of zero or more messages. this allows you to schedule it in cron to run, say, every 5 minutes with a statement like:

# m h  dom mon dow   command
0-59/5 * * * * /usr/bin/syslogMailer < /etc/syslog.pipes/criticalMessages > /dev/null 2>&1

alternatively, if you'd like to turn this script into a log-running daemon that will sit in an endless loop and send out messages as soon as log statements arrive, remove the timeout line and surround the read statement with an while-true loop i.e.

# process each line of input and produce an error message
while :
do
   while read line
   do
      [...]
      # send the alert
      echo "${line}" | mailx -s "critical error on syslog" ${alertRecipient}
   done
done

the daemon approach is a little more efficient and sends out emails synchronously. it has the disadvantage that if your daemon terminates unexpectedly, alerts will stop until the daemon is restarted. the cron based implementation is arguably more robust in this regard. the cron approach also allows you to batch up notifications into n minute chunks. 5 minutes in our example cron file above.