Nagging Notifications

In the February 2011 issue, I wrote about screen, the console window manager, and how I configure its hardstatus line to show notifications along the bottom of my terminal window. Although some people like their desktop environment to fire up notifications when they have a new e-mail or IM, because I spend a good deal of my time within screen, it has my focus, and it makes sense to put important notifications there. In that February 2011 article, I introduced how to set up the hardstatus line and demonstrated a custom script I use to show when I have new e-mail.

For this article, I expand on the topic of screen notifications with a new notification script I've found incredibly useful. Ever since I've had more than a handful of servers, I've relied on monitoring programs like Nagios to keep track of server health. Although monitoring software has its own method of notifications via e-mail or SMS, I've found it valuable to have my current Nagios health right there in my screen session. It not only provides a backup to my mail notifications, it also saves me from having a Nagios window open in my browser all the time.

If you are new to screen and haven't set up a custom hardstatus line, check out my February 2011 article first to get up to speed. Instead of revisiting how to configure a .screenrc file from scratch, I'm assuming you already have a basic .screenrc set up, and instead, I'm skipping ahead to how to add this Nagios script to your existing screen session.

Screen Scraping for Screen

When I set about writing this script, I realized there are a number of different ways to capture the current health of Nagios. Although I didn't spend a lot of time looking into it, I imagine there are lower-level APIs I could query, but honestly, all I really wanted was to know if Nagios was all green (okay) or had any warnings or critical alerts (yellow or red), and if so, how many. To accomplish that, I decided the simplest method was to scrape one of the Nagios status pages for the information I needed. Honestly, this same method should work pretty well for just about any monitoring program you might use, as long as it has a Web interface and you have enough regex-fu to parse the HTML for the data you need.

I originally wrote the script so that if the Nagios status was okay, it would print that, and if there were any critical or warning alerts, it would output those statistics instead. I realized that I wanted screen to print okay in green, warnings in yellow and critical alerts in red. That way, I might notice problems even if I wasn't looking directly at my terminal at the time. To accomplish this, I actually needed to run the script three different times within screen.

The script below takes just two arguments: the Nagios host to poll (with an optional user name and password if you use one) and the type of status to report. I chose the color codes green, yellow and red to represent okay, warning and critical statuses, respectively. I found the http://nagioshostname/cgi-bin/nagios3/tac.cgi page was the simplest to scrape and had all of the information I needed for the script:


# usage: [user:password@]nagios_host STATUS
# where STATUS is green, red, yellow, or all


open TAC, "wget --timeout=2 -q -O -
 ↪http://$nagios_host/cgi-bin/nagios3/tac.cgi |"; @tac = <TAC>;
close TAC;

foreach $line (@tac){
   if   ($line =~ /(\d+) Down/){        $hosts_down = $1; }
   elsif($line =~ /(\d+) Unreachable/){ $hosts_unreachable = $1; }
   elsif($line =~ /(\d+) Up/){          $hosts_up = $1; }
   elsif($line =~ /(\d+) Pending/){     $hosts_pending = $1; }
   elsif($line =~ /(\d+) Critical/){    $services_critical = $1; }
   elsif($line =~ /(\d+) Warning/){     $services_warning = $1; }
   elsif($line =~ /(\d+) Unknown/){     $services_unknown = $1; }
   elsif($line =~ /(\d+) Ok/){          $services_ok = $1; }
   elsif($line =~ /(\d+) Pending/){     $services_pending = $1; }

# remove the username and password from the output
$nagios_host =~ s/.*\@//;

if($show eq "green" && ($hosts_down == 0 && $services_critical == 0
 ↪&& $services_warning == 0)){
   print "$nagios_host: OK";
elsif($show eq "red" && ($hosts_down > 0 || $services_critical > 0)){
   print "$nagios_host: ${hosts_down}D ${services_critical}C ";
elsif($show eq "yellow" && $services_warning > 0){
   print "$nagios_host: ${services_warning}W ";
elsif($show eq "all"){
   print "${hosts_down}D ${hosts_up}U ${services_critical}C
   ↪${services_warning}W ${services_ok}OK";

As you can see, I actually collect a lot more statistics than I ultimately use, just in case I want to refer to them later. The important thing to note in this script is that in each of the green, red and yellow statuses, I print something only if there's something of that status to print. This is crucial, because I don't want to clutter my hardstatus line, and I want to see yellow or red text only if it truly needs my attention.


Kyle Rankin is Chief Security Officer at Purism, a company focused on computers that respect your privacy, security, and freedom. He is the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu