Fixing Perl strict errors in HAProxy monitoring plugin check_haproxy

Written by Claudio Kuenzler - 0 comments

Published on September 12th 2025 - Listed in HAProxy Perl Coding Monitoring Observability

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To integrate HAProxy into a classic system monitoring, such as Nagios or Icinga, one very handy way is to use the monitoring plugin "check_haproxy.pl". 

I've been using this monitoring for the past 10 years and also contributed to it a while ago, adding performance data for the sessions and traffic per HAProxy backend. This allows to create nice graphs in Grafana for observability:

Bareword X not allowed while strict subs in use 

But lately I came across some Perl errors, when the plugin was executed on an Ubuntu 24.04 machine with Perl 5.38 and libmonitoring-plugin-perl 0.40-1:

ck@ubuntu-noble:~$ /usr/lib/nagios/plugins/check_haproxy.pl -u "http://mylovelyhaproxy.example.com:8099?stats;csv"
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 184.
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 187.
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 200.
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 209.
Bareword "OK" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 229.
Bareword "OK" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 241.
Bareword "UNKNOWN" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 254.
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 300.
Bareword "WARNING" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 303.
Bareword "CRITICAL" not allowed while "strict subs" in use at /usr/lib/nagios/plugins/check_haproxy.pl line 306.
/usr/lib/nagios/plugins/check_haproxy.pl has too many errors. 

Missing string "doublequotes"

The Perl errors are very helpful here; they precisely show the string (BAREWORD) causing the problem in which line. Example in the mentioned line 184:

$np->add_message(CRITICAL, _gt("HTTP error: ").$err );

By simply putting the message (CRITICAL) into a valid string fixes the strict subs error:

$np->add_message("CRITICAL", _gt("HTTP error: ").$err );

The plugin is working nicely again:

ck@ubuntu-noble:~$ /usr/lib/nagios/plugins/check_haproxy.pl -u "http://mylovelyhaproxy.example.com:8099?stats;csv"
HAPROXY OK -  myapp-out (Active: 1/1) | t=0.047756s;2;10;0; sess_myapp-out=1;;;0;26212 bytes_in_myapp-out=72341102B;;;0; bytes_out_myapp-out=1188288893B;;;0;

The fixed version is check_haproxy.pl v1.2 and can be found here. 

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