check_links.pl

Perl script to monitor resources on the internet.

check_links is quit powerful, as it can monitor

• availability of webpages
• content delivery
• uptime of web servers
• content negotiation
• etc

Requirements

The Perl script requires

• the Nagios plugin library in /usr/lib/nagios/plugins/utils.pm
• LWP::UserAgent
• Getopt::Long
• HTTP::Cookies
• URI

USAGE

perl -w check_links.pl [OPTIONS] -u URL

Here, OPTIONS may be:

• --url URL or -u URL the URL to the site
• ---status STATUS or -s STATUS you expect STATUS to be the returned HTTP status code (default: 200)
• ---no-status do not check the HTTP status
• ---content "CONTENT" or -c "CONTENT" you expect CONTENT to appear within the response
• ---referer "REFERER" or -r "REFERER" use REFERER when sending the request
• ---user-agent "USER AGENT" pretend to be the user agent USER AGENT
• ---header "KEY=VALUE" expected HTTP header value as KEY=VALUE, multiple options are possible and VALUE may be a regex
• ---cookie "NAME=VALUE" sent a cookie NAME=VALUE, multiple options possible
• ---accept "MIME" or -a "MIME" ask for a result of content type MIME, especially useful to check for content negotiation
• ---follow or -f should we follow redirects?
• ---timeout INT or -t INT wait INT seconds before timeout
• ---help or -h show this help

The script’s return code and message is to be interpreted by your monitoring infrastructure.

EXAMPLES

Check a URL is accessible

perl -w check_links.pl -u https://binfalse.de

Check a URL-shortener does the proper redirection

perl -w check_links.pl -u http://goo.gl/33e3Lb -s 301 --header location=https://binfalse.de/

Check that your webserver properly redirects HTTP to HTTPS

perl -w check_links.pl -u http://binfalse.de/ -s 301 --header location=https://binfalse.de/

Check that a web resource has a specific size and that the webserver is NGINX

perl -w check_links.pl -h content-length=191406 -h "server=nginx.*" -u https://binfalse.de/assets/media/pics/2016/drm-inchains.png

Here, the image needs to be exactly of size 191406 Bytes and the regex nginx.* matches to NGINX in any version.

Perform a check using a cookie

perl -w check_links.pl -c userid=karl -c secret=passwd123456 -u https://secret.site

Test content negotiation

Check that you get XML when asking for XML:

check_links.pl -u http://purl.uni-rostock.de/comodi/comodi#Attribution -f -a text/xml --header content-type=xml

Check that you get HTML when asking for HTML:

check_links.pl -u http://purl.uni-rostock.de/comodi/comodi#Attribution -f -a text/html --header content-type=text/html

TESTS

There is a python tool in src/test.py that performs some basic test to verify that check_links.pl is working correctly. I’d like to encourage you to add further test when extending check_links.pl!

Download and install

Download the tool at check_links.pl (or see the project on GitHub)

Please consider to take a look at my general monitoring setup notes.