Category: linux

  • SSL Certificate Expiry Warning Script

    With the increasing trend of SSL on the web, where Google values SSL sites higher and you can have your site be added to the HSTS preload list (the browser will first try HTTPS before trying HTTP), it is a good idea to start using SSL yourself.

    The downside: you need to get a certificate through a (pre-trusted by the browser) CA, or certificate authority. This usually costs money, though there are some services that give you a certificate for free. The free certificates only last for one year or less, this means you need to request and install a new certificate frequently, especially when you have multiple domains.

    Now it can happen to anyone, even Microsoft (Windows Azure Service Disruption from Expired Certificate), that you forget to renew (and update) your certificate in time.

    There is a nice service called (interestingly enough not over HTTPS) that will send you an e-mail when a certificate is due to be updated. But as with any web service, unfortunately you can never be sure how long it’s going to live.

    So, I have created a script that I run through a cronjob every day that will send me a notification e-mail several times in advance (1 day and 2 7 14 30 60 days ahead), so that you are not dependent on a third party to get notified about expiries. As it is supposed to be with cronjobs, there is no output when there is nothing to report (thus no e-mail).

    Here is the script (download

    for i in /etc/certificates/*.pem; do
    	echo $(basename $i): $(openssl x509 -in $i -inform PEM -text -noout -enddate | grep "Not After" | tail -1 | awk '{print $4, $5, $7}') >> $CertExpiries
    Date=$(date -ud "+1 day" | awk '{print $2, $3, $6}')
    Expiries=$(grep "$Date" $CertExpiries)
    if [ $? -eq 0 ]; then 
    	echo These Certificates expire TOMORROW!
    	echo $Expiries
    for i in 2 7 14 30 60; do
    	Date=$(date -ud "+$i day" | awk '{print $2, $3, $6}')
    	Expiries=$(grep "$Date" $CertExpiries)
    	if [ $? -eq 0 ]; then 
    		echo These Certificates expire in $i days:
    		echo $Expiries
    rm $CertExpiries;

  • munin smart plugin: ignore error in the past

    As a hard drive in my server failed, my hosting provider exchanged the drive with another one which obviously had some sort of error in its past, but now seems to be fully ok again. I would have wished to receive a drive without any problems but as my server is RAID 1, I can live with that.

    I do my monitoring with Munin and for monitoring my hard drives I use the smart plugin. Now this plugin also monitors the exit code of smartctl, where smartctl sets bit no 6 if there was an error in the past, so now while everything is alright, the exit code is always numeric 64.

    Now the smart plugin reports this as an error, if the exit code is > 0, i.e. now it always reports a problem.

    I could set the threshold to 65, but then I wouldn’t be notified of other errors which essentially makes the plugin useless.

    I asked at Serverfault but no one seems to have a solution for that.

    So I attacked the problem on my own and patched the plugin. In the source code the important line is here:

    if exit_status!=None :
    # smartctl exit code is a bitmask, check man page.

    which I have modified to look like this:

    if exit_status!=None :
    # smartctl exit code is a bitmask, check man page.
    # filter out bit 6
    num_exit_status &= 191
    if num_exit_status<=2 : exit_status=None if exit_status!=None :

    Now it doesn't bug me anymore when bit 6 is set, but if any other bit goes on again, I will still be notified. The most interesting part is the line where there is a bitwise operation with 191: this is 0x11011111 in binary, so doing an AND operation with the current value it will just set bit no 6 to 0 while letting the other values untouched.

    Therefore a value of 64 (as mine does) will be reported as 0 while a value of 8 would remain at 8. But also, very importantly, a value of 72 (bit 6 set as always and bit 3 set because the disk is failing) it would also report 8.

    And there we have another reason, why it is good to be firm with knowledge about how bits and bytes behave in a computer. Saved me from a warning message every 5 minutes :-)

  • Setting up exim4 on Debian

    i have been dealing with setting up a mail server, lately. debian seems to have a preference for exim. most of the mail admins i know also support this. so i had a look at this.

    as it always happens to me, i start with a not so easy scenario, but after some figuring i got it, mostly by following this guide: Configuring Exim4 and Courier IMAP under Debian GNU/Linux.

    what the article does not say (and might be common knowledge — i will still describe this here for anyone who does this the first time) to have mails addressed to domain be sent to the appropriate mail server, there needs to be a so called MX entry (mail exchanger). See also section 5 of RFC 2821.

    I’ve got some more useful links on this topic:
    Virtual Domains with Exim + Courier-IMAP + MySQL
    Eleven Examples for Configuring Exim
    Secure Mail Relaying with Exim and OpenSSL

    there also is the Exim FAQ and the Exim documentation, but I’m not to fond of lots of on-screen reading.

    exim, courier imap, debian