networked day to day technical issues


Multiple domain selfsigned ssl/tls certificates for Apache (namebased ssl/tls vhosts)

This is an old problem: how to have ssl/tls name based virtual hosts with Apache .
The issue is that the ssl/tls connection is established before Apache even receives a HTTP request.When Apache receives the request already the SSL connection is established with a particular hostname - ip & ssl certificate combination so this means that it is capable of serving NameBased virtual hosts only for that particular ssl/tls certificate.

There are two possible solutions here:

  • Multi domain or wildcard SSL/TLS certificates. Those are certificates which are configured with more than one name so you can create virtual hosts (in case of apache) for those domains. This is fairly easy to set up and at least for me it has worked ok in the past.
  • Server Name Indication (SNI) which is an extension to the SSL/TLS protocol and allows the client to specify the desired domain earlier and the server to be notified so it supplies the correct SSL/TLS certificate depending on the requested hostname. The problem is SNI is fairly new and few server side software supports it, also client side software needs to be fairly new. On the long run this is going to be the best solution as it has been designed to overcome this specific problem
Filed under: Apache, Linux, ssl, Web Continue reading

KSM (Kernel Samepage Merging) status

KSM allows physical memory de-duplication in Linux, so basically you can get a lot more out of your memory at expense of some cpu usage (because there is a thread which scans memory for duplicate pages). Typical usage is for servers running virtual machines on top of KVM but applications aware of this capability could also use it even on OS instances which aren't VMs running on KVM.
The requirements are a kernel version of at least 2.6.32 and CONFIG_KSM=y. For more details you can check the official documentation and a tutorial on how to enable it.

Below is a small script (called ksm_stat) which I wrote in order to see how much memory is "shared" and how much memory is actually being saved by using this feature.

if [ "`cat /sys/kernel/mm/ksm/run`" -ne 1 ] ; then
       echo 'KSM is not enabled. Run echo 1 > /sys/kernel/mm/ksm/run' to enable it.
       exit 1
echo Shared memory is $((`cat /sys/kernel/mm/ksm/pages_shared`*`getconf PAGE_SIZE`/1024/1024)) MB
echo Saved memory is $((`cat /sys/kernel/mm/ksm/pages_sharing`*`getconf PAGE_SIZE`/1024/1024)) MB
if ! `type bc &>/dev/null`  ; then
        echo "bc is missing or not in path, skipping ratio calculation"
        exit 1
if [ "`cat /sys/kernel/mm/ksm/pages_sharing`" -ne 0 ] ; then
        echo -n "Shared pages usage ratio is ";echo "scale=2;`cat /sys/kernel/mm/ksm/pages_sharing`/`cat /sys/kernel/mm/ksm/pages_shared`"|bc -q
        echo -n "Unshared pages usage ratio is ";echo "scale=2;`cat /sys/kernel/mm/ksm/pages_unshared`/`cat /sys/kernel/mm/ksm/pages_sharing`"|bc -q

Example of a machine where it just has been enabled, so it takes a while until all pages are scanned

# ksm_stat
Shared memory is 67 MB
Saved memory is 328 MB
Shared pages usage ratio is 4.87
Unshared pages usage ratio is 17.04


Zarafa templates for Zabbix

Recently i had to create Zabbix templates in order to monitor Zarafa Collaboration Platform installations. My employer was kind enough to make them available .

Some screenshots follow below, you can get the templates from Accelcloud's site.