How to fix SSLSessionCache shmcb session cache not supported

SSLSessionCache cache not supported error occurs when there is an issue with Apache configuration file after an upgrade from version 2.2 to 2.4.

Here are Ibmi Media, we regularly help numerous customers fix Apache related issues as part of our Server Support Services.

In this context, we shall put your through the steps taken to solve this issue in different control panels such as Plesk, Windows and Linux.

More about SSLSessionCache shmcb error and tips to fix it?

Most Apache web server users encounter Apache SSLSessionCache error experienced after upgrading Apache from version 2.2 to 2.4. An error message such as listed below is seen;

SSLSessionCache: 'shmcb' session cache not supported (known names: ). Maybe you need to load the appropriate socache module (mod_socache_shmcb?).

In a shared memory segment (shmcb:/path/to/datafile(512000)), a shared object cache provider known as "mod_socache_shmcb" helps to manage the creation and access to a cache system which enhances performance of the cyclic buffer.

In here, we shall look into some scenarios where this error occurs in Windows, Linux and in Plesk.

In Windows

When you upgrade Apache from version 2.2 to 2.4, you will discover that the "mod_socache_shmcb" module is not enabled by default. However, in Apache 2.2, the apache configuration file (apache/conf/httpd.conf) file has in it the "LoadModule" uncommented as show below;

LoadModule socache_shmcb_module modules/mod_socache_shmcb.so

But note that in Apache 2.4, it is commented by default.

An easy way to fix this error is to simply uncomment the line as stated above in the configuration file at apache/conf/httpd.conf. You should remove the # sign.

Additionally, check that the servername in the SSL attribute line matches the SSL Certificate as well as its location in the server.

If the Windows operating system is 64 bit, ensure that you get a corresponding Apache 64 bit installed for it to work.

In Linux

After the upgrade process of Apache from 2.2 to 2.4, the socache_shmcb.load module might be missing in the Mods-enabled file at /etc/apache2/mods-enabled.

To fix this error in Linux, simply create a symbolic link referencing the module and implement it in the /etc/apache2/mods-enabled as shown below using the command;

ln -s /etc/apache2/mods-available/socache_shmcb.load /etc/apache2/mods-enabled/

Next, do a restart of the Apache web server with any of the following command;

service apache2 restart
/etc/init.d/apache2 restart
systemctl restart apache2
apache2 -k restart

In Plesk

In plesk control panel, after Apache upgrade to version 2.4, you will notice that the configuration files is not completely generated.

To resolve this, inspect the custom templates available in the /usr/local/psa/admin/conf/templates folder. Replace the Include to IncludeOptional  in evert line containing a Wildcart. You can observer the following lines;


Additionally, enable a few Apache modules with the command below;

a2enmod authz_core authz_host access_compat socache_shmcb slotmem_shm mpm_worker unixd php5 dir

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SSLSessionCache: 'shmcb' error is usually triggered after upgrading the Apache from 2.2. to 2.4 version.