You're looking for an iSCSI Target
The Linux kernel has supported the LIO Target
since it was merged in January 2011. The choice if LIO over other competing targets was controversial, and the official Wiki page is full of warnings not to use supposedly supported features.
You'll need to download the necessary userspace tools, which right now seems to be targetcli
. Some distributions offer it as a package, but AFAIK this is not the case for Slackware.
Alternatively, you could use iSCSI-SCST
. It supports several of the features marked as "do not use" in LIO and has been considered stable for years, but to achieve optimal performance you'll unfortunately have to patch the kernel.
(Disclaimer: I'm a longtime SCST user and no fan of LIO.)