There is source code available for both the iSCSI and iSNS protocols.

iSCSI Source Code
There are two versions, one written by Intel, and another written by employees of Cisco Systems. Both run under Linux. If you have a choice, you should use the Linux 2.4 kernel because the 2.2 kernel has a number of significant SCSI-related bugs. Both of these implementations are a bit behind the draft standard, typically lagging by about six months.

Intel iSCSI Reference Implementation
(v18, 98KB)
This includes both initiator and target implementations for Linux versions based on the 2.2 or 2.4 kernels. At this point the Intel software is a bit dated, having been last updated in December of 2001. You can download a local copy of the code from here, or go directly to the master repository at Source Forge:
http://sourceforge.net/projects/intel-iscsi/

Linux iSCSI Source Code (Cisco)
(v3.1.0.19, 250KB)
This is an initiator-only implementation for Linux versions based on the 2.2 or 2.4 kernels. You can download a local copy of the code from here, or go directly to the master repository at Source Forge:
http://sourceforge.net/projects/linux-iscsi/

iSNS Source Code
Nishan has written a reference implementation of iSNS that they freely distribute. The code can be downloaded from Source Forge, or directly from Nishan. There are two versions, one for Linux and one for Windows NT. They also have a client package for Linux.

Nishan iSNS Source Code
http://www.nishansystems.com/iSNS/

Source Forge location
http://sourceforge.net/projects/linuxisns