The core dumps can be activated/deactivated via "ulimit" (if you use bash). Try
ulimit -a on your system. Most likely, you will see a "core file size" of 0, which means "please, no core dumps"; set it to "unlimited" in the shell script, where you start the server. Then restart the server and you will get a core dump in the directory, where the scripts runs (on doubts, do in the script a "cd SOMEPATH" before invoking nsd). Once you have the core file, check the link provided earlier how to analyze the core dump using gdb.