First, some minor details you may or may not already be aware of:
  . Have you seen the NetBSD netbooting documentation?
    http://www.netbsd.org/Documentation/network/netboot/index.html
  . Also, do you know what model of NeXT these are?  NetBSD doesn't currently
    run on the 33 Mhz "turbo" machines due to the undocumented DMA controller.
    (I usually tell by looking inside the box.  I _think_ all of the turbo machines
    had socketed CPUs and the non-turbo machines had soldered down CPUs.)

Michael Jinks <mjinks@bsd.uchicago.edu> writes:
> Howdy.  Some friends and I have assembled a big stack of 68040 slabs and
> we'd like to get some kind of *n*x running on them.  So far we've tried
> to boot Linux and NetBSD, with similar results: tftp loads the kernel,
> no hitch.  NFS _appears_ to work fine (server claims to have allowed the
> mount, screen output on the NeXT looks like it has the mount), then
> "something" causes a crash right after the root filesystem is mounted.
tftp should be loading the primary bootloader.  A copy of which can be found in
the NetBSD/next68k tree as /usr/mdec/boot.  This bootloader can then correctly
use NFS to mount and retreive the kernel from the root filesystem.

> 
> I'll spare you the Linux details ;-) but here's what's happened so far
> with NetBSD.  There are subtle differences in screen output depending on
> whether we cold boot, use a plain "b en" boot command, or use (from the
> NetBSD net boot web page) "ben()netbsd".
(Looks like you have seen the netboot web page.  Good)

> If I cold boot the NeXT it flops with the line:
> 
> Load of en()diagnostics: No such file or directory
You should be able to reset the boot rom to look for something other
than "diagnostics".  Check the "p" boot rom command.
> 
> 
> If I use "ben()netbsd" it flops with the line:
> 
> Load of en()netbsd: No such file or directory
This should be the right command.  You need to have bootp set up
correctly to tell it where to look though.

I find tcpdump a useful tool while debugging a booting configuration.
That way I can tell at each step what the NeXT is requesting and what
the host is returning.

If you are still having trouble, try to give us more details about
what your tftp, bootp, and nfs configurations are and what your host
platform is, as well as verbatim console output from just before you
get the lines you listed above.

Hope we can get you up and running,

Darrin