You can try holding "option" when turning the computer on. This will bring up a menu and you should be able to select "Faux Netboot" or something along those lines.

I expect your netboot set is 10.10.5? that should support the iMac. (in fact every Mac sold today)
If not: which model iMac? which OS version in netboot set?
The option key is the best, it shows all boot options available.

@Poseiden I second @mjsanders recommendation of option-N booting.. Also, are there other NetBoot Images being offered? Which is the default?

@LyndhurstSchools

Holding down the option key doesn't sow the Netboot server.

@mjsanders @bentoms

We have one Netboot image which is on OS X 10.10.5 (created with AutoCasperNBI) served over HTTP.

Could the reason it's not showing be that the Netboot server is on a different VLAN and we're using IP Helpers to boot to it?

@Poseiden The IP Helpers should actually show the NBI...

What's the NetBoot server? Anything in it's logs?

@bentoms

2009 Mac Pro running OS X 10.10.5

The console logs on the server shows that the iMac14,3 connected and packets were sent to it multiple times. But I get a quick flash of the famous "Folder with a question mark" then straight to the Apple logo were it boots to the setup assistant.

Booting the machine up once, then shutting it down and turning it back on works.

@Poseiden Is the NBI you're trying to boot to the default?

Hmmm. The only variable that I see (when comparing your setup to mine) is that you're serving your NetBoot image over HTTP. Maybe try toggling that to NFS? Maybe there are keys of some sort that don't get created until first boot that prevent the HTTP option from working? That's a total guess, but I would wager that if you polled people, most use the NFS option because it has proven (historically speaking) to be less problematic in general.

@bentoms It was never marked as "default". I'll give this a try

@bmarks I'll try an NFS NBI and see if I get the same issue.

@bentoms

Marking it as "default" worked. Wow, thanks Ben!

*Tested with another brand new iMac

@Poseiden What NBI was set to default? or was there none?

I think setting to default will resolve this for you.

@bentoms

One NBI. The only change I made was marking it as default.

So the trick I use is this:
First let's assume your netboot server is at the ip address 10.0.1.2 for fun.
Also let's assume your using *NFS and your netboot images are in standard place.
Finally assume the netboot image is named netboot.nbi

Create yourself a shell script like the example at the end of this post and put it in /Library/NetBoot/NetBootSP0
on your netboot server.

Now on the target device:
Command R to boot to recovery partition.
Select Terminal from Utilities menu and type the following 3 commands:

mount_nfs netbootservername:/private/tftpboot/NetBoot/NetBootSP0 /Volumes/Macintosh HD
/Volumes/Macintosh HD/netboot.sh
reboot

This should get you booting from about anywhere that the firewalls let you.
* So I have not done it but guessing that one could use
mount_webdav instead but I don't know what the url would be. Would have to change the options parms to in the script. If anyone knows the right syntax chip in.
EGA

#!/bin/sh

##### Example netboot.sh script####

##Uncomment for post 10.10.x 

#csrutil netboot add 10.0.1.2

# the next bless command is all one line

bless --verbose --netboot --booter tftp://10.0.1.2/NetBoot/NetBootSP0/netboot.nbi/i386/booter --kernelcache tftp://10.0.1.2/NetBoot/NetBootSP0/netboot.nbi/i386/x86_64/kernelcache --options "rp=nfs:10.0.1.2:/private/tftpboot/NetBoot/NetBootSP0:netboot.nbi/NetInstall.dmg" --nextonly

#

## Uncomment for verbose text output at boot

#nvram boot-args="-v"

@ega You're one OS ahead. :)

Or not. Sorry saw mention of csrutil & assumed 10.11.

My bad @ega

@bentoms I'd like to be one OS ahead at this point but still dealing with 2 behind!
:-)