Netboot Across Subnets

Not applicable

IS there a way to Netboot across subnets for casper imaging?


Glenn J Burns
Computer Technician
Snyder ISD
gburns at
• Certified Mac Technician


New Contributor II

Yes, depending on your network equipment... You will need to set up an "IP Helper" address.

Bombich has a very nice article:

Apple's is cool too:


Honored Contributor

If you look at the bless command you can tell the machine to boot to
any subnet. From the man page

bless --netboot --server url [--verbose]

so for example: `bless --netboot --server`

Would tell the unit to netboot to that server. Your network
hardware/infrastructure must be set up to allow this of course

Contributor III

So what you really want is to netboot across subnets without any helpers etc.
There are a couple of things to know: 1) The full bless command has changed in 10.7+. The form is:
bless --verbose --netboot --booter tftp:// --kernelcache tftp:// --options "rp=nfs:" --nextonly
(NOTE: This uses NFS to mount the disk )
In the pre-10.7 bless command the second switch (--kernelcache) was --kernel and pointed to the mach.macosx file in the .nbi like this:
--kernel tftp://

2) The Configure Manually part of setting up a NetBoot Server in Casper Setting web page dose not seem to honor this change to bless.

The long and short of this is that you will have to send the bless and reboot command in some other way than simply assigning a netboot server to computers in Casper. ARD, SSH, or create a script on a trigger will all work. Note too that the netboot sets created for 10.7+ are different structure and you will need a separate netboot set (.nbi) for machines that require 10.7.

Also if you are interested, bless has been moved to the open source and you can look at the current source at:

New Contributor II

How we did it in our environment was to enable PXE boot, as recommend by our JAMF rep. We have a separate team that manages our PC's and network, but there were able to setup for us. So, not sure the technical steps they went through but I think it's very straight forward if your router(s) support PXE boot. We just gave them our NetBoot server IP address and network segments we needed NetBoot.

Contributor III

Yes, setting the Helper IP is the most straightforward way of doing this (so long as someone can program your switches!) Here, we have a network group that handles the infrastructure so I just submit the request to them & they do the actual programming. Once it's set, though, it works great (as long as no one overwrites the switch configuration, that is).

Interesting info about bless, ega.

Contributor II

We were able to netboot without bothering with IP Helpers or bless commands. Those may work but we were able to work around them for now. We are still new to JAMF and learning the ropes, trying to figure things out. So far this seemed to work for us.

We have multiple buildings but currently only have two netboot servers, both in our main location. However, we have several labs of machines that needed to be imaged at different locations. What we found to work is this:

(1) Leave netboot server #1 in place at central location for regular maintenance and imaging.
(2) Take netboot server #2 to other location.
(3) IP netboot server #2 to that location.
(4) Update netboot server #2 IP address in JAMF web Casper Admin setting for Settings > Servers > Netboot Servers to point to the new IP address. (Note: entry at bottom for "boot-device" updates automatically.)
(5) Update DNS entry for server #2 to new IP address. We deleted the existing DNS entry and added a new one.
(6) Option-boot or Netboot a machine, select the Netboot option that appears.
(7) We did not do prestaging, but you might be able to. We just manually entered info and selected the distribution point to be server#1 (note: that is server #1 at our central location!).

For whatever reason, it doesn't want to image from the netboot server located at the new location. Instead it seems to act as some sort of gateway back to the netboot server in our main location. It's a bit slower but it works. We came back the next day and they were all done. Repeated at next building lab. Unfortunately these were older macs so only had thunderbolt display ports, so we couldn't use our thunderbolt drives to one-off image them. We had to find a way to netboot.