Re: diskless system's limited dhcp support

[Dan Ritter <dsr@randomstring.org>]

Ross Boylan wrote: 
> On Sun, Sep 6, 2020 at 3:37 PM Dan Ritter <dsr@randomstring.org> wrote:
> 
> > The usual way is:
> >
> > 1. DNS record tied to a static IP address
> > 2. IP address handed out by DHCP server based on MAC address of
> >    the interface
> 
> 
> > This is especially normal when the root is served by NFS, so PXE
> > needs to figure out the right root to hand out -- PXE is
> > governed by MAC address, and then you really want the kernel's
> > conception of its IP address to remain the same.
> >
> I don't follow that last part.  I thought PXE was irrelevant once the
> system was up.  And the IP address of the root fs is distinct from the IP
> address of the client.

You made me double-check my belief here. I thought that the
kernel inherited the IP address from the PXE booter, but I
was wrong; there's no mechanism for that. It's an artifact of
my own setup, where individual MAC addresses are used to return
specific IP addreses.

I would still argue that this is a good practice, because it
makes debugging easier, but it's not necessary.

> > > bit of a hack.  Also, I'd like the DNS entry for the system to appear
> > only
> > > while it is up, and without the client sending a host name that's harder.
> >
> > That requires a server that listens to a dynamic DNS protocol,
> > and a dynamic DNS client on the client system.
> >
> My server is set for dynamic updates.  Since ipconfig sets up the
> interface, the usual dhcp client that manages such stuff doesn't come up.
> 
> > Why would you care about the DNS name not being available when
> > the machine isn't up?
> >
> Mostly because the machine might be up, but running a different OS
> instance.  This applies to non-diskless, non-PXE clients as well.  So the
> MAC address does not determine a unique system.  PXE booting (and grub for
> regular systems) provides a menu of possible systems from which to boot, so
> I may not know which of them is running until someone makes a selection
> from the menu.
> 
> I've been shooting for using the same IP regardless of the OS, but maybe
> that's inadvisable. OTOH, for PXE the machine gets an IP before the
> selection is made.

It depends on the semantics that you're assigning to names and
IPs and MACs. IMHO, the physical hardware is important because
if there's a problem, I need to know what piece of equipment I
will have to walk over to and debug.


-dsr-