[CALUG] Booting back to Linux

Bryan J Smith b.j.smith at ieee.org
Mon Nov 26 16:01:53 EST 2012

On Mon, Nov 26, 2012 at 11:12 AM,  <jecottrell3 at comcast.net> wrote:
> No. Any DHCP *should* see the box as The Same Client. Hosts frequently
> crash and are given the same address, especially if the Lease is Active.

Because the client requests the same IP from its existing client
leases store (e.g., files under /var/lib/dhclient/)  .  Some DHCP
services can be configured to be very strict about requests, including
if a request is made for the same MAC and same IP in an existing
leases window.  Some vendors have their own extensions beyond IETF RFC
and STD.

Those are not shared in client lease stores between OSes.  I.e., I've
even seen the same issue with dual-booting different Windows
instances.  tcpdump confirms many issues.  ;)

> The other possibility is that the Windows Client and the Linux Client look
> different.

Some "vendor integrated" DHCP services can have additional
requirements, including authentication, DDNS handshake and the like.
There are some UPnP extensions that are not IETF Zeroconf compliant or
other IETF RFC and STD.

> A popular misconception is that DHCP looks at the MAC Address. In reality,
> it looks at the Client ID, which can be anything from a Host Name to a
> Random Number, but if not given, it Defaults to the MAC.

And a popular misconception is that a DHCP client cannot request the
same IP and send other information during the negotiation, beyond the
initial RARP.  DHCP does far more than BOOTP ever did with RARP.  DHCP
clients often request/send specifics, especially if they have an
existing, known lease.

The fact that a reboot of the DHCP server (Router) seems to clear it
suggests this is related.

> Certain Routers make very poor DHCP Servers. Windows Servers are OK, but you probably
> have to play their game. Linux DHCP is more flexible, but it may take you awhile to find the
> right options.

Define "are OK" and "have to play their game."  Windows Server DHCP,
DDNS, etc... gets rather interesting.

> On Linux, do a man on dhclient and see if there is a file that lists your hostname.

Again, Linux don't have the same dhclient leases file as Windows.
This is commonly an issue, one that affects multiple installs of
Windows as well.

> A packet trace may have to be done.

Indeed.  I'd like to see what NT6.2 is sending.

Because according to the OP, NT6.1 didn't have an issue.

Bryan J Smith - Professional, Technical Annoyance
b.j.smith at ieee.org - http://www.linkedin.com/in/bjsmith
Computers are precise, but not accurate, and make mistakes
due to lack of input, as lack of awareness and observation

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