Conflicts and network boot
Simon Hobson
dhcp at thehobsons.co.uk
Mon Mar 20 15:24:40 UTC 2006
Bruno Gola wrote:
>I've already look this "authoritative" issue, but the problem is not here. I
>wanna my dhcpd to be able to boot the machine before the other dhcp server (
>172.16.0.1) send its information.
BAD BAD BAD idea !
You really should NOT ever have more than one DHCP server (or set of
servers in a true redundant setup) serving a network. You most
definitely should not have two servers, acting independently, able to
offer different leases to one client.
Ignore this and you will have strange, random, and non-reproducable
problems appearing at random times.
If you MUST have two different servers servicing the same network,
then you should make sure that they will only respond to mutually
exclusive sets of clients. Ie, so any client, one server should be
set to service it, the other server should be set to ignore it. This
is harder said than done !
Probably the easiest way to do it is to have one server that responds
only to a set list of hosts (by MAC address) - eg :
ignore unknown clients ;
subnet a.b.c.0 ... {
...
}
host a { hardware aa:bb:cc:dd:ee:ff ; }
host ...
And at the same time, set the other server to ignore those clients - eg :
subnet a.b.c.0 ... {
...
}
group {
ignore booting ;
host a { hardware aa:bb:cc:dd:ee:ff ; }
host ...
}
What you absolutely CANNOT do is rely on "getting your reply back
first" as a means of getting one offer accepted in place of another.
Firstly you cannot guarantee this (at least, not without sufficient
control of the "other" server in which case you could configure it to
ignore the client). Secondly, even if you do contrive that you offer
gets to the client first, the client won't automatically accept it -
the spec specifically allows for multiple offers to be received and
for the client to select one (perhaps by selecting the offer that
satisfies the largest number of requested options).
Finally, please correct me if I'm wrong, but it sounds like you are
trying to use DCHP on the network without the permission (or
assistance) of the network administrators. IF this is the case, then
I strongly advise against it, as a network admin it's the sort of
thing I take very seriously and would have no hesitation in removing
(by unorthodox means* if neccessary) such offending equipment and
users.
Simon
* "unorthodox means" would at one extreme simply mean going round and
physically unplugging equipment. Repeat offenders may find equipment
rendered permanently inoperative (eg by wire cutters, hammer, or even
the crusher !), or in extreme cases may find themselves having their
exit interview with HR.
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