No Free Addresses?
dhcp1 at thehobsons.co.uk
Tue Dec 14 08:03:12 UTC 2010
Bob Proulx wrote:
> > When the communication between them breaks they do not automatically
>> assume the other server is down,
>Right. It could be a network split. Each server might continue to be
>up and online on an isolated segment of the network.
More seriously, there are people running failover pairs with out of
band communication - ie the server-server traffic follows a different
route to server-client traffic. In this situation (and one or two
other corner cases such as routing errors) it's possible to have both
servers still able to serve clients ON THE SAME SUBNET while unable
to communicate with each other. You wouldn't want either of them
going into partner down state in that situation, so it's always been
left to the admin to decide.
If you wanted it to be automatic, you could always script a "is
partner there ? If not put myself into partner down state" process -
external to the DHCP service itself.
> > it is your responsibility to tell the remaining server that the
>> other is really dead, not just unable to communicate.
>It died today. But it will be back tomorrow. I am provisioning a new
>server for it right now. It isn't permanently gone. It isn't even
>going to be gone long term.
Doesn't matter - just put the remaining one into partner down state.
When the other recovers (or is replaced) they will sort themselves
> > Until the server gets the information that its partner really is dead,
>> it will not hand out leases belonging to the partner -> effectively you
>> are missing half your address space until then.
>Correct. That is why you need twice the available address pool.
You don't have to, it's a design decision you've made that imposes
that. Running a network is always a matter of being pragmatic about
these things - at least with RFC1918 addresses there are plenty to go
The fun thing is when you come to try and explain to (supposedly)
technical peers that x.x.1.1/23 really is not a ".1" address (but
rather think of it as .257 relative to the network address) which in
their addressing plan is reserved for the primary router :-/
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