static reservation from dynamic pool
dhcp1 at thehobsons.co.uk
Wed Oct 13 20:43:29 UTC 2010
Top posting fixed ...
Adam Moffett wrote:
>> The answer is yes. It can cause problems. The IP address can be given
>>to another system other than the one in the fixed address one. You have to
>>break up you pool by having different ranges.
> I wonder I haven't had a problem yet.
>Once the host "foo" has the lease and keeps renewing it, I assume
>any old host requesting an IP address isn't going to get the IP that
>"foo" already has.
>Is the danger that when "foo" is offline, someone else can get the IP?
The address can be assigned to another host even if host "foo" is
online ! Host declarations do NOT go through the normal lease
processing, and so the server doesn't actually keep any record of
having given the address to "foo". therefore, it can, and will, at
some point select that address to give to another client.
By default, the server will do a ping check to see if the address if
free - but many devices now default to not answering pings because
some people believe that it somehow enhances security. If the device
doesn't answer, then the address will be offered to another client.
The client itself may well, and should, check to see if the address
is in use - and decline it if it is. However, if the client declines
the offer, the server will keep offering it over and over again.
Obviously, if "foo" is down at the time then another device will be
offered, and accept, the address. Once "foo" comes back online, it
too will be offered the address - which it will probably decline.
Since it's configured to only get that one address, "foo" will then
simply not get onto the network.
Two solutions :
1) As already stated, split your range so that the fixed address is
not within any range statement. This is known functionality and a
known requirement. Eg :
range 10.1.1.1 10.1.1.141 ;
range 10.1.1.143 10.1.1.250 ;
2) In newer versions, you can create a reserved lease instead of
using a host statement with fixed address. This lease is then
"locked" to a specific client, but otherwise it goes through the
normal lifecycle - including appearing in the leases file and
undergoing dynamic DNS.
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