randomly(!) assign ip's from dynamic address range

[Glenn Satchell]

Most dhcp clients also store the last address. So even if you configure
the server to offer different IPs to the client, the client could keep
asking for the old address and the server would accept that.

regards,
-glenn

On Tue, June 9, 2015 12:58 am, Bob Harold wrote:
> A Windows 8 client appears to keep the last IP in the registry in:
> HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{...<Network
> Adapter>...}\DhcpIPAddress
>
> Although many years ago I remember having clients that remembered the last
> 5 or 10 IP's (in different subnets).
>
>
>
> --
> Bob Harold
> hostmaster, UMnet, ITcom
> Information and Technology Services (ITS)
> rharolde at umich.edu
> 734-647-6524 desk
>
> On Mon, Jun 8, 2015 at 9:11 AM, Glenn Satchell
> <glenn.satchell at uniq.com.au>
> wrote:
>
>>
>> On Mon, June 8, 2015 7:29 pm, Arne Baeumler wrote:
>> > Hi Simon,
>> >
>> > thank you for your reply.
>> >
>> > On 2015-06-05 15:05, Simon Hobson wrote:
>> >> Hmm, that's a variation I don't think we've seen before ;-)
>> >> What you are seeing is correct operation according to the RFCs - the
>> >> server is required to keep the address stable as far as is possible,
>> and
>> >> that means the client can come back after an arbitrary length of time
>> >> and as long as the address has not been re-used then the client
>> *must*
>> >> get the same address.
>> >
>> > Would you please point me to the RFC you are referring to?
>> > Can't find any requirements for the server to maintain some kind of IP
>> > history in RFC2131.
>> >
>>
>> I'm looking at https://www.ietf.org/rfc/rfc2131.txt, there are several
>> references to this feature within the document. Section 4.3.1 is the
>> clearest on this, even though it says it "SHOULD" rather than "MUST" be
>> done this way.
>>
>> 1.6 Design goals
>>
>>       o Retain DHCP client configuration across DHCP client reboot.  A
>>         DHCP client should, whenever possible, be assigned the same
>>         configuration parameters (e.g., network address) in response
>>         to each request,
>>
>>       o Retain DHCP client configuration across server reboots, and,
>>         whenever possible, a DHCP client should be assigned the same
>>         configuration parameters despite restarts of the DHCP mechanism,
>>
>> 2.2 Dynamic allocation of network addresses
>>
>>    The
>>    allocation mechanism (the collection of DHCP servers) guarantees not
>>    to reallocate that address within the requested time and attempts to
>>    return the same network address each time the client requests an
>>    address.
>>
>> 4.3.1 DHCPDISCOVER message
>>
>>    When a server receives a DHCPDISCOVER message from a client, the
>>    server chooses a network address for the requesting client.  If no
>>    address is available, the server may choose to report the problem to
>>    the system administrator. If an address is available, the new address
>>    SHOULD be chosen as follows:
>>
>>       o The client's current address as recorded in the client's current
>>         binding, ELSE
>>
>>       o The client's previous address as recorded in the client's (now
>>         expired or released) binding, if that address is in the server's
>>         pool of available addresses and not already allocated, ELSE
>>
>>       o The address requested in the 'Requested IP Address' option, if
>> that
>>         address is valid and not already allocated, ELSE
>>
>>       o A new address allocated from the server's pool of available
>>         addresses; the address is selected based on the subnet from
>> which
>>         the message was received (if 'giaddr' is 0) or on the address of
>>         the relay agent that forwarded the message ('giaddr' when not
>> 0).
>>
>> I hope this helps.
>>
>> I realise this doesn't help with your original question :) In terms of
>> privacy there is an IPV6 feature that assigns a new IP address each day.
>> This is part of the client functionality defined in RFC 4941 "Privacy
>> Extensions for Stateless Address Autoconfiguration in IPv6" but doesn't
>> help with your current IPv4 issue.
>>
>> regards,
>> -glenn
>>
>>
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>>
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