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

Roberto Innocente inno at sissa.it
Tue Jun 9 09:27:37 UTC 2015


Don't forget also rfc 4436 (Detect network attachment)
used in particular by iphones and ipads that are so nice to switch
almost instantaneously from a wifi ap to another (less than 1 second).
This is done (when the lease is still valid) without re-negotiating
anything with the dhcp server and , provided they recognize the mac 
address
of the default router, continue to use the old lease address.
 From some checks they remember 4/5 ap.

roberto



On 2015-06-08 17:24, Glenn Satchell wrote:
> 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
>>> 
>>> 
>>> _______________________________________________
>>> dhcp-users mailing list
>>> dhcp-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>>> 
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> 
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-- 
Roberto Innocente - SISSA
inno at sissa.it - +39 40 3787541


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