MAC randomisation and DHCP pools

Mike Richardson mike.richardson at
Wed Jul 29 10:00:16 UTC 2020

On Mon, Jul 27, 2020 at 07:36:36PM +0200, Sten Carlsen wrote:
>    From reading the links provided by Matt, I see a somewhat better
>    situation. Thanks Matt for providing this information.
>    I may not have read all the information correctly, so no guarantee.

One of the articles I posted states that the MAC will change every 24 hours,
which means that things could change mid-lease, between renewals, if
correct. The Apple site mentions 'periodically', which could mean anything



>    Inline below:
>    --
>    Best regards
>    Sten Carlsen
>    For every problem, there is a solution that
>    is simple, elegant, and wrong.
>    HL Mencken
>    On 27 Jul 2020, at 15.08, [1]glenn.satchell at wrote:
>    Hi Mike,
>    Going back to the original question where you have a pool of 100 leases
>    and 50 clients with a 7 day lease time. Here is what I think might
>    happen.
>    On day 1 the 50 clients each take one lease. 50 in use, 50 free.
>    On day 2 the 50 clients all have a new MAC address, now we assume that
>    once the new MAC switches over the next time the client tries to renew
>    it will not match the old lease but will get a new lease. With a 7 day
>    lease the usual renewal time is half way through the lease, so none of
>    these 50 clients try to renew until 3.5 days after initially getting
>    the lease. So no problems for days 2 or 3 until later in the day.
>    For IOS the MAC stays constant until it detaches from that network, so
>    renewal is not an issue. Going away and returning later might be but
>    then the old lease should be free - for each network the user can chose
>    to keep a constant MAC, some will, most will not is my guess.
>    So now we have 50 old leases and 50 new leases. Of course some systems
>    may have been shutdown and released their lease, so maybe less than 50
>    leases in use initially so <50 old leases and 50 new leases.
>    On day 4 the first few clients to renew with a new MAC address use up
>    the previous few free leases. Other clients get "no free leases". The
>    dhcp server can't revoke a lease it has already legitimately given to a
>    client. I would expect this behaviour to continue until the first of
>    the 7 day leases expire.
>    Now the question is, for a client with a new MAC address, but possibly
>    the same dhcp-identifier, will it match the existing lease? If it does
>    match,then no problem. Behaviour will be much the same as previously.
>    AFAIK in the RFC, the ClientID is to main ID, MAC is not used by
>    default, only as a second option, so fixed ID should be fine.
>    The other thing with this is that if the client gets a new IP address,
>    all existing sessions break, so apps and webpages may have to reload or
>    may not pass authentication. So there could be a noticeable
>    interruption.
>    Since at least IOS seems to keep the MAC while connected, this is not a
>    problem, The new address comes with the next discover in dhcpd
>    The above is what I think will happen based on my understanding of ISC
>    dhcpd. I don't really know exactly how the new IOS version will behave.
>    I would suggest setting up a trial and testing with one of these new
>    devices and see what actually happens. There are too many variables to
>    predict what will happen exactly.
>    It seems that IOS would change addresses between networks but not
>    across renewals. That will reduce the traceability quite much with
>    little harm. If needed the IOS can be told to not change the MAC for
>    any particular network.
>    regards,
>    -glenn
>    On 2020-07-27 19:34, Mike Richardson wrote:
>      On Sun, Jul 26, 2020 at 03:13:16PM -0400, Bill Shirley wrote:
>        Did you see my reply [2]about:?
>        adaptive-lease-time-threshold       75;       # use min-lease-time
>      when
>        pool is above this percent
>      I did and thanks for the information, that sounds very useful in the
>      circumstances but I'm not after a solution to a problem, I'm just
>      trying to
>      understand the behaviour of the server in a given configuration.  I
>      have to
>      write up a 'these are the implications' type summary to be sent to a
>      large
>      number of different organisations and knowing what happens when
>      using longer
>      leases will help.  I don't know their configurations and can't
>      dictate to
>      them.  All I can do is say "if you're doing X then Y happens".
>      Thanks,
>      Mike
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> References
>    1. mailto:glenn.satchell at
>    2. about:?
>    3.
>    4. mailto:dhcp-users at

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Mike Richardson

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