ISC DHCP 4.4.0 Operational Notification - Lease database may not be saved

Brian Conry bconry at
Thu Feb 8 00:34:17 UTC 2018

To the users of ISC DHCP:

ISC would like to make you aware of an Operational Notification for a
just-discovered defect in ISC DHCP 4.4.0.

This is a serious defect such that we have already withdrawn the
download for 4.4.0 from our website and are recommending that it not be

Brian Conry
ISC Support



   A serious flaw has been discovered in the recent release DHCP
   4.4.0 that may cause the server to not properly write lease
   changes to the lease file. As a result, after restart the server
   may incorrectly treat leases that are assigned as available for
   allocation and eventually report them as abandoned.

Posting date:        07 February 2018
Program Impacted:    DHCP
Versions affected:   DHCP 4.4.0 (including development releases
                     DHCP 4.4.0b1 and DHCP 4.4.0rc1)


   DHCP 4.4.0 is the first release of DHCP 4.4, a new branch of
   DHCP.   DHCP 4.4 is the first branch to have the delayed-ack
   feature compiled on by default.  By default, the value of the
   delayed-ack parameter is 0.  Due to a logic error, when these
   two conditions are satisfied (delayed-ack is enabled and the
   value for the delayed-ack parameter is 0) the server will
   incorrectly assume that the lease writes are to be delayed but
   never actually write them to a lease file. The server may appear
   to operate normally, responding to requests and granting leases,
   but the lease file will not be updated properly.  This problem
   will become most apparent after the server is restarted, whereupon
   the addresses that had been earlier leased to clients will be
   incorrectly treated as available for allocation.

   In the DHCP 4.4 default configuration, servers will confirm that
   an address is not in use before assigning it to a client
   (ping-check.) Since some addresses will have been assigned before
   the restart, the ping check test for those addresses is likely
   to indicate a conflict, causing the server to abandon it, choose
   another address and try again.  This will cause unexpectedly
   high numbers of leases being marked as abandoned.

   If the ping check feature is disabled, this can result in the
   server assigning an address to a new client despite it already
   being in use,


   All 4.4.0 installations that do not have delayed-ack parameter
   set to non-zero value are affected. After restart the server may
   incorrectly report significant number of addresses being used
   by unknown devices and mark those leases as abandoned.


   First and most importantly: if you have not already upgraded to
   DHCP 4.4.0, avoid doing so until after DHCP 4.4.1 (which will
   correct the problem) is made available.

   If you HAVE upgraded to 4.4.0 already, you may have some options
   for mitigating the severity of the problem.  To begin with:  if
   you performed a backup on your leases database before upgrading,
   you may be able to recover information for most of your clients,
   especially those that have been active and renewing since the
   upgrade.  Even if you did NOT back up your leases database before
   the upgrade it may still contain information which will limit
   the scope of the issue.  Back it up now.

   While your server is running it will be keeping track of granted
   leases in volatile memory; the problem is that it is probably
   not saving new leases to non-volatile storage.  The severity of
   this problem will depend to some extent on the amount of time
   since you upgraded and the rate at which new devices enter your
   network and request new leases.  In network environments where
   client assignments are very stable the problem will develop
   slowly and most information is probably still in the existing
   copy of the leases file which was written before the upgrade.
   If your network changes slowly, you should pick a time to restart
   the server which will minimize disruption and restart the server
   after either:

   -  downgrading to a version without this issue,
   -  recompiling 4.4.0 without delayed-ack (i.e. use --disable-delayed-ack
      as a configure-time option when rebuilding ISC DHCP), or
   -  setting delayed-ack to a non-zero value using the delayed-ack
      configuration statement in dhcpd.conf

   Any of those alternatives will prevent the problem from getting
   worse but your server will still have to recover from those
   leases which were assigned to new clients but not recorded in
   non-volatile storage.

   The greatest problems will be experienced by those operating
   networks in which clients frequently need new lease assignments.
   In such networks the longer you run an affected build of DHCP
   4.4.0 the more your network state will diverge from the state
   information which is recorded in non-volatile storage.  As soon
   as a convenient time can be found to restart the server (after
   applying one of the three options above to prevent recurrence
   after restart) the server should be restarted.  Expect for it
   to take some time for the network to recover and be forewarned
   that conflicting leases may be assigned due to the failure of
   the server to record them before the restart.  It may help to
   add an abandon-lease-time setting to your config (e.g.
   "abandon-lease-time 30;" to aid in recovery of leases marked as
   abandoned.  You should also ensure that that ping-check will be
   turned on after any restart, as this will help to minimize the
   chance of handing out conflicting leases.


   Do not upgrade to 4.4.0; wait for 4.4.1 to be released. If you
   have already deployed 4.4.0 use one of the mitigations described

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