option dhcp-server-identifier

Thomas Markwalder tmark at isc.org
Wed Oct 12 09:56:44 UTC 2016

On 10/11/16 10:39 PM, John Ratliff wrote:
> I have a pair of isc dhcp servers, version 4.3.1 on Debian 8 Jessie.
> I do not want to use the DHCP failover protocol. Instead, I want to use a
> virtual IP on the primary active server, and have the backup server
> shutdown. I have a cronjob to sync the lease file automatically, but
> failover will be strictly a manual process.
> It seems that the running server always sends packets from the primary IP
> on the NIC, and sets the dhcp-server-identifier option to this IP. So when
> a DHCP client tries to renew, if the server has changed, it takes quite a
> while before the client realizes this. If I could change the packet and
> the dhcp-server-identifier to be the virtual IP, the clients wouldn't have
> this problem.
> Is there a way to configure this in isc-dhcp?
> Thanks.
> --John
> _______________________________________________
> dhcp-users mailing list
> dhcp-users at lists.isc.org
> https://lists.isc.org/mailman/listinfo/dhcp-users

Hello John:

You should be able to use the "server-identifier" configuration
parameter to fix the value used to the desired IP address.  From

"The server-identifier statement

    server-identifier hostname;

    The  server-identifier  statement  can be used to define the value
    that is sent in the DHCP Server  Identifier  option  for  a  given
    scope.   The  value  specified  must be an IP address for the DHCP
    server, and must be reachable by all clients served by a  particu-
    lar scope.

    The  use  of  the server-identifier statement is not recommended -
    the only reason to use it is to  force  a  value  other  than  the
    default  value  to  be  sent  on occasions where the default value
    would be incorrect.  The default value is  the  first  IP  address
    associated  with  the  physical  network  interface  on  which the
    request arrived.

    The usual case where the server-identifier statement needs  to  be
    sent  is  when  a physical interface has more than one IP address,
    and the one being sent by default isn't appropriate  for  some  or
    all clients served by that interface.  Another common case is when
    an alias is defined for the purpose  of  having  a  consistent  IP
    address  for  the  DHCP server, and it is desired that the clients
    use this IP address when contacting the server.

    Supplying a value for the dhcp-server-identifier option is equiva-
    lent to using the server-identifier statement."


Thomas Markwalder

ISC Software Engineering

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