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Every home router I've setup/used issues addresses from low to high.

However, dhcpd from the Internet Systems Consortium seems to issue addresses in reverse order - ie, high to low.

Is there a way to change this so that addresses are issued starting at the bottom of the range, and progress higher?

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  • 2
    Stupid question on my part, but as long as the server keeps track of the leases, why does it matter?
    – David Mackintosh
    May 5, 2010 at 15:37
  • Merely so that I have a better idea of when lease space is running low - watching numbers count down vs up makes less sense to me
    – warren
    May 5, 2010 at 15:43
  • You can check the dhcpd.leases file (in /var/lib/dhcp3 on Debian) to see how many dynamic addresses are currently in use (look for leases with an end date that is in the future).
    – Phil Ross
    May 5, 2010 at 15:49
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    I don't know enough about dhcpd, but are the number of active leases exported via SNMP at all? Looks like it should be possible: net-track.ch/opensource/dhcpd-snmp/README.php If you were to throw this into (for example) MRTG or some other graphing utility, then you could quite easily see your historical usage
    – Ben Quick
    May 5, 2010 at 16:26

3 Answers 3

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It isn't possible to control the order dynamic addresses are allocated with ISC dhcpd version 3 or later. From the dhcpd.conf man page:

The DHCP server generates the list of available IP addresses from a hash table. This means that the addresses are not sorted in any particular order, and so it is not possible to predict the order in which the DHCP server will allocate IP addresses. Users of previous versions of the ISC DHCP server may have become accustomed to the DHCP server allocating IP addresses in ascending order, but this is no longer possible, and there is no way to configure this behavior with version 3 of the ISC DHCP server.

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From the ISC dhcp.conf (5) manpage 

   The DHCP server generates the list of available  IP  addresses  from  a
   hash  table.   This means that the addresses are not sorted in any par-
   ticular order, and so it is not possible to predict the order in  which
   the  DHCP  server  will allocate IP addresses.   Users of previous ver-
   sions of the ISC DHCP server may have become  accustomed  to  the  DHCP
   server  allocating  IP  addresses  in  ascending  order, but this is no
   longer possible, and there is no way to configure  this  behavior  with
   version 3 of the ISC DHCP server.
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2

No, according to the docs for the current version (3.x) of ISC DHCPd. (If you're using a different version, the answer may be different.)

From the man page ('man 5 dhcpd.conf', available here: http://linux.die.net/man/5/dhcpd.conf):

"The DHCP server generates the list of available IP addresses from a hash table. This means that the addresses are not sorted in any particular order, and so it is not possible to predict the order in which the DHCP server will allocate IP addresses. Users of previous versions of the ISC DHCP server may have become accustomed to the DHCP server allocating IP addresses in ascending order, but this is no longer possible, and there is no way to configure this behavior with version 3 of the ISC DHCP server."

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