Subnet Selection Option not working

Colin Jenkinson C.Jenkinson at
Mon Apr 2 04:07:48 UTC 2007

 I need to have Subnet Selection (Option 118) working for an outdoor
wireless mesh
 network we are putting in. We are running ISC 3.0.5 in failover
configuration on
 a pair of SUN UltraAX-i2 computers running SunOS 5.9. The problem I am
having is
 the clients do not receive an IP address and debug of the DHCP packet
 "Unrecognized Option = 118" yet my understanding of the code is that
this is supported.

 Any help would be greatly appreciated.

 According to "dhcp-options_5.txt":

option subnet-selection string;

         Sent  by  the client if an address is required in a subnet
other than
         the one that would  normally  be  selected  (based  on  the
         address  of  the  connected subnet the request is obtained
from). See
         RFC3011. Note that the option number used by this server is
118; this
         has  not  always  been the defined number, and some clients may
use a
         different value. Use of this option should be  regarded  as

       This option is not user configurable in the server.

If I do "snoop -v dhcp" i get the following:

DHCP: ----- Dynamic Host Configuration Protocol -----
DHCP: Hardware address type (htype) =  1 (Ethernet (10Mb))
DHCP: Hardware address length (hlen) = 6 octets
DHCP: Relay agent hops = 0
DHCP: Transaction ID = 0x345
DHCP: Time since boot = 0 seconds
DHCP: Flags = 0x0000
DHCP: Client address (ciaddr) =
DHCP: Your client address (yiaddr) =
DHCP: Next server address (siaddr) =
DHCP: Relay agent address (giaddr) =
DHCP: Client hardware address (chaddr) = 00:0E:35:53:D1:68
DHCP: ----- (Options) field options -----
DHCP: Unrecognized Option = 118, length = 4 octets
DHCP:   Value = 0xAC 0x19 0x00 0x00 (unprintable)
DHCP: Client Identifier =       0x01 0x00 0x0E 0x35 0x53 0xD1 0x68

Relevant part of configuration file:

  subnet netmask {
    option routers;
    option broadcast-address;
    pool {
      failover peer "dhcp";
      deny members of "MacWirelessClients";
      deny dynamic bootp clients;
    pool {
      failover peer "dhcp";
      allow members of "MacWirelessClients";
      deny dynamic bootp clients;
    option mobile-ip-home-agent;


Colin E Jenkinson
Team Member Networking
Information Technology Directorate
University of Western Sydney mailto:C.Jenkinson at

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