User-class and DHCPINFORM

Glenn Satchell Glenn.Satchell at
Thu Oct 15 22:20:48 UTC 2009

Hi Thomas

Usually it is an application, eg IE, causingthe DHCPINFORMs to try to
get proxy info. A workaround is to add the proxy info to the request
parameters so no DHCPINFORM is sent. This was sent a whilke ago, and I
use it in my dhcpd.conf:

# Special config for Windows ("MSFT 5.0") systems.
# Note this does not catch Windows CE.
class "MSFT" {
  match if substring(option vendor-class-identifier, 0, 4) = "MSFT";
  # They put 252 on the DHCPINFORM's, but not on the DHCPREQUEST's
  # PRL.  So we over-ride the PRL to include 252 = 0xFC, which will
  # send the option at DHCPDISCOVER/DHCPREQUEST time, and wil also
  # suppress the DHCPINFORMS!  They seem to cache it.
  option dhcp-parameter-request-list = concat(option 
dhcp-parameter-request-list, fc);
# Windows systems accept the WPAD option for setting up their proxy server
# (Autodetect proxy settings):
# (According to Ted Lemon, "text" will get the Microsoft
# NUL termination bug heuristic applied, and "string" won't!
option wpad code 252 = text;
option wpad "http://proxy/proxy.pac\000";

The \000 adds a null byte on the end. In some unspecified
configurations windows chops off the last byte.

Glenn Satchell   mailto:glenn.satchell at | Miss 9: What do you
Uniq Advances Pty Ltd | do at work Dad?
PO Box 70 Paddington NSW Australia 2021            | Miss 6: He just
tel:0409-458-580     fax:02-9380-6416              | types random stuff.

>From: "DELFAIRIERE Thomas" <thomas.delfairiere at>
>I've got XP workstations and DHCP servers configured with a user class like 
>class "PCs_User_Class_Chrul" {	
>    match if (option  user-class = "mycompany");	
>    option domain-name "";	
>In default network, the default domain-name option is "".
>Some workstations with good settings (user class "mycompany" in the paramaters 
of the network interface) don't have the good domain-name  (they have 
instead of
>I realize a sniff : 
>- when a workstation (with the user-class "mycompany") send DHCPREQUEST or 
DHCPDISCOVER, the dhcp server respond with the good domain-name "".
>- when the same workstation send DHCPINFORM, the dhcp server respond with the 
default domain-name "".
>The rfc 2131 (4.3.1) explains that the user-class is  interpreted by dhcp  
server when receiving DHCPREQUEST or DHCPDISCOVER. Why the client send 
DHCPINFORM and DHCP server responds without interpreting the user class?
>Is there a way to force the DHCP server to interpret the user class in the 
>Thanks for your ansvers

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