Experience with answering option 252 with simple newline?

Jay Ford jay-ford at uiowa.edu
Mon Sep 23 20:01:11 UTC 2013

On Mon, 23 Sep 2013, John Wobus wrote:
> We're considering replying to DHCPINFORMs
> with option 252 with a newline to induce
> clients to stop asking.
> Anyone doing this on a large scale, i.e.
> while trying to serve all sorts of clients?
> In theory, something could break.
> What I anticipate doing (globally):
> option msft-wpad code 252 = text;
> if substring (option vendor-class-identifier, 0, 8) = "MSFT 5.0" {
> 	option msft-wpad "\n";
> 	}

We do this on all of our normal DHCP servers to quell useless DHCPINFORM
chatter by Microsoft devices, with good results:

    # add option sequence for suppressing DHCPINFORM
    #   Client browsers typically looking for wpad proxy sites
    option wpad code 252 = text;
    option wpad "\n\000";
    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 = null, which will
    # send the option at DHCPDISCOVER/DHCPREQUEST time, and will also
    # suppress the DHCPINFORMS!  They seem to cache it.
     option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list, fc);

Jay Ford, Network Engineering Group, Information Technology Services
University of Iowa, Iowa City, IA 52242
email: jay-ford at uiowa.edu, phone: 319-335-5555, fax: 319-335-2951

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