Can ISC's dhcpd be setup to respond to DHCPINFORM only

Steve Davies steve at
Tue Mar 15 17:21:35 UTC 2011

Hi list members.

I'm trying to setup dhcpd to stay silent except in the case where it
receives a DHCPINFORM meeting specific criteria - in which case I'd like to
ACK with the relevant info.

The situation is as follows:  we are trying to integrate phones for a hosted
PBX solution into a customer's existing LAN.

For elegant auto-provisioning of the phones they need to receive a boot
server via DHCP option 66.

If the customer adds that option to their DHCP setup then things go
swimmingly.  Unfortunately the customer is often unable or unwilling to
adjust their existing DHCP server setup.

Conveniently, the Polycom phone has a feature where it will broadcast a
DHCPINFORM looking for option 66 in the event that the network's primary
DHCP server(s) do not provide this option.  The phone will search all
DHCPACK replies looking for the option.

So we'd like our outbound SIP proxy device to lay in wait on the network and
when we receive such a DHCPINFORM to send the response.

But I have been unable to figure out how to configure ISC dhcpd without a
pool but still have it respond to the inform.

Here's what I tried so far:

ddns-update-style interim;
ignore client-updates;

not authoritative; # as required

class "phones" {
match if
    substring(option vendor-class-identifier, 0, 4) = "snom" or
    substring(hardware, 1, 3) = 00:04:13 or
    substring(option vendor-class-identifier, 0, 4) = 00:00:36:3d or
    substring(hardware, 1, 3) = 00:04:f2;

subnet netmask {
pool {
allow members of "phones";
        option tftp-server-name         "";
option bootfile-name            "cgi-bin/snom_settings.cgi?mac={mac}";

This results in a complaint that the pool must have a range.  But I
deliberately don't want the dhcp server to be offering addresses.

Any suggestions on how to proceed?

If dhcpd can't do this, I'd also be interested in other suggestions.

Any pointers much appreciated,
Steve Davies
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