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List:       busybox
Subject:    udhcp proxy support
From:       Chris McClimans <chris () hippiehacker ! org>
Date:       2014-11-13 15:16:06
Message-ID: CAABZ_wqFXgMr+G+2hB6CdwV5c_WEnzsugNo8AvCpV=cn93pYKA () mail ! gmail ! com
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I'm looking to get proxy-DHCP support into udhcp[cd].

With udhcpc the client takes the very first OFFER, rather than the
'may choose to wait for multiple responses' option from rfc2131:

     The client receives one or more DHCPOFFER messages from one or more
     servers.  The client may choose to wait for multiple responses.
     The client chooses one server from which to request configuration
     parameters, based on the configuration parameters offered in the
     DHCPOFFER messages.

I suggest that if udhcpc has --request-options, that the behavior
should be to at least entertain other offers and not take the first
one that comes along. Maybe with a --prefer-my-options-timeout?

On the udhcpd side, proxying is not supported, but a currenty working
implementation is provided by dnsmasq:

From http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html

Dnsmasq supports PXE "proxy-DHCP", in this case another DHCP server on
the network is responsible for allocating IP addresses, and dnsmasq
simply provides the information given in pxe-prompt and pxe-service to
allow netbooting. This mode is enabled using the proxy keyword in
dhcp-range.

It is enabled with: dhcp-range=1.1.1.10,proxy,255.255.255.0

Where 1.1.1.10 is the local address that listens for OFFERs and uses
them as a base to send a modified OFFER with extra options.

In udcpcd we could use the first OFFER as the defaults and set any
desired options in our config.
start and end options would be ignored in proxy mode, because we
wouldn't be selecting IPs to hand out.

I haven't looked  very far into the code base, but the client seems
like it would be pretty straight forward.
I'd be thankful for any direction or thoughts on how to go about this.

Cheers,
-chris
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