Re: [lwip-users] DHCP requesting additional options via option 55

[Craig McQueen]

I wrote:
> 
> I have tried using LWIP_HOOK_DHCP_APPEND_OPTIONS() and
> LWIP_HOOK_DHCP_PARSE_OPTION() to request additional DHCP options
> from the server, using DHCP option 55
> (DHCP_OPTION_PARAMETER_REQUEST_LIST).
> 
> At first I thought it was working fine. But on closer examination, it seems 
> the
> server is not sending all requested options (e.g. options 3 and 6). That is
> evidently because the DHCP request contains the option 55 twice (once from
> the lwIP core code, a second time from my
> LWIP_HOOK_DHCP_APPEND_OPTIONS()). According to the DHCP RFC 2131, "
> Options may appear only once, unless otherwise specified in the options
> document." So I guess the DHCP server (Microsoft in this case) is ignoring the
> first occurrence of option 55.
> 
> So, it would be good to improve the DHCP hook mechanism so that user code
> can request additional options via option 55, without duplicating the option
> contrary to the RFC. Ideally, it could be dynamic not a static array, so the 
> user
> code could decide at run-time which additional options are requested.
> 
> What would be a good way to improve the hook mechanism?
> 
> (I'm interested in contributing some code back to the lwIP project, as long as
> I have a rough idea what would be accepted.)

I worked out that in LWIP_HOOK_DHCP_APPEND_OPTIONS(), I can actually do a 
search through the existing options in msg->options[], looking for any 
pre-existing DHCP_OPTION_PARAMETER_REQUEST_LIST. Then I can insert the extra 
options into it. It's a bit fiddly, but I've got it working. Does that sound 
like a reasonable thing to do in LWIP_HOOK_DHCP_APPEND_OPTIONS()?

-- 
Craig McQueen