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

[Craig McQueen]

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.)

-- 
Craig McQueen