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

[goldsimon]

Am 13. November 2018 06:10:32 MEZ schrieb Craig McQueen <address@hidden>:
>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()?

No. That seems too complicated. I would prefer a seaparate method.

Simon