[Dnsmasq-discuss] pxe boot problem

[Simon Kelley]

stevea wrote:
> I am attempting to use dnsmasq-2.38 to support pxe (etherboot) for 
> LynxOS and have run onto a problem.
> 
> The bootfile name is supplied by the server in the rfc2132 option field, 
> but does not appear in the main packet body (similarly the servername 
> appears only in the option field, but this doesn't matter to me).  The 
> following hack solves my problem and always inserts the bootfile name in 
> the packet body (and optionally in the option field).
> 
> --- rfc2131.c.orig      2007-10-06 10:04:58.000000000 -0400
> +++ rfc2131.c   2007-09-25 09:46:14.000000000 -0400
> @@ -1347,7 +1347,7 @@
>        {
>          if (req_options && in_list(req_options, OPTION_FILENAME))
>            option_put_string(mess, end, OPTION_FILENAME, boot->file, 1);
> -         else
> +         //sja- else
>            strncpy((char *)mess->file, boot->file, sizeof(mess->file)-1);
>        }
> 
> 
> dnsmasq.conf contains:
> 
>>dhcp-boot=targe.0,lycoperdon,192.168.133.1
> 
> 
> Is there a configuration file parameter I've missed to place the 
> bootfile name in main packet body or is this controlled by the 
> target/requester ?  If the latter could someone point me to the rfc that 
> describes this please.
> 

It's controlled by the requester. If the requester explicitly puts 
option 67 into the list of options it requests, then dnsmasq assumes 
that the client knows about the use of option 67 as an alternative to 
the fixed "filename" field, so it sends the filename using an option-67 
option. That leaves the 128 byte filename field available as more option 
space.

If the client doesn't request option 67 then dnsmasq sends the filename 
in the old way, using the 128 byte filename field.

There's an agument that a client which asks for option 67 but then 
doesn't use it is broken. Is there some whay to stop your client from 
requesting option 67?

The only RFC which deals with this, AFAIK, is 2132, the relevent bits are:

9.3. Option Overload

    This option is used to indicate that the DHCP 'sname' or 'file'
    fields are being overloaded by using them to carry DHCP options. A
    DHCP server inserts this option if the returned parameters will
    exceed the usual space allotted for options.

    If this option is present, the client interprets the specified
    additional fields after it concludes interpretation of the standard
    option fields.

    The code for this option is 52, and its length is 1.  Legal values
    for this option are:

            Value   Meaning
            -----   --------
              1     the 'file' field is used to hold options
              2     the 'sname' field is used to hold options
              3     both fields are used to hold options

     Code   Len  Value
    +-----+-----+-----+
    |  52 |  1  |1/2/3|
    +-----+-----+-----+

and

9.5 Bootfile name

    This option is used to identify a bootfile when the 'file' field in
    the DHCP header has been used for DHCP options.

    The code for this option is 67, and its minimum length is 1.

        Code  Len   Bootfile name
       +-----+-----+-----+-----+-----+---
       | 67  |  n  |  c1 |  c2 |  c3 | ...
       +-----+-----+-----+-----+-----+---




Cheers,

Simon.