Re: [lwip-users]lwip and DHCP

[Bob Brusa]

Am 25.09.2009, 10:19 Uhr, schrieb Kieran Mansley <address@hidden>:

> On Wed, 2009-09-23 at 14:52 +0200, Bob Brusa wrote:
> > Hi
> > Further to my problem report "problem with recent port of lwip" I have
> > found the that most probably there is something wrong with the checksum
> > calculation of the incoming DHCPoffer. From the debug-output I see that
> > lwip combines several fragments and then comes up with a checksum error.
> > It drops the packet and DHCP remains without success.
> >
> > I attach the "erroneous" packet as seen by wireshark (txt-mode)
> > 090923_xx_1.out and an excerpt of the debug output of the lwip-stack
> > 090922_3.txt. As the name might make you think - the packages are not of
> > the same session. But the picture does not change.
> > Can somebody guide me how to fix this problem? Just a few words where  
> > this
> > calculation in the code is done. Thanks and best regards.
> >    Robert
>
> Have you considered getting wireshark to validate the checksum?  In the
> trace you provide you've got checksum validation disabled.
>
> Kieran
Hi
Well, its a long time ago that we talked about this dhcp-problem. In the  
meantime, my app works fine - but still without DHCP, using a fixed  
IP-address. So I come back to the issue to make it work also with DHCP.

I have looked at the computation of the checksum of the DHCP-offer  
(because it is reported as wrong). The packet comes in (at  
inet_chksum_pseudo in inet_chksum.c) fractioned in 4 pbufs. The first one  
is ok, but as of the 2nd one, pbufs contain all zeros only - although I  
see from wireshark that the 2nd one includes a few nonzero bytes.  
Interestingly, the tot_len and len fields of all pbufs look very  
reasonable.
1st: 556 150
2nc: 406 184
3rd: 222 184
4th:  38  38
Hence the problem is not the chksum, but somewhere there is a loss of  
data. Now I am trying to find out where the true contents of the pubs gets  
lost. Unfortunately I also am lost: The internal working of the lwip-stack  
is not easy to grasp. Best thing would be if someone already has a fix for  
the problem. Or good advice on how to takle the issue. Thanks for help
  Robert