hi,

> > > +#include <netinet/tcp.h>
> > > +#include <netinet/udp.h>
> > 
> > why you need these includes?  isn't m_pkthdr.csum_data enough?
> 
> Aren't these necessary for struct tcphdr and udphdr?

i think all you need is offset of th_sum (or uh_sum),
which is stored in csum_data.

> > > +				th = (struct tcphdr *) (mtod(m, caddr_t) +
> > > +				     hlen);
> > > +				th->th_sum = in_cksum_phdr(ip->ip_src.s_addr,
> > 
> > i don't think you can assume m->m_len is large enough.
> 
> I thought that might not be a good assumption.  so, I have to use
> m_copyback.  Is that right?

yes.  see in_delayed_cksum.

YAMAMOTO Takashi