[Click] Stupid question: Crafting packets with correct checksums...

[Nicholas Weaver]

I'm doing a little hacking where I'm crafting raw TCP packets.

Things seem to be going well looking at the data received on the other  
side looking at TCPdump, except I can't seem to generate proper TCP  
checksums, either with raw code or a pipeline of

SetTCPChecksum->SetIPChecksum

or with code in the file.  If it helps, this is on OS-X.  I feel like  
I'm doing something incredibly stupid here, and I'm just missing  
something obvious.

Suggestions?



I'm using the following code (taken from SetTCPChecksum and  
SetIPChecksum), where
dst_iph and dst_tcph as appropriate are pointing into a buffer of  
bytes I'm using to construct the packet.


     dst_iph->ip_len = htons(strlen(reply_string) + (dst_tcph->th_off  
<< 2) +
                             (dst_iph->ip_hl << 2));

     unsigned plen, csum;

     dst_tcph->th_sum = 0;

     plen = ntohs(dst_iph->ip_len) - (dst_iph->ip_hl << 2);
     csum = click_in_cksum((unsigned char *) tcph, plen);

     dst_tcph->th_sum = click_in_cksum_pseudohdr(csum, dst_iph, plen);


     dst_iph->ip_sum = 0;
     dst_iph->ip_sum = click_in_cksum((unsigned char *)dst_iph,
                                      dst_iph->ip_hl << 2);

     WritablePacket * dst = Packet::make(data, ntohs(dst_iph->ip_len)  
+ 14);
     dst->set_ip_header(dst_iph, dst_iph->ip_hl << 2);

     output(0).push(dst);