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

[Nicholas Weaver]

Both the sending and receiving side show the click calculated  
(incorrectly) checksum, whether to a file or to the network.

I suspect I'm just doing something INCREDIBLY dumb.

On May 18, 2008, at 3:32 PM, Matthias Petschick wrote:

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> Hey,
>
> are you looking at your checksums on the receiving or on the sending  
> side?
> This may be caused by your OS skipping the checksum calculation and
> leaving it to your network card (tcp checksum offloading), if it's the
> latter. Afaik OSX does that, no idea if it can be disabled or not.
>
> - - Matthias
>
>
> Nicholas Weaver wrote:
> | 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);
> |
> |
> | _______________________________________________
> | click mailing list
> | click at amsterdam.lcs.mit.edu
> | https://amsterdam.lcs.mit.edu/mailman/listinfo/click
>
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