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

[Matthias Petschick]

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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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