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

[Eddie Kohler]

Well, without seeing more of your code it'd be hard to tell.  I think the 
SetTCPChecksum->SetIPChecksum pipeline should work, assuming the IP header 
anntoation has been correctly set; tested:

-----/tmp/x------
!data src dst proto sport dport payload
1.0.0.1 2.0.0.2 T 10 20 "abcdefghijklmn"

-----without checksum------
% click -e 'FromIPSummaryDump(/tmp/x, STOP true) -> ToDump(-, ENCAP IP)' | 
tcpdump -vnr -
reading from file -, link-type RAW (Raw IP)
18:12:24.969786 IP (tos 0x0, id 0, offset 0, flags [none], proto TCP (6), 
length 54, bad cksum 0 (->b7c0)!) 1.0.0.1.10 > 2.0.0.2.ftp-data: ., cksum 
0x0000 (incorrect (-> 0xd8db), 0:14(14) win 0

-----with checksum------
% click -e 'FromIPSummaryDump(/tmp/x, STOP true) -> SetTCPChecksum -> 
SetIPChecksum -> ToDump(-, ENCAP IP)' | tcpdump -vnr -
reading from file -, link-type RAW (Raw IP)
18:12:34.788939 IP (tos 0x0, id 0, offset 0, flags [none], proto TCP (6), 
length 54) 1.0.0.1.10 > 2.0.0.2.ftp-data: ., cksum 0xd8db (correct), 0:14(14) 
win 0

Eddie


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);
> 
> 
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