[lwip-devel] [bug #21107] LwIP doesn't reset the TTL in ICMP Echo Replies

[Tom Evans]

Follow-up Comment #2, bug #21107 (project lwip):

> But I'm not really sure what you mean with: 
>
>> The LwIP code should set the TTL and then recalculate 
>> (rather than just "adjust") the TTL.

Oops, typo. I meant "Recalculate the IP Checksum".

The ICMP code has the following, so I assumed it was in
charge of the checksum. It is likely that ip_output()
actually recalculates, in which case the following could
be removed:

    /* adjust the checksum */
    if (iecho->chksum >= htons(0xffff - (ICMP_ECHO << 8))) {
      iecho->chksum += htons(ICMP_ECHO << 8) + 1;
    } else {
      iecho->chksum += htons(ICMP_ECHO << 8);
    }

In my code (version 1.2) the call to ip_output_if() is:

    ip_output_if(p, &(iphdr->src), IP_HDRINCL,
     IPH_TTL(iphdr), 0, IP_PROTO_ICMP, inp);

That passes in "IPH_TTL(iphdr)" which looks to me to be
the TTL in the inbound packet and not a request to set to
the default (pcb->ttl) or probably (ICMP_TTL).


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