[Feature] recalculate IPv6 Fragment header checksum #897

ChuckCottrill · 2024-07-16T18:25:09Z

Overview

The tcpreplay program tcprewrite does not support IPv6 FRAG(mented) header checksum recalculation. The protocol 0x2c is not included in the switch statement, and thus a TCPEDIT_WARN is generated and the header checksum is not recalculated.

The option --fixcsum does not work against the original 11465.0-0.unknown_1.v6.pcap pcap (and others) due to how the protocol value supplied to do_checksum() is handled. The protocol is IPV6-FRAG(mented). There is a switch statement where the protocol value (== 0x2c) is not one of the recognized values. The tcprewrite code explicitly returns indication (TCPEDIT_WARN) that certain packets (both TCP and HTTP) do not have their checksum recalculated.
Since the checksum needs to be correctly calculated, this incorrect checksum causes the pcap to fail testing.

Expected Behavior

An IPV6 FRAGmented packet should have correct checksum calculated when packet is rewritten.

Description of desired solution

Since tcpreplay does not support IPv6-FRAG(ment) header checksum recalculation. The protocol 0x2c is not included in the switch statement, and thus a TCPEDIT_WARN is generated and the header checksum is not recalculated.

What is desired is that the protocol 0x2c be recognized and the correct header checksum be calculated and replaced, rather than the warning TCPEDIT_WARN generated, and the generated pcap file not be corrupted with an invalid checksum.

Add the protocol 0x2c to the constants src/tcpr.h

#ifndef IPPROTO_TCP_V6FRAG
#define IPPROTO_TCP_V6FRAG 0x2c
#endif
#ifndef IPPROTO_HTTP_V6FRAG
#define IPPROTO_HTTP_V6FRAG 0x2c
#endif

And the protocol added to the switch statement in src/tcpedit/checksum.c

        case IPPROTO_TCP:
        case IPPROTO_TCP_V6FRAG:
        // case IPPROTO_HTTP_V6FRAG:
protoname = "IPPROTO_TCP";
            if (len < (int)sizeof(tcp_hdr_t)) {
                tcpedit_setwarn(tcpedit, "%s", "Unable to checksum TCP with insufficient L4 data");
                return TCPEDIT_WARN;
            }

            tcp = (tcp_hdr_t *)(data + ip_hl);
#ifdef STUPID_SOLARIS_CHECKSUM_BUG
            tcp->th_sum = tcp->th_off << 2;
            return (TCPEDIT_OK);
#endif
            tcp->th_sum = 0;

            /* Note, we do both src & dst IP's at the same time, that's why the
             * length is 2x a single IP
             */
            if (ipv6 != NULL) {
                sum = do_checksum_math((uint16_t *)&ipv6->ip_src, 32);
            } else {
                sum = do_checksum_math((uint16_t *)&ipv4->ip_src, 8);
            }
            sum += ntohs(IPPROTO_TCP + len);
            sum += do_checksum_math((uint16_t *)tcp, len);
            tcp->th_sum = CHECKSUM_CARRY(sum);
            break;

see:

To Reproduce
Steps to reproduce the behavior:

have pcap with protocol == 0x2c (IPV6 FRAG(mented))
run tcpprep on pcap
run tcprewrite on pcap (change src/dest ip address to precipitate checksum recalculation)
examine rewritten pcap in wireshark, observe that checksum is incorrect

Describe alternatives you've considered

The header checksum is not correctly rewritten by tcpreplay in this situation.
Either:

a) do nothing (unacceptable),
b) rewrite checksum using some external program,
c) recalculate header checksum for these packets (this solution)
d) ideas? suggestions? how else can we recalculate header checksums?

Additional context
The TCP case can be fixed with a patch to allow protocol value 0x2c to be handled the same way as IPPROTO_TCP, but this patch does not work for (HTTP protocol). Further investigation is needed.
Screenshots
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System (please complete the following information):