Fix IPv4/IPv6 fragment continuation data being decoded as fake transport headers

A real correctness bug, not just missing visibility: a non-first
fragment's payload is pure continuation data with no TCP/UDP/ICMP
header in it at all, but it was being handed to the transport parsers
unconditionally, which could misread arbitrary payload bytes as port
numbers, sequence numbers, etc. and print a plausible-looking but
entirely fake decode.

IPv4 gained identification/more_fragments/fragment_offset fields; a
nonzero offset now stops summarize_packet before transport dispatch,
reporting the fragment instead. IPv6 expresses fragmentation as an
extension header instead, so the fix lives in
walk_ipv6_extension_headers(): it already walked past Fragment
headers, but never checked the offset before continuing on as if a
transport header followed - the same bug, reached through a
different path. Fixed with an ESP-style hard stop on a nonzero offset.

Caught and fixed a second bug while writing the first fix, before it
ever ran: the fragment's Identification field was a loop-local
variable, discarded the moment the walk continued past a *first*
fragment (offset zero) to keep decoding the real payload underneath --
every later return reported no fragment id even though one applied.
Fixed by hoisting it to a variable that persists across iterations,
the same category of mistake as an earlier IGMPv3 bug.

Fuzzed afterward regardless (fuzz_ipv4, fuzz_ipv6, fuzz_summarize,
~16.8M combined runs) - clean. Live-verified with a real 4000-byte
ping to the local gateway over the actual 1500-MTU interface: both
directions fragmented into 3 pieces each, the first decoded normally
with a fragmentation note, and the continuation fragments correctly
showed only fragment metadata, no fake ICMP decode attempted.

1 files changed, 28 insertions, 0 deletions

diff --git a/tests/test_summarize.cpp b/tests/test_summarize.cpp
index f180f87..baf4ed4 100644
--- a/tests/test_summarize.cpp
+++ b/tests/test_summarize.cpp
@@ -186,6 +186,34 @@ TEST_CASE("summarize_packet decodes an ARP request end to end") {
           "ARP who-has 10.0.0.2 tell 10.0.0.1 (aa:bb:cc:dd:ee:ff)");
 }
 
+TEST_CASE("summarize_packet reports a non-first IPv4 fragment without decoding fake TCP/UDP") {
+    // Payload here is arbitrary bytes - if this were mistakenly
+    // handed to a transport parser it would produce a plausible-
+    // looking but entirely fake TCP/UDP line. The point of this test
+    // is that it must not.
+    std::vector<unsigned char> fake_continuation_data = {0xDE, 0xAD, 0xBE, 0xEF, 0x00, 0x01, 0x02, 0x03};
+
+    std::vector<unsigned char> ip(20, 0);
+    ip[0] = 0x45;
+    ip[4] = 0x00; ip[5] = 0x7B;  // identification = 123
+    ip[6] = 0x00; ip[7] = 0x08;  // fragment_offset = 8 (byte offset 64), MF=0
+    ip[9] = packeteer::net::kProtoTcp;
+    ip[12] = 10; ip[13] = 0; ip[14] = 0; ip[15] = 1;
+    ip[16] = 10; ip[17] = 0; ip[18] = 0; ip[19] = 2;
+
+    std::vector<unsigned char> eth = {
+        0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF, 0x08, 0x00,
+    };
+
+    std::vector<unsigned char> frame = eth;
+    frame.insert(frame.end(), ip.begin(), ip.end());
+    frame.insert(frame.end(), fake_continuation_data.begin(), fake_continuation_data.end());
+
+    auto line = packeteer::summarize_packet(frame, DLT_EN10MB);
+    CHECK(line.find("fragment id=123 offset=64") != std::string::npos);
+    CHECK(line.find("TCP") == std::string::npos);  // must not have decoded the fake continuation data
+}
+
 TEST_CASE("summarize_packet falls back to the RTCP heuristic on an unmatched UDP port") {
     std::vector<unsigned char> rtcp = {0x80, 0xC9, 0x00, 0x01, 0, 0, 0, 0};  // RR, len=1 -> 8 bytes