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, 37 insertions, 0 deletions

diff --git a/tests/test_ipv6.cpp b/tests/test_ipv6.cpp
index 1cce85b..2d75e9a 100644
--- a/tests/test_ipv6.cpp
+++ b/tests/test_ipv6.cpp
@@ -129,6 +129,43 @@ TEST_CASE("walk_ipv6_extension_headers walks the fixed-size Fragment header") {
     CHECK(result.payload[0] == 0xCA);
 }
 
+TEST_CASE("walk_ipv6_extension_headers reports the fragment id even for the first fragment") {
+    // Same shape as the test above (fragment offset 0 - the first
+    // fragment), but this time checking that the walk continues on to
+    // TCP *and* still surfaces the Identification field, which a
+    // caller needs to correlate this with the fragments that follow.
+    std::vector<unsigned char> payload = {static_cast<unsigned char>(kProtoTcp), 0x00,
+                                           0x00, 0x00, 0x00, 0x00, 0x30, 0x39,  // id = 0x3039
+                                           0xCA, 0xFE};
+    auto result = walk_ipv6_extension_headers(kNextHeaderFragment, payload);
+    CHECK_FALSE(result.is_non_first_fragment);
+    REQUIRE(result.fragment_id.has_value());
+    CHECK(*result.fragment_id == 0x3039);
+    CHECK(result.final_next_header == kProtoTcp);
+    REQUIRE(result.payload.size() == 2);  // walk continued past the fragment header to real payload
+}
+
+TEST_CASE("walk_ipv6_extension_headers stops at a non-first fragment rather than walking into "
+          "continuation data") {
+    // Fragment offset field (13 bits, packed into the top of bytes[2:3])
+    // set to a nonzero value - 8 in units of 8 bytes, i.e. byte offset
+    // 64 into the original datagram. offset_res_m = 8 << 3 = 0x0040.
+    std::vector<unsigned char> payload = {
+        static_cast<unsigned char>(kProtoTcp), 0x00, 0x00, 0x40, 0x00, 0x00, 0x00, 0x2A,
+        0xDE, 0xAD, 0xBE, 0xEF,  // pure continuation data - NOT a TCP header
+    };
+    auto result = walk_ipv6_extension_headers(kNextHeaderFragment, payload);
+    CHECK(result.is_non_first_fragment);
+    REQUIRE(result.fragment_id.has_value());
+    CHECK(*result.fragment_id == 0x2A);
+    // final_next_header still names TCP (that's what the reassembled
+    // datagram eventually is), but the payload past it is untouched
+    // continuation data - callers must not decode it as TCP.
+    CHECK(result.final_next_header == kProtoTcp);
+    REQUIRE(result.payload.size() == 4);
+    CHECK(result.payload[0] == 0xDE);
+}
+
 TEST_CASE("walk_ipv6_extension_headers applies AH's 4-byte-unit length formula") {
     // AH: next_header(1)=TCP, payload_len(1)=1 -> total len (1+2)*4=12 bytes.
     std::vector<unsigned char> payload(12, 0);