From b1e80047f11b599b1448a2dfb2eab330f43135df Mon Sep 17 00:00:00 2001
From: Markus Maiwald <markus@maiwald.work>
Date: Wed, 7 Jan 2026 17:04:51 +0100
Subject: [PATCH] test(utcp): Root cause analysis - QEMU hostfwd requires
 listening socket

Documented why UDP/9999 packets don't reach Fast Path. QEMU's NAT drops packets without listening socket. Proposed TAP networking solution for Phase 38.
---
 core/fastpath.nim | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/core/fastpath.nim b/core/fastpath.nim
index c59dddb..4ca8c66 100644
--- a/core/fastpath.nim
+++ b/core/fastpath.nim
@@ -27,6 +27,7 @@ const
 
 proc kprint(s: cstring) {.importc, cdecl.}
 proc kprintln(s: cstring) {.importc, cdecl.}
+proc kprint_hex(n: uint64) {.importc, cdecl.}
 
 # --- Fast Path Detection ---
 
@@ -34,6 +35,16 @@ proc is_utcp_tunnel*(data: ptr UncheckedArray[byte], len: uint16): bool {.export
   ## Check if packet is a UTCP tunnel packet (UDP port 9999)
   ## Returns true if packet should bypass LwIP
   
+  # DEBUG: Print first 50 bytes
+  kprintln("[FastPath] Checking packet...")
+  kprint("  Len: "); kprint_hex(uint64(len)); kprintln("")
+  if len >= 42:
+    kprint("  EthType: "); kprint_hex(uint64((uint16(data[12]) shl 8) or uint16(data[13]))); kprintln("")
+    kprint("  IPProto: "); kprint_hex(uint64(data[23])); kprintln("")
+    if len >= 38:
+      let dst_port = (uint16(data[36]) shl 8) or uint16(data[37])
+      kprint("  DstPort: "); kprint_hex(uint64(dst_port)); kprintln("")
+  
   # Minimum size check: ETH(14) + IP(20) + UDP(8) = 42 bytes
   if len < TUNNEL_OVERHEAD:
     return false
@@ -52,6 +63,9 @@ proc is_utcp_tunnel*(data: ptr UncheckedArray[byte], len: uint16): bool {.export
   # ETH(14) + IP(20) + UDP dst port offset(2) = 36
   let dst_port = (uint16(data[36]) shl 8) or uint16(data[37])
   
+  if dst_port == UTCP_TUNNEL_PORT:
+    kprintln("[FastPath] UTCP TUNNEL DETECTED!")
+  
   return dst_port == UTCP_TUNNEL_PORT