net: IP stack milestone B - UDP echo (pseudo-header checksum)

Refactor netd into a protocol dispatcher (IP -> ICMP/UDP) and add UDP echo:
swap addresses + ports and recompute the UDP checksum over the pseudo-header
(src/dst IP + proto + length) plus the datagram. This is the same pseudo-
header TCP uses, so it de-risks the next milestone.

tools/gateway.py --mode udp sends a datagram and verifies the echo.
Verified: 'hello udp gbos' echoes back with cksum=ok.

1 files changed, 27 insertions, 1 deletions

diff --git a/tools/gateway.py b/tools/gateway.py
index 590526e..f5fc701 100644
--- a/tools/gateway.py
+++ b/tools/gateway.py
@@ -126,9 +126,19 @@ def icmp_echo(seq, payload=b"gbos-ping-0123456789"):
     ip = ip[:10] + struct.pack("!H", inet_cksum(ip)) + ip[12:]
     return ip + icmp
 
+def udp_packet(sport, dport, payload):
+    ulen = 8 + len(payload)
+    udp = struct.pack("!HHHH", sport, dport, ulen, 0) + payload
+    pseudo = GW_IP + GB_IP + bytes([0, 17]) + struct.pack("!H", ulen)
+    ck = inet_cksum(pseudo + udp) or 0xffff
+    udp = udp[:6] + struct.pack("!H", ck) + udp[8:]
+    ip = struct.pack("!BBHHHBBH", 0x45, 0, 20+ulen, 7, 0, 64, 17, 0) + GW_IP + GB_IP
+    ip = ip[:10] + struct.pack("!H", inet_cksum(ip)) + ip[12:]
+    return ip + udp
+
 def main():
     ap = argparse.ArgumentParser()
-    ap.add_argument("--mode", default="echo", choices=["echo", "http", "chat", "irc", "ping"])
+    ap.add_argument("--mode", default="echo", choices=["echo", "http", "chat", "irc", "ping", "udp"])
     ap.add_argument("--cmd", default="necho")
     ap.add_argument("--nick", default="gameboy")
     ap.add_argument("--channel", default="#sl0p")
@@ -159,6 +169,17 @@ def main():
             m = bytes(payload).decode("latin1", "replace")
             sys.stderr.write("\n[gw] GB says: %s\n" % m)
             return [("<bot> " + m).encode()]
+    elif a.mode == "udp":
+        def on_frame(payload):
+            p = bytes(payload)
+            if len(p) >= 28 and p[9] == 17:
+                ihl = (p[0] & 0x0f) * 4
+                sport, dport, ulen = struct.unpack("!HHH", p[ihl:ihl+6])
+                data = p[ihl+8:ihl+8+ulen-8]
+                ck_ok = inet_cksum(GB_IP + GW_IP + bytes([0,17]) + p[ihl+4:ihl+6] + p[ihl:]) == 0
+                sys.stderr.write("\n[gw] UDP echo %d->%d: %r cksum=%s\n"
+                                 % (sport, dport, data, "ok" if ck_ok else "BAD"))
+            return []
     elif a.mode == "ping":
         def on_frame(payload):
             p = bytes(payload)
@@ -183,6 +204,11 @@ def main():
             except Exception: pass
         irc_holder[0] = IRC(a.host, a.port, a.nick, a.channel, to_gb)
         threading.Thread(target=irc_holder[0].loop, daemon=True).start()
+    if a.mode == "udp":
+        time.sleep(1.0)
+        p.stdin.write(slip_encode(udp_packet(5000, 7, b"hello udp gbos"))); p.stdin.flush()
+        sys.stderr.write("[gw] sent UDP :5000->:7 'hello udp gbos'\n")
+        time.sleep(2.0)
     if a.mode == "ping":
         time.sleep(1.0)
         for seq in range(1, 5):