vpp: Workaround scapy bfd issue

ansible/roles/test/files/ptftests/py3/bfd_responder.py

@@ -8,6 +8,8 @@
 import ptf.packet as scapy
 from ptf.base_tests import BaseTest
 from scapy.contrib.bfd import BFD
+from scapy.layers.inet import IP
+from scapy.layers.inet6 import IPv6
 from ptf.testutils import (send_packet, test_params_get)
 from ipaddress import ip_address, IPv4Address, IPv6Address
 session_timeout = 1
@@ -161,19 +163,37 @@ def craft_bfd_packet(self,
                          bfd_remote_disc,
                          bfd_state):
         ethpart = scapy.Ether(data)
-        bfdpart = BFD(bytes(ethpart.payload.payload.payload))
+
+        # Parse only the mandatory 24-byte BFD header; ignore any
+        # trailing data such as Simple Password authentication.
+        bfdpart = BFD(bytes(ethpart.payload.payload.payload)[:24])
         bfdpart.my_discriminator = my_discriminator
         bfdpart.your_discriminator = bfd_remote_disc
         bfdpart.sta = bfd_state
 
-        ethpart.payload.payload.payload = bfdpart
-        ethpart.src = mac_dst
-        ethpart.dst = mac_src
-        ethpart.payload.src = ip_dst
-        ethpart.payload.dst = ip_src
-
-        # recompute UDP checksum
-        ethpart.payload.payload.chksum = None
-        ethpart.show()
-
-        return ethpart
+        # If the A (Auth Present) flag is set, scapy auto-creates a
+        # phantom OptionalAuth with default auth_key=b'password' even
+        # when no auth bytes exist on the wire.  Clear both to prevent
+        # the 11-byte auth trailer from being serialized into the
+        # response packet.
+        bfdpart.flags = bfdpart.flags & ~0x04
+        bfdpart.optional_auth = None
+
+        udp_sport = ethpart[scapy.UDP].sport
+        udp_dport = ethpart[scapy.UDP].dport
+
+        # Build the response packet from scratch so no cached raw
+        # bytes (e.g. auth trailing data) leak from the parsed packet.
+        if ethpart.type == 0x86dd:  # IPv6
+            pkt = (scapy.Ether(src=mac_dst, dst=mac_src, type=0x86dd) /
+                   IPv6(src=ip_dst, dst=ip_src, hlim=255) /
+                   scapy.UDP(sport=udp_sport, dport=udp_dport) /
+                   bfdpart)
+        else:  # IPv4
+            pkt = (scapy.Ether(src=mac_dst, dst=mac_src, type=0x0800) /
+                   IP(src=ip_dst, dst=ip_src, ttl=255) /
+                   scapy.UDP(sport=udp_sport, dport=udp_dport) /
+                   bfdpart)
+
+        pkt.show()
+        return pkt