vnet: fix ARP for unnumbered

On unnumbered interfaces, ARP fails because there is no attached route.
Allow replies to peer-to-peer addresses on unnumbered interfaces:
  eg. 192.0.2.1/32 <-> 192.0.2.2/32

Type: fix
Change-Id: Ibeb8d8ebc8d58d5bfb0724739a17694e0217356e
Signed-off-by: Pim van Pelt <pim@ipng.nl>

2 files changed, 30 insertions, 21 deletions

diff --git a/src/vnet/arp/arp.c b/src/vnet/arp/arp.c
index cacdd71e381..43b2a93a7b3 100644
--- a/src/vnet/arp/arp.c
+++ b/src/vnet/arp/arp.c
@@ -423,6 +423,10 @@ arp_reply (vlib_main_t * vm, vlib_node_runtime_t * node, vlib_frame_t * frame)
 
 	    }
 
+	  dst_fei = ip4_fib_table_lookup (ip4_fib_get (fib_index0),
+					  &arp0->ip4_over_ethernet[1].ip4, 32);
+	  conn_sw_if_index0 = fib_entry_get_any_resolving_interface (dst_fei);
+
 	  {
 	    /*
 	     * we're looking for FIB entries that indicate the source
@@ -509,24 +513,20 @@ arp_reply (vlib_main_t * vm, vlib_node_runtime_t * node, vlib_frame_t * frame)
 	    while (!attached &&
 		   !fib_entry_is_sourced (src_fei, FIB_SOURCE_DEFAULT_ROUTE));
 
-	    if (!attached)
+	    if (!attached &&
+		!arp_unnumbered (p0, sw_if_index0, conn_sw_if_index0))
 	      {
 		/*
-		 * the matching route is a not attached, i.e. it was
-		 * added as a result of routing, rather than interface/ARP
-		 * configuration. If the matching route is not a host route
-		 * (i.e. a /32)
+		 * the matching route is a not attached and not unnumbered,
+		 * i.e. it was added as a result of routing, rather than
+		 * interface/ARP configuration. If the matching route is not
+		 * a host route (i.e. a /32)
 		 */
 		error0 = ARP_ERROR_L3_SRC_ADDRESS_NOT_LOCAL;
 		goto drop;
 	      }
 	  }
 
-	  dst_fei = ip4_fib_table_lookup (ip4_fib_get (fib_index0),
-					  &arp0->ip4_over_ethernet[1].ip4,
-					  32);
-	  conn_sw_if_index0 = fib_entry_get_any_resolving_interface (dst_fei);
-
 	  switch (arp_dst_fib_check (dst_fei, &dst_flags))
 	    {
 	    case ARP_DST_FIB_ADJ:
@@ -619,9 +619,9 @@ arp_reply (vlib_main_t * vm, vlib_node_runtime_t * node, vlib_frame_t * frame)
 	      sw_if_index0 != fib_entry_get_resolving_interface (src_fei))
 	    {
 	      /*
-	       * The interface the ARP is sent to or was received on is not the
-	       * interface on which the covering prefix is configured.
-	       * Maybe this is a case for unnumbered.
+	       * The interface the ARP is sent to or was received on is
+	       * not the interface on which the covering prefix is
+	       * configured. Maybe this is a case for unnumbered.
 	       */
 	      if (!arp_unnumbered (p0, sw_if_index0, conn_sw_if_index0))
 		{
@@ -636,8 +636,7 @@ arp_reply (vlib_main_t * vm, vlib_node_runtime_t * node, vlib_frame_t * frame)
 	      goto drop;
 	    }
 
-	  next0 = arp_mk_reply (vnm, p0, sw_if_index0,
-				if_addr0, arp0, eth_rx);
+	  next0 = arp_mk_reply (vnm, p0, sw_if_index0, if_addr0, arp0, eth_rx);
 
 	  /* We are going to reply to this request, so, in the absence of
 	     errors, learn the sender */
diff --git a/test/test_neighbor.py b/test/test_neighbor.py
index 7338eff86d3..6fcf13f8261 100644
--- a/test/test_neighbor.py
+++ b/test/test_neighbor.py
@@ -432,14 +432,24 @@ class ARPTestCase(VppTestCase):
         self.assertEqual(unnum[0].ip_sw_if_index, self.pg1.sw_if_index)
         self.assertEqual(unnum[0].sw_if_index, self.pg2.sw_if_index)
 
-        #
-        # We should respond to ARP requests for the unnumbered to address
-        # once an attached route to the source is known
-        #
-        self.send_and_assert_no_replies(
-            self.pg2, p, "ARP req for unnumbered address - no source"
+        # Allow for ARP requests from point-to-point ethernet neighbors
+        # without an attached route on pg2
+        self.pg2.add_stream(p)
+        self.pg_enable_capture(self.pg_interfaces)
+        self.pg_start()
+
+        rx = self.pg2.get_capture(1)
+        self.verify_arp_resp(
+            rx[0],
+            self.pg2.local_mac,
+            self.pg2.remote_mac,
+            self.pg1.local_ip4,
+            self.pg2.remote_hosts[3].ip4,
         )
 
+        #
+        # Allow for ARP requests from neighbors on unnumbered with
+        # an attached route on pg2
         attached_host = VppIpRoute(
             self,
             self.pg2.remote_hosts[3].ip4,