dns: handle multiple replies for single requests

The world is a mess. A single DNS request may yield multiple, subtly
different responses; all with the same DNS protocol-level ID.

Last response wins in terms of what ends up in the cache.

First response wins in terms of the response sent to the client. Hard
to do otherwise since we have no clue that more than one answer will
be forthcoming.

Type: fix
Ticket: VPP-1749
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I3175a40eb1fea237048d16b852a430f5ab51eaef
(cherry picked from commit e8d2dcb6a619f0884ece2284a286f21b3aa77e5a)

4 files changed, 126 insertions, 59 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 5b7ca88cd19..7e4a9d01fea 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -245,6 +245,11 @@ M:	Dave Barach <dave@barachs.net>
 M:	Neale Ranns <nranns@cisco.com>
 F:	src/vnet/dhcp/
 
+VNET DNS
+I:	dns
+M:	Dave Barach <dave@barachs.net>
+F:	src/vnet/dns/
+
 VNET TLS and TLS engine plugins
 I:	tls
 M:	Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/dns/dns.c b/src/vnet/dns/dns.c
index 3285bedc903..eac95450485 100644
--- a/src/vnet/dns/dns.c
+++ b/src/vnet/dns/dns.c
@@ -248,7 +248,8 @@ vnet_dns_send_dns4_request (dns_main_t * dm,
   fib_index = fib_table_find (prefix.fp_proto, 0 /* default VRF for now */ );
   if (fib_index == (u32) ~ 0)
     {
-      clib_warning ("no fib table");
+      if (0)
+	clib_warning ("no fib table");
       return;
     }
 
@@ -257,7 +258,8 @@ vnet_dns_send_dns4_request (dns_main_t * dm,
   /* Couldn't find route to destination. Bail out. */
   if (fei == FIB_NODE_INDEX_INVALID)
     {
-      clib_warning ("no route to DNS server");
+      if (0)
+	clib_warning ("no route to DNS server");
       return;
     }
 
@@ -265,9 +267,10 @@ vnet_dns_send_dns4_request (dns_main_t * dm,
 
   if (sw_if_index == ~0)
     {
-      clib_warning
-	("route to %U exists, fei %d, get_resolving_interface returned"
-	 " ~0", format_ip4_address, &prefix.fp_addr, fei);
+      if (0)
+	clib_warning
+	  ("route to %U exists, fei %d, get_resolving_interface returned"
+	   " ~0", format_ip4_address, &prefix.fp_addr, fei);
       return;
     }
 
@@ -365,7 +368,8 @@ vnet_dns_send_dns6_request (dns_main_t * dm,
   fib_index = fib_table_find (prefix.fp_proto, 0 /* default VRF for now */ );
   if (fib_index == (u32) ~ 0)
     {
-      clib_warning ("no fib table");
+      if (0)
+	clib_warning ("no fib table");
       return;
     }
 
@@ -1061,6 +1065,7 @@ vnet_dns_cname_indirection_nolock (dns_main_t * dm, u32 ep_index, u8 * reply)
 	goto found_last_request;
       }
   clib_warning ("pool elt %d supposedly pending, but not found...", ep_index);
+  return -1;
 
 found_last_request:
 
@@ -2206,46 +2211,51 @@ format_dns_cache (u8 * s, va_list * args)
       return s;
     }
 
-  /* *INDENT-OFF* */
-  pool_foreach (ep, dm->entries,
-  ({
-    if (ep->flags & DNS_CACHE_ENTRY_FLAG_VALID)
-      {
-        ASSERT (ep->dns_response);
-        if (ep->flags & DNS_CACHE_ENTRY_FLAG_STATIC)
-          ss = "[S] ";
-        else
-          ss = "    ";
+  s = format (s, "DNS cache contains %d entries\n", pool_elts (dm->entries));
 
-        if (verbose < 2 && ep->flags & DNS_CACHE_ENTRY_FLAG_CNAME)
-          s = format (s, "%s%s -> %s", ss, ep->name, ep->cname);
-        else
-          s = format (s, "%s%s -> %U", ss, ep->name,
-                      format_dns_reply,
-                      ep->dns_response,
-                      verbose);
-        if (!(ep->flags & DNS_CACHE_ENTRY_FLAG_STATIC))
+  if (verbose > 0)
+    {
+      /* *INDENT-OFF* */
+      pool_foreach (ep, dm->entries,
+      ({
+        if (ep->flags & DNS_CACHE_ENTRY_FLAG_VALID)
           {
-            f64 time_left = ep->expiration_time - now;
-            if (time_left > 0.0)
-              s = format (s, "  TTL left %.1f", time_left);
+            ASSERT (ep->dns_response);
+            if (ep->flags & DNS_CACHE_ENTRY_FLAG_STATIC)
+              ss = "[S] ";
             else
-              s = format (s, "  EXPIRED");
+              ss = "    ";
 
-            if (verbose > 2)
-              s = format (s, "    %d client notifications pending\n",
-                          vec_len(ep->pending_requests));
+            if (verbose < 2 && ep->flags & DNS_CACHE_ENTRY_FLAG_CNAME)
+              s = format (s, "%s%s -> %s", ss, ep->name, ep->cname);
+            else
+              s = format (s, "%s%s -> %U", ss, ep->name,
+                          format_dns_reply,
+                          ep->dns_response,
+                          verbose);
+            if (!(ep->flags & DNS_CACHE_ENTRY_FLAG_STATIC))
+              {
+                f64 time_left = ep->expiration_time - now;
+                if (time_left > 0.0)
+                  s = format (s, "  TTL left %.1f", time_left);
+                else
+                  s = format (s, "  EXPIRED");
+
+                if (verbose > 2)
+                  s = format (s, "    %d client notifications pending\n",
+                              vec_len(ep->pending_requests));
+              }
           }
-      }
-    else
-      {
-        ASSERT (ep->dns_request);
-        s = format (s, "[P] %U", format_dns_reply, ep->dns_request,
-                    verbose);
-      }
-    vec_add1 (s, '\n');
-  }));
-  /* *INDENT-ON* */
+        else
+          {
+            ASSERT (ep->dns_request);
+            s = format (s, "[P] %U", format_dns_reply, ep->dns_request,
+                        verbose);
+          }
+        vec_add1 (s, '\n');
+      }));
+      /* *INDENT-ON* */
+    }
 
   dns_cache_unlock (dm);
 
@@ -2764,6 +2774,7 @@ vnet_send_dns4_reply (dns_main_t * dm, dns_pending_request_t * pr,
   dns_rr_t *rr;
   u8 *rrptr;
   int is_fail = 0;
+  int is_recycle = (b0 != 0);
 
   ASSERT (ep && ep->dns_response);
 
@@ -2804,6 +2815,16 @@ vnet_send_dns4_reply (dns_main_t * dm, dns_pending_request_t * pr,
 	return;
       b0 = vlib_get_buffer (vm, bi);
     }
+  else
+    {
+      /* Use the buffer we were handed. Reinitialize it... */
+      vlib_buffer_t bt = { };
+      /* push/pop the reference count */
+      u8 save_ref_count = b0->ref_count;
+      vlib_buffer_copy_template (b0, &bt);
+      b0->ref_count = save_ref_count;
+      bi = vlib_get_buffer_index (vm, b0);
+    }
 
   if (b0->flags & VLIB_BUFFER_NEXT_PRESENT)
     vlib_buffer_free_one (vm, b0->next_buffer);
@@ -2817,9 +2838,6 @@ vnet_send_dns4_reply (dns_main_t * dm, dns_pending_request_t * pr,
    */
   b0->flags |= (VNET_BUFFER_F_LOCALLY_ORIGINATED
 		| VLIB_BUFFER_TOTAL_LENGTH_VALID);
-  b0->current_data = 0;
-  b0->current_length = 0;
-  b0->total_length_not_including_first_buffer = 0;
   vnet_buffer (b0)->sw_if_index[VLIB_RX] = 0;	/* "local0" */
   vnet_buffer (b0)->sw_if_index[VLIB_TX] = 0;	/* default VRF for now */
 
@@ -2971,12 +2989,18 @@ found_src_address:
   udp->checksum = 0;
   vec_free (reply);
 
-  /* Ship it to ip4_lookup */
-  f = vlib_get_frame_to_node (vm, ip4_lookup_node.index);
-  to_next = vlib_frame_vector_args (f);
-  to_next[0] = bi;
-  f->n_vectors = 1;
-  vlib_put_frame_to_node (vm, ip4_lookup_node.index, f);
+  /*
+   * Ship pkts made out of whole cloth to ip4_lookup
+   * Caller will ship recycled dns reply packets to ip4_lookup
+   */
+  if (is_recycle == 0)
+    {
+      f = vlib_get_frame_to_node (vm, ip4_lookup_node.index);
+      to_next = vlib_frame_vector_args (f);
+      to_next[0] = bi;
+      f->n_vectors = 1;
+      vlib_put_frame_to_node (vm, ip4_lookup_node.index, f);
+    }
 }
 
 /*
diff --git a/src/vnet/dns/dns.h b/src/vnet/dns/dns.h
index e6944de0ca9..6d0901020f9 100644
--- a/src/vnet/dns/dns.h
+++ b/src/vnet/dns/dns.h
@@ -150,7 +150,9 @@ _(DISABLED, "DNS pkts punted (feature disabled)")       \
 _(PROCESSED, "DNS reply pkts processed")                \
 _(NO_ELT, "No DNS pool element")                        \
 _(FORMAT_ERROR, "DNS format errors")                    \
-_(TEST_DROP, "DNS reply pkt dropped for test purposes")
+_(TEST_DROP, "DNS reply pkt dropped for test purposes") \
+_(MULTIPLE_REPLY, "DNS multiple reply packets")	        \
+_(NO_UNRESOLVED_ENTRY, "No unresolved entry for pkt")
 
 typedef enum
 {
diff --git a/src/vnet/dns/resolver_process.c b/src/vnet/dns/resolver_process.c
index 220a4907963..4ed7c799ce2 100644
--- a/src/vnet/dns/resolver_process.c
+++ b/src/vnet/dns/resolver_process.c
@@ -59,6 +59,8 @@ resolve_event (dns_main_t * dm, f64 now, u8 * reply)
   u16 flags;
   u16 rcode;
   int i;
+  int entry_was_valid;
+  int remove_count;
   int rv = 0;
 
   d = (dns_header_t *) reply;
@@ -72,6 +74,8 @@ resolve_event (dns_main_t * dm, f64 now, u8 * reply)
   if (pool_is_free_index (dm->entries, pool_index))
     {
       vec_free (reply);
+      if (0)
+	clib_warning ("pool index %d is free", pool_index);
       vlib_node_increment_counter (vm, dns46_reply_node.index,
 				   DNS46_REPLY_ERROR_NO_ELT, 1);
       dns_cache_unlock (dm);
@@ -149,8 +153,29 @@ resolve_event (dns_main_t * dm, f64 now, u8 * reply)
 reply:
   /* Save the response */
   ep->dns_response = reply;
-  /* Pick some sensible default. */
+
+  /*
+   * Pick a sensible default cache entry expiration time.
+   * We don't play the 10-second timeout game.
+   */
   ep->expiration_time = now + 600.0;
+
+  if (0)
+    clib_warning ("resolving '%s', was %s valid",
+		  ep->name, (ep->flags & DNS_CACHE_ENTRY_FLAG_VALID) ?
+		  "already" : "not");
+  /*
+   * The world is a mess. A single DNS request sent to e.g. 8.8.8.8
+   * may yield multiple, subtly different responses - all with the same
+   * DNS protocol-level ID.
+   *
+   * Last response wins in terms of what ends up in the cache.
+   * First response wins in terms of the response sent to the client.
+   */
+
+  /* Strong hint that we may not find a pending resolution entry */
+  entry_was_valid = (ep->flags & DNS_CACHE_ENTRY_FLAG_VALID) ? 1 : 0;
+
   if (vec_len (ep->dns_response))
     ep->flags |= DNS_CACHE_ENTRY_FLAG_VALID;
 
@@ -218,17 +243,27 @@ reply:
     }
   vec_free (ep->pending_requests);
 
+  remove_count = 0;
   for (i = 0; i < vec_len (dm->unresolved_entries); i++)
     {
       if (dm->unresolved_entries[i] == pool_index)
 	{
 	  vec_delete (dm->unresolved_entries, 1, i);
-	  goto found;
+	  remove_count++;
+	  i--;
 	}
     }
-  clib_warning ("pool index %d AWOL from unresolved vector", pool_index);
+  /* See multiple response comment above... */
+  if (remove_count == 0)
+    {
+      u32 error_code = entry_was_valid ? DNS46_REPLY_ERROR_MULTIPLE_REPLY :
+	DNS46_REPLY_ERROR_NO_UNRESOLVED_ENTRY;
+
+      vlib_node_increment_counter (vm, dns46_reply_node.index, error_code, 1);
+      dns_cache_unlock (dm);
+      return;
+    }
 
-found:
   /* Deal with bogus names, server issues, etc. */
   switch (rcode)
     {
@@ -240,13 +275,13 @@ found:
     case DNS_RCODE_NOT_IMPLEMENTED:
     case DNS_RCODE_REFUSED:
       if (ep->server_af == 0)
-	clib_warning ("name server %U backfire",
+	clib_warning ("name server %U can't resolve '%s'",
 		      format_ip4_address,
-		      dm->ip4_name_servers + ep->server_rotor);
+		      dm->ip4_name_servers + ep->server_rotor, ep->name);
       else
-	clib_warning ("name server %U backfire",
+	clib_warning ("name server %U can't resolve '%s'",
 		      format_ip6_address,
-		      dm->ip6_name_servers + ep->server_rotor);
+		      dm->ip6_name_servers + ep->server_rotor, ep->name);
       /* FALLTHROUGH */
     case DNS_RCODE_NAME_ERROR:
     case DNS_RCODE_FORMAT_ERROR:
@@ -255,6 +290,7 @@ found:
       break;
     }
 
+
   dns_cache_unlock (dm);
   return;
 }