ip: source address selection

Implement a simple source address selection algorithm
for IPv4 and IPv6.
IPv6 does not yet implement RFC6724 but supports link-locals.
ping now chooses correct source address for link-local destination.
Added ping support for link-local multicast (e.g. allnodes).

Type: feature
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: I1a3382c1f7d4ace0386c2c19e4e47b045b73a3ed
Signed-off-by: Ole Troan <ot@cisco.com>

1 files changed, 214 insertions, 0 deletions

diff --git a/src/vnet/ip/ip_sas.c b/src/vnet/ip/ip_sas.c
new file mode 100644
index 00000000000..7d3632d95ed
--- /dev/null
+++ b/src/vnet/ip/ip_sas.c
@@ -0,0 +1,214 @@
+/*
+ * Copyright (c) 2021 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "ip_sas.h"
+#include <vppinfra/types.h>
+#include <vnet/ip/ip_interface.h>
+#include <vnet/fib/fib_table.h>
+#include <vnet/ip/ip6_link.h>
+#include <vppinfra/byte_order.h>
+
+/*
+ * This file implement source address selection for VPP applications
+ * (e.g. ping, DNS, ICMP)
+ * It does not yet implement full fledged RFC6724 SAS.
+ * SAS assumes every IP enabled interface has an address. The algorithm will
+ * not go and hunt for a suitable IP address on other interfaces than the
+ * output interface or the specified preferred sw_if_index.
+ * That means that an interface with just an IPv6 link-local address must also
+ * be configured with an unnumbered configuration pointing to a numbered
+ * interface.
+ */
+
+static int
+ip6_sas_commonlen (const ip6_address_t *a1, const ip6_address_t *a2)
+{
+  u64 fa = clib_net_to_host_u64 (a1->as_u64[0]) ^
+	   clib_net_to_host_u64 (a2->as_u64[0]);
+  if (fa == 0)
+    {
+      u64 la = clib_net_to_host_u64 (a1->as_u64[1]) ^
+	       clib_net_to_host_u64 (a2->as_u64[1]);
+      if (la == 0)
+	return 128;
+      return 64 + __builtin_clzll (la);
+    }
+  else
+    {
+      return __builtin_clzll (fa);
+    }
+}
+
+static int
+ip4_sas_commonlen (const ip4_address_t *a1, const ip4_address_t *a2)
+{
+  u64 a =
+    clib_net_to_host_u32 (a1->as_u32) ^ clib_net_to_host_u32 (a2->as_u32);
+  if (a == 0)
+    return 32;
+  return __builtin_clz (a);
+}
+
+/*
+ * walk all addresses on an interface:
+ *  - prefer a source matching the scope of the destination address.
+ *  - last resort pick the source address with the longest
+ *    common prefix with destination
+ * NOTE: This should at some point implement RFC6724.
+ */
+bool
+ip6_sas_by_sw_if_index (u32 sw_if_index, const ip6_address_t *dst,
+			ip6_address_t *src)
+{
+  ip_interface_address_t *ia = 0;
+  ip_lookup_main_t *lm6 = &ip6_main.lookup_main;
+  ip6_address_t *tmp, *bestsrc = 0;
+  int bestlen = 0, l;
+
+  if (ip6_address_is_link_local_unicast (dst) ||
+      dst->as_u32[0] == clib_host_to_net_u32 (0xff020000))
+    {
+      ip6_address_copy (src, ip6_get_link_local_address (sw_if_index));
+      return true;
+    }
+
+  foreach_ip_interface_address (
+    lm6, ia, sw_if_index, 1, ({
+      if (ia->flags & IP_INTERFACE_ADDRESS_FLAG_STALE)
+	continue;
+      tmp = ip_interface_address_get_address (lm6, ia);
+      l = ip6_sas_commonlen (tmp, dst);
+      if (l > bestlen || bestsrc == 0)
+	{
+	  bestsrc = tmp;
+	  bestlen = l;
+	}
+    }));
+  if (bestsrc)
+    {
+      ip6_address_copy (src, bestsrc);
+      return true;
+    }
+  return false;
+}
+
+/*
+ * walk all addresses on an interface and pick the source address with the
+ * longest common prefix with destination.
+ */
+bool
+ip4_sas_by_sw_if_index (u32 sw_if_index, const ip4_address_t *dst,
+			ip4_address_t *src)
+{
+  ip_interface_address_t *ia = 0;
+  ip_lookup_main_t *lm4 = &ip4_main.lookup_main;
+  ip4_address_t *tmp, *bestsrc = 0;
+  int bestlen = 0, l;
+
+  foreach_ip_interface_address (
+    lm4, ia, sw_if_index, 1, ({
+      if (ia->flags & IP_INTERFACE_ADDRESS_FLAG_STALE)
+	continue;
+      tmp = ip_interface_address_get_address (lm4, ia);
+      l = ip4_sas_commonlen (tmp, dst);
+      if (l > bestlen || bestsrc == 0)
+	{
+	  bestsrc = tmp;
+	  bestlen = l;
+	}
+    }));
+  if (bestsrc)
+    {
+      src->as_u32 = bestsrc->as_u32;
+      return true;
+    }
+  return false;
+}
+
+/*
+ * table_id must be set. Default = 0.
+ * sw_if_index is the interface to pick SA from otherwise ~0 will pick from
+ * outbound interface.
+ *
+ * NOTE: What to do if multiple output interfaces?
+ *
+ */
+bool
+ip6_sas (u32 table_id, u32 sw_if_index, const ip6_address_t *dst,
+	 ip6_address_t *src)
+{
+  fib_prefix_t prefix;
+  u32 if_index = sw_if_index;
+
+  /* If sw_if_index is not specified use the output interface. */
+  if (sw_if_index == ~0)
+    {
+      clib_memcpy (&prefix.fp_addr.ip6, dst, sizeof (*dst));
+      prefix.fp_proto = FIB_PROTOCOL_IP6;
+      prefix.fp_len = 128;
+
+      u32 fib_index = fib_table_find (prefix.fp_proto, table_id);
+      if (fib_index == (u32) ~0)
+	return false;
+
+      fib_node_index_t fei = fib_table_lookup (fib_index, &prefix);
+      if (fei == FIB_NODE_INDEX_INVALID)
+	return false;
+
+      u32 output_sw_if_index = fib_entry_get_resolving_interface (fei);
+      if (output_sw_if_index == ~0)
+	return false;
+      if_index = output_sw_if_index;
+    }
+  return ip6_sas_by_sw_if_index (if_index, dst, src);
+}
+
+/*
+ * table_id must be set. Default = 0.
+ * sw_if_index is the interface to pick SA from otherwise ~0 will pick from
+ * outbound interface.
+ *
+ * NOTE: What to do if multiple output interfaces?
+ *
+ */
+bool
+ip4_sas (u32 table_id, u32 sw_if_index, const ip4_address_t *dst,
+	 ip4_address_t *src)
+{
+  fib_prefix_t prefix;
+  u32 if_index = sw_if_index;
+
+  /* If sw_if_index is not specified use the output interface. */
+  if (sw_if_index == ~0)
+    {
+      clib_memcpy (&prefix.fp_addr.ip4, dst, sizeof (*dst));
+      prefix.fp_proto = FIB_PROTOCOL_IP4;
+      prefix.fp_len = 32;
+
+      u32 fib_index = fib_table_find (prefix.fp_proto, table_id);
+      if (fib_index == (u32) ~0)
+	return false;
+
+      fib_node_index_t fei = fib_table_lookup (fib_index, &prefix);
+      if (fei == FIB_NODE_INDEX_INVALID)
+	return false;
+
+      u32 output_sw_if_index = fib_entry_get_resolving_interface (fei);
+      if (output_sw_if_index == ~0)
+	return false;
+      if_index = output_sw_if_index;
+    }
+  return ip4_sas_by_sw_if_index (if_index, dst, src);
+}