net: if: Add helper to get src interface and address from dst address

Instead of calling various network interface API functions to get
the network interface and related source IP address, have a single
function that can return both data.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

include/zephyr/net/net_if.h

@@ -2207,6 +2207,32 @@ static inline struct net_if *net_if_ipv6_select_src_iface(
 }
 #endif
 
+/**
+ * @brief Get a network interface that should be used when sending
+ * IPv6 network data to destination. Also return the source IPv6 address from
+ * that network interface.
+ *
+ * @param dst IPv6 destination address
+ * @param src_addr IPv6 source address. This can be set to NULL if the source
+ *                 address is not needed.
+ *
+ * @return Pointer to network interface to use, NULL if no suitable interface
+ * could be found.
+ */
+#if defined(CONFIG_NET_IPV6)
+struct net_if *net_if_ipv6_select_src_iface_addr(const struct in6_addr *dst,
+						 const struct in6_addr **src_addr);
+#else
+static inline struct net_if *net_if_ipv6_select_src_iface_addr(
+	const struct in6_addr *dst, const struct in6_addr **src_addr)
+{
+	ARG_UNUSED(dst);
+	ARG_UNUSED(src_addr);
+
+	return NULL;
+}
+#endif /* CONFIG_NET_IPV6 */
+
 /**
  * @brief Get a IPv6 link local address in a given state.
  *
@@ -2588,6 +2614,32 @@ static inline struct net_if *net_if_ipv4_select_src_iface(
 }
 #endif
 
+/**
+ * @brief Get a network interface that should be used when sending
+ * IPv4 network data to destination. Also return the source IPv4 address from
+ * that network interface.
+ *
+ * @param dst IPv4 destination address
+ * @param src_addr IPv4 source address. This can be set to NULL if the source
+ *                 address is not needed.
+ *
+ * @return Pointer to network interface to use, NULL if no suitable interface
+ * could be found.
+ */
+#if defined(CONFIG_NET_IPV4)
+struct net_if *net_if_ipv4_select_src_iface_addr(const struct in_addr *dst,
+						 const struct in_addr **src_addr);
+#else
+static inline struct net_if *net_if_ipv4_select_src_iface_addr(
+	const struct in_addr *dst, const struct in_addr **src_addr)
+{
+	ARG_UNUSED(dst);
+	ARG_UNUSED(src_addr);
+
+	return NULL;
+}
+#endif /* CONFIG_NET_IPV4 */
+
 /**
  * @brief Get a IPv4 source address that should be used when sending
  * network data to destination.

subsys/net/ip/net_if.c

@@ -3275,7 +3275,8 @@ const struct in6_addr *net_if_ipv6_select_src_addr(struct net_if *dst_iface,
 						IPV6_PREFER_SRC_PUBTMP_DEFAULT);
 }
 
-struct net_if *net_if_ipv6_select_src_iface(const struct in6_addr *dst)
+struct net_if *net_if_ipv6_select_src_iface_addr(const struct in6_addr *dst,
+						 const struct in6_addr **src_addr)
 {
 	struct net_if *iface = NULL;
 	const struct in6_addr *src;
@@ -3285,6 +3286,10 @@ struct net_if *net_if_ipv6_select_src_iface(const struct in6_addr *dst)
 		net_if_ipv6_addr_lookup(src, &iface);
 	}
 
+	if (src_addr != NULL) {
+		*src_addr = src;
+	}
+
 	if (iface == NULL) {
 		iface = net_if_get_default();
 	}
@@ -3292,6 +3297,11 @@ struct net_if *net_if_ipv6_select_src_iface(const struct in6_addr *dst)
 	return iface;
 }
 
+struct net_if *net_if_ipv6_select_src_iface(const struct in6_addr *dst)
+{
+	return net_if_ipv6_select_src_iface_addr(dst, NULL);
+}
+
 #if defined(CONFIG_NET_NATIVE_IPV6)
 
 uint32_t net_if_ipv6_calc_reachable_time(struct net_if_ipv6 *ipv6)
@@ -3600,7 +3610,8 @@ out:
 	return ret;
 }
 
-struct net_if *net_if_ipv4_select_src_iface(const struct in_addr *dst)
+struct net_if *net_if_ipv4_select_src_iface_addr(const struct in_addr *dst,
+						 const struct in_addr **src_addr)
 {
 	struct net_if *selected = NULL;
 	const struct in_addr *src;
@@ -3614,9 +3625,18 @@ struct net_if *net_if_ipv4_select_src_iface(const struct in_addr *dst)
 		selected = net_if_get_default();
 	}
 
+	if (src_addr != NULL) {
+		*src_addr = src;
+	}
+
 	return selected;
 }
 
+struct net_if *net_if_ipv4_select_src_iface(const struct in_addr *dst)
+{
+	return net_if_ipv4_select_src_iface_addr(dst, NULL);
+}
+
 static uint8_t get_diff_ipv4(const struct in_addr *src,
 			  const struct in_addr *dst)
 {