On Mon, Mar 04, 2024 at 04:09:05PM +0100, Antonio Quartulli wrote:

In a multi-peer scenario there are a number of situations when a
specific peer needs to be looked up.

We may want to lookup a peer by:
1. its ID
2. its VPN destination IP
3. its tranport IP/port couple

nit: transport

     checkpatch.pl --codespell is your friend here.

For each of the above, there is a specific routing table referencing all
peers for fast look up.

Case 2. is a bit special in the sense that an outgoing packet may not be
sent to the peer VPN IP directly, but rather to a network behind it. For
this reason we first perform a nexthop lookup in the system routing
table and then we use the retrieved nexthop as peer search key.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>

...

+/**
+ * ovpn_nexthop_lookup4() - looks up the IP of the nexthop for the given destination
+ *
+ * Looks up in the IPv4 system routing table the IP of the nexthop to be used
+ * to reach the destination passed as argument. If no nexthop can be found, the
+ * destination itself is returned as it probably has to be used as nexthop.
+ *
+ * @ovpn: the private data representing the current VPN session
+ * @dst: the destination to be looked up

I think you need to document @src instead of @dst here.

+ *
+ * Return the IP of the next hop if found or the dst itself otherwise
+ */
+static __be32 ovpn_nexthop_lookup4(struct ovpn_struct *ovpn, __be32 src)
+{
+	struct rtable *rt;
+	struct flowi4 fl = {
+		.daddr = src
+	};
+
+	rt = ip_route_output_flow(dev_net(ovpn->dev), &fl, NULL);
+	if (IS_ERR(rt)) {
+		net_dbg_ratelimited("%s: no nexthop found for %pI4\n", ovpn->dev->name, &src);
+		/* if we end up here this packet is probably going to be
+		 * thrown away later
+		 */
+		return src;
+	}
+
+	if (!rt->rt_uses_gateway)
+		goto out;
+
+	src = rt->rt_gw4;
+out:
+	return src;
+}
+
+/**
+ * ovpn_nexthop_lookup6() - looks up the IPv6 of the nexthop for the given destination
+ *
+ * Looks up in the IPv6 system routing table the IP of the nexthop to be used
+ * to reach the destination passed as argument. If no nexthop can be found, the
+ * destination itself is returned as it probably has to be used as nexthop.
+ *
+ * @ovpn: the private data representing the current VPN session
+ * @dst: the destination to be looked up

And here.

+ *
+ * Return the IP of the next hop if found or the dst itself otherwise
+ */
+static struct in6_addr ovpn_nexthop_lookup6(struct ovpn_struct *ovpn, struct in6_addr addr)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	struct rt6_info *rt;
+	struct flowi6 fl = {
+		.daddr = addr,
+	};
+
+	rt = (struct rt6_info *)ipv6_stub->ipv6_dst_lookup_flow(dev_net(ovpn->dev), NULL, &fl,
+								NULL);
+	if (IS_ERR(rt)) {
+		net_dbg_ratelimited("%s: no nexthop found for %pI6\n", ovpn->dev->name, &addr);
+		/* if we end up here this packet is probably going to be thrown away later */
+		return addr;
+	}
+
+	if (rt->rt6i_flags & RTF_GATEWAY)
+		addr = rt->rt6i_gateway;
+
+	dst_release((struct dst_entry *)rt);
+#endif
+	return addr;
+}

...