kernel-pfroute: Route via interface if we don't have a next hop

Compared to RTA_IFP, the format is different when passing an interface
as RTA_GATEWAY, it requires the interface's address and not its name.

src/libcharon/plugins/kernel_pfroute/kernel_pfroute_net.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2009-2016 Tobias Brunner
+ * Copyright (C) 2009-2024 Tobias Brunner
  *
  * Copyright (C) secunet Security Networks AG
  *
@@ -1443,6 +1443,35 @@ static void add_rt_ifname(struct rt_msghdr *hdr, int type, char *name)
 	}
 }
 
+/**
+ * Append an interface address sockaddr_dl to routing message
+ */
+static void add_rt_ifaddr(struct rt_msghdr *hdr, int type, char *name)
+{
+	struct ifaddrs *ifap, *ifa;
+	struct sockaddr_dl *sdl;
+
+	if (getifaddrs(&ifap) < 0)
+	{
+		return;
+	}
+
+	for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next)
+	{
+		if (!ifa->ifa_addr || ifa->ifa_addr->sa_family != AF_LINK ||
+			!streq(name, ifa->ifa_name))
+		{
+			continue;
+		}
+		sdl = (struct sockaddr_dl*)ifa->ifa_addr;
+		memcpy((char*)hdr + hdr->rtm_msglen, sdl, sdl->sdl_len);
+		hdr->rtm_msglen += SA_LEN(sdl->sdl_len);
+		hdr->rtm_addrs |= type;
+		break;
+	}
+	freeifaddrs(ifap);
+}
+
 /**
  * Add or remove a route
  */
@@ -1519,6 +1548,10 @@ static status_t manage_route(private_kernel_pfroute_net_t *this, int op,
 				{
 					add_rt_addr(&msg.hdr, RTA_GATEWAY, gateway);
 				}
+				else if (if_name)
+				{
+					add_rt_ifaddr(&msg.hdr, RTA_GATEWAY, if_name);
+				}
 				break;
 			default:
 				break;