[RFC 10/12] netlink: Add host-side route monitoring and propagation

[Jon Maloy <jmaloy@redhat.com>]

We extend host-side netlink monitoring to also track default route
changes on the template interface and propagate them to the namespace.

- Subscribe to RTMGRP_IPV4_ROUTE and RTMGRP_IPV6_ROUTE groups on the
  host-side netlink socket
- Handle RTM_NEWROUTE/RTM_DELROUTE events for default routes.
- Support late binding via routes: if no template interface is bound
  yet, adopt the interface in question when a default route appears
  on it.
- When a default route is added, set guest_gw/our_tap_addr and
  propagate the route to the namespace via nl_route_set_def()
- When a default route is removed, clear guest_gw/our_tap_addr

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
---
 netlink.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 97 insertions(+), 3 deletions(-)

diff --git a/netlink.c b/netlink.c
index 583ada8..d049239 100644
--- a/netlink.c
+++ b/netlink.c
@@ -199,7 +199,7 @@ static bool nl_addr6_add(struct ctx *c, const struct in6_addr *addr,
 	idx = c->ip6.addr_count++;
 	c->ip6.addrs[idx].addr = *addr;
 	c->ip6.addrs[idx].prefix_len = prefix_len;
-	c->ip6.addrs[idxyes].permanent = 0;
+	c->ip6.addrs[idx].permanent = 0;
 	return true;
 }
 
@@ -254,7 +254,7 @@ static bool nl_addr6_del(struct ctx *c, const struct in6_addr *addr)
 }
 
 /**
- * nl_linkaddr_host_msg_read() - Handle host-side link/addr changes
+ * nl_linkaddr_host_msg_read() - Handle host-side link/addr/route changes
  * @c:		Execution context
  * @nh:	Netlink message header
  *
@@ -420,6 +420,99 @@ static void nl_linkaddr_host_msg_read(struct ctx *c, const struct nlmsghdr *nh)
 		}
 		return;
 	}
+
+	if (nh->nlmsg_type == RTM_NEWROUTE || nh->nlmsg_type == RTM_DELROUTE) {
+		bool is_new = (nh->nlmsg_type == RTM_NEWROUTE);
+		const struct rtmsg *rtm = NLMSG_DATA(nh);
+		struct rtattr *rta = RTM_RTA(rtm);
+		size_t na = RTM_PAYLOAD(nh);
+		unsigned int template_ifi;
+		char ifname[IFNAMSIZ];
+		unsigned int oif = 0;
+		void *gw = NULL;
+		bool is_default;
+		bool is_match;
+		bool unbound;
+
+		/* Only interested in default routes */
+		if (rtm->rtm_dst_len != 0)
+			return;
+
+		for (; RTA_OK(rta, na); rta = RTA_NEXT(rta, na)) {
+			if (rta->rta_type == RTA_GATEWAY)
+				gw = RTA_DATA(rta);
+			else if (rta->rta_type == RTA_OIF)
+				oif = *(unsigned int *)RTA_DATA(rta);
+		}
+
+		if (!gw || !oif)
+			return;
+
+		/* Get interface name for late binding check */
+		if (!if_indextoname(oif, ifname))
+			return;
+
+		/* Check for late binding conditions */
+		is_default = !strcmp(c->pasta_ifn, pasta_default_ifn);
+		is_match = !strcmp(ifname, c->pasta_ifn);
+
+		if (rtm->rtm_family == AF_INET)
+			template_ifi = c->ifi4;
+		else if (rtm->rtm_family == AF_INET6)
+			template_ifi = c->ifi6;
+		else
+			return;
+
+		unbound = (rtm->rtm_family == AF_INET) ?
+			  (int)c->ifi4 <= 0 : (int)c->ifi6 <= 0;
+
+		if (unbound && (is_default || is_match)) {
+			debug("Late binding (route): using %s as %s template",
+			      ifname,
+			      rtm->rtm_family == AF_INET ? "IPv4" : "IPv6");
+
+			if (rtm->rtm_family == AF_INET) {
+				c->ifi4 = oif;
+				template_ifi = c->ifi4;
+			} else {
+				c->ifi6 = oif;
+				template_ifi = c->ifi6;
+			}
+
+			if (is_default)
+				snprintf(c->pasta_ifn, sizeof(c->pasta_ifn),
+					 "%s", ifname);
+		}
+
+		if (oif != template_ifi)
+			return;
+
+		if (rtm->rtm_family == AF_INET) {
+			char buf[INET_ADDRSTRLEN];
+
+			if (!is_new) {
+				c->ip4.guest_gw = (struct in_addr){ 0 };
+				c->ip4.our_tap_addr = (struct in_addr){ 0 };
+				return;
+			}
+			c->ip4.guest_gw = *(struct in_addr *)gw;
+			c->ip4.our_tap_addr = c->ip4.guest_gw;
+			nl_route_set_def(nl_sock_ns, c->pasta_ifi, AF_INET, gw);
+			inet_ntop(AF_INET, &c->ip4.guest_gw, buf, sizeof(buf));
+			debug("Set IPv4 default route via %s", buf);
+		} else if (rtm->rtm_family == AF_INET6) {
+			char buf[INET6_ADDRSTRLEN];
+
+			if (!is_new) {
+				c->ip6.guest_gw = (struct in6_addr){ 0 };
+				return;
+			}
+			c->ip6.guest_gw = *(struct in6_addr *)gw;
+			nl_route_set_def(nl_sock_ns, c->pasta_ifi, AF_INET6, gw);
+			inet_ntop(AF_INET6, &c->ip6.guest_gw, buf, sizeof(buf));
+			debug("Set IPv6 default route via %s", buf);
+		}
+	}
 }
 
 /**
@@ -676,7 +769,8 @@ static int nl_linkaddr_init_do(void *arg)
 static int nl_linkaddr_host_init_do(void *arg)
 {
 	struct sockaddr_nl addr = { .nl_family = AF_NETLINK,
-		.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR };
+		.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR |
+			     RTMGRP_IPV4_ROUTE | RTMGRP_IPV6_ROUTE };
 
 	(void)arg;
 
-- 
2.51.1