netlink: Add support to fetch default gateway from multipath routes

If the default route for a given IP version is a multipath one,
instead of refusing to start because there's no RTA_GATEWAY attribute
in the set returned by the kernel, we can just pick one of the paths.

To make this somewhat less arbitrary, pick the path with the highest
weight, if weights differ.

Reported-by: Ed Santiago <santiago@redhat.com>
Link: https://github.com/containers/podman/issues/20927
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

2 files changed, 52 insertions, 5 deletions

diff --git a/netlink.c b/netlink.c
index bf79dd4..f0b04cb 100644
--- a/netlink.c
+++ b/netlink.c
@@ -271,12 +271,17 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 
 		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
-			if (rta->rta_type != RTA_OIF)
-				continue;
+			if (rta->rta_type == RTA_OIF) {
+				ifi = *(unsigned int *)RTA_DATA(rta);
+			} else if (rta->rta_type == RTA_MULTIPATH) {
+				struct rtnexthop *rtnh;
 
-			ifi = *(unsigned int *)RTA_DATA(rta);
+				rtnh = (struct rtnexthop *)RTA_DATA(rta);
+				ifi = rtnh->rtnh_ifindex;
+			}
 		}
 	}
+
 	if (status < 0)
 		warn("netlink: RTM_GETROUTE failed: %s", strerror(-status));
 
@@ -284,6 +289,43 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 }
 
 /**
+ * nl_route_get_def_multipath() - Get lowest-weight route from nexthop list
+ * @rta:	Routing netlink attribute with type RTA_MULTIPATH
+ * @gw:		Default gateway to fill
+ *
+ * Return: true if a gateway was found, false otherwise
+ */
+bool nl_route_get_def_multipath(struct rtattr *rta, void *gw)
+{
+	struct rtnexthop *rtnh;
+	bool found = false;
+	int hops = -1;
+
+	for (rtnh = (struct rtnexthop *)RTA_DATA(rta);
+	     RTNH_OK(rtnh, RTA_PAYLOAD(rta)); rtnh = RTNH_NEXT(rtnh)) {
+		size_t len = rtnh->rtnh_len - sizeof(*rtnh);
+		struct rtattr *rta_inner;
+
+		if (rtnh->rtnh_hops < hops)
+			continue;
+
+		hops = rtnh->rtnh_hops;
+
+		for (rta_inner = RTNH_DATA(rtnh); RTA_OK(rta_inner, len);
+		     rta_inner = RTA_NEXT(rta_inner, len)) {
+
+			if (rta_inner->rta_type != RTA_GATEWAY)
+				continue;
+
+			memcpy(gw, RTA_DATA(rta_inner), RTA_PAYLOAD(rta_inner));
+			found = true;
+		}
+	}
+
+	return found;
+}
+
+/**
  * nl_route_get_def() - Get default route for given interface and address family
  * @s:		Netlink socket
  * @ifi:	Interface index
@@ -326,6 +368,9 @@ int nl_route_get_def(int s, unsigned int ifi, sa_family_t af, void *gw)
 
 		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
+			if (rta->rta_type == RTA_MULTIPATH)
+				found = nl_route_get_def_multipath(rta, gw);
+
 			if (rta->rta_type != RTA_GATEWAY)
 				continue;
 
diff --git a/passt.1 b/passt.1
index efd6bb7..cc678ed 100644
--- a/passt.1
+++ b/passt.1
@@ -171,8 +171,10 @@ Assign IPv4 \fIaddr\fR as default gateway via DHCP (option 3), or IPv6
 \fIaddr\fR as source for NDP Router Advertisement and DHCPv6 messages.
 This option can be specified zero (for defaults) to two times (once for IPv4,
 once for IPv6).
-By default, IPv4 and IPv6 addresses are taken from the host interface with the
-first default route for the corresponding IP version.
+By default, IPv4 and IPv6 gateways are taken from the host interface with the
+first default route for the corresponding IP version. If the default route is a
+multipath one, the gateway is the first nexthop router returned by the kernel
+which has the highest weight in the set of paths.
 
 Note: these addresses are also used as source address for packets directed to
 the guest or to the target namespace having a loopback or local source address,