conf, netlink: Don't require a default route to start

There might be isolated testing environments where default routes and
global connectivity are not needed, a single interface has all
non-loopback addresses and routes, and still passt and pasta are
expected to work.

In this case, it's pretty obvious what our upstream interface should
be, so go ahead and select the only interface with at least one
route, disabling DHCP and implying --no-map-gw as the documentation
already states.

If there are multiple interfaces with routes, though, refuse to start,
because at that point it's really not clear what we should do.

Reported-by: Martin Pitt <mpitt@redhat.com>
Link: https://github.com/containers/podman/issues/21896
Signed-off-by: Stefano brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 28 insertions, 3 deletions

diff --git a/netlink.c b/netlink.c
index 20de9b3..f93f377 100644
--- a/netlink.c
+++ b/netlink.c
@@ -254,6 +254,7 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 		.rtm.rtm_type	 = RTN_UNICAST,
 		.rtm.rtm_family	 = af,
 	};
+	bool default_only = true;
 	unsigned int ifi = 0;
 	struct nlmsghdr *nh;
 	struct rtattr *rta;
@@ -262,21 +263,40 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 	uint32_t seq;
 	size_t na;
 
+again:
+	/* Look for an interface with a default route first, failing that, look
+	 * for any interface with a route, and pick it only if it's the only
+	 * interface with a route.
+	 */
 	seq = nl_send(s, &req, RTM_GETROUTE, NLM_F_DUMP, sizeof(req));
 	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWROUTE) {
 		struct rtmsg *rtm = (struct rtmsg *)NLMSG_DATA(nh);
 
-		if (ifi || rtm->rtm_dst_len || rtm->rtm_family != af)
-			continue;
+		if (default_only) {
+			if (ifi || rtm->rtm_dst_len || rtm->rtm_family != af)
+				continue;
+		} else {
+			if (rtm->rtm_family != af)
+				continue;
+		}
 
 		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
 			if (rta->rta_type == RTA_OIF) {
+				if (!default_only && ifi &&
+				    ifi != *(unsigned int *)RTA_DATA(rta))
+					return 0;
+
 				ifi = *(unsigned int *)RTA_DATA(rta);
 			} else if (rta->rta_type == RTA_MULTIPATH) {
 				const struct rtnexthop *rtnh;
 
 				rtnh = (struct rtnexthop *)RTA_DATA(rta);
+
+				if (!default_only && ifi &&
+				    (int)ifi != rtnh->rtnh_ifindex)
+					return 0;
+
 				ifi = rtnh->rtnh_ifindex;
 			}
 		}
@@ -285,6 +305,11 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 	if (status < 0)
 		warn("netlink: RTM_GETROUTE failed: %s", strerror(-status));
 
+	if (!ifi && default_only) {
+		default_only = false;
+		goto again;
+	}
+
 	return ifi;
 }
 
@@ -332,7 +357,7 @@ bool nl_route_get_def_multipath(struct rtattr *rta, void *gw)
  * @af:		Address family
  * @gw:		Default gateway to fill on NL_GET
  *
- * Return: 0 on success, negative error code on failure
+ * Return: error on netlink failure, or 0 (gw unset if default route not found)
  */
 int nl_route_get_def(int s, unsigned int ifi, sa_family_t af, void *gw)
 {