zebra, lib: add recursive support for protocol nhid

In sharpd, configuring a nexthop-group with an IP nexthop that is not
directly connected does not create a NHG context in zebra:

> ubuntu2204(config)# interface loop1
> ubuntu2204(config-if)# ip address 192.0.2.1/24
> ubuntu2204(config-if)# exi
> ubuntu2204(config)# ip route 192.168.0.0/24 192.0.2.100
> ubuntu2204(config)# nexthop-group ABCD
> ubuntu2204(config-nh-group)# nexthop 192.168.0.100
> 2024/01/17 16:36:23 SHARP: [Y2G2F-ZTW6M] nhg_add: nhg 181818169 not sent: no valid nexthops
> ubuntu2204(config-nh-group)# do show nexthop-group rib 181818169
> Nexthop Group ID: 181818169 does not exist

Nexthops with an undefined interface index are neither handled
in ZEBRA, nor in SHARPD.

On the other hand, if we had created a route pointing to the same
nexthop (by using ZEBRA_ROUTE_ADD zapi), the next-hop would have been
installed, thanks to the ALLOW_RECURSION flag embedded in the zapi_route
structure.

Add the support for recursivity in nexthop groups by introducing
a flags attribute in the nexthop group structure. Define the
NEXTHOP_GROUP_ALLOW_RECURSION value, which will be used by ZEBRA to
check for the next-hop group recursivity. This flag is not used by
default.

This flag control is mandatory, as the recursivity may not be allowed
by protocols like eBGP single-hop.

Signed-off-by: Philippe Guibert <[email protected]>

lib/nexthop_group.h

@@ -34,6 +34,10 @@ struct nexthop_group {
 	struct nexthop *nexthop;
 
 	struct nhg_resilience nhgr;
+
+	/* nexthop group flags */
+#define NEXTHOP_GROUP_ALLOW_RECURSION (1 << 1)
+	uint8_t flags;
 };
 
 struct nexthop_group *nexthop_group_new(void);

lib/zclient.c

@@ -1173,6 +1173,8 @@ static int zapi_nhg_encode(struct stream *s, int cmd, struct zapi_nhg *api_nhg)
 	stream_putl(s, api_nhg->resilience.idle_timer);
 	stream_putl(s, api_nhg->resilience.unbalanced_timer);
 
+	stream_putc(s, api_nhg->flags);
+
 	if (cmd == ZEBRA_NHG_ADD) {
 		/* Nexthops */
 		zapi_nexthop_group_sort(api_nhg->nexthops,

lib/zclient.h

@@ -486,6 +486,9 @@ struct zapi_nhg {
 
 	uint16_t backup_nexthop_num;
 	struct zapi_nexthop backup_nexthops[MULTIPATH_NUM];
+
+	/* nexthop group flags */
+	uint8_t flags;
 };
 
 /*

sharpd/sharp_zebra.c

@@ -539,6 +539,7 @@ void nhg_add(uint32_t id, const struct nexthop_group *nhg,
 
 	api_nhg.id = id;
 
+	api_nhg.flags = nhg->flags;
 	api_nhg.resilience = nhg->nhgr;
 
 	for (ALL_NEXTHOPS_PTR(nhg, nh)) {

zebra/zapi_msg.c

@@ -1895,6 +1895,8 @@ static int zapi_nhg_decode(struct stream *s, int cmd, struct zapi_nhg *api_nhg)
 	STREAM_GETL(s, api_nhg->resilience.idle_timer);
 	STREAM_GETL(s, api_nhg->resilience.unbalanced_timer);
 
+	STREAM_GETC(s, api_nhg->flags);
+
 	/* Nexthops */
 	STREAM_GETW(s, api_nhg->nexthop_num);
 
@@ -2013,6 +2015,7 @@ static void zread_nhg_add(ZAPI_HANDLER_ARGS)
 	nhe = zebra_nhg_alloc();
 	nhe->id = api_nhg.id;
 	nhe->type = api_nhg.proto;
+	nhe->nhg.flags = api_nhg.flags;
 	nhe->zapi_instance = client->instance;
 	nhe->zapi_session = client->session_id;
 

zebra/zebra_nhg.c

@@ -2346,8 +2346,10 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
 		 * if specified) - i.e., we cannot have a nexthop NH1 is
 		 * resolved by a route NH1. The exception is if the route is a
 		 * host route.
+		 * This control will not work by using nexthop groups, and will
+		 * have to be handled at protocol level
 		 */
-		if (prefix_same(&rn->p, top))
+		if (top && prefix_same(&rn->p, top))
 			if (((afi == AFI_IP)
 			     && (rn->p.prefixlen != IPV4_MAX_BITLEN))
 			    || ((afi == AFI_IP6)
@@ -3365,7 +3367,7 @@ struct nhg_hash_entry *zebra_nhg_proto_add(struct nhg_hash_entry *nhe,
 	struct nexthop *newhop;
 	bool replace = false;
 	int ret = 0, type;
-	uint32_t id, session;
+	uint32_t id, session, api_message = 0;
 	uint16_t instance;
 
 	id = nhe->id;
@@ -3413,14 +3415,23 @@ struct nhg_hash_entry *zebra_nhg_proto_add(struct nhg_hash_entry *nhe,
 			return NULL;
 		}
 
-		if (!newhop->ifindex) {
+		if (!newhop->ifindex &&
+		    !CHECK_FLAG(nhg->flags, NEXTHOP_GROUP_ALLOW_RECURSION)) {
 			if (IS_ZEBRA_DEBUG_NHG)
-				zlog_debug(
-					"%s: id %u, nexthop without ifindex is not supported",
-					__func__, id);
+				zlog_debug("%s: id %u, nexthop without ifindex and allow-recursion is not supported",
+					   __func__, id);
 			return NULL;
 		}
-		SET_FLAG(newhop->flags, NEXTHOP_FLAG_ACTIVE);
+
+		/* Check that the route may be recursively resolved */
+		if (CHECK_FLAG(nhg->flags, NEXTHOP_GROUP_ALLOW_RECURSION))
+			api_message = ZEBRA_FLAG_ALLOW_RECURSION;
+
+		if (nexthop_active(newhop, nhe, NULL, 0, api_message, NULL,
+				   newhop->vrf_id))
+			SET_FLAG(newhop->flags, NEXTHOP_FLAG_ACTIVE);
+		else
+			UNSET_FLAG(newhop->flags, NEXTHOP_FLAG_ACTIVE);
 	}
 
 	zebra_nhe_init(&lookup, afi, nhg->nexthop);