bgpd: Allow overriding default distance if set to 0

`set distance` with route-maps allow setting it to zero. Relax zebra to
use distance as 0 as well.

Or imagine a case when you have a static kernel route with distance 0, but
you need to override it from BGP with 0 distance as well.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

bgpd/bgp_route.c

@@ -15115,8 +15115,9 @@ static int bgp_distance_unset(struct vty *vty, const char *distance_str,
 }
 
 /* Apply BGP information to distance method. */
-uint8_t bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
-			   afi_t afi, safi_t safi, struct bgp *bgp)
+bool bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
+			afi_t afi, safi_t safi, struct bgp *bgp,
+			uint8_t *distance)
 {
 	struct bgp_dest *dest;
 	struct prefix q = {0};
@@ -15127,12 +15128,15 @@ uint8_t bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
 	struct bgp_path_info *bpi_ultimate;
 
 	if (!bgp)
-		return 0;
+		return false;
 
-	peer = pinfo->peer;
+	if (pinfo->attr->distance != ZEBRA_EBGP_DISTANCE_DEFAULT ||
+	    pinfo->attr->distance != ZEBRA_IBGP_DISTANCE_DEFAULT) {
+		*distance = pinfo->attr->distance;
+		return true;
+	}
 
-	if (pinfo->attr->distance)
-		return pinfo->attr->distance;
+	peer = pinfo->peer;
 
 	/* get peer origin to calculate appropriate distance */
 	if (pinfo->sub_type == BGP_ROUTE_IMPORTED) {
@@ -15145,7 +15149,7 @@ uint8_t bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
 	 */
 	if (pinfo->sub_type != BGP_ROUTE_AGGREGATE
 	    && !sockunion2hostprefix(&peer->su, &q))
-		return 0;
+		return false;
 
 	dest = bgp_node_match(bgp_distance_table[afi][safi], &q);
 	if (dest) {
@@ -15154,11 +15158,15 @@ uint8_t bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
 
 		if (bdistance->access_list) {
 			alist = access_list_lookup(afi, bdistance->access_list);
-			if (alist
-			    && access_list_apply(alist, p) == FILTER_PERMIT)
-				return bdistance->distance;
-		} else
-			return bdistance->distance;
+			if (alist &&
+			    access_list_apply(alist, p) == FILTER_PERMIT) {
+				*distance = bdistance->distance;
+				return true;
+			}
+		} else {
+			*distance = bdistance->distance;
+			return true;
+		}
 	}
 
 	/* Backdoor check. */
@@ -15168,26 +15176,31 @@ uint8_t bgp_distance_apply(const struct prefix *p, struct bgp_path_info *pinfo,
 		bgp_dest_unlock_node(dest);
 
 		if (bgp_static->backdoor) {
-			if (bgp->distance_local[afi][safi])
-				return bgp->distance_local[afi][safi];
-			else
-				return ZEBRA_IBGP_DISTANCE_DEFAULT;
+			*distance = (bgp->distance_local[afi][safi])
+					    ? bgp->distance_local[afi][safi]
+					    : ZEBRA_IBGP_DISTANCE_DEFAULT;
+			return true;
 		}
 	}
 
 	if (peer->sort == BGP_PEER_EBGP) {
-		if (bgp->distance_ebgp[afi][safi])
-			return bgp->distance_ebgp[afi][safi];
-		return ZEBRA_EBGP_DISTANCE_DEFAULT;
+		*distance = (bgp->distance_ebgp[afi][safi])
+				    ? bgp->distance_ebgp[afi][safi]
+				    : ZEBRA_EBGP_DISTANCE_DEFAULT;
+		return true;
 	} else if (peer->sort == BGP_PEER_IBGP) {
-		if (bgp->distance_ibgp[afi][safi])
-			return bgp->distance_ibgp[afi][safi];
-		return ZEBRA_IBGP_DISTANCE_DEFAULT;
+		*distance = (bgp->distance_ibgp[afi][safi])
+				    ? bgp->distance_ibgp[afi][safi]
+				    : ZEBRA_IBGP_DISTANCE_DEFAULT;
+		return true;
 	} else {
-		if (bgp->distance_local[afi][safi])
-			return bgp->distance_local[afi][safi];
-		return ZEBRA_IBGP_DISTANCE_DEFAULT;
+		*distance = (bgp->distance_local[afi][safi])
+				    ? bgp->distance_local[afi][safi]
+				    : ZEBRA_IBGP_DISTANCE_DEFAULT;
+		return true;
 	}
+
+	return false;
 }
 
 /* If we enter `distance bgp (1-255) (1-255) (1-255)`,

bgpd/bgp_route.h

@@ -776,9 +776,9 @@ extern void bgp_aggregate_decrement(struct bgp *bgp, const struct prefix *p,
 				    struct bgp_path_info *path, afi_t afi,
 				    safi_t safi);
 
-extern uint8_t bgp_distance_apply(const struct prefix *p,
-				  struct bgp_path_info *path, afi_t afi,
-				  safi_t safi, struct bgp *bgp);
+extern bool bgp_distance_apply(const struct prefix *p,
+			       struct bgp_path_info *path, afi_t afi,
+			       safi_t safi, struct bgp *bgp, uint8_t *distance);
 
 extern afi_t bgp_node_afi(struct vty *);
 extern safi_t bgp_node_safi(struct vty *);

bgpd/bgp_zebra.c

@@ -1286,7 +1286,7 @@ void bgp_zebra_announce(struct bgp_dest *dest, const struct prefix *p,
 	int nh_family;
 	unsigned int valid_nh_count = 0;
 	bool allow_recursion = false;
-	uint8_t distance;
+	uint8_t distance = ZEBRA_EBGP_DISTANCE_DEFAULT;
 	struct peer *peer;
 	struct bgp_path_info *mpinfo;
 	struct bgp *bgp_orig;
@@ -1613,9 +1613,11 @@ void bgp_zebra_announce(struct bgp_dest *dest, const struct prefix *p,
 		api.tag = tag;
 	}
 
-	distance = bgp_distance_apply(p, info, afi, safi, bgp);
-	if (distance) {
+	if (bgp_distance_apply(p, info, afi, safi, bgp, &distance)) {
 		SET_FLAG(api.message, ZAPI_MESSAGE_DISTANCE);
+		if (!distance)
+			SET_FLAG(api.flags,
+				 ZEBRA_FLAG_DEFAULT_DISTANCE_OVERRIDE);
 		api.distance = distance;
 	}
 
@@ -1630,9 +1632,10 @@ void bgp_zebra_announce(struct bgp_dest *dest, const struct prefix *p,
 
 		zlog_debug(
 			"Tx route %s VRF %u %pFX metric %u tag %" ROUTE_TAG_PRI
-			" count %d nhg %d",
+			" count %d nhg %d distance %u",
 			is_add ? "add" : "delete", bgp->vrf_id, &api.prefix,
-			api.metric, api.tag, api.nexthop_num, nhg_id);
+			api.metric, api.tag, api.nexthop_num, nhg_id,
+			api.distance);
 		for (i = 0; i < api.nexthop_num; i++) {
 			api_nh = &api.nexthops[i];
 

lib/zclient.h

@@ -533,13 +533,18 @@ struct zapi_route {
  * offload situation.
  */
 #define ZEBRA_FLAG_OFFLOAD_FAILED     0x200
-
 /*
  * This flag lets us know that we think the route entry
  * received has caused us to be out of sync with the
  * kernel (NLM_F_APPEND at the very least )
  */
 #define ZEBRA_FLAG_OUTOFSYNC          0x400
+/*
+ * This flag tells to override a default distance for
+ * the routes that has distance set to 0. For example,
+ * via route-maps `set distance 0`.
+ */
+#define ZEBRA_FLAG_DEFAULT_DISTANCE_OVERRIDE 0x800
 
 	/* The older XXX_MESSAGE flags live here */
 	uint32_t message;

tests/topotests/bgp_distance_zero/r1/bgpd.conf

@@ -0,0 +1,9 @@
+!
+router bgp 65001
+ no bgp ebgp-requires-policy
+ no bgp network import-check
+ neighbor 192.168.1.2 remote-as external
+ address-family ipv4 unicast
+  network 10.10.10.1/32
+ exit-address-family
+!

tests/topotests/bgp_distance_zero/r1/zebra.conf

@@ -0,0 +1,4 @@
+!
+int r1-eth0
+ ip address 192.168.1.1/24
+!

tests/topotests/bgp_distance_zero/r2/bgpd.conf

@@ -0,0 +1,10 @@
+!
+router bgp 65002
+ neighbor 192.168.1.1 remote-as external
+ address-family ipv4 unicast
+  neighbor 192.168.1.1 route-map r1 in
+ exit-address-family
+!
+route-map r1 permit 10
+ set distance 0
+exit

tests/topotests/bgp_distance_zero/r2/zebra.conf

@@ -0,0 +1,4 @@
+!
+int r2-eth0
+ ip address 192.168.1.2/24
+!

tests/topotests/bgp_distance_zero/test_bgp_distance_zero.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python
+# SPDX-License-Identifier: ISC
+
+# Copyright (c) 2023 by
+# Donatas Abraitis <donatas@opensourcerouting.org>
+#
+
+"""
+Test if distance can be set to zero via route-maps.
+"""
+
+import os
+import sys
+import json
+import pytest
+import functools
+
+pytestmark = pytest.mark.bgpd
+
+CWD = os.path.dirname(os.path.realpath(__file__))
+sys.path.append(os.path.join(CWD, "../"))
+
+# pylint: disable=C0413
+from lib import topotest
+from lib.topogen import Topogen, TopoRouter, get_topogen
+
+pytestmark = [pytest.mark.bgpd]
+
+
+def setup_module(mod):
+    topodef = {"s1": ("r1", "r2")}
+    tgen = Topogen(topodef, mod.__name__)
+    tgen.start_topology()
+
+    router_list = tgen.routers()
+
+    for i, (rname, router) in enumerate(router_list.items(), 1):
+        router.load_config(
+            TopoRouter.RD_ZEBRA, os.path.join(CWD, "{}/zebra.conf".format(rname))
+        )
+        router.load_config(
+            TopoRouter.RD_BGP, os.path.join(CWD, "{}/bgpd.conf".format(rname))
+        )
+
+    tgen.start_router()
+
+
+def teardown_module(mod):
+    tgen = get_topogen()
+    tgen.stop_topology()
+
+
+def test_bgp_distance_zero():
+    tgen = get_topogen()
+
+    if tgen.routers_have_failure():
+        pytest.skip(tgen.errors)
+
+    r2 = tgen.gears["r2"]
+
+    def _bgp_converge_table():
+        output = json.loads(r2.vtysh_cmd("show bgp ipv4 unicast 10.10.10.1/32 json"))
+        expected = {"paths": [{"valid": True}]}
+        return topotest.json_cmp(output, expected)
+
+    test_func = functools.partial(_bgp_converge_table)
+    _, result = topotest.run_and_expect(test_func, None, count=30, wait=1)
+    assert result is None, "Can't see distance 0 in BGP table"
+
+    def _bgp_converge_rib():
+        output = json.loads(r2.vtysh_cmd("show ip route 10.10.10.1/32 json"))
+        expected = {
+            "10.10.10.1/32": [
+                {"protocol": "bgp", "distance": 0, "selected": True, "installed": True}
+            ]
+        }
+        return topotest.json_cmp(output, expected)
+
+    test_func = functools.partial(_bgp_converge_rib)
+    _, result = topotest.run_and_expect(test_func, None, count=30, wait=1)
+    assert result is None, "Can't see distance 0 in RIB"
+
+
+if __name__ == "__main__":
+    args = ["-s"] + sys.argv[1:]
+    sys.exit(pytest.main(args))

zebra/zebra_rib.c

@@ -2781,8 +2781,11 @@ static void process_subq_early_route_add(struct zebra_early_route *ere)
 	if (re->distance == 0) {
 		if (same && !zebra_router_notify_on_ack())
 			re->distance = same->distance;
-		else
-			re->distance = route_distance(re->type);
+		else {
+			if (!CHECK_FLAG(re->flags,
+					ZEBRA_FLAG_DEFAULT_DISTANCE_OVERRIDE))
+				re->distance = route_distance(re->type);
+		}
 	}
 
 	if (re->metric == ROUTE_INSTALLATION_METRIC &&