bgpd: Set NO_ADVERTISE community if blackhole community received

rfc7999:
A BGP speaker receiving an announcement tagged with the BLACKHOLE
community SHOULD add the NO_ADVERTISE or NO_EXPORT community as
defined in [RFC1997], or a similar community, to prevent propagation
of the prefix outside the local AS.  The community to prevent
propagation SHOULD be chosen according to the operator's routing
policy.

Sent:
```
router bgp 65534
 no bgp ebgp-requires-policy
 neighbor 192.168.0.2 remote-as 65030
 !
 address-family ipv4 unicast
  redistribute connected
  neighbor 192.168.0.2 route-map spine out
 exit-address-family
 !
!
ip prefix-list self seq 5 permit 192.168.100.1/32
!
route-map spine permit 10
 match ip address prefix-list self
 set community blackhole
!
```

Received:
```
spine1-debian-9# show ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32
Paths: (1 available, best #1, table default, inform peer to blackhole prefix)
  Not advertised to any peer
  65534
    192.168.0.1 from 192.168.0.1 (192.168.100.1)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Community: blackhole no-advertise
      Last update: Thu Jan 21 12:56:39 2021
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

bgpd/bgp_community.c

@@ -56,7 +56,7 @@ void community_free(struct community **com)
 }
 
 /* Add one community value to the community. */
-static void community_add_val(struct community *com, uint32_t val)
+void community_add_val(struct community *com, uint32_t val)
 {
 	com->size++;
 	if (com->val)

bgpd/bgp_community.h

@@ -88,6 +88,7 @@ extern struct community *community_delete(struct community *,
 					  struct community *);
 extern struct community *community_dup(struct community *);
 extern bool community_include(struct community *, uint32_t);
+extern void community_add_val(struct community *com, uint32_t val);
 extern void community_del_val(struct community *, uint32_t *);
 extern unsigned long community_count(void);
 extern struct hash *community_hash(void);

bgpd/bgp_route.c

@@ -3505,6 +3505,34 @@ bool bgp_update_martian_nexthop(struct bgp *bgp, afi_t afi, safi_t safi,
 	return ret;
 }
 
+static void bgp_attr_add_no_advertise_community(struct attr *attr)
+{
+	struct community *old;
+	struct community *new;
+	struct community *merge;
+	struct community *noadv;
+
+	old = attr->community;
+	noadv = community_str2com("no-advertise");
+
+	if (old) {
+		merge = community_merge(community_dup(old), noadv);
+
+		if (!old->refcnt)
+			community_free(&old);
+
+		new = community_uniq_sort(merge);
+		community_free(&merge);
+	} else {
+		new = community_dup(noadv);
+	}
+
+	community_free(&noadv);
+
+	attr->community = new;
+	attr->flag |= ATTR_FLAG_BIT(BGP_ATTR_COMMUNITIES);
+}
+
 int bgp_update(struct peer *peer, const struct prefix *p, uint32_t addpath_id,
 	       struct attr *attr, afi_t afi, safi_t safi, int type,
 	       int sub_type, struct prefix_rd *prd, mpls_label_t *label,
@@ -3697,6 +3725,20 @@ int bgp_update(struct peer *peer, const struct prefix *p, uint32_t addpath_id,
 
 	if (peer->sort == BGP_PEER_EBGP) {
 
+		/* rfc7999:
+		 * A BGP speaker receiving an announcement tagged with the
+		 * BLACKHOLE community SHOULD add the NO_ADVERTISE or
+		 * NO_EXPORT community as defined in RFC1997, or a
+		 * similar community, to prevent propagation of the
+		 * prefix outside the local AS. The community to prevent
+		 * propagation SHOULD be chosen according to the operator's
+		 * routing policy.
+		 */
+		if (new_attr.community
+		    && community_include(new_attr.community,
+					 COMMUNITY_BLACKHOLE))
+			bgp_attr_add_no_advertise_community(&new_attr);
+
 		/* If we receive the graceful-shutdown community from an eBGP
 		 * peer we must lower local-preference */
 		if (new_attr.community

bgpd/bgp_routemap.c

@@ -4822,6 +4822,11 @@ DEFUN (set_community,
 			buffer_putstr(b, "no-export");
 			continue;
 		}
+		if (strncmp(argv[i]->arg, "blackhole", strlen(argv[i]->arg))
+		    == 0) {
+			buffer_putstr(b, "blackhole");
+			continue;
+		}
 		if (strncmp(argv[i]->arg, "graceful-shutdown",
 			    strlen(argv[i]->arg))
 		    == 0) {