bgpd: fix ipv4-mapped ipv6 on non 6pe

0325116 ("bgpd: fix 6vpe nexthop") enforced sending an IPv4-mapped
IPv6 address as nexthop when the BGP session is on IPv4 and address
family IPv6. This is not always correct. In some cases, sending an
IPv6 global address is valid on an IPv4 session:
- MP-BGP is used and the sending router has an IPv4 and a global IPv6
  address on the interface towards the peer.
- nexthop is set by a route-map, nexthop-self...
- router that is sending the UPDATE is a route-reflector or a
  route-server

IPv4-mapped IPv6 nexthop should be limited to the 6PE /6vPE cases where
the prefix is reachable via an IPv4 only MPLS backbone.

Do not replace the global IPv6 address when sending UPDATE in the cases
described above. Also only set the IPv4-mapped IPv6 address as nexthop
when only a link-local IPv6 address or no IPv6 address is found on the
interface towards the peer. It assumes a 6PE router has a no configured
IPv6 address on the interface.

Fixes: 0325116 ("bgpd: fix 6vpe nexthop")
Signed-off-by: Louis Scalbert <[email protected]>

bgpd/bgp_updgrp_packet.c

@@ -525,11 +525,26 @@ struct stream *bpacket_reformat_for_peer(struct bpacket *pkt,
 
 		if (peer->nexthop.v4.s_addr != INADDR_ANY &&
 		    (IN6_IS_ADDR_UNSPECIFIED(mod_v6nhg) ||
-		     (peer->connection->su.sa.sa_family == AF_INET &&
-		      paf->afi == AFI_IP6))) {
-			/* set a IPv4 mapped IPv6 address if no global IPv6
-			 * address is found or if announcing IPv6 prefix
-			 * over an IPv4 BGP session.
+		     (IN6_IS_ADDR_LINKLOCAL(mod_v6nhg) &&
+		      peer->connection->su.sa.sa_family == AF_INET &&
+		      paf->afi == AFI_IP6 && !route_map_sets_nh &&
+		      !peer_af_flag_check(peer, paf->afi, paf->safi,
+					  PEER_FLAG_NEXTHOP_UNCHANGED) &&
+		      !CHECK_FLAG(vec->flags,
+				  BPKT_ATTRVEC_FLAGS_RMAP_NH_UNCHANGED) &&
+			  !CHECK_FLAG(vec->flags,
+					  PEER_FLAG_NEXTHOP_SELF) &&
+		      !peer_af_flag_check(peer, paf->afi, paf->safi,
+					  PEER_FLAG_REFLECTOR_CLIENT) &&
+		      !peer_af_flag_check(peer, paf->afi, paf->safi,
+					  PEER_FLAG_RSERVER_CLIENT)))) {
+			/* set a IPv4 mapped IPv6 address
+			 * - if no global IPv6 address is found
+			 * - or if a Link-Local is found (because no global is present)
+			 *   and the announced prefix is IPv6 over an IPv4 BGP session
+			 *   and no specific instructions for nexthop override or unchanged
+			 *   and not a route-reflector
+			 *   and not a route-server
 			 */
 			ipv4_to_ipv4_mapped_ipv6(mod_v6nhg, peer->nexthop.v4);
 			gnh_modified = 1;