From 15fb7209eede6226bbe15a9618a8d06b88d44a08 Mon Sep 17 00:00:00 2001
From: Louis Scalbert <louis.scalbert@6wind.com>
Date: Mon, 22 Jul 2024 10:57:21 +0200
Subject: [PATCH] bgpd: fix ipv4-mapped ipv6 for ipv6 over ipv4 peer

When IPv6 prefixes are sent over an IPv4 session, the UPDATE is sent
with a link-local nexthop as global nexthop instead of a IPv4-mapped
IPv6 address. If the peer interface has no IPv6 address, routes are
not installed.

Seen with bgp_nexthop_mp_ipv4_6 topotests on step2:

> root@r2:/# vtysh -c 'show bgp ipv6 fd00:100::/64 json' | jq .
> {
>   "prefix": "fd00:100::/64",
>   "paths": [
>     {
>       "nexthops": [
>         {
>           "ip": "fe80::449a:f8ff:fe67:1f93",
>           "hostname": "r1",
>           "afi": "ipv6",
>           "scope": "global",
>           "metric": 0,
>           "accessible": true
>         },
>         {
>           "ip": "fe80::449a:f8ff:fe67:1f93",
>           "hostname": "r1",
>           "afi": "ipv6",
>           "scope": "link-local",
>           "accessible": true,
>           "used": true
>         }
>       ],
>     }
>   ]
> }

Now:

> root@r2:/# vtysh -c 'show bgp ipv6 fd00:100::/64 json' | jq .
> {
>   "prefix": "fd00:100::/64",
>   "paths": [
>       "nexthops": [
>         {
>           "ip": "::ffff:172.16.0.1",
>           "hostname": "r1",
>           "afi": "ipv6",
>           "scope": "global",
>           "metric": 0,
>           "accessible": true
>         },
>         {
>           "ip": "fe80::3842:28ff:fe90:f815",
>           "hostname": "r1",
>           "afi": "ipv6",
>           "scope": "link-local",
>           "accessible": true,
>           "used": true
>         }
>       ],
>     }
>   ]
> }

Note that the link-local is still preferred over the global address.

Fixes: 25995695f5 ("bgpd: set ipv4-mapped ipv6 for ipv4 with ipv6 nexthop")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
---
 bgpd/bgp_updgrp_packet.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/bgpd/bgp_updgrp_packet.c b/bgpd/bgp_updgrp_packet.c
index 6e30d4f846..468bf47715 100644
--- a/bgpd/bgp_updgrp_packet.c
+++ b/bgpd/bgp_updgrp_packet.c
@@ -526,8 +526,10 @@ struct stream *bpacket_reformat_for_peer(struct bpacket *pkt,
 		if (peer->nexthop.v4.s_addr != INADDR_ANY &&
 		    (IN6_IS_ADDR_UNSPECIFIED(mod_v6nhg) ||
 		     (IN6_IS_ADDR_LINKLOCAL(mod_v6nhg) &&
-		      peer->connection->su.sa.sa_family == AF_INET6 &&
-		      paf->afi == AFI_IP))) {
+		      ((peer->connection->su.sa.sa_family == AF_INET6 &&
+			paf->afi == AFI_IP) ||
+		       (peer->connection->su.sa.sa_family == AF_INET &&
+			paf->afi == AFI_IP6))))) {
 			ipv4_to_ipv4_mapped_ipv6(mod_v6nhg, peer->nexthop.v4);
 			gnh_modified = 1;
 		}