Babel: Implement IPv4 via IPv6 extension (RFC 9229)

The patch implements an IPv4 via IPv6 extension (RFC 9229) to the Babel
routing protocol (RFC 8966) that allows annoncing routes to an IPv4
prefix with an IPv6 next hop, which makes it possible for IPv4 traffic
to flow through interfaces that have not been assigned an IPv4 address.

The implementation is compatible with the current Babeld version.

Thanks to Toke Høiland-Jørgensen for early review on this work.

Minor changes from committer.

1 files changed, 23 insertions, 6 deletions

diff --git a/proto/babel/babel.c b/proto/babel/babel.c
index becff6d0..a9a08e31 100644
--- a/proto/babel/babel.c
+++ b/proto/babel/babel.c
@@ -1003,8 +1003,18 @@ babel_send_update_(struct babel_iface *ifa, btime changed, struct fib *rtable)
     msg.update.router_id = e->router_id;
     net_copy(&msg.update.net, e->n.addr);
 
-    msg.update.next_hop = ((e->n.addr->type == NET_IP4) ?
-			   ifa->next_hop_ip4 : ifa->next_hop_ip6);
+    if (e->n.addr->type == NET_IP4)
+    {
+      /* Always prefer IPv4 nexthop if set */
+      if (ipa_nonzero(ifa->next_hop_ip4))
+        msg.update.next_hop = ifa->next_hop_ip4;
+
+      /* Only send IPv6 nexthop if enabled */
+      else if (ifa->cf->ext_next_hop)
+        msg.update.next_hop = ifa->next_hop_ip6;
+    }
+    else
+      msg.update.next_hop = ifa->next_hop_ip6;
 
     /* Do not send route if next hop is unknown, e.g. no configured IPv4 address */
     if (ipa_zero(msg.update.next_hop))
@@ -1263,6 +1273,13 @@ babel_handle_update(union babel_msg *m, struct babel_iface *ifa)
     return;
   }
 
+  /* Reject IPv4 via IPv6 routes if disabled */
+  if ((msg->net.type == NET_IP4) && ipa_is_ip6(msg->next_hop) && !ifa->cf->ext_next_hop)
+  {
+    DBG("Babel: Ignoring disabled IPv4 via IPv6 route.\n");
+    return;
+  }
+
   /* Retraction */
   if (msg->metric == BABEL_INFINITY)
   {
@@ -1729,7 +1746,7 @@ babel_iface_update_addr4(struct babel_iface *ifa)
   ip_addr addr4 = ifa->iface->addr4 ? ifa->iface->addr4->ip : IPA_NONE;
   ifa->next_hop_ip4 = ipa_nonzero(ifa->cf->next_hop_ip4) ? ifa->cf->next_hop_ip4 : addr4;
 
-  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel)
+  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel && !ifa->cf->ext_next_hop)
     log(L_WARN "%s: Missing IPv4 next hop address for %s", p->p.name, ifa->ifname);
 
   if (ifa->up)
@@ -1806,8 +1823,8 @@ babel_add_iface(struct babel_proto *p, struct iface *new, struct babel_iface_con
   ifa->next_hop_ip4 = ipa_nonzero(ic->next_hop_ip4) ? ic->next_hop_ip4 : addr4;
   ifa->next_hop_ip6 = ipa_nonzero(ic->next_hop_ip6) ? ic->next_hop_ip6 : ifa->addr;
 
-  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel)
-    log(L_WARN "%s: Missing IPv4 next hop address for %s", p->p.name, new->name);
+  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel && !ic->ext_next_hop)
+    log(L_WARN "%s: Missing IPv4 next hop address for %s", p->p.name, ifa->ifname);
 
   init_list(&ifa->neigh_list);
   ifa->hello_seqno = 1;
@@ -1927,7 +1944,7 @@ babel_reconfigure_iface(struct babel_proto *p, struct babel_iface *ifa, struct b
   if ((new->auth_type != BABEL_AUTH_NONE) && (new->auth_type != old->auth_type))
     babel_auth_reset_index(ifa);
 
-  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel)
+  if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel && !new->ext_next_hop)
     log(L_WARN "%s: Missing IPv4 next hop address for %s", p->p.name, ifa->ifname);
 
   if (ifa->next_hello > (current_time() + new->hello_interval))