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.

5 files changed, 118 insertions, 41 deletions

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 001fcbd4..6f3fcfc6 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1929,6 +1929,7 @@ protocol babel [<name>] {
 		check link <switch>;
 		next hop ipv4 <address>;
 		next hop ipv6 <address>;
+		extended next hop <switch>;
 		authentication none|mac [permissive];
 		password "&lt;text&gt;";
 		password "&lt;text&gt;" {
@@ -2033,6 +2034,11 @@ protocol babel [<name>] {
       source for Babel packets will be used. In normal operation, it should not
       be necessary to set this option.
 
+      <tag><label id="babel-extended-next-hop">extended next hop <m/switch/</tag>
+      If enabled, BIRD will accept and emit IPv4 routes with an IPv6 next
+      hop when IPv4 addresses are absent from the interface as described in
+      <rfc id="9229">. Default: yes.
+
       <tag><label id="babel-authentication">authentication none|mac [permissive]</tag>
       Selects authentication method to be used. <cf/none/ means that packets
       are not authenticated at all, <cf/mac/ means MAC authentication is
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))
diff --git a/proto/babel/babel.h b/proto/babel/babel.h
index da8386b3..6699127e 100644
--- a/proto/babel/babel.h
+++ b/proto/babel/babel.h
@@ -114,6 +114,7 @@ enum babel_ae_type {
   BABEL_AE_IP4			= 1,
   BABEL_AE_IP6			= 2,
   BABEL_AE_IP6_LL		= 3,
+  BABEL_AE_IP4_VIA_IP6		= 4,
   BABEL_AE_MAX
 };
 
@@ -147,8 +148,9 @@ struct babel_iface_config {
 
   ip_addr next_hop_ip4;
   ip_addr next_hop_ip6;
+  u8 ext_next_hop;			/* Enable IPv4 via IPv6 */
 
-  u8 auth_type;			/* Authentication type (BABEL_AUTH_*) */
+  u8 auth_type;				/* Authentication type (BABEL_AUTH_*) */
   u8 auth_permissive;			/* Don't drop packets failing auth check */
   uint mac_num_keys;			/* Number of configured HMAC keys */
   uint mac_total_len;			/* Total digest length for all configured keys */
diff --git a/proto/babel/config.Y b/proto/babel/config.Y
index 05210fa4..1b4dc6f5 100644
--- a/proto/babel/config.Y
+++ b/proto/babel/config.Y
@@ -25,7 +25,8 @@ CF_DECLS
 CF_KEYWORDS(BABEL, INTERFACE, METRIC, RXCOST, HELLO, UPDATE, INTERVAL, PORT,
 	TYPE, WIRED, WIRELESS, RX, TX, BUFFER, PRIORITY, LENGTH, CHECK, LINK,
 	NEXT, HOP, IPV4, IPV6, BABEL_METRIC, SHOW, INTERFACES, NEIGHBORS,
-	ENTRIES, RANDOMIZE, ROUTER, ID, AUTHENTICATION, NONE, MAC, PERMISSIVE)
+	ENTRIES, RANDOMIZE, ROUTER, ID, AUTHENTICATION, NONE, MAC, PERMISSIVE,
+	EXTENDED)
 
 CF_GRAMMAR
 
@@ -67,6 +68,7 @@ babel_iface_start:
   BABEL_IFACE->tx_tos = IP_PREC_INTERNET_CONTROL;
   BABEL_IFACE->tx_priority = sk_priority_control;
   BABEL_IFACE->check_link = 1;
+  BABEL_IFACE->ext_next_hop = 1;
 };
 
 
@@ -143,6 +145,7 @@ babel_iface_item:
  | CHECK LINK bool { BABEL_IFACE->check_link = $3; }
  | NEXT HOP IPV4 ipa { BABEL_IFACE->next_hop_ip4 = $4; if (!ipa_is_ip4($4)) cf_error("Must be an IPv4 address"); }
  | NEXT HOP IPV6 ipa { BABEL_IFACE->next_hop_ip6 = $4; if (!ipa_is_ip6($4)) cf_error("Must be an IPv6 address"); }
+ | EXTENDED NEXT HOP bool { BABEL_IFACE->ext_next_hop = $4; }
  | AUTHENTICATION NONE { BABEL_IFACE->auth_type = BABEL_AUTH_NONE; }
  | AUTHENTICATION MAC { BABEL_IFACE->auth_type = BABEL_AUTH_MAC; BABEL_IFACE->auth_permissive = 0; }
  | AUTHENTICATION MAC PERMISSIVE { BABEL_IFACE->auth_type = BABEL_AUTH_MAC; BABEL_IFACE->auth_permissive = 1; }
diff --git a/proto/babel/packets.c b/proto/babel/packets.c
index d4acc170..28bf9f63 100644
--- a/proto/babel/packets.c
+++ b/proto/babel/packets.c
@@ -166,10 +166,12 @@ struct babel_parse_state {
   ip_addr next_hop_ip6;
   u64 router_id;		/* Router ID used in subsequent updates */
   u8 def_ip6_prefix[16];	/* Implicit IPv6 prefix in network order */
-  u8 def_ip4_prefix[4];		/* Implicit IPv4 prefix in network order */
+  u8 def_ip4_prefix[4];		/* Implicit IPv4 prefix (AE 1) in network order */
+  u8 def_ip4_via_ip6_prefix[4];	/* Implicit IPv4 prefix (AE 4) in network order */
   u8 router_id_seen;		/* router_id field is valid */
   u8 def_ip6_prefix_seen;	/* def_ip6_prefix is valid */
   u8 def_ip4_prefix_seen;	/* def_ip4_prefix is valid */
+  u8 def_ip4_via_ip6_prefix_seen; /* def_ip4_via_ip6_prefix is valid */
   u8 current_tlv_endpos;	/* End of self-terminating TLVs (offset from start) */
   u8 sadr_enabled;
   u8 is_unicast;
@@ -515,9 +517,6 @@ babel_read_ihu(struct babel_tlv *hdr, union babel_msg *m,
   msg->addr = IPA_NONE;
   msg->sender = state->saddr;
 
-  if (msg->ae >= BABEL_AE_MAX)
-    return PARSE_IGNORE;
-
   /*
    * We only actually read link-local IPs. In every other case, the addr field
    * will be 0 but validation will succeed. The handler takes care of these
@@ -526,17 +525,20 @@ babel_read_ihu(struct babel_tlv *hdr, union babel_msg *m,
    */
   switch (msg->ae)
   {
+  case BABEL_AE_WILDCARD:
+    return PARSE_SUCCESS;
+
   case BABEL_AE_IP4:
     if (TLV_OPT_LENGTH(tlv) < 4)
       return PARSE_ERROR;
     state->current_tlv_endpos += 4;
-    break;
+    return PARSE_SUCCESS;
 
   case BABEL_AE_IP6:
     if (TLV_OPT_LENGTH(tlv) < 16)
       return PARSE_ERROR;
     state->current_tlv_endpos += 16;
-    break;
+    return PARSE_SUCCESS;
 
   case BABEL_AE_IP6_LL:
     if (TLV_OPT_LENGTH(tlv) < 8)
@@ -544,10 +546,17 @@ babel_read_ihu(struct babel_tlv *hdr, union babel_msg *m,
 
     msg->addr = ipa_from_ip6(get_ip6_ll(&tlv->addr));
     state->current_tlv_endpos += 8;
-    break;
+    return PARSE_SUCCESS;
+
+  /* RFC 9229 2.4 - IHU TLV MUST NOT carry the AE 4 (IPv4-via-IPv6) */
+  case BABEL_AE_IP4_VIA_IP6:
+    return PARSE_ERROR;
+
+  default:
+    return PARSE_IGNORE;
   }
 
-  return PARSE_SUCCESS;
+  return PARSE_IGNORE;
 }
 
 static uint
@@ -640,6 +649,10 @@ babel_read_next_hop(struct babel_tlv *hdr, union babel_msg *m UNUSED,
     state->current_tlv_endpos += 8;
     return PARSE_IGNORE;
 
+  /* RFC 9229 2.4 - Next Hop TLV MUST NOT carry the AE 4 (IPv4-via-IPv6) */
+  case BABEL_AE_IP4_VIA_IP6:
+    return PARSE_ERROR;
+
   default:
     return PARSE_IGNORE;
   }
@@ -692,6 +705,42 @@ babel_write_next_hop(struct babel_tlv *hdr, ip_addr addr,
   return 0;
 }
 
+/* This is called directly from babel_read_update() to handle
+   both BABEL_AE_IP4 and BABEL_AE_IP4_VIA_IP6 encodings */
+static int
+babel_read_ip4_prefix(struct babel_tlv_update *tlv, struct babel_msg_update *msg,
+		     u8 *def_prefix, u8 *def_prefix_seen, ip_addr next_hop, int len)
+{
+    if (tlv->plen > IP4_MAX_PREFIX_LENGTH)
+      return PARSE_ERROR;
+
+    /* Cannot omit data if there is no saved prefix */
+    if (tlv->omitted && !*def_prefix_seen)
+      return PARSE_ERROR;
+
+    /* Update must have next hop, unless it is retraction */
+    if (ipa_zero(next_hop) && msg->metric != BABEL_INFINITY)
+      return PARSE_ERROR;
+
+    /* Merge saved prefix and received prefix parts */
+    u8 buf[4] = {};
+    memcpy(buf, def_prefix, tlv->omitted);
+    memcpy(buf + tlv->omitted, tlv->addr, len);
+
+    ip4_addr prefix4 = get_ip4(buf);
+    net_fill_ip4(&msg->net, prefix4, tlv->plen);
+
+    if (tlv->flags & BABEL_UF_DEF_PREFIX)
+    {
+      put_ip4(def_prefix, prefix4);
+      *def_prefix_seen = 1;
+    }
+
+    msg->next_hop = next_hop;
+
+    return PARSE_SUCCESS;
+}
+
 static int
 babel_read_update(struct babel_tlv *hdr, union babel_msg *m,
                   struct babel_parse_state *state)
@@ -706,11 +755,11 @@ babel_read_update(struct babel_tlv *hdr, union babel_msg *m,
 
   /* Length of received prefix data without omitted part */
   int len = BYTES(tlv->plen) - (int) tlv->omitted;
-  u8 buf[16] = {};
 
   if ((len < 0) || ((uint) len > TLV_OPT_LENGTH(tlv)))
     return PARSE_ERROR;
 
+  int rc;
   switch (tlv->ae)
   {
   case BABEL_AE_WILDCARD:
@@ -724,31 +773,20 @@ babel_read_update(struct babel_tlv *hdr, union babel_msg *m,
     break;
 
   case BABEL_AE_IP4:
-    if (tlv->plen > IP4_MAX_PREFIX_LENGTH)
-      return PARSE_ERROR;
-
-    /* Cannot omit data if there is no saved prefix */
-    if (tlv->omitted && !state->def_ip4_prefix_seen)
-      return PARSE_ERROR;
-
-    /* Update must have next hop, unless it is retraction */
-    if (ipa_zero(state->next_hop_ip4) && (msg->metric != BABEL_INFINITY))
-      return PARSE_IGNORE;
-
-    /* Merge saved prefix and received prefix parts */
-    memcpy(buf, state->def_ip4_prefix, tlv->omitted);
-    memcpy(buf + tlv->omitted, tlv->addr, len);
+    rc = babel_read_ip4_prefix(tlv, msg, state->def_ip4_prefix,
+			       &state->def_ip4_prefix_seen,
+			       state->next_hop_ip4, len);
+    if (rc != PARSE_SUCCESS)
+      return rc;
 
-    ip4_addr prefix4 = get_ip4(buf);
-    net_fill_ip4(&msg->net, prefix4, tlv->plen);
-
-    if (tlv->flags & BABEL_UF_DEF_PREFIX)
-    {
-      put_ip4(state->def_ip4_prefix, prefix4);
-      state->def_ip4_prefix_seen = 1;
-    }
+    break;
 
-    msg->next_hop = state->next_hop_ip4;
+  case BABEL_AE_IP4_VIA_IP6:
+    rc = babel_read_ip4_prefix(tlv, msg, state->def_ip4_via_ip6_prefix,
+			       &state->def_ip4_via_ip6_prefix_seen,
+			       state->next_hop_ip6, len);
+    if (rc != PARSE_SUCCESS)
+      return rc;
 
     break;
 
@@ -761,6 +799,7 @@ babel_read_update(struct babel_tlv *hdr, union babel_msg *m,
       return PARSE_ERROR;
 
     /* Merge saved prefix and received prefix parts */
+    u8 buf[16] = {};
     memcpy(buf, state->def_ip6_prefix, tlv->omitted);
     memcpy(buf + tlv->omitted, tlv->addr, len);
 
@@ -863,7 +902,7 @@ babel_write_update(struct babel_tlv *hdr, union babel_msg *m,
   }
   else if (msg->net.type == NET_IP4)
   {
-    tlv->ae = BABEL_AE_IP4;
+    tlv->ae = ipa_is_ip4(msg->next_hop) ? BABEL_AE_IP4 : BABEL_AE_IP4_VIA_IP6;
     tlv->plen = net4_pxlen(&msg->net);
     put_ip4_px(tlv->addr, &msg->net);
   }
@@ -931,7 +970,12 @@ babel_read_route_request(struct babel_tlv *hdr, union babel_msg *m,
     msg->full = 1;
     return PARSE_SUCCESS;
 
+  /*
+   * RFC 9229 2.3 - When receiving requests, AE 1 (IPv4) and AE 4
+   * (IPv4-via-IPv6) MUST be treated in the same manner.
+   */
   case BABEL_AE_IP4:
+  case BABEL_AE_IP4_VIA_IP6:
     if (tlv->plen > IP4_MAX_PREFIX_LENGTH)
       return PARSE_ERROR;
 
@@ -1032,7 +1076,12 @@ babel_read_seqno_request(struct babel_tlv *hdr, union babel_msg *m,
   case BABEL_AE_WILDCARD:
     return PARSE_ERROR;
 
+  /*
+   * RFC 9229 2.3 - When receiving requests, AE 1 (IPv4) and AE 4
+   * (IPv4-via-IPv6) MUST be treated in the same manner.
+   */
   case BABEL_AE_IP4:
+  case BABEL_AE_IP4_VIA_IP6:
     if (tlv->plen > IP4_MAX_PREFIX_LENGTH)
       return PARSE_ERROR;