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, 82 insertions, 33 deletions

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;