4 files changed, 26 insertions, 6 deletions
diff --git a/proto/bgp/attrs.c b/proto/bgp/attrs.c
index 9b243763..39297dd7 100644
--- a/proto/bgp/attrs.c
+++ b/proto/bgp/attrs.c
@@ -404,6 +404,15 @@ bgp_format_origin(eattr *a, byte *buf, uint size UNUSED)
 }
 
 
+static inline int
+bgp_as_path_first_as_equal(const byte *data, uint len, u32 asn)
+{
+  return (len >= 6) &&
+    ((data[0] == AS_PATH_SEQUENCE) || (data[0] == AS_PATH_CONFED_SEQUENCE)) &&
+    (data[1] > 0) &&
+    (get_u32(data+2) == asn);
+}
+
 static int
 bgp_encode_as_path(struct bgp_write_state *s, eattr *a, byte *buf, uint size)
 {
@@ -433,11 +442,6 @@ bgp_decode_as_path(struct bgp_parse_state *s, uint code UNUSED, uint flags, byte
   if (!as_path_valid(data, len, as_length, as_sets, as_confed, err, sizeof(err)))
     WITHDRAW("Malformed AS_PATH attribute - %s", err);
 
-  /* In some circumstances check for initial AS_CONFED_SEQUENCE; RFC 5065 5.0 */
-  if (p->is_interior && !p->is_internal &&
-      ((len < 2) || (data[0] != AS_PATH_CONFED_SEQUENCE)))
-    WITHDRAW("Malformed AS_PATH attribute - %s", "missing initial AS_CONFED_SEQUENCE");
-
   if (!s->as4_session)
   {
     /* Prepare 32-bit AS_PATH (from 16-bit one) in a temporary buffer */
@@ -446,6 +450,16 @@ bgp_decode_as_path(struct bgp_parse_state *s, uint code UNUSED, uint flags, byte
     len = as_path_16to32(data, src, len);
   }
 
+  /* In some circumstances check for initial AS_CONFED_SEQUENCE; RFC 5065 5.0 */
+  if (p->is_interior && !p->is_internal &&
+      ((len < 2) || (data[0] != AS_PATH_CONFED_SEQUENCE)))
+    WITHDRAW("Malformed AS_PATH attribute - %s", "missing initial AS_CONFED_SEQUENCE");
+
+  /* Reject routes with first AS in AS_PATH not matching neighbor AS; RFC 4271 6.3 */
+  if (!p->is_internal && p->cf->enforce_first_as &&
+      !bgp_as_path_first_as_equal(data, len, p->remote_as))
+    WITHDRAW("Malformed AS_PATH attribute - %s", "First AS differs from neigbor AS");
+
   bgp_set_attr_data(to, s->pool, BA_AS_PATH, flags, data, len);
 }
 
diff --git a/proto/bgp/bgp.c b/proto/bgp/bgp.c
index 053016dd..2a64958a 100644
--- a/proto/bgp/bgp.c
+++ b/proto/bgp/bgp.c
@@ -1936,6 +1936,9 @@ bgp_postconfig(struct proto_config *CF)
   if (!cf->gr_mode && cf->llgr_mode)
     cf_error("Long-lived graceful restart requires basic graceful restart");
 
+  if (internal && cf->enforce_first_as)
+    cf_error("Enforce first AS check is requires EBGP sessions");
+
 
   struct bgp_channel_config *cc;
   WALK_LIST(cc, CF->channels)
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index 1270b8f3..d336132b 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -108,6 +108,7 @@ struct bgp_config {
   int allow_local_as;			/* Allow that number of local ASNs in incoming AS_PATHs */
   int allow_local_pref;			/* Allow LOCAL_PREF in EBGP sessions */
   int allow_as_sets;			/* Allow AS_SETs in incoming AS_PATHs */
+  int enforce_first_as;			/* Enable check for neighbor AS as first AS in AS_PATH */
   int gr_mode;				/* Graceful restart mode (BGP_GR_*) */
   int llgr_mode;			/* Long-lived graceful restart mode (BGP_LLGR_*) */
   int setkey;				/* Set MD5 password to system SA/SP database */
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index 090ba24e..7279560b 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -30,7 +30,8 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY, KEEPALIVE,
 	GRACEFUL, RESTART, AWARE, CHECK, LINK, PORT, EXTENDED, MESSAGES, SETKEY,
 	STRICT, BIND, CONFEDERATION, MEMBER, MULTICAST, FLOW4, FLOW6, LONG,
 	LIVED, STALE, IMPORT, IBGP, EBGP, MANDATORY, INTERNAL, EXTERNAL, SETS,
-	DYNAMIC, RANGE, NAME, DIGITS, BGP_AIGP, AIGP, ORIGINATE, COST)
+	DYNAMIC, RANGE, NAME, DIGITS, BGP_AIGP, AIGP, ORIGINATE, COST, ENFORCE,
+	FIRST)
 
 %type <i> bgp_nh
 %type <i32> bgp_afi
@@ -191,6 +192,7 @@ bgp_proto:
  | bgp_proto CHECK LINK bool ';' { BGP_CFG->check_link = $4; }
  | bgp_proto BFD bool ';' { BGP_CFG->bfd = $3; cf_check_bfd($3); }
  | bgp_proto BFD GRACEFUL ';' { BGP_CFG->bfd = BGP_BFD_GRACEFUL; cf_check_bfd(1); }
+ | bgp_proto ENFORCE FIRST AS bool ';' { BGP_CFG->enforce_first_as = $5; }
  ;
 
 bgp_afi: