From 23ee6b1cd6dc597876d91db9a015f7a633764808 Mon Sep 17 00:00:00 2001
From: "Ondrej Zajicek (work)" <santiago@crfreenet.org>
Date: Wed, 3 Apr 2019 15:54:50 +0200
Subject: BGP: Promiscuous ASN mode

Allow to specify just 'internal' or 'external' for remote neighbor
instead of specific ASN. In the second case that means BGP peers with
any non-local ASNs are accepted.
---
 doc/bird.sgml       | 11 +++++++----
 proto/bgp/bgp.c     | 25 ++++++++++++++++++++-----
 proto/bgp/bgp.h     |  5 +++++
 proto/bgp/config.Y  |  4 +++-
 proto/bgp/packets.c |  9 +++++++--
 5 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 52c9d6fd..9675caf0 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -2195,12 +2195,15 @@ using the following configuration parameters:
 	<cf/local 10.0.0.1; local as 65000;/ are valid). This parameter is
 	mandatory.
 
-	<tag><label id="bgp-neighbor">neighbor [<m/ip/] [port <m/number/] [as <m/number/]</tag>
+	<tag><label id="bgp-neighbor">neighbor [<m/ip/] [port <m/number/] [as <m/number/] [internal|external]</tag>
 	Define neighboring router this instance will be talking to and what AS
 	it is located in. In case the neighbor is in the same AS as we are, we
-	automatically switch to iBGP. Optionally, the remote port may also be
-	specified. Like <cf/local/ parameter, this parameter may also be used
-	multiple times with different sub-options. This parameter is mandatory.
+	automatically switch to IBGP. Alternatively, it is possible to specify
+	just <cf/internal/ or </cf/external/ instead of AS number, in that case
+	either local AS number, or any external AS number is accepted.
+	Optionally, the remote port may also be specified. Like <cf/local/
+	parameter, this parameter may also be used multiple times with different
+	sub-options. This parameter is mandatory.
 
 	<tag><label id="bgp-iface">interface <m/string/</tag>
 	Define interface we should use for link-local BGP IPv6 sessions.
diff --git a/proto/bgp/bgp.c b/proto/bgp/bgp.c
index 756f0326..92f41ef6 100644
--- a/proto/bgp/bgp.c
+++ b/proto/bgp/bgp.c
@@ -507,6 +507,10 @@ bgp_conn_enter_established_state(struct bgp_conn *conn)
   if (ipa_zero(p->local_ip))
     p->local_ip = conn->sk->saddr;
 
+  /* For promiscuous sessions */
+  if (!p->remote_as)
+    p->remote_as = conn->received_as;
+
   /* In case of LLv6 is not valid during BGP start */
   if (ipa_zero(p->link_addr) && p->neigh && p->neigh->iface && p->neigh->iface->llv6)
     p->link_addr = p->neigh->iface->llv6->ip;
@@ -1754,14 +1758,19 @@ void
 bgp_postconfig(struct proto_config *CF)
 {
   struct bgp_config *cf = (void *) CF;
-  int internal = (cf->local_as == cf->remote_as);
-  int interior = internal || cf->confederation_member;
 
   /* Do not check templates at all */
   if (cf->c.class == SYM_TEMPLATE)
     return;
 
 
+  /* Handle undefined remote_as, zero should mean unspecified external */
+  if (!cf->remote_as && (cf->peer_type == BGP_PT_INTERNAL))
+    cf->remote_as = cf->local_as;
+
+  int internal = (cf->local_as == cf->remote_as);
+  int interior = internal || cf->confederation_member;
+
   /* EBGP direct by default, IBGP multihop by default */
   if (cf->multihop < 0)
     cf->multihop = internal ? 64 : 0;
@@ -1781,8 +1790,14 @@ bgp_postconfig(struct proto_config *CF)
   if (ipa_zero(cf->remote_ip))
     cf_error("Neighbor must be configured");
 
-  if (!cf->remote_as)
-    cf_error("Remote AS number must be set");
+  if (!cf->remote_as && !cf->peer_type)
+    cf_error("Remote AS number (or peer type) must be set");
+
+  if ((cf->peer_type == BGP_PT_INTERNAL) && !internal)
+    cf_error("IBGP cannot have different ASNs");
+
+  if ((cf->peer_type == BGP_PT_EXTERNAL) &&  internal)
+    cf_error("EBGP cannot have the same ASNs");
 
   if (!cf->iface && (ipa_is_link_local(cf->local_ip) ||
 		     ipa_is_link_local(cf->remote_ip)))
@@ -2242,7 +2257,7 @@ bgp_show_proto_info(struct proto *P)
 
   cli_msg(-1006, "  BGP state:          %s", bgp_state_dsc(p));
   cli_msg(-1006, "    Neighbor address: %I%J", p->cf->remote_ip, p->cf->iface);
-  cli_msg(-1006, "    Neighbor AS:      %u", p->cf->remote_as);
+  cli_msg(-1006, "    Neighbor AS:      %u", p->cf->remote_as ?: p->remote_as);
 
   if (p->gr_active_num)
     cli_msg(-1006, "    Neighbor graceful restart active");
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index 512410d4..fd515cbb 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -83,6 +83,7 @@ struct bgp_config {
   struct iface *iface;			/* Interface for link-local addresses */
   u16 local_port;			/* Local listening port */
   u16 remote_port; 			/* Neighbor destination port */
+  int peer_type;			/* Internal or external BGP (BGP_PT_*, optional) */
   int multihop;				/* Number of hops if multihop */
   int strict_bind;			/* Bind listening socket to local address */
   int ttl_security;			/* Enable TTL security [RFC 5082] */
@@ -152,6 +153,9 @@ struct bgp_channel_config {
   struct rtable_config *igp_table_ip6;	/* Table for recursive IPv6 next hop lookups */
 };
 
+#define BGP_PT_INTERNAL		1
+#define BGP_PT_EXTERNAL		2
+
 #define NH_NO			0
 #define NH_ALL			1
 #define NH_IBGP			2
@@ -237,6 +241,7 @@ struct bgp_conn {
   u8 state;				/* State of connection state machine */
   u8 as4_session;			/* Session uses 4B AS numbers in AS_PATH (both sides support it) */
   u8 ext_messages;			/* Session uses extended message length */
+  u32 received_as;			/* ASN received in OPEN message */
 
   struct bgp_caps *local_caps;
   struct bgp_caps *remote_caps;
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index 9dea78ca..c9a6af96 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -29,7 +29,7 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY, KEEPALIVE,
 	SECURITY, DETERMINISTIC, SECONDARY, ALLOW, BFD, ADD, PATHS, RX, TX,
 	GRACEFUL, RESTART, AWARE, CHECK, LINK, PORT, EXTENDED, MESSAGES, SETKEY,
 	STRICT, BIND, CONFEDERATION, MEMBER, MULTICAST, FLOW4, FLOW6, LONG,
-	LIVED, STALE, IMPORT, IBGP, EBGP, MANDATORY)
+	LIVED, STALE, IMPORT, IBGP, EBGP, MANDATORY, INTERNAL, EXTERNAL)
 
 %type <i> bgp_nh
 %type <i32> bgp_afi
@@ -82,6 +82,8 @@ bgp_nbr_opts:
    /* empty */
  | bgp_nbr_opts PORT expr { BGP_CFG->remote_port = $3; if (($3<1) || ($3>65535)) cf_error("Invalid port number"); }
  | bgp_nbr_opts AS expr { BGP_CFG->remote_as = $3; }
+ | bgp_nbr_opts INTERNAL { BGP_CFG->peer_type = BGP_PT_INTERNAL; }
+ | bgp_nbr_opts EXTERNAL { BGP_CFG->peer_type = BGP_PT_EXTERNAL; }
  ;
 
 bgp_cease_mask:
diff --git a/proto/bgp/packets.c b/proto/bgp/packets.c
index fc572a21..0e142a7a 100644
--- a/proto/bgp/packets.c
+++ b/proto/bgp/packets.c
@@ -732,13 +732,18 @@ bgp_rx_open(struct bgp_conn *conn, byte *pkt, uint len)
     if ((as4 != asn) && (asn != AS_TRANS))
       log(L_WARN "%s: Peer advertised inconsistent AS numbers", p->p.name);
 
-    if (as4 != p->remote_as)
+    /* When remote ASN is unspecified, it must be external one */
+    if (p->remote_as ? (as4 != p->remote_as) : (as4 == p->local_as))
     { as4 = htonl(as4); bgp_error(conn, 2, 2, (byte *) &as4, 4); return; }
+
+    conn->received_as = as4;
   }
   else
   {
-    if (asn != p->remote_as)
+    if (p->remote_as ? (asn != p->remote_as) : (asn == p->local_as))
     { bgp_error(conn, 2, 2, pkt+20, 2); return; }
+
+    conn->received_as = asn;
   }
 
   /* Check the other connection */
-- 
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