From 33b6c292c3e3a8972d0b9f43d156aae50db65720 Mon Sep 17 00:00:00 2001
From: "Ondrej Zajicek (work)" <santiago@crfreenet.org>
Date: Tue, 14 Mar 2017 12:56:47 +0100
Subject: [PATCH] BGP: Allow to specify interface for regular sessions

This may be useful if multple interfaces share the same network range.

Thanks to Fritz Grimpen for the original patch.
---
 doc/bird.sgml   |  5 +++--
 proto/bgp/bgp.c | 12 ++++++------
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index dd4472ae..ffd28964 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1883,8 +1883,9 @@ using the following configuration parameters:
 	<tag><label id="bgp-iface">interface <m/string/</tag>
 	Define interface we should use for link-local BGP IPv6 sessions.
 	Interface can also be specified as a part of <cf/neighbor address/
-	(e.g., <cf/neighbor fe80::1234%eth0 as 65000;/). It is an error to use
-	this parameter for non link-local sessions.
+	(e.g., <cf/neighbor fe80::1234%eth0 as 65000;/). The option may also be
+	used for non link-local sessions when it is necessary to explicitly
+	specify an interface, but only for direct (not multihop) sessions.
 
 	<tag><label id="bgp-direct">direct</tag>
 	Specify that the neighbor is directly connected. The IP address of the
diff --git a/proto/bgp/bgp.c b/proto/bgp/bgp.c
index 0f1c9446..f706e76e 100644
--- a/proto/bgp/bgp.c
+++ b/proto/bgp/bgp.c
@@ -790,7 +790,7 @@ bgp_find_proto(sock *sk)
       {
 	struct bgp_proto *p = (struct bgp_proto *) pc->proto;
 	if (ipa_equal(p->cf->remote_ip, sk->daddr) &&
-	    (!ipa_is_link_local(sk->daddr) || (p->cf->iface == sk->iface)))
+	    (!p->cf->iface || (p->cf->iface == sk->iface)))
 	  return p;
       }
 
@@ -1324,11 +1324,8 @@ bgp_check_config(struct bgp_config *c)
   if (!c->remote_as)
     cf_error("Remote AS number must be set");
 
-  // if (ipa_is_link_local(c->remote_ip) && !c->iface)
-  //   cf_error("Link-local neighbor address requires specified interface");
-
-  if (!ipa_is_link_local(c->remote_ip) != !c->iface)
-    cf_error("Link-local address and interface scope must be used together");
+  if (ipa_is_link_local(c->remote_ip) && !c->iface)
+    cf_error("Link-local neighbor address requires specified interface");
 
   if (!(c->capabilities && c->enable_as4) && (c->remote_as > 0xFFFF))
     cf_error("Neighbor AS number out of range (AS4 not available)");
@@ -1346,6 +1343,9 @@ bgp_check_config(struct bgp_config *c)
 		      ipa_is_link_local(c->source_addr)))
     cf_error("Multihop BGP cannot be used with link-local addresses");
 
+  if (c->multihop && c->iface)
+    cf_error("Multihop BGP cannot be bound to interface");
+
   if (c->multihop && c->check_link)
     cf_error("Multihop BGP cannot depend on link state");