From 6e37ee384fecdec5af1df06fd94c5c350f2b4747 Mon Sep 17 00:00:00 2001
From: Radu Carpa <radu.carpa@cern.ch>
Date: Tue, 17 Jan 2023 18:13:37 +0100
Subject: [PATCH] Filter: Allow setting the 'onlink' route attribute in filters

Add static route attribute to set onlink flag for route next hop. Can be
used to build a dynamically routed IP-in-IP overlay network. Usage:

     ifname = "tunl0";
     onlink = true;
     gw = bgp_next_hop;
---
 doc/bird.sgml   |  8 ++++++++
 filter/config.Y |  3 ++-
 filter/data.h   |  1 +
 filter/f-inst.c | 14 ++++++++++++--
 4 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 50657ebf..001fcbd4 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -1816,6 +1816,14 @@ Common route attributes are:
 	creation/removal. Zero is returned for routes with undefined outgoing
 	interfaces. Read-only.
 
+	<tag><label id="rta-onlink"><m/bool/ onlink</tag>
+	Onlink flag means that the specified nexthop is accessible on the
+	interface regardless of IP prefixes configured on the interface.
+	The attribute can be used to configure such next hops by first setting
+	<cf/onlink = true/ and <cf/ifname/, and then setting <cf/gw/. Possible
+	use case for setting this flag is to automatically build overlay IP-IP
+	networks on linux.
+
 	<tag><label id="rta-weight"><m/int/ weight</tag>
 	Multipath weight of route next hops. Valid values are 1-256. Reading
 	returns the weight of the first next hop, setting it sets weights of all
diff --git a/filter/config.Y b/filter/config.Y
index 1d9d9aa9..a1e5e9f1 100644
--- a/filter/config.Y
+++ b/filter/config.Y
@@ -305,7 +305,7 @@ CF_KEYWORDS(FUNCTION, PRINT, PRINTN, UNSET, RETURN,
 	IF, THEN, ELSE, CASE,
 	FOR, IN, DO,
 	TRUE, FALSE, RT, RO, UNKNOWN, GENERIC,
-	FROM, GW, NET, MASK, PROTO, SOURCE, SCOPE, DEST, IFNAME, IFINDEX, WEIGHT, GW_MPLS,
+	FROM, GW, NET, MASK, PROTO, SOURCE, SCOPE, DEST, IFNAME, IFINDEX, WEIGHT, GW_MPLS, ONLINK,
 	PREFERENCE,
 	ROA_CHECK, ASN, SRC, DST,
 	IS_V4, IS_V6,
@@ -795,6 +795,7 @@ static_attr:
  | WEIGHT  { $$ = f_new_static_attr(T_INT,        SA_WEIGHT,	0); }
  | PREFERENCE { $$ = f_new_static_attr(T_INT,	  SA_PREF,	0); }
  | GW_MPLS { $$ = f_new_static_attr(T_INT,        SA_GW_MPLS,	0); }
+ | ONLINK  { $$ = f_new_static_attr(T_BOOL,       SA_ONLINK,	0); }
  ;
 
 term:
diff --git a/filter/data.h b/filter/data.h
index 700609e9..b3767f7b 100644
--- a/filter/data.h
+++ b/filter/data.h
@@ -102,6 +102,7 @@ enum f_sa_code {
   SA_WEIGHT,
   SA_PREF,
   SA_GW_MPLS,
+  SA_ONLINK,
 } PACKED;
 
 /* Static attribute definition (members of struct rta) */
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 2d2a30e4..e4b47ff4 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -694,6 +694,7 @@
       case SA_WEIGHT:	RESULT(sa.f_type, i, rta->nh.weight + 1); break;
       case SA_PREF:	RESULT(sa.f_type, i, rta->pref); break;
       case SA_GW_MPLS:	RESULT(sa.f_type, i, rta->nh.labels ? rta->nh.label[0] : MPLS_NULL); break;
+      case SA_ONLINK:	RESULT(sa.f_type, i, rta->nh.flags & RNF_ONLINK ? 1 : 0); break;
 
       default:
 	bug("Invalid static attribute access (%u/%u)", sa.f_type, sa.sa_code);
@@ -720,8 +721,8 @@
       case SA_GW:
 	{
 	  ip_addr ip = v1.val.ip;
-	  struct iface *ifa = ipa_is_link_local(ip) ? rta->nh.iface : NULL;
-	  neighbor *n = neigh_find((*fs->rte)->src->proto, ip, ifa, 0);
+	  struct iface *ifa = ipa_is_link_local(ip) || (rta->nh.flags & RNF_ONLINK) ? rta->nh.iface : NULL;
+	  neighbor *n = neigh_find((*fs->rte)->src->proto, ip, ifa, (rta->nh.flags & RNF_ONLINK) ? NEF_ONLINK : 0);
 	  if (!n || (n->scope == SCOPE_HOST))
 	    runtime( "Invalid gw address" );
 
@@ -801,6 +802,15 @@
 	rta->pref = v1.val.i;
 	break;
 
+      case SA_ONLINK:
+	{
+	  if (v1.val.i)
+	    rta->nh.flags |= RNF_ONLINK;
+	  else
+	    rta->nh.flags &= ~RNF_ONLINK;
+	}
+	break;
+
       default:
 	bug("Invalid static attribute access (%u/%u)", sa.f_type, sa.sa_code);
       }