BGP: Allow exchanging LOCAL_PREF with eBGP peers

Adds option 'allow bgp_local_pref' to override the usual restriction of
LOCAL_PREF on eBGP sessions.

Thanks to Lennert Buytenhek for the patch.

doc/bird.sgml

@@ -2043,6 +2043,14 @@ using the following configuration parameters:
 	TX direction. When active, all available routes accepted by the export
 	filter are advertised to the neighbor. Default: off.
 
+	<tag><label id="bgp-allow-local-pref">allow bgp_local_pref <m/switch/</tag>
+	A standard BGP implementation do not send the Local Preference attribute
+	to eBGP neighbors and ignore this attribute if received from eBGP
+	neighbors, as per <rfc id="4271">.  When this option is enabled on an
+	eBGP session, this attribute will be sent to and accepted from the peer,
+	which is useful for example if you have a setup like in <rfc id="7938">.
+	The option does not affect iBGP sessions. Default: off.
+
 	<tag><label id="bgp-allow-local-as">allow local as [<m/number/]</tag>
 	BGP prevents routing loops by rejecting received routes with the local
 	AS number in the AS path. This option allows to loose or disable the

proto/bgp/attrs.c

@@ -306,7 +306,7 @@ static struct attr_desc bgp_attr_table[] = {
     bgp_check_next_hop, bgp_format_next_hop },
   { "med", 4, BAF_OPTIONAL, EAF_TYPE_INT, 1,					/* BA_MULTI_EXIT_DISC */
     NULL, NULL },
-  { "local_pref", 4, BAF_TRANSITIVE, EAF_TYPE_INT, 0,				/* BA_LOCAL_PREF */
+  { "local_pref", 4, BAF_TRANSITIVE, EAF_TYPE_INT, 1,				/* BA_LOCAL_PREF */
     NULL, NULL },
   { "atomic_aggr", 0, BAF_TRANSITIVE, EAF_TYPE_OPAQUE, 1,			/* BA_ATOMIC_AGGR */
     NULL, NULL },
@@ -821,8 +821,13 @@ bgp_get_bucket(struct bgp_proto *p, net *n, ea_list *attrs, int originate)
       code = EA_ID(a->id);
       if (ATTR_KNOWN(code))
 	{
-	  if (!bgp_attr_table[code].allow_in_ebgp && !p->is_internal)
+	  if (!p->is_internal)
-	    continue;
+	    {
+	      if (!bgp_attr_table[code].allow_in_ebgp)
+		continue;
+	      if ((code == BA_LOCAL_PREF) && !p->cf->allow_local_pref)
+		continue;
+	    }
 	  /* The flags might have been zero if the attr was added by filters */
 	  a->flags = (a->flags & BAF_PARTIAL) | bgp_attr_table[code].expected_flags;
 	  if (code < 32)
@@ -1776,8 +1781,13 @@ bgp_decode_attrs(struct bgp_conn *conn, byte *attr, uint len, struct linpool *po
 	    { errcode = 5; goto err; }
 	  if ((desc->expected_flags ^ flags) & (BAF_OPTIONAL | BAF_TRANSITIVE))
 	    { errcode = 4; goto err; }
-	  if (!desc->allow_in_ebgp && !bgp->is_internal)
+	  if (!bgp->is_internal)
-	    continue;
+	    {
+	      if (!desc->allow_in_ebgp)
+		continue;
+	      if ((code == BA_LOCAL_PREF) && !bgp->cf->allow_local_pref)
+		continue;
+	    }
 	  if (desc->validate)
 	    {
 	      errcode = desc->validate(bgp, z, l);

proto/bgp/bgp.h

@@ -50,6 +50,7 @@ struct bgp_config {
   int secondary;			/* Accept also non-best routes (i.e. RA_ACCEPTED) */
   int add_path;				/* Use ADD-PATH extension [RFC7911] */
   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 gr_mode;				/* Graceful restart mode (BGP_GR_*) */
   int setkey;				/* Set MD5 password to system SA/SP database */
   unsigned gr_time;			/* Graceful restart timeout */

proto/bgp/config.Y

@@ -126,6 +126,7 @@ bgp_proto:
  | bgp_proto ADD PATHS RX ';' { BGP_CFG->add_path = ADD_PATH_RX; }
  | bgp_proto ADD PATHS TX ';' { BGP_CFG->add_path = ADD_PATH_TX; }
  | bgp_proto ADD PATHS bool ';' { BGP_CFG->add_path = $4 ? ADD_PATH_FULL : 0; }
+ | bgp_proto ALLOW BGP_LOCAL_PREF bool ';' { BGP_CFG->allow_local_pref = $4; }
  | bgp_proto ALLOW LOCAL AS ';' { BGP_CFG->allow_local_as = -1; }
  | bgp_proto ALLOW LOCAL AS expr ';' { BGP_CFG->allow_local_as = $5; }
  | bgp_proto GRACEFUL RESTART bool ';' { BGP_CFG->gr_mode = $4; }