skip asn negotiation for BGP unnumbered peers

since BGP unnumbered peers are typically trusted, skip asn negotiation
for easy configuration.

from configuration file:

```toml
[[neighbors]]
[[neighbors.config]]
neighbor-interface = "eth0"
```

from cli:

```
$ gobgp neighbor add interface eth0
```

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>

4 files changed, 57 insertions, 31 deletions

diff --git a/server/fsm.go b/server/fsm.go
index e680f8ec..f623f399 100644
--- a/server/fsm.go
+++ b/server/fsm.go
@@ -474,9 +474,9 @@ func (h *FSMHandler) active() (bgp.FSMState, FsmStateReason) {
 				break
 			}
 			fsm.conn = conn
-			if fsm.pConf.Config.PeerType == config.PEER_TYPE_EXTERNAL {
+			if fsm.pConf.Config.PeerAs != 0 && fsm.pConf.Config.PeerType == config.PEER_TYPE_EXTERNAL {
 				ttl := 1
-				if fsm.pConf.EbgpMultihop.Config.Enabled == true {
+				if fsm.pConf.EbgpMultihop.Config.Enabled {
 					ttl = int(fsm.pConf.EbgpMultihop.Config.MultihopTtl)
 				}
 				if ttl != 0 {
@@ -844,11 +844,29 @@ func (h *FSMHandler) opensent() (bgp.FSMState, FsmStateReason) {
 				if m.Header.Type == bgp.BGP_MSG_OPEN {
 					fsm.recvOpen = m
 					body := m.Body.(*bgp.BGPOpen)
-					err := bgp.ValidateOpenMsg(body, fsm.pConf.Config.PeerAs)
+					peerAs, err := bgp.ValidateOpenMsg(body, fsm.pConf.Config.PeerAs)
 					if err != nil {
 						fsm.sendNotificationFromErrorMsg(err.(*bgp.MessageError))
 						return bgp.BGP_FSM_IDLE, FSM_INVALID_MSG
 					}
+
+					// ASN negotiation was skipped
+					if fsm.pConf.Config.PeerAs == 0 {
+						typ := config.PEER_TYPE_EXTERNAL
+						if fsm.peerInfo.LocalAS == peerAs {
+							typ = config.PEER_TYPE_INTERNAL
+						}
+						fsm.pConf.State.PeerType = typ
+						log.WithFields(log.Fields{
+							"Topic": "Peer",
+							"Key":   fsm.pConf.Config.NeighborAddress,
+							"State": fsm.state.String(),
+						}).Infof("skiped asn negotiation: peer-as: %d, peer-type: %s", peerAs, typ)
+					} else {
+						fsm.pConf.State.PeerType = fsm.pConf.Config.PeerType
+					}
+					fsm.pConf.State.PeerAs = peerAs
+					fsm.peerInfo.AS = peerAs
 					fsm.peerInfo.ID = body.ID
 					fsm.capMap, fsm.rfMap = open2Cap(body, fsm.pConf)
 
diff --git a/server/peer.go b/server/peer.go
index 96e014db..15029bd6 100644
--- a/server/peer.go
+++ b/server/peer.go
@@ -69,7 +69,7 @@ func (peer *Peer) TableID() string {
 }
 
 func (peer *Peer) isIBGPPeer() bool {
-	return peer.fsm.pConf.Config.PeerAs == peer.fsm.gConf.Config.As
+	return peer.fsm.pConf.State.PeerAs == peer.fsm.gConf.Config.As
 }
 
 func (peer *Peer) isRouteServerClient() bool {
diff --git a/server/server.go b/server/server.go
index 8260e566..5238c33b 100644
--- a/server/server.go
+++ b/server/server.go
@@ -288,7 +288,7 @@ func sendFsmOutgoingMsg(peer *Peer, paths []*table.Path, notification *bgp.BGPMe
 
 func isASLoop(peer *Peer, path *table.Path) bool {
 	for _, as := range path.GetAsList() {
-		if as == peer.fsm.pConf.Config.PeerAs {
+		if as == peer.fsm.pConf.State.PeerAs {
 			return true
 		}
 	}
@@ -329,7 +329,7 @@ func filterpath(peer *Peer, path, old *table.Path) *table.Path {
 			ignore = true
 			info := path.GetSource()
 			//if the path comes from eBGP peer
-			if info.AS != peer.fsm.pConf.Config.PeerAs {
+			if info.AS != peer.fsm.pConf.State.PeerAs {
 				ignore = false
 			}
 			// RFC4456 8. Avoiding Routing Information Loops
@@ -509,18 +509,19 @@ func createWatchEventPeerState(peer *Peer) *WatchEventPeerState {
 	sentOpen := buildopen(peer.fsm.gConf, peer.fsm.pConf)
 	recvOpen := peer.fsm.recvOpen
 	return &WatchEventPeerState{
-		PeerAS:       peer.fsm.peerInfo.AS,
-		LocalAS:      peer.fsm.peerInfo.LocalAS,
-		PeerAddress:  peer.fsm.peerInfo.Address,
-		LocalAddress: net.ParseIP(laddr),
-		PeerPort:     rport,
-		LocalPort:    lport,
-		PeerID:       peer.fsm.peerInfo.ID,
-		SentOpen:     sentOpen,
-		RecvOpen:     recvOpen,
-		State:        peer.fsm.state,
-		AdminState:   peer.fsm.adminState,
-		Timestamp:    time.Now(),
+		PeerAS:        peer.fsm.peerInfo.AS,
+		LocalAS:       peer.fsm.peerInfo.LocalAS,
+		PeerAddress:   peer.fsm.peerInfo.Address,
+		LocalAddress:  net.ParseIP(laddr),
+		PeerPort:      rport,
+		LocalPort:     lport,
+		PeerID:        peer.fsm.peerInfo.ID,
+		SentOpen:      sentOpen,
+		RecvOpen:      recvOpen,
+		State:         peer.fsm.state,
+		AdminState:    peer.fsm.adminState,
+		Timestamp:     time.Now(),
+		PeerInterface: peer.fsm.pConf.Config.NeighborInterface,
 	}
 }
 
@@ -687,6 +688,10 @@ func (server *BgpServer) handleFSMMessage(peer *Peer, e *FsmMsg) {
 			peer.prefixLimitWarned = make(map[bgp.RouteFamily]bool)
 			peer.DropAll(drop)
 			server.dropPeerAllRoutes(peer, drop)
+			if peer.fsm.pConf.Config.PeerAs == 0 {
+				peer.fsm.pConf.State.PeerAs = 0
+				peer.fsm.peerInfo.AS = 0
+			}
 		} else if peer.fsm.pConf.GracefulRestart.State.PeerRestarting && nextState == bgp.BGP_FSM_IDLE {
 			if peer.fsm.pConf.GracefulRestart.State.LongLivedEnabled {
 				llgr, no_llgr := peer.llgrFamilies()
@@ -2114,18 +2119,19 @@ type WatchEventUpdate struct {
 }
 
 type WatchEventPeerState struct {
-	PeerAS       uint32
-	LocalAS      uint32
-	PeerAddress  net.IP
-	LocalAddress net.IP
-	PeerPort     uint16
-	LocalPort    uint16
-	PeerID       net.IP
-	SentOpen     *bgp.BGPMessage
-	RecvOpen     *bgp.BGPMessage
-	State        bgp.FSMState
-	AdminState   AdminState
-	Timestamp    time.Time
+	PeerAS        uint32
+	LocalAS       uint32
+	PeerAddress   net.IP
+	LocalAddress  net.IP
+	PeerPort      uint16
+	LocalPort     uint16
+	PeerID        net.IP
+	SentOpen      *bgp.BGPMessage
+	RecvOpen      *bgp.BGPMessage
+	State         bgp.FSMState
+	AdminState    AdminState
+	Timestamp     time.Time
+	PeerInterface string
 }
 
 type WatchEventAdjIn struct {
diff --git a/server/server_test.go b/server/server_test.go
index 3c343b9e..8f5fcc4d 100644
--- a/server/server_test.go
+++ b/server/server_test.go
@@ -185,9 +185,11 @@ func TestNumGoroutineWithAddDeleteNeighbor(t *testing.T) {
 }
 
 func newPeerandInfo(myAs, as uint32, address string, rib *table.TableManager) (*Peer, *table.PeerInfo) {
+	nConf := &config.Neighbor{Config: config.NeighborConfig{PeerAs: as, NeighborAddress: address}}
+	config.SetDefaultNeighborConfigValues(nConf, myAs)
 	p := NewPeer(
 		&config.Global{Config: config.GlobalConfig{As: myAs}},
-		&config.Neighbor{Config: config.NeighborConfig{PeerAs: as, NeighborAddress: address}},
+		nConf,
 		rib,
 		&table.RoutingPolicy{})
 	for _, f := range rib.GetRFlist() {