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>

1 files changed, 6 insertions, 6 deletions

diff --git a/packet/bgp/validate.go b/packet/bgp/validate.go
index 5cf2afbb..13c66a77 100644
--- a/packet/bgp/validate.go
+++ b/packet/bgp/validate.go
@@ -230,9 +230,9 @@ func ValidateBGPMessage(m *BGPMessage) error {
 	return nil
 }
 
-func ValidateOpenMsg(m *BGPOpen, expectedAS uint32) error {
+func ValidateOpenMsg(m *BGPOpen, expectedAS uint32) (uint32, error) {
 	if m.Version != 4 {
-		return NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_UNSUPPORTED_VERSION_NUMBER, nil, fmt.Sprintf("upsuppored version %d", m.Version))
+		return 0, NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_UNSUPPORTED_VERSION_NUMBER, nil, fmt.Sprintf("upsuppored version %d", m.Version))
 	}
 
 	as := uint32(m.MyAS)
@@ -248,12 +248,12 @@ func ValidateOpenMsg(m *BGPOpen, expectedAS uint32) error {
 			}
 		}
 	}
-	if as != expectedAS {
-		return NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_BAD_PEER_AS, nil, fmt.Sprintf("as number mismatch expected %d, received %d", expectedAS, as))
+	if expectedAS != 0 && as != expectedAS {
+		return 0, NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_BAD_PEER_AS, nil, fmt.Sprintf("as number mismatch expected %d, received %d", expectedAS, as))
 	}
 
 	if m.HoldTime < 3 && m.HoldTime != 0 {
-		return NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_UNACCEPTABLE_HOLD_TIME, nil, fmt.Sprintf("unacceptable hold time %d", m.HoldTime))
+		return 0, NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_UNACCEPTABLE_HOLD_TIME, nil, fmt.Sprintf("unacceptable hold time %d", m.HoldTime))
 	}
-	return nil
+	return as, nil
 }