Add FQDN capability

It's not kinda RFC (draft), but it's implemented and used in various
other open-source software like FRRouting, Bird, ExaBGP.

It's very handy when dealing with lots of peers.

Exampe between GoBGP and FRRouting:
```
% ./cmd/gobgp/gobgp neighbor 192.168.10.123 | grep -A4 fqdn:
    fqdn:       advertised and received
      Local:
         name: donatas-pc, domain:
      Remote:
         name: exit1-debian-9, domain:
```

```
% vtysh -c 'show bgp neighbors 192.168.10.17 json' | jq .'"192.168.10.17".neighborCapabilities.hostName'
{
  "advHostName": "exit1-debian-9",
  "advDomainName": "n/a",
  "rcvHostName": "donatas-pc",
  "rcvDomainName": "n/a"
}
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

7 files changed, 185 insertions, 36 deletions

diff --git a/api/attribute.pb.go b/api/attribute.pb.go
index 3113bce6..2adb2f88 100644
--- a/api/attribute.pb.go
+++ b/api/attribute.pb.go
@@ -136,6 +136,7 @@ It has these top-level messages:
 	LongLivedGracefulRestartCapabilityTuple
 	LongLivedGracefulRestartCapability
 	RouteRefreshCiscoCapability
+	FQDNCapability
 	UnknownCapability
 	StartBgpRequest
 	StopBgpRequest
diff --git a/api/capability.pb.go b/api/capability.pb.go
index 957b30b4..7fe4a7cc 100644
--- a/api/capability.pb.go
+++ b/api/capability.pb.go
@@ -298,6 +298,46 @@ func (m *RouteRefreshCiscoCapability) String() string            { return proto.
 func (*RouteRefreshCiscoCapability) ProtoMessage()               {}
 func (*RouteRefreshCiscoCapability) Descriptor() ([]byte, []int) { return fileDescriptor1, []int{13} }
 
+type FQDNCapability struct {
+	HostNameLen   uint32 `protobuf:"varint,1,opt,name=host_name_len,json=hostNameLen" json:"host_name_len,omitempty"`
+	HostName      string `protobuf:"bytes,2,opt,name=host_name,json=hostName" json:"host_name,omitempty"`
+	DomainNameLen uint32 `protobuf:"varint,3,opt,name=domain_name_len,json=domainNameLen" json:"domain_name_len,omitempty"`
+	DomainName    string `protobuf:"bytes,4,opt,name=domain_name,json=domainName" json:"domain_name,omitempty"`
+}
+
+func (m *FQDNCapability) Reset()                    { *m = FQDNCapability{} }
+func (m *FQDNCapability) String() string            { return proto.CompactTextString(m) }
+func (*FQDNCapability) ProtoMessage()               {}
+func (*FQDNCapability) Descriptor() ([]byte, []int) { return fileDescriptor1, []int{14} }
+
+func (m *FQDNCapability) GetHostNameLen() uint32 {
+	if m != nil {
+		return m.HostNameLen
+	}
+	return 0
+}
+
+func (m *FQDNCapability) GetHostName() string {
+	if m != nil {
+		return m.HostName
+	}
+	return ""
+}
+
+func (m *FQDNCapability) GetDomainNameLen() uint32 {
+	if m != nil {
+		return m.DomainNameLen
+	}
+	return 0
+}
+
+func (m *FQDNCapability) GetDomainName() string {
+	if m != nil {
+		return m.DomainName
+	}
+	return ""
+}
+
 type UnknownCapability struct {
 	Code  uint32 `protobuf:"varint,1,opt,name=code" json:"code,omitempty"`
 	Value []byte `protobuf:"bytes,2,opt,name=value,proto3" json:"value,omitempty"`
@@ -306,7 +346,7 @@ type UnknownCapability struct {
 func (m *UnknownCapability) Reset()                    { *m = UnknownCapability{} }
 func (m *UnknownCapability) String() string            { return proto.CompactTextString(m) }
 func (*UnknownCapability) ProtoMessage()               {}
-func (*UnknownCapability) Descriptor() ([]byte, []int) { return fileDescriptor1, []int{14} }
+func (*UnknownCapability) Descriptor() ([]byte, []int) { return fileDescriptor1, []int{15} }
 
 func (m *UnknownCapability) GetCode() uint32 {
 	if m != nil {
@@ -337,6 +377,7 @@ func init() {
 	proto.RegisterType((*LongLivedGracefulRestartCapabilityTuple)(nil), "gobgpapi.LongLivedGracefulRestartCapabilityTuple")
 	proto.RegisterType((*LongLivedGracefulRestartCapability)(nil), "gobgpapi.LongLivedGracefulRestartCapability")
 	proto.RegisterType((*RouteRefreshCiscoCapability)(nil), "gobgpapi.RouteRefreshCiscoCapability")
+	proto.RegisterType((*FQDNCapability)(nil), "gobgpapi.FQDNCapability")
 	proto.RegisterType((*UnknownCapability)(nil), "gobgpapi.UnknownCapability")
 	proto.RegisterEnum("gobgpapi.AddPathMode", AddPathMode_name, AddPathMode_value)
 }
@@ -344,38 +385,42 @@ func init() {
 func init() { proto.RegisterFile("capability.proto", fileDescriptor1) }
 
 var fileDescriptor1 = []byte{
-	// 520 bytes of a gzipped FileDescriptorProto
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-	0x3b, 0xec, 0xd3, 0x4d, 0x34, 0x05, 0x00, 0x00,
+	// 590 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xb4, 0x54, 0xc1, 0x6e, 0xd3, 0x40,
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+	0x9f, 0x00, 0x00, 0x00, 0xff, 0xff, 0x07, 0x77, 0x70, 0x63, 0xd1, 0x05, 0x00, 0x00,
 }
diff --git a/api/capability.proto b/api/capability.proto
index 78504b5d..5bab741e 100644
--- a/api/capability.proto
+++ b/api/capability.proto
@@ -94,7 +94,14 @@ message LongLivedGracefulRestartCapability {
 message RouteRefreshCiscoCapability {
 }
 
+message FQDNCapability {
+    uint32 host_name_len = 1;
+    string host_name = 2;
+    uint32 domain_name_len = 3;
+    string domain_name = 4;
+}
+
 message UnknownCapability {
     uint32 code = 1;
     bytes value = 2;
-}
\ No newline at end of file
+}
diff --git a/cmd/gobgp/neighbor.go b/cmd/gobgp/neighbor.go
index 8c80338b..8964cf9e 100644
--- a/cmd/gobgp/neighbor.go
+++ b/cmd/gobgp/neighbor.go
@@ -452,6 +452,16 @@ func showNeighbor(args []string) error {
 					fmt.Printf("         %s:\t%s\n", item.RouteFamily, item.Mode)
 				}
 			}
+		case bgp.BGP_CAP_FQDN:
+			fmt.Printf("    %s:\t%s\n", c.Code(), support)
+			if m := lookup(c, lcaps); m != nil {
+				fmt.Println("      Local:")
+				fmt.Printf("         name: %s, domain: %s\n", m.(*bgp.CapFQDN).HostName, m.(*bgp.CapFQDN).DomainName)
+			}
+			if m := lookup(c, rcaps); m != nil {
+				fmt.Println("      Remote:")
+				fmt.Printf("         name: %s, domain: %s\n", m.(*bgp.CapFQDN).HostName, m.(*bgp.CapFQDN).DomainName)
+			}
 		default:
 			fmt.Printf("    %s:\t%s\n", c.Code(), support)
 		}
diff --git a/internal/pkg/apiutil/capability.go b/internal/pkg/apiutil/capability.go
index d135713a..125427db 100644
--- a/internal/pkg/apiutil/capability.go
+++ b/internal/pkg/apiutil/capability.go
@@ -110,6 +110,15 @@ func NewRouteRefreshCiscoCapability(a *bgp.CapRouteRefreshCisco) *api.RouteRefre
 	return &api.RouteRefreshCiscoCapability{}
 }
 
+func NewFQDNCapability(a *bgp.CapFQDN) *api.FQDNCapability {
+	return &api.FQDNCapability{
+		HostNameLen:   uint32(a.HostNameLen),
+		HostName:      a.HostName,
+		DomainNameLen: uint32(a.DomainNameLen),
+		DomainName:    a.DomainName,
+	}
+}
+
 func NewUnknownCapability(a *bgp.CapUnknown) *api.UnknownCapability {
 	return &api.UnknownCapability{
 		Code:  uint32(a.CapCode),
@@ -140,6 +149,8 @@ func MarshalCapability(value bgp.ParameterCapabilityInterface) (*any.Any, error)
 		m = NewLongLivedGracefulRestartCapability(n)
 	case *bgp.CapRouteRefreshCisco:
 		m = NewRouteRefreshCiscoCapability(n)
+	case *bgp.CapFQDN:
+		m = NewFQDNCapability(n)
 	case *bgp.CapUnknown:
 		m = NewUnknownCapability(n)
 	default:
@@ -227,6 +238,8 @@ func unmarshalCapability(a *any.Any) (bgp.ParameterCapabilityInterface, error) {
 		return bgp.NewCapLongLivedGracefulRestart(tuples), nil
 	case *api.RouteRefreshCiscoCapability:
 		return bgp.NewCapRouteRefreshCisco(), nil
+	case *api.FQDNCapability:
+		return bgp.NewCapFQDN(a.HostName, a.DomainName), nil
 	case *api.UnknownCapability:
 		return bgp.NewCapUnknown(bgp.BGPCapabilityCode(a.Code), a.Value), nil
 	}
diff --git a/pkg/packet/bgp/bgp.go b/pkg/packet/bgp/bgp.go
index 15e15362..5403a5f3 100644
--- a/pkg/packet/bgp/bgp.go
+++ b/pkg/packet/bgp/bgp.go
@@ -295,6 +295,7 @@ const (
 	BGP_CAP_ADD_PATH                    BGPCapabilityCode = 69
 	BGP_CAP_ENHANCED_ROUTE_REFRESH      BGPCapabilityCode = 70
 	BGP_CAP_LONG_LIVED_GRACEFUL_RESTART BGPCapabilityCode = 71
+	BGP_CAP_FQDN                        BGPCapabilityCode = 73
 	BGP_CAP_ROUTE_REFRESH_CISCO         BGPCapabilityCode = 128
 )
 
@@ -309,6 +310,7 @@ var CapNameMap = map[BGPCapabilityCode]string{
 	BGP_CAP_ENHANCED_ROUTE_REFRESH:      "enhanced-route-refresh",
 	BGP_CAP_ROUTE_REFRESH_CISCO:         "cisco-route-refresh",
 	BGP_CAP_LONG_LIVED_GRACEFUL_RESTART: "long-lived-graceful-restart",
+	BGP_CAP_FQDN:                        "fqdn",
 }
 
 func (c BGPCapabilityCode) String() string {
@@ -916,6 +918,71 @@ func NewCapLongLivedGracefulRestart(tuples []*CapLongLivedGracefulRestartTuple)
 	}
 }
 
+type CapFQDN struct {
+	DefaultParameterCapability
+	HostNameLen   uint8
+	HostName      string
+	DomainNameLen uint8
+	DomainName    string
+}
+
+func (c *CapFQDN) DecodeFromBytes(data []byte) error {
+	c.DefaultParameterCapability.DecodeFromBytes(data)
+	data = data[2:]
+	if len(data) < 2 {
+		return NewMessageError(BGP_ERROR_OPEN_MESSAGE_ERROR, BGP_ERROR_SUB_UNSUPPORTED_CAPABILITY, nil, "Not all CapabilityFQDN bytes allowed")
+	}
+	hostNameLen := uint8(data[0])
+	c.HostNameLen = hostNameLen
+	c.HostName = string(data[1 : c.HostNameLen+1])
+	domainNameLen := uint8(data[c.HostNameLen+1])
+	c.DomainNameLen = domainNameLen
+	c.DomainName = string(data[c.HostNameLen+2:])
+	return nil
+}
+
+func (c *CapFQDN) Serialize() ([]byte, error) {
+	buf := make([]byte, c.HostNameLen+c.DomainNameLen+2)
+	buf[0] = c.HostNameLen
+	copy(buf[1:c.HostNameLen+1], c.HostName)
+	buf[c.HostNameLen+1] = c.DomainNameLen
+	copy(buf[c.HostNameLen+2:], c.DomainName)
+	c.DefaultParameterCapability.CapValue = buf
+	return c.DefaultParameterCapability.Serialize()
+}
+
+func (c *CapFQDN) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		HostNameLen   uint8  `json:"hostname_len"`
+		HostName      string `json:"hostname"`
+		DomainNameLen uint8  `json:"domainname_len"`
+		DomainName    string `json:"domainname"`
+	}{
+		HostNameLen:   c.HostNameLen,
+		HostName:      c.HostName,
+		DomainNameLen: c.DomainNameLen,
+		DomainName:    c.DomainName,
+	})
+}
+
+func NewCapFQDN(hostname string, domainname string) *CapFQDN {
+	if len(hostname) > 64 {
+		hostname = hostname[:64]
+	}
+	if len(domainname) > 64 {
+		domainname = domainname[:64]
+	}
+	return &CapFQDN{
+		DefaultParameterCapability{
+			CapCode: BGP_CAP_FQDN,
+		},
+		uint8(len(hostname)),
+		hostname,
+		uint8(len(domainname)),
+		domainname,
+	}
+}
+
 type CapUnknown struct {
 	DefaultParameterCapability
 }
@@ -955,6 +1022,8 @@ func DecodeCapability(data []byte) (ParameterCapabilityInterface, error) {
 		c = &CapRouteRefreshCisco{}
 	case BGP_CAP_LONG_LIVED_GRACEFUL_RESTART:
 		c = &CapLongLivedGracefulRestart{}
+	case BGP_CAP_FQDN:
+		c = &CapFQDN{}
 	default:
 		c = &CapUnknown{}
 	}
diff --git a/pkg/server/fsm.go b/pkg/server/fsm.go
index b024cf3e..abd94ed9 100644
--- a/pkg/server/fsm.go
+++ b/pkg/server/fsm.go
@@ -21,6 +21,7 @@ import (
 	"io"
 	"math/rand"
 	"net"
+	"os"
 	"strconv"
 	"sync"
 	"syscall"
@@ -714,8 +715,11 @@ func capAddPathFromConfig(pConf *config.Neighbor) bgp.ParameterCapabilityInterfa
 }
 
 func capabilitiesFromConfig(pConf *config.Neighbor) []bgp.ParameterCapabilityInterface {
+	fqdn, _ := os.Hostname()
 	caps := make([]bgp.ParameterCapabilityInterface, 0, 4)
 	caps = append(caps, bgp.NewCapRouteRefresh())
+	caps = append(caps, bgp.NewCapFQDN(fqdn, ""))
+
 	for _, af := range pConf.AfiSafis {
 		caps = append(caps, bgp.NewCapMultiProtocol(af.State.Family))
 	}