Add disconnect callback

server.go

@@ -45,6 +45,7 @@ type Server struct {
 	PublicKeyHandler              PublicKeyHandler              // public key authentication handler
 	PtyCallback                   PtyCallback                   // callback for allowing PTY sessions, allows all if nil
 	ConnCallback                  ConnCallback                  // optional callback for wrapping net.Conn before handling
+	DisconnectCallback            DisconnectCallback            // optional callback executed when net.Conn disconnects
 	LocalPortForwardingCallback   LocalPortForwardingCallback   // callback for allowing local port forwarding, denies all if nil
 	ReversePortForwardingCallback ReversePortForwardingCallback // callback for allowing reverse port forwarding, denies all if nil
 	ServerConfigCallback          ServerConfigCallback          // callback for configuring detailed SSH options
@@ -307,6 +308,11 @@ func (srv *Server) HandleConn(newConn net.Conn) {
 	conn.updateDeadline()
 	srv.trackConn(sshConn, true)
 	defer srv.trackConn(sshConn, false)
+	defer func() {
+		if srv.DisconnectCallback != nil {
+			srv.DisconnectCallback(ctx, conn)
+		}
+	}()
 
 	ctx.SetValue(ContextKeyConn, sshConn)
 	applyConnMetadata(ctx, sshConn)

ssh.go

@@ -71,6 +71,11 @@ type ServerConfigCallback func(ctx Context) *gossh.ServerConfig
 // Please note: the net.Conn is likely to be closed at this point
 type ConnectionFailedCallback func(conn net.Conn, err error)
 
+// DisconnectCallback is a hook for reporting closed connections
+// (that is, connection that did not fire ConnectionFailedCallback)
+// Please note: the net.Conn is likely closed or unusable at this point
+type DisconnectCallback func(ctx Context, conn net.Conn)
+
 // Window represents the size of a PTY window.
 type Window struct {
 	Width  int