code: func (a *LocalKeyAgent) CheckHostSignature(addr string, remote net.Addr, key ssh.PublicKey) error {
  	certChecker := utils.CertChecker{
  		CertChecker: ssh.CertChecker{
  			IsHostAuthority: a.checkHostCertificate,
  			HostKeyFallback: a.checkHostKey,
  		},
  	}
  	err := certChecker.CheckHostKey(addr, remote, key)
  	if err != nil {
  		a.log.Debugf("Host validation failed: %v.", err)
  		return trace.Wrap(err)
  	}
  	a.log.Debugf("Validated host %v.", addr)
  	return nil
  }
docs: // CheckHostSignature checks if the given host key was signed by a Teleport
  // certificate authority (CA) or a host certificate the user has seen before.