kubernetes/kops · error

error from callback challenge: %w

Error message

error from callback challenge: %w

What it means

The gRPC call to the callback server's Challenge RPC returned an error; it is wrapped with this message. This is a server-side or RPC-level failure (unauthenticated TLS client, missing required subject, server rejecting the request, deadline exceeded), not a local validation problem.

Source

Thrown at pkg/bootstrap/challenge_client.go:122

	req := &pb.ChallengeRequest{
		ChallengeId:     challenge.ChallengeID,
		ChallengeRandom: kospControllerNonce,
	}

	expectedChallengeResponse := buildChallengeResponse(challenge.ChallengeSecret, kospControllerNonce)

	var opts []grpc.DialOption
	opts = append(opts, grpc.WithTransportCredentials(credentials.NewTLS(tlsConfig)))
	conn, err := grpc.DialContext(ctx, targetEndpoint, opts...)
	if err != nil {
		return fmt.Errorf("error dialing target %q: %w", targetEndpoint, err)
	}
	defer conn.Close()
	client := pb.NewCallbackServiceClient(conn)

	response, err := client.Challenge(ctx, req)
	if err != nil {
		return fmt.Errorf("error from callback challenge: %w", err)
	}

	if subtle.ConstantTimeCompare(response.GetChallengeResponse(), expectedChallengeResponse) != 1 {
		return fmt.Errorf("callback challenge returned wrong result")
	}
	return nil
}

View on GitHub (pinned to 4c8573c808)

Solutions

1. Check kops-controller logs for the corresponding Challenge RPC error
2. Verify the client certificate subject matches challengeKopsControllerSubject for the cluster
3. Retry with a longer context deadline if it was a timeout
4. Re-issue the client certificate from the cluster CA if the subject/CA is wrong

Example fix

null

// ensure client cert subject matches expected controller subject
if clientCert.Subject.CommonName != expectedSubject {
	return fmt.Errorf("client cert subject %q != required %q", clientCert.Subject.CommonName, expectedSubject)
}

if err := client.DoCallbackChallenge(ctx, clusterName, ch); err != nil {
	var st status.Status
	if errors.As(err, &st) && st.Code() == codes.DeadlineExceeded {
		// retry with longer deadline
	}
	return fmt.Errorf("challenge RPC failed: %w", err)
}

When it happens

Trigger: Calling DoCallbackChallenge when the server rejects the Challenge RPC: mTLS client cert not accepted, node not registered, context deadline exceeded, or the server returns a gRPC status error.

Common situations: Client certificate subject not matching the required kops-controller subject; server-side authorization denying the node; network interruption mid-RPC; server logs containing the matching underlying error.

Related errors

• error reading client keypair: %v
• no TLS connection
• no client certificate presented
• failed to verify client certificate chain: %w
• client certificate chain does not match universe ID

AI-assisted analysis of kubernetes/kops@4c8573c808 (2026-09-05). Data as JSON: /api/errors/13a39cef5b221b08. Report an issue: GitHub.