cloudflare/cloudflared · error

datagram response message is an invalid size

Error message

datagram response message is an invalid size

What it means

ErrDatagramResponseMsgInvalidSize is returned by UDPSessionRegistrationResponseDatagram.MarshalBinary when the ErrorMsg field exceeds maxResponseErrorMessageLen. The response datagram format reserves a fixed-size field for the error message, so longer messages cannot be encoded. The library refuses to marshal rather than silently truncating the message.

Solutions

1. Truncate ErrorMsg to maxResponseErrorMessageLen bytes before setting it on the datagram
2. Replace long error text with a short reason code and log the full message locally instead
3. Use errors.Is(err, v3.ErrDatagramResponseMsgInvalidSize) after MarshalBinary to catch and shorten the message

Example fix

// before
resp := &v3.UDPSessionRegistrationResponseDatagram{ErrorMsg: veryLongErrorString}
data, err := resp.MarshalBinary() // -> ErrDatagramResponseMsgInvalidSize
// after
msg := veryLongErrorString
if len(msg) > maxResponseErrorMessageLen {
    msg = msg[:maxResponseErrorMessageLen]
}
resp := &v3.UDPSessionRegistrationResponseDatagram{ErrorMsg: msg}
data, err := resp.MarshalBinary()

if len(errMsg) > maxResponseErrorMessageLen {
    errMsg = errMsg[:maxResponseErrorMessageLen]
}

if _, err := resp.MarshalBinary(); errors.Is(err, v3.ErrDatagramResponseMsgInvalidSize) {
    resp.ErrorMsg = "error details too long"
    _, err = resp.MarshalBinary()
}

When it happens

Trigger: Marshaling a UDPSessionRegistrationResponseDatagram whose ErrorMsg string is longer than maxResponseErrorMessageLen bytes.

Common situations: Propagating raw origin/registration errors (long Go error strings, wrapped chain text) into the response ErrorMsg without trimming; embedding stack traces or URLs in ErrorMsg.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

• payload length is too large to be bundled in datagram
• datagram icmp payload is missing
• datagram marshal error
• datagram provided is an invalid size
• datagram response is an invalid size

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/39f1448bdc36dd3d. Report an issue: GitHub.

Appendix: source

Thrown at quic/v3/datagram_errors.go:14

package v3

import (
	"errors"
	"fmt"
)

var (
	ErrInvalidDatagramType                 error = errors.New("invalid datagram type expected")
	ErrDatagramHeaderTooSmall              error = fmt.Errorf("datagram should have at least %d byte", datagramTypeLen)
	ErrDatagramPayloadTooLarge             error = errors.New("payload length is too large to be bundled in datagram")
	ErrDatagramPayloadHeaderTooSmall       error = errors.New("payload length is too small to fit the datagram header")
	ErrDatagramPayloadInvalidSize          error = errors.New("datagram provided is an invalid size")
	ErrDatagramResponseMsgInvalidSize      error = errors.New("datagram response message is an invalid size")
	ErrDatagramResponseInvalidSize         error = errors.New("datagram response is an invalid size")
	ErrDatagramResponseMsgTooLargeMaximum  error = fmt.Errorf("datagram response error message length exceeds the length of the datagram maximum: %d", maxResponseErrorMessageLen)
	ErrDatagramResponseMsgTooLargeDatagram error = fmt.Errorf("datagram response error message length exceeds the length of the provided datagram")
	ErrDatagramICMPPayloadTooLarge         error = fmt.Errorf("datagram icmp payload exceeds %d bytes", maxICMPPayloadLen)
	ErrDatagramICMPPayloadMissing          error = errors.New("datagram icmp payload is missing")
)

func wrapMarshalErr(err error) error {
	return fmt.Errorf("datagram marshal error: %w", err)
}

func wrapUnmarshalErr(err error) error {
	return fmt.Errorf("datagram unmarshal error: %w", err)
}

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