cloudflare/cloudflared · error

datagram response is an invalid size

Error message

datagram response is an invalid size

What it means

ErrDatagramResponseInvalidSize is returned by UDPSessionRegistrationResponseDatagram.UnmarshalBinary when the input slice is shorter than the fixed datagramSessionRegistrationResponseLen. The response format has a fixed layout (type byte, request ID, response code, error message field), so undersized input cannot be parsed. It signals a truncated or non-response datagram.

Source

Thrown at quic/v3/datagram_errors.go:15

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)
}

View on GitHub (pinned to 2253eeeb25)

Solutions

1. Check len(data) >= datagramSessionRegistrationResponseLen before unmarshaling
2. Dispatch on the datagram type byte so only response datagrams reach this parser
3. Verify the peer version produces the same fixed response layout (protocol/version mismatch)

Example fix

// before
unmarshaled := &v3.UDPSessionRegistrationResponseDatagram{}
err := unmarshaled.UnmarshalBinary(payload) // too short
// after
if len(payload) < v3.DatagramSessionRegistrationResponseLen {
    return fmt.Errorf("response truncated: %d bytes", len(payload))
}
err := unmarshaled.UnmarshalBinary(payload)

if len(data) < datagramSessionRegistrationResponseLen {
    return ErrTruncatedResponse
}

if err := resp.UnmarshalBinary(data); errors.Is(err, v3.ErrDatagramResponseInvalidSize) {
    return fmt.Errorf("registration response truncated (%d bytes)", len(data))
}

When it happens

Trigger: Calling UnmarshalBinary on bytes with len(data) < datagramSessionRegistrationResponseLen, e.g. payload[:15] in tests, or feeding a payload-type datagram into the response parser.

Common situations: Reading a short QUIC stream/datagram chunk; dispatching datagrams to the wrong unmarshaler by type; a peer sending a corrupted or truncated registration response.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• datagram provided is an invalid size
• invalid datagram type expected
• payload length is too large to be bundled in datagram
• payload length is too small to fit the datagram header
• datagram response message is an invalid size

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/62aebb358897ed0f. Report an issue: GitHub.