XTLS/Xray-core · error

write login start: %w

Error message

write login start: %w

What it means

Wraps a writePacket failure when sending the Minecraft Login Start packet (packet ID 0x00 carrying the selected profile's username and UUID) right after the protocol handshake. Like error 853, the %w chains the underlying conn write error; this is the second packet on the wire, so failures here mean the connection degraded between the two writes.

Source

Thrown at transport/internet/finalmask/xmc/client.go:117

		}
	}

	err = writePacket(c.writer, 0x00, &protocolVersion, &serverAddress, &serverPort, &nextState)
	if err != nil {
		return fmt.Errorf("write handshake packet: %w", err)
	}

	// Login Start
	randomProfile, err := rand.Int(rand.Reader, big.NewInt(int64(len(c.profiles))))
	if err != nil {
		return fmt.Errorf("select profile: %w", err)
	}
	selectedProfile := c.profiles[randomProfile.Int64()]
	username := String(selectedProfile.Username)

	err = writePacket(c.writer, 0x00, &username, &selectedProfile.UUID)
	if err != nil {
		return fmt.Errorf("write login start: %w", err)
	}

	// Encryption Request
	pkt, err := readPacket(c.reader)
	if err != nil {
		return fmt.Errorf("read encryption request: %w", err)
	}

	if pkt.packetID != 0x01 {
		return fmt.Errorf("bad encrypt request packet id")
	}

	var (
		serverId    String
		publicKey   Bytes
		verifyToken Bytes
	)

View on GitHub (pinned to 7d214f8b09)

Solutions

1. Confirm the target is a Minecraft login server accepting protocol 775 clients.
2. Unwrap the error to distinguish timeout vs. ECONNRESET.
3. Retry with a fresh connection once transient network issues are ruled out.

if err := cc.Handshake(); err != nil {
	if isWriteErr(err) { // timeout, reset, broken pipe
		return retryDial(ctx) // new conn, restart from handshake packet
	}
	return err
}

When it happens

Trigger: Server closing the connection after receiving the handshake packet (wrong next-state, protocol version mismatch), write deadline expiry, or a reset from a middlebox. The username/UUID fields are small, so partial-write issues are rare.

Common situations: Protocol version 775 not accepted by the target server; connecting to a Minecraft server in a state that rejects logins (whitelist, full server); network path resetting mid-handshake.

Related errors

• write handshake packet: %w
• read encryption request: %w
• bad encrypt request packet id
• LRU size is bigger than subnet size
• set deadline: %w

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/30435f74fbe9f4be. Report an issue: GitHub.