oracle data type negotiation expected message 2, got

Error message

oracle data type negotiation expected message 2, got %d

What it means

In the second phase of the raw protocol handshake, the library expects the server's data type negotiation (dty) message with message type 2. A different message byte was received, so the handshake aborts with the actual value. This indicates the server or intermediary deviated from the expected TNS message sequence.

Solutions

Read the actual server message — often an ORA- error about login/DB state; fix the underlying login problem (credentials, account locked, database not open).
Verify listener redirects aren't rewriting the stream; connect to the node the listener would redirect to.
Capture TNS traffic and compare the message sequence against the library's expectations; report a server-version incompatibility if it's a consistent order difference.

Example fix

// Diagnostic: log server-side ORA error before dty stage
// before: handshake fails with 'expected message 2, got 4'
// after: check alert.log / listener.log for ORA- errors at connect time and fix e.g. locked account:
//   ALTER USER app_user ACCOUNT UNLOCK;

Defensive patterns

Strategy: try-catch

Try / catch

if err := connect(); err != nil {
    if strings.Contains(err.Error(), "expected message 2") {
        // server refused/redirected mid-handshake: check credentials and listener.log
    }
}

Prevention

Keep credentials valid and accounts unlocked to avoid error-packet responses mid-handshake
Watch listener.log for refusals during deploy windows
Use supported Oracle protocol versions (client negotiates 4/5/6)

When it happens

Trigger: The data-type negotiation step of the connect sequence reads a message byte that is not 2 — e.g. the server sent an error/redirect/refuse packet (type 4/9/11) instead of proceeding to dty negotiation.

Common situations: Server refuses the connection (wrong credentials, DB not open) and sends an error message where dty was expected; listener redirect responses; protocol version mismatch between client's negotiated version (4/5/6) and what the server then sends.

Related errors

AI-assisted analysis of shadow1ng/fscan@95cc12e753 (2026-09-06). Data as JSON: /api/errors/0dfc13a5bc52efbd. Report an issue: GitHub.

Appendix: source

Thrown at plugins/services/oracle_raw.go:1121

		for _, v := range typeReps {
			s.putBytes(uint8(v))
		}
		s.putBytes(0)
	} else {
		for _, v := range typeReps {
			s.putInt(v, 2, true, false)
		}
		s.putBytes(0, 0)
	}
	if err := s.writeData(); err != nil {
		return err
	}
	msg, err := s.getByte()
	if err != nil {
		return err
	}
	if msg != 2 {
		return fmt.Errorf("oracle data type negotiation expected message 2, got %d", msg)
	}
	if runtimeCaps[1] == 1 {
		if _, err = s.getBytes(11); err != nil {
			return err
		}
		if compileCaps[37]&2 == 2 {
			if _, err = s.getInt(4, false, true); err != nil {
				return err
			}
		}
	}
	level := 0
	for {
		var n int
		if compileCaps[27] == 0 {
			n, err = s.getInt(1, false, false)
		} else {
			n, err = s.getInt(2, false, true)

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