no PostgreSQL user name specified in startup packet

Symptoms

A connection’s startup packet did not specify a PostgreSQL user name. PostgreSQL rejects it with SQLSTATE 28000 (invalid_authorization_specification).

• The startup packet omitted the user.
• Usually a malformed client or proxy.
• A user name is mandatory to connect.

What the server log shows

FATAL:  no PostgreSQL user name specified in startup packet

Why PostgreSQL raises this — what the manual says

Section 54.2.1 Start-up:

“To begin a session, a frontend opens a connection to the server and sends a startup message.”

The protocol’s startup message must carry a user name to establish the session’s authorization identity. A startup packet without one cannot be authenticated, so PostgreSQL reports 28000.

Common causes

• A misconfigured client/driver not sending a user.
• A proxy/pooler mangling the startup packet.
• A non-PostgreSQL client speaking the protocol incorrectly.

How to fix it

1. Provide a user in the connection string/driver configuration.
2. Fix the proxy/pooler so it forwards the user name.
3. Verify the client speaks the PostgreSQL protocol correctly.

Related & next steps

• SQLSTATE 28000 — Invalid Authorization Specification (full reference)
• Message: received invalid response to SSL negotiation
• Glossary: Wire protocol

Reference: PostgreSQL 18 Section 53.2 “Message Flow”.