What does [unknown]@[unknown] LOG: incomplete startup packet mean?

Question

I am having troubles with starting Kong with Postgres. I inspected my desktop's log

root@miki:/var/log/postgresql#
2019-04-22 07:35:28.040 CEST [1181] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2019-04-22 07:35:28.070 CEST [1181] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2019-04-22 07:35:28.438 CEST [1426] LOG:  database system was shut down at 2019-04-21 20:13:57 CEST
2019-04-22 07:35:28.546 CEST [1181] LOG:  database system is ready to accept connections
2019-04-22 07:35:29.175 CEST [1690] [unknown]@[unknown] LOG:  incomplete startup packet

What does this mean?

My postgresql is running

/etc/init.d/postgresql status
● postgresql.service - PostgreSQL RDBMS
   Loaded: loaded (/lib/systemd/system/postgresql.service; enabled; vendor preset: enabled)
   Active: active (exited) since Mon 2019-04-22 07:35:30 CEST; 1h 45min ago
  Process: 1939 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
 Main PID: 1939 (code=exited, status=0/SUCCESS)

апр 22 07:35:30 miki systemd[1]: Starting PostgreSQL RDBMS...
апр 22 07:35:30 miki systemd[1]: Started PostgreSQL RDBMS.

Solution

2019-04-22 07:35:28.040 CEST [1181] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2019-04-22 07:35:28.070 CEST [1181] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2019-04-22 07:35:28.438 CEST [1426] LOG:  database system was shut down at 2019-04-21 20:13:57 CEST
2019-04-22 07:35:28.546 CEST [1181] LOG:  database system is ready to accept connections

This means your database started just fine.

2019-04-22 07:35:29.175 CEST [1690] [unknown]@[unknown] LOG:  incomplete startup packet

This means that someone initiated a TCP or Unix domain socket connection to the 5432 port but did not follow on with an authentication attempt. This is harmless. Some legitimate startup scripts do this at every start of PostgreSQL, probably to check that it's started.

Whatever is Kong and the trouble you have starting it, these messages tell nothing about it.