How to clear local ssh port for forwarding to remote?

I'm using ssh forwarding local port to a bastion server(10.20.30.40), connect to remote RDS database.

ssh -i ~/.ssh/id_rsa -f -N -L 5432:db1.cluster-1.region.rds.amazonaws.com:5432 10.20.30.40 -v
...
Authenticated to 10.20.30.40 ([10.20.30.40]:22).
debug1: Local connections to LOCALHOST:5432 forwarded to remote address db1.cluster-1.region.amazonaws.com:5432
debug1: Local forwarding listening on ::1 port 5432.
debug1: channel 0: new [port listener]
debug1: Local forwarding listening on 127.0.0.1 port 5432.
debug1: channel 1: new [port listener]
debug1: Requesting [email protected]
debug1: forking to background
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype [email protected] want_reply 0

The 5432 on local port will be used. If create a new forwarding

ssh -i ~/.ssh/id_rsa -f -N -L 5433:db1.cluster-1.region.rds.amazonaws.com:5432 10.20.30.40 -v

The 5433 port will be used.

If start 5432 in a new terminal, it will be failed because already in use.

Authenticated to 10.20.30.40 ([10.20.30.40]:22).
debug1: Local connections to LOCALHOST:5432 forwarded to remote address db1.cluster-1.region.rds.amazonaws.com:5432
debug1: Local forwarding listening on ::1 port 5432.
bind [::1]:5432: Address already in use
debug1: Local forwarding listening on 127.0.0.1 port 5432.
bind [127.0.0.1]:5432: Address already in use
channel_setup_fwd_listener_tcpip: cannot listen to port: 5432
Could not request local forwarding.
debug1: Requesting [email protected]
debug1: forking to background
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype [email protected] want_reply 0

How to release these ports connection?

As others have said in-comments... that port is bound to the process that launched it. It will stay open until that process exits.

You can use the -p argument to netstat to see what processes are holding which ports open. I'd highly suggest you limit that to only TCP ports with the -t flag, as well... and finally, turn off DNS lookups with the -n flag.

So, you're looking at something like:

netstat -npt

That should "list" all ports currently in the state table, along with the process ID or program name. That last part will likely require privilege escalation through something like sudo (or Cygwin "Run as Administrator")

For example:

$ sudo netstat -npt
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:8651          127.0.0.1:50644         TIME_WAIT   -
tcp        0      0 127.0.0.1:8649          127.0.0.1:53761         TIME_WAIT   -
tcp        0      0 127.0.0.1:80            127.0.0.1:58488         TIME_WAIT   -
tcp        0    348 172.16.XXX.YYY:22       172.16.XXX.ZZZ:54272    ESTABLISHED 21020/sshd: russell
...