Change ssh tunnel direction with the same port? - shell

Question

I'm using an ssh tunnel to forward a port to a db server.
Let's say I'm using mysql, so my ssh command would be something along the lines of
ssh -fqTN -L 12345:127.0.0.1:3306 user#server.com
based on the method of transfer (sync from or sync to) I want to use either the -L or -R flags.
I do need the -L flag at first though, so I open the tunnel above anyway.
My question is though -
If after now run
ssh -fqTN -R 12345:127.0.0.1:3306 user#server.com
Will it replace the above command and make a reverse tunnel on the same port?

Answer 1

The second command will not "replace" the first command, but it will work just fine.
You started with:
ssh -fqTN -L 12345:127.0.0.1:3306 user#server.com
This opens port 12345 on your local system and forwards it to 127.0.0.1:3306 from the perspective of the remote system, so that you can access the mysql server on the remote system using local port 12345.
The second command...
ssh -fqTN -R 12345:127.0.0.1:3306 user#server.com
...opens port 12345 on the remote system and forwards it to 127.0.0.1:3306 from the perspective of your local system, allowing the remote system to access a mysql server on your local host via port 12345.
This doesn't conflict with the original command, so these can both be run at the same time.
Update
Responding to your comment here, because I want to quote some command output:
If I run:
ssh -R 12345:127.0.0.1:3306 remote_system
Then on remote_system I run lsof -i -n, I see:
sshd 23280 lars 10u IPv6 37263762 0t0 TCP [::1]:italk (LISTEN)
sshd 23280 lars 11u IPv4 37263763 0t0 TCP 127.0.0.1:italk (LISTEN)
And from /etc/services, we see that italk is port 12345. If you add -P to your lsof command line it will not try to translate port numbers to service names:
# lsof -i -n -P | grep 12345
sshd 23280 lars 10u IPv6 37263762 0t0 TCP [::1]:12345 (LISTEN)
sshd 23280 lars 11u IPv4 37263763 0t0 TCP 127.0.0.1:12345 (LISTEN)