I have linux box A on network 1
I have port 123 forwarded to linux box A on network 1.

Currently Linux box A is not listening for port 123

I have linux box B on network 2
Linux box B is listening for port 123, and port 123 is forwarded to Linux box B on network 2.
and I can currently connect using Network2:123 from an outside network, but I need to be able to connect to the same box using network1:123.

What I need to do is tell Linux box A to forward all incoming traffic on network 1 on port 123 to network 2 on port 123 and be a middle man for communications, but only port 123.

I've looked into SSH tunneling and iptables but I can't quite seem to get it right, could someone help me out?

incoming packets need to do this: Traffic -> Network1:123 -> Linux Box A -> Network2:123 -> Linux Box B
Outgoing packets need to do this: Linux Box B -> Network2:123 -> Linux Box A -> Network1:123 -> Traffic

I hope this makes sense...

looks like there are some confusions ....
network has no port, you cannot forward that anywhere. You always must have a host and hosts have ports (router is a host too).

no theres a way to do this. I have two buildings two isps.
Im the client connecting to network 1 on port 123. I just want network 1 to send that connection to network 2 in another building.
This way the client can communicate with network 2 on port 123 but only by telling network 1.

Pretend the buidlings have no inter linked networking harware the only way they can communicate is by external wan ip