I have an Xapp that we need to view remotely for trouble shooting.
The Xapp runs on IRIX server.
We connect to the IRIX server from a CentOS 6.8 desktop via ssh to a RHEL 6.5 gateway server, and then via telnet to the IRIX server.
We can see the data coming into the gateway server, but never leaving.
The iptables on the RHEL gateway accept all input traffic from the IRIX server interface and allows all output traffic to the interface that we are accessing from our CentOS desktops.
tshark packet captures from the gateway server show the Xapp session sending data on port 6000 of the interface connecting to the IRIX server.
tshark packet captures from the gateway server show NO data on port 6000 of the interface connecting to our workstations.
We were able to do this a last year and nothing within our servers has been changed since then.
And, due to our linux guru retiring out of the country, we no longer have his talents available.
Any assistance would be greatly appreciated.

The X session is presumably initiating from your IRIX server and attempting to connect to the RHEL server since you say you're doing telnet from RHEL to IRIX (i.e. it is not using ssh X tunneling but actually initiating a new X session from IRIX).

This means your RHEL server has to accept the port 6000 range connection for X. You should look at the xhost command on RHEL for that. Doing "xhost +" on the RHEL server generally would allow all X connections there.

The ssh from your CentOS workstation to your RHEL may or may not be relying on ssh X tunneling. Look at the "-x", "-X" and "-y" options of ssh by typing "man ssh".