Long story short:
I have a CentOS 4.3 x86_64 full install. Ssh with XForwarding works and I'm not interested in that solution because I want to export from a Solaris 5.7 which doesn't have ssh. (No I will not install ssh-server on it, or maybe I will but that is not the issue here).

The problem: I telnet to any kind of machine (Linux/Solaris) from the CentOS box. I issue xhost + <remote machine> on the CentOS machine and set the display on the remote machine properly. When I try to run a graphic application on the remote I get Cannot open display when I should get the result on the CentOS display. The telnet server on the remote is ok because with the same procedure as above I run graphical apps on a suse 10 box.

The problem is with the CentOS. The same problem I have it on a FC3 x86_64 installation.

And BTW if I telnet to the localhost on the CentOS works like a charm. So my guess is that this is a setting somewhere that doesn't let external tcp connections to the X server but I couldn't google my way out of this.

The strange thing is if I issue a telnet <CentOSbox> 6001 it connects.

I don't know if this is a "bug" or a "feature" Any help?

PS: iptables is clean

The problem is probably being caused by gdm. There is a line in gdm.conf that will probably look like this:

This is blocking normal TCP from getting in. SSH works because the TCP stuff is tunneled through SSH.

You may also find this line in /usr/share/gdm/defaults.conf. A lot depends on the distribution.