Problem Using Rsync Through Local Firewall

[Hal Vaughan <hal@thresholddigital.com>]

I have 2 LANs, the old one, which is connected to the Internet through a 
firewall, and the new one, which is separated from the old one by a firewall 
(which will one day be the main firewall, when the new LAN replaces the old 
one).

Right now I am trying to run daily backups (with rsync) from the old (and 
currently working) systems, through the firewall, to the new systems on the 
new LAN.  When I first set it up, everything worked.  I have one port on the 
inner firewall forwarding rsync to port 873 on one computer, and another port 
forwarding rsync to port 873 another computer.  All this worked fine, until I 
had to re-install the main server (which handled DNS, NIS, etc. for the new 
LAN).  I have the server reinstalled, with DNS and NIS working.  Now, when I 
try to connect, from the old LAN, through the firewall, to the new LAN, I get 
an error:

rsync error: error in socket IO (code 10) at clientserver.c"(94)

(In some cases, it simply timed out.)

I haven't touched the firewall in between when it worked and now, so I figure 
this has to be something going on with the computers running rsync (both 
Debian based Linux systems), especially since the error message isn't always 
a "timed out" problem.

Any idea what's going on here or what I need to look at for this?

Thanks!

Hal