503 Service Unavailable when using firewall

[Ivan Martinez]

Hello all,
In a CentOS 6 server, I'm running Varnish in port 80 serving pages from 
a Zotonic site in port 8000. It works fine when I leave all the ports 
open. However, if I close all ports from 0 to 631 and explicitly open 80 
and 8000, the following happens:

- I can see the website in server:8000.
- In server:80, I get the following after some seconds:

Error 503 Service Unavailable

Service Unavailable
Guru Meditation:

XID: 544990083

Varnish cache server

I have read about the sysctl issues with firewalls but it seems I have 
it right:

[root at server ivanmr]# sysctl -A | grep tw
net.ipv4.tcp_max_tw_buckets = 2048
net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_tw_reuse = 0

varnish.log is empty. What can be the problem?. Thank you.

Ivan