iptables sporadic "sendmsg: operation not permitted" problem and packet loss

[Dennis Jacobfeuerborn <d.jacobfeuerborn@conversis.de>]

Hi,

I'm running into a problem on a machine that right now acts as a simple 
gateway but is supposed to become a firewall too. When I start iptables 
using "/etc/init.d/iptables start" on the Centos 5.2 machine first 
everything works fine but after about 30 seconds I'm seeing packet loss and 
running a ping outputs "sendmsg: operation not permitted" sporadically.
The moment I stop iptables again everything returns to normal. What is 
consufing to me is that I don't even have any rules defined so far. This is 
what my "/etc/sysconfig/iptables" file looks like:

# Generated by iptables-save v1.3.5 on Thu Mar  5 17:40:28 2009
*filter
:INPUT ACCEPT [26715202:4750206096]
:FORWARD ACCEPT [1382646771:1563210213960]
:OUTPUT ACCEPT [22930985:6256734041]
COMMIT

iptables -L says:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Does anyone have an idea why that would have such a severe impact on the 
traffic? The fact that it takes a moment for the problems to show up makes 
me suspect some kind of buffer issue so that the packet loss only begins to 
occur after some buffer begins to overflow. That just a guess though and I 
have no idea what buffer that could be.

Regards,
   Dennis