Pinging Internet from client computers behind this gateway works and then stops working, unpredictabely.

This is where you should be focusing your efforts.

Watch for ICMP traffic while logged into to a shell on your server/gateway:

tcpdump -i <LANNET> icmp

Where LANNET is your lan interface (eg. eth1, enp15s0 etc.).

You should both the request and the response.

If you don't see the request on occasion...start looking upstream...cables, hubs, switches etc.

If you don't see the response, check to see if you see the request on the WAN interface...eg., instead of LANNET = to your LAN interface, change it to your WAN interface (eg. ppp0 enp15s1 etc.), and see if those packets are making it across to your WAN interface when you are getting failures...eg. is blocked or do you see the responses and it's getting lost (eg. NATing issue) when the packet comes back.

Why any of this would be affected when the filter is on or off is beyond me...I suspect that that might be bad/misleading information that needs to be confirmed.

B.

Hmm, I'm not a expert with content filter to be honest I never used it.., maybe someone else can jump in?

What is the output of:

lspci -k | grep -i eth -A 3

Marcel, thank you for your answer.
I have seen and (partly) read ClearOS user guide, and have experience configuring various kinds of routers/gateways/firewalls.
After more thorough inspection of this problem looks like traffic flow through this ClearOS machine computer is not stable.
Pinging Internet from client computers behind this gateway works and then stops working, unpredictabely.
It does not always depend on me turning on/off content filter plugin.
One more thing, pinging Internet from the ClearOS gateway works all the time.
How can I troubleshoot this issue?

Hi Drazen Petrek,

First welcome to the ClearOS forums.

Have you seen the ClearOS user guide? There is also a part about the Content Filter. Maybe this points you to the right direction.