Thank you for your considerations tekhead2. You are right and I am just having no idea what's going wrong.

pc1/eth0, pc2/eth0, and pc3/eth0(although not in used in my case), are all working fine. All of them can access the Internet and one another through the switch.

Anything else can you think of?

Oh, sorry I don't quite get you. Why are you saying we need to send a packet from pc2/eth0 to its own eth1? How?

On the other hand, you are right that I get lost because of the problem between pc2/eth1 and pc3/eth1.

Well man , this connection is by far the most simple in your network, but its taking the whole thing down. So lets get down to the basics, you have two nics. Both appear to be enable and working. We have lights, we have ip addresses. SO as far as I can see from here its not a physical issue. We just need to get out the white glove and get anal retentive about this.

Did you say that you were able to ping from the local ips ie. 192.168.0.1 to the global ips ie. 144.34.x.x? I may be mistaken , i just could have sworn you said you could do that.

Man I guess Im just trying to understand what your wanting to accomplish. Where are you wanting the packets to go? Basically from what I can see, you are trying to setup some kinda router. I guess we are just losing each other in all the numbers and interfaces. So tell me in a perfect world, what would this accomplish for you? What should this setup do? If the nics were working where would that get you. I guess If I knew what you were doing I could get you there faster! Sorry If Im losing you on this.

tekhead2 why complicate things so much?
The problem is to get a simple cross-link up and going.

enjoyzj: Yes I would try to put in manual arp entries just to check if it works.
arp entries should come up automatically when you ping but if the arp protocol somehow messes up then we can try setting it manually. If can ping after that, then we know the problem to be arp or broadcasting.

What did you see from the iptables -L -n command?
Does the counters increase? And what chains get packets?
Do you get packets to the OUTPUT chain?
If you get then we can probably say that packet are leaving. At least they leave the iptables system.

Hi ugge and tekhead2,

Thank you so much for your time on this simplest issue. After I manually set an arp entry, everything works fine! It's strange that after this when I reboot the PCs, sometimes they are automatically connected after ping, and sometimes it needs manually setting arp entries. It seems the system is not quite stable. But anyway, it works now. Thank you again!

Great, I thought you already had the arp entered manually