Can't access internal network but Internet

Question

I have a strange problem with one Ubuntu-system in my network. As well as some other Linux-boxes it is located in an internal network in IP range 192.168.1.x with full access to all other systems in this network and with access to the internet via a separate firewall (IP/Gateway 192.168.1.1).

All Linux-boxes are working fine except this one box (let's name it X): Internet access works properly but it can't ping or even SSH an other system in same network. On the other hand when I ping from an other system in this network to X, ping works but SSH not. Funny again: ping or SSH from X to some other server in Internet works!

Network configuration on this one system X is default/the same as on all other (as far as I can tell) Any idea what could cause this or where I could look for a reason for this strange behaviour?

Thanks!

Answer 1

Here are some possibilities assuming the system firewall is configured identically to that on your other systems.

1. You're running a VPN on the machine in question and the VPN is routing the traffic you intend for you local network to a different network behind the VPN.
2. A hardware firewall is configured on your local network that is blocking traffic that originates from the computer in question, preventing it from accessing any other systems on your network.
3. The system in question is behaving in a questionable manner that your network or system IDS/IPS (assuming you have one) is picking up on and the IDS/IPS is throwing up a firewall to block outgoing traffic from the system in question to the rest of the local network
4. You have put the system in question behind a secondary NAT router that serves addresses in the same address space as your local network, and you have configured the system in question as the DMZ on said NAT router. Hence, when you try to access systems in your intranet, you're not making it past the secondary router you're sitting behind.