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I intend to pass Internet from openSUSE Leap 15.1 to CentOS 7:

  • openSUSE Leap 15.1 machine with access to Internet through a wireless router
  • CentOS 7 machine without access to Internet due to lack of proper wireless chip driver

I connect openSUSE machine to CentOS by an Ethernet cable. My understanding is that I should have two different sub-nets:

  • openSUSE
    • Has static IP address of 192.168.2.252/24
    • Has DHCP IP address of 192.168.1.51/24
      • Access to Internet through 192.168.1.1
  • CentOS
    • Has static IP address of 192.168.2.251/24

Observations

  • Inside CentOS 192.168.2.251 I can ping openSUSE 192.168.2.252
  • Inside CentOS 192.168.2.251 I can ping openSUSE 192.168.1.51
  • Inside CentOS 192.168.2.251 I can not ping internet router 192.168.1.1
  • Inside CentOS 192.168.2.251 I can not ping 1.1.1.1
  • Inside CEntOS 192.168.2.251 I do not have access to Internet

Routing on openSUSE

> ip route
default via 192.168.1.1 dev wlan0 proto dhcp metric 600 
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown 
192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.51 metric 600 
192.168.2.0/24 dev eth0 proto kernel scope link src 192.168.2.252 metric 100

Question

I cannot figure out what is wrong with routing. It's strange that from CentOS I can ping openSUSE on both sub-nets, but I cannot ping internet router!

Megidd
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    You want openSUSE to act as a router. This requires it to forward traffic that is not destined to itself. Most Linux distros are configured to ignore such traffic. Ensure that IP forwarding is enabled. Use sysctl to enable it permanently. – berndbausch Mar 13 '21 at 03:12
  • @berndbausch When I check with sudo sysctl net.ipv4.ip_forward I'm getting net.ipv4.ip_forward = 1. I think IP forwarding is enabled. – Megidd Mar 13 '21 at 14:09
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    Yes it is. A different thought: What's the routing table on Centos? – berndbausch Mar 13 '21 at 14:32
  • @berndbausch CentOS ip route output summary is default via 192.168.1.1 and 192.168.1.1 and 192.168.2.0/24 src 192.168.2.251 – Megidd Mar 13 '21 at 14:58
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    That's the problem. There is no connectivity to the default router. Make 192.168.2.252 the default. I don't understand the rest of your routing table; can't you just add ip route to the question? – berndbausch Mar 13 '21 at 15:03

1 Answers1

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The problem is that your internet router does not know about the network 192.168.2.0/24. The openSUSE machine knows that network and therefore answers even when accessed with the other address.

If you have access to the internet router and the address 192.168.1.51 is a fixed DHCP lease, you could add a route at the internet router pointing to 192.168.2.0/24 via 192.168.1.51.

A probably easier way would be to add masquerading to the wifi interface of the openSUSE machine like in this question at superuser.

Mathias Weidner
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