for school we have to do a linux routing vm between 4 subnet but the routing only works in one way. Let me explain, here is a schematic:

enter image description here The router in the center is a CentOS 7 fully up-to-date with 4 nic, each one in a different subnet: (top) -> lan of client pc and esxi server. (left) -> lan of a first windowsServer2016 AD/Exchange (bottom) -> lan of a nagios server (right) -> lan of a second windowsServer2016 AD/Exchange
The default gateway is

After enabling ip forwarding in centOS, I can access the nagios web interface from my client so the routing in that way is ok.

But with the nagios server, when i try to quit his subnet and ping my client, esxi, windowsServer1 or 2 -> nothing

Since the router has access to all subnet directly, i did not create any static route. Am i wrong ?

I think it isn't a routing problem but a "software" problem... Firewalld and NetworkManager are disabled.


ip route list ->
default via dev ens13 proto static metric 100 dev ens13 proto kernel scope link src dev ens14 proto kernel scope link src dev ens15 proto kernel scope link src dev ens16 proto kernel scope link src

  • Did you modify any other kernel parameters for network traffic handling, routing, forwarding, etc?
    – 0xSheepdog
    Commented May 2, 2018 at 21:28
  • no, each time i've tried a fix, i rollback to a snapshot of a clean install
    – F.Devilez
    Commented May 2, 2018 at 21:41
  • Okay, good. You need to specify the net.ipv4.ip_forward=1 kernel parameter, as I'm sure you know. There are others that may or may not impact this setup, but most can probably be safely ignored.
    – 0xSheepdog
    Commented May 2, 2018 at 21:53
  • It may also be helpful to look at the Cent7 routing table. netstat -nr
    – 0xSheepdog
    Commented May 2, 2018 at 21:55
  • don't have the netstat command and we can't have internet right now so here is the output of ip route list above.
    – F.Devilez
    Commented May 2, 2018 at 22:09

1 Answer 1


You cannot specify a gateway address for an interface that is not in the local network (its broadcast domain). E.G. would contain all IPs from through If your system was assigned, it can only talk directly to through

When it needs to talk to an address outside the local network, one of the 253 others must forward the traffic. Specifying a gateway address is how your system knows WHICH of the other IPs will forward traffic for it (this is a very basic description, btw.)

From your example, each system (AD1, AD2, Nagios, etc.) should use the IP address from the Cent7 on its own network as its gateway setting. So, the gateway is going to be different for each of those systems...

AD1 should have a gateway of ...

(ifcfg file entries)

IPADDR=200.200.0.x (IP of AD1)
  • Yes that's what i did. If not i would not be able to access my nagios server from outside or even ping it.
    – F.Devilez
    Commented May 2, 2018 at 21:56

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