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I have a CentOS 7.2 host running Virtual Machine Manager, there are three guest VMs on this host (all are CentOS 7.2)

Two servers (A and B) are on the same network 192.168.122.0/24, they have the same Virtual NIC configuration set up with NAT (virbr0). Guest A has an address of 192.168.122.50 and Guest B has address of 192.168.122.150, both have static IPs configured. Guest A can ping/ssh to Guest B or vice versa.

Guest C is on a different subnet 192.168.100.0/24, Virtual NIC configuration also set up with NAT (virbr1). Guest C has a static IP address of 192.168.100.100.

Guest C cannot ping or ssh to either guest A or B. However, both A and B can ping/ssh to C. No routes have been explicitly defined. Firewalld configuration is default for all guests and host machines.

All three guests are able to get out to the internet, get updates, etc. I'm thinking I was supposed to set something up in order for C to ping/ssh A and B. But I can't think of what that should be. Any ideas?

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There are many ways:

  1. Put C guest machine into the same subnet as on A and B guests.

  2. Use bridge connection instead NAT for C guest machine. Then you will have access to C guest from A and B guests, but C guest won't have access to A and B guests.

  3. Allow forwarding on host server between interfaces virbr0 and virbr1. Run these commands:

    iptables -I FORWARD -i virbr0 -o virbr1 -s 192.168.122.0/24 -d 192.168.100.0/24 -j ACCEPT

    iptables -I FORWARD -i virbr1 -o virbr0 -s 192.168.100.0/24 -d 192.168.122.0/24 -j ACCEPT

    /etc/init.d/iptables save

  4. Simply add second virtual network card to C machine and connect it to virbr0.

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  • I'd like to keep C in a different subnet for now, while I look for a way to address the communication problem. The bridge connection vs NAT still leaves me in the same situation. I am currently able to ping/ssh C from both A and B and the 122.0/24 subnet but cannot ping/ssh A and C from C and the 100.0/24 subnet. Same with the third suggestion. I want to be able to ssh/communication to/from both subnets.
    – jwhstman
    Oct 19, 2016 at 12:23
  • To add some details:
    – jwhstman
    Oct 19, 2016 at 12:24
  • Firewall configuration on all hosts has not been modified, host IP configuration has 2 virtual switches (bridges) virbr1 and virbr0 for the 192.168.100.0/24 and 192.168.100.0/24 network created in Virtual Machine Manager. Host ip route command shows the following: $ ip route default via 192.168.1.1 dev enp3s0 proto static metric 100 192.168.1.0/24 dev enp3s0 proto kernel scope link src 192.168.1.100 metric 100 192.168.100.0/24 dev virbr1 proto kernel scope link src 192.168.100.1 192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1
    – jwhstman
    Oct 19, 2016 at 12:31
  • Please show iptables rules for forwarding and nat on the virtualization server.
    – Khirgiy Mikhail
    Oct 20, 2016 at 17:16
  • I update my answer.
    – Khirgiy Mikhail
    Oct 23, 2016 at 19:23

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