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I have a cluster with 1 login node and 100 computation nodes.

I can ping the external networks including google.com from the login node. However, I cannot from the computation nodes.

The computation nodes are all reachable (ping) from each other. They can also ping the login node. (and vice versa)

Where should I check first? (I am using CentOS 7)

The below is some info of my network status

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Is net.ipv4.ip_forward set to 1 on the login node (which I assume is acting as a router for the compute nodes)? Have you set up NATing (i.e. is iptables -t nat -L empty)?

Check /etc/sysctl.conf for net.ipv4.ip_forward=1, and if it's not there, add it and run sysctl -p to source the changes you've made.

  • yes right. the login node acts as a route for compute nodes. I found that net.ipv4.ip_forward was set to 0. I changed it to 1 and the computes node can access the external networks now! (I don't know how it worked before). – syko Jan 16 '17 at 2:40
  • Additionally, 'iptables -t nat -L' shows no entry. But all the policy for (PREROUTING, INPUT, OUTPUT, POSTROUTING) is set to ACCEPT. – syko Jan 16 '17 at 2:41
  • You might also want to add -t nat -A POSTROUTING -o ens3 -j MASQUERADE to your iptables where ens3 is your public facing interface. – Travis DePrato Jan 16 '17 at 3:37

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