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I have two machines:

  • Ubuntu 16.04 server VM (172.18.6.10)
  • Proxmox VE5 station (192.168.6.30)

they are communicating through a third machine that forwards packets between the two. I want to create a gre tunnel between the two machines and to do that and make it persistent I have edited the /etc/network/interfaces and added a gre interface and tunnel to be made on boot up as the following:

Ubuntu Server config

Proxmox Config

After they were created I have tried to ping one machine from the other to check connectivity, pinging the gre interface IP address (10.10.10.1 and 10.10.10.2). The issue is that when I ping the Proxmox machine from Ubuntu I get no feedback, but when I run tcpdump on gre1 on Porxmox I see that the packets are received and there is a ICMP reply outgoing: tcp dump output

When I run the ping the other way around and check it with tcpdump on the Ubuntu machine I get nothing. I understand that the issue is when packets leave Proxmox to Ubuntu via gre1 and get lost or blocked because Ubuntu can clearly send Proxmox packets but the reply never comes back. How can I fix this?

Itay Gurvich
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1 Answers1

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Check if you have packet forwarding enabled for the kernel of the 3rd machine that you user for the communication of the other 2 machines

Check /etc/sysctl.conf and see if you have this:

net.ipv4.ip_forward = 1

if it's commented (#) uncomment it, save the file and issue a:

sysctl -p

Then try again the pings...

Bogdan Stoica
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