GRE tunnel between two VPS

Step 1: Install Necessary Software

Ensure that both VPS servers have the required software installed for GRE tunneling. This typically involves iproute2 which is usually pre-installed on most Linux distributions. If it’s not installed, they can install it using the package manager specific to their Linux distribution (e.g., apt-get install iproute2 on Debian/Ubuntu).

Step 2: Load the GRE Module

The customer needs to load the GRE module on both VPS servers. This can be done with the following command:

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modprobe ip_gre

They might also want to ensure that this module loads on boot by adding it to the /etc/modules file.

Step 3: Configure the GRE Tunnel

On the first server (let’s call it Server A), they will need to execute commands similar to the following, replacing IP addresses and names as necessary:

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ip tunnel add gre1 mode gre remote [Server_B_Public_IP] local [Server_A_Public_IP] ttl 255 ip addr add 10.0.0.1/30 dev gre1 ip link set gre1 up

On the second server (Server B), they should execute:

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ip tunnel add gre1 mode gre remote [Server_A_Public_IP] local [Server_B_Public_IP] ttl 255 ip addr add 10.0.0.2/30 dev gre1 ip link set gre1 up

Step 4: Configure Routing (if necessary)

Depending on what they want to achieve with the GRE tunnel, they may need to configure routing rules to direct traffic appropriately. This could involve adding specific routes or modifying existing ones.

Step 5: Adjust Firewall Settings

Both servers may have firewalls that could block GRE traffic. They will need to allow this traffic through the firewall on both ends. This typically involves allowing traffic on protocol 47 (GRE). The exact commands will depend on the firewall they are using. For example, if using iptables, they might add a rule like:

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iptables -A INPUT -p 47 -j ACCEPT

Step 6: Testing the Tunnel

They can test the tunnel by pinging the internal IP address of the tunnel from one server to the other. For example, from Server A:

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ping 10.0.0.2

If they receive replies, the tunnel is up and running.

Troubleshooting:

If the customer is having issues, here are a few things they should check:

1. IP Addresses: Make sure they are using the correct IP addresses for each server.
2. Firewall/Security Group Settings: Ensure that both the GRE protocol and the necessary IP addresses are allowed through any firewalls or security groups.
3. GRE Module: Confirm that the GRE module is loaded correctly on both servers.
4. Network Configuration: Double-check the network configuration and routing rules to ensure they are correct.

If your customer is following these steps and still experiencing issues, they might need to provide more specific error messages or descriptions of what’s going wrong for further assistance.