Guard Your Network with IPCop, Part 5: Your First Site-to-Site VPN

Your First Site-to-Site VPN

Your First Site-to-Site VPN

The first article in this series discussed the basic installation and configuration of IPCop 1.4.21. Subsequent articles covered six important add-ons: Advanced Proxy, URL Filter, Update Accelerator, Calamaris, Block Outgoing Traffic and Zerina. In this article, we will explore IPCop’s built-in VPN functionality, and implement a site-to-site tunnel-mode VPN using two IPCop.

Imagine a company with two offices at different locations, connected via the Internet. Here, organisational traffic travels over an “open” channel, risking confidentiality (unauthorised snooping of data) and integrity (unauthorised tampering of data). To overcome these risks, one of the best ways is to encrypt traffic over the Internet, which is what a VPN does.

Site-to-site VPN
Figure 1: Site-to-site VPN

Refer to Figure 1. The two private networks, Office 1 and Office 2, are connected using inexpensive Internet bandwidth. For data security, VPN Gateway 1 and VPN Gateway 2 establish a tunnel — all traffic flowing through it is encrypted, to ensure confidentiality and integrity.

To avoid firewalls at either end blocking this encrypted traffic as malicious, VPN functionality is incorporated into the firewall itself (or by using a separate gateway, and bypassing the firewall for VPN traffic). IPCop readily supports IPSec site-to-site (also called net-to-net) VPN.

The actual transfer involves the following steps: Gateway 1 and Gateway 2 establish a VPN tunnel. When a host in Office 1 sends data to a host in Office 2, VPN Gateway 1 encrypts the data, encapsulates the encrypted data in an IP packet, and sends this packet to VPN Gateway 2. VPN Gateway 2 de-encapsulates the packet, decrypts the data, and delivers it to the desired host in Office 2. This mechanism ensures confidentiality of data travelling over the Internet. Also note that internal IP addresses of Office 1 and Office 2 are not disclosed to the Internet.

The IPCop site-to-site VPN setup

Here’s an overview of the network used for the trial setup:

  • Two IPCop firewalls, one for Office 1 (Sankalp) and Office 2 (Vasantika)
  • The respective IP address ranges are and
  • The IPCop green addresses are and
  • The setup is carried out from Office 1.
  • For identifying IPCop boxes easily, Office 1 is the left side, and Office 2 is the right side

Configure the boxes as follows: IPCop GUI –> VPNs –> VPNs menu, Configure Public IP or FQDN or red interface, and select the Enabled check-box (see Figure 1).

Configuring site-to-site VPNs
Figure 2: Configuring site-to-site VPNs

Select ADD under Connection status and control. Select Net-to-Net Virtual Private Network, and fill in the following parameters:

Parameters Value for left box Value for right box
Name TowardsVasantika TowardsSankalp
Remote host/IP
Remote subnet
Host IP address Office 1 RED IP Address Office 2 RED IP Address
Local subnet
Use a Pre-Shared Key (PSK) Thisisatestpresharedkey Thisisatestpresharedkey

Scroll down, then click on Save to complete both configurations. Successful completion will open the tunnels immediately, as seen in Figures 3 and 4. Now, all hosts from either office can access hosts from the other office, using their IP address.

Left VPN Tunnel Open
Figure 3: Left VPN Tunnel Open

Right VPN Tunnel Open
Figure 4: Right VPN Tunnel Open

The security of PSK-based VPNs depends on the strength and confidentiality of the key. The key is also required to be distributed securely to the remote gateway location(s). If the PSK is compromised, the whole network may be under threat. To overcome this problem, certificate-based VPNs can be employed, using the inbuilt certificate generator provided with the IPCop distro, as follows: Generate a Root/Host certificate on both the IPCop boxes — scroll to the bottom of VPNs –> VPNs, give a CA name, and click the Generate Root/Host Certificates button. Fill in the required details. Download the Host and Root certificates of both the boxes.

On the left IPCop, go to the VPNs –> VPNs menu and add a new Net-to-Net VPN connection. Set the Remote Host IP as the RED IP of the Right IPCop, and Remote Subnet as Upload the Right IPCop’s host certificate. Save this setup. Upload the Right IPCop’s root certificate via the VPNs –> VPNs menu, giving the correct CA name used while creating the certificate.

Repeat the same process on the Right IPCop box, using the corresponding Left IPCop host and root certificates.

That is all! If the configuration is correct, the tunnel will immediately open. You may verify this from Connection status and Control under the VPNs –> VPNs menu.

IPCop offers an excellent cost-effective solution to establish site-to-site VPNs. Once established, the connectivity is very stable, and requires attention only on rare occasions.

Important notes and troubleshooting guidelines

  • Make sure that different IP subnets are used in the offices.
  • The default value in the “Public IP or FQDN for RED interface” VPN setup field could have the service provider’s name, such as Make sure you remove non-IP-address text.
  • IPCop RED with private IP addresses (such as 192.168.x.x or 10.x.x.x) makes VPN configuration really difficult. This happens when IPCop is installed inside a router configured in route mode. Here, reconfigure the router in bridge mode, so that IPCop RED will get a public IP.
  • Refresh the browser a few times if tunnel “open” status is not displayed.
  • Ensure correct uploading of root and host certificates generated Ensure that the root and host certificates generated on the right box are uploaded correctly on the left box and vice versa.
  • If the tunnel does not open, verify the parameters on both boxes. You can also check Logs –> System logs –> IPSec logs for further troubleshooting.
  • Additional tunnels can be created to network multiple offices; the most important point to remember is to use different IP subnets for different offices.
  • IKE aggressive mode (available under advanced options) is definitely not preferred; it transmits a pre-shared key in clear text.
  • Though IPCop has an inbuilt certificate generator, you may also purchase and use certificates from various certifying authorities.


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