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IPsec Site-to-Site VPN Example with Pre-Shared Keys¶
A site-to-site IPsec tunnel interconnects two networks as if they were directly connected by a router. Systems at Site A can reach servers or other systems at Site B, and vice versa. This traffic may also be regulated via firewall rules, as with any other network interface. If more than one client will be connecting to another site from the same controlled location, a site-to-site tunnel will likely be more efficient, not to mention more convenient and easier to support.
With a site-to-site tunnel, the systems on either network need not have any knowledge that a VPN exists. No client software is needed, and all of the tunnel work is handled by the tunnel endpoints. This is also a good solution for devices that have network support but do not handle VPN connections such as printers, cameras, HVAC systems, and other embedded hardware.
Site-to-site example configuration¶
The key to making a working IPsec tunnel is to ensure that both sides have matching settings for authentication, encryption, and so on. Before starting, make a note of the local and remote WAN IP addresses, as well as the local and remote internal subnets that will be carried across the tunnel. An IP address from the remote subnet to ping is optional, but recommended to keep the tunnel alive. The firewall doesn’t check for replies, as any traffic initiated to an IP address on the remote network will trigger IPsec negotiation, so it doesn’t matter if the host actually responds or not as long as it is an IP address on the other side of the connection. Aside from the cosmetic tunnel Description and these pieces of information, the other connection settings will be identical.
In this example and some of the subsequent examples in this chapter, the following settings will be assumed:
Start with Site A. Create the tunnel by clicking Add P1. The phase 1 configuration page for the tunnel is shown. Many of these settings may be left at their default values.
First, fill in the top section that holds the general phase 1 information, shown in Figure figure-vpn-tunnel-settings. Items in bold are required. Fill in the settings as described:
Uncheck this box so that the tunnel will be operational.
- Key Exchange version
Specifies whether to use IKEv1 or IKEv2. For this example, IKEv2 is used, but if one side does not support IKEv2, use IKEv1 instead.
- Internet Protocol
Will be IPv4 in most cases unless both WANs have IPv6, in which case either type may be used.
Most likely set to WAN , but see the note at Interface Selection on selecting the proper interface when unsure.
- Remote Gateway
The WAN address at Site B,
203.0.113.5in this example.
Some text to state the purpose or identity of the tunnel. It is a good idea to put the name of Site B in this box, and some detail about the tunnel’s purpose to help with future administration. For this example
ExampleCo London Officeis used for the Description to identify where the tunnel terminates.
The next section controls IPsec phase 1, or Authentication. It is shown in Figure Site A Phase 1 Settings. The defaults are desirable for most of these settings, and simplifies the process.
- Authentication Method
The default, Mutual PSK, is used for this example.
- My Identifier
The default, My IP Address, is kept.
- Peer Identifier
The default, Peer IP Address, is kept.
- Pre-Shared Key
This is the most important setting to get correct. As mentioned in the VPN overview, IPsec using pre-shared keys can be broken if a weak key is used. Use a strong key, at least 10 characters in length containing a mix of upper and lowercase letters, numbers and symbols. The same exact key will need to be entered into the tunnel configuration for Site B later, so note it down or copy and paste it elsewhere. Copy and paste may come in handy, especially with a complex key like
- Encryption Algorithm
Use AES with a key length of 256 bits.
- Hash Algorithm
Use SHA256 if both sides support it, otherwise use the default SHA1.
- DH Group
The default of 2 (1024 bit) is OK, higher values are more secure but use more CPU.
May also be specified, otherwise the default value of 28800 will be used.
- Disable rekey
- Responder only
- NAT Traversal
Leave on Auto, since in this example neither firewall is behind NAT.
- Dead Peer Detection
Leave checked, the default Delay of
10seconds and Max Failures of
5is adequate. Depending on the needs at a site a higher value may be better, such as
6retries, but a problematic WAN connection on either side may make that too low.
Click Save to complete the phase 1 setup.
After the phase 1 has been added, add a new phase 2 definition to the VPN:
Click Show Phase 2 Entries as seen in Figure Site A Phase 2 List (Empty) to expand the phase 2 list for this VPN.
Click Add P2 to add a new phase 2 entry, as seen in Figure Adding a Phase 2 entry to Site A.
Now add settings for phase 2 on this VPN. The settings for phase 2 (Figure Site A Phase 2 General Settings) can vary more than phase 1.
Since tunneling traffic is desired, select Tunnel IPv4
- Local Subnet
Best to leave this as LAN Subnet, but it could also be changed to Network with the proper subnet value filled in, in this case 10.3.0.0/24. Leaving it as LAN Subnet will ensure that if the network is renumbered, this end of the tunnel will follow. If that does happen, the other end must be changed manually.
Set to None.
- Remote Subnet
Set to the network at Site B, in this case 10.5.0.0/24.
The remainder of the phase 2 settings, seen in Figure Site A Phase 2 Settings, cover the encryption of the traffic. Encryption algorithms and Hash algorithms can both be set to allow multiple options in phase 2, and both sides will negotiate and agree upon the settings so long as each side has at least one of each in common. In some cases that may be a good thing, but it is usually better to restrict this to the single specific options desired on both sides.
Set to ESP for encryption.
- Encryption algorithm
Ideally, if both sides support it, select AES256-GCM with a 128 bit key length. Otherwise, use AES 256, or whichever cipher both ends will support.
- Hash algorithm
With AES-GCM in use, no hash is required. Otherwise, use SHA 256 or SHA 1. Avoid MD5 when possible.
Perfect Forward Secrecy can help protect against certain key attacks, but is optional. In this example, it is disabled.
3600for this example.
Lastly, an IP address can be entered for a system on the remote LAN that will be periodically sent an ICMP ping, as in Figure Site A Keep Alive. The return value of the ping is not checked, this will only send traffic the tunnel so that it will stay established. In this setup, the LAN IP address of the pfSense® firewall at Site B, 10.5.0.1, is used.
To finalize the settings and put them into action: * Click Save * Click Apply Changes on the IPsec Tunnels screen, as seen in Figure Apply IPsec Settings.
The tunnel for Site A is finished, but now firewall rules are needed to allow
traffic from the network at Site B to enter through the IPsec tunnel. These
rules must be added to the IPsec tab under Firewall Rules. See
Firewall for specifics on adding rules. Rules may be as
permissive as desired, (allow any protocol from anywhere to anywhere), or
restrictive (allow TCP from a certain host on Site B to a certain host at Site A
on a certain port). In each case, make sure the Source address(es) are Site B
addresses, such as
10.5.0.0/24 . The destination addresses will be the Site
Now that Site A is configured, it is time to tackle Site B. Repeat the process on Site B’s endpoint to add a tunnel.
The phase 1 settings for WAN address and Description
The phase 2 tunnel networks
The keep alive setting
Add a Phase 1 to the Site B firewall using identical settings used on Site A but with the following differences:
- Remote Gateway
The WAN address at Site A,
ExampleCo Austin Office.
Add a phase 2 entry to the Site B firewall using identical settings used on Site A but with the following differences.
- Remote Subnet
The network at Site A, in this case
- Automatically ping host
(Figure Site B Keep Alive). The LAN IP address of the pfSense firewall at Site A,
Click Apply changes on the IPsec Tunnels screen.
As with Site A, firewall rules must also be added to allow traffic on the tunnel to cross from Site A to Site B. Add these rules to the IPsec tab under Firewall Rules. For more details, see IPsec and firewall rules. This time, the source of the traffic would be Site A, destination Site B.
Both tunnels are now configured and are now active. Check the IPsec status by visiting Status > IPsec. A description of the tunnel is shown along with its status.
If the tunnel is not listed as Established, there may be a problem establishing the tunnel. This soon, the most likely reason is that no traffic has attempted to cross the tunnel. Since the local network includes an address that the firewall has, a connect button is offered on this screen that will initiate a ping to the remote phase 2. Click the Connect VPN button to attempt to bring up the tunnel, as seen in Figure Site A IPsec Status. If the connect button does not appear, try to ping a system in the remote subnet at Site B from a device inside of the phase 2 local network at Site A (or vice versa) and see if the tunnel establishes. Look at Testing IPsec Connectivity for other means of testing a tunnel.
Failing that, the IPsec logs will offer an explanation. They are located under Status > System Logs on the IPsec tab. Be sure to check the status and logs at both sites. For more troubleshooting information, check the Troubleshooting IPsec VPNs section later in this chapter.
Routing and gateway considerations¶
When the VPN endpoint, in this case a pfSense firewall, is the default gateway for a network there are normally no problems with routing. As a client PC sends traffic, it will go to the pfSense firewall, over the tunnel, and out the other end. However, if the pfSense firewall is not the default gateway for a given network, then other routing measures will need to be taken.
As an example, imagine that the pfSense firewall is the gateway at Site B, but not Site A, as illustrated in Figure Site-to-Site IPsec Where pfSense is not the Gateway. A client, PC1 at Site B sends a ping to PC2 at Site A. The packet leaves PC1, then through the pfSense firewall at Site B, across the tunnel, out the pfSense firewall at Site A, and on to PC2. But what happens on the way back? The gateway on PC2 is another router entirely. The reply to the ping will be sent to the gateway router and most likely be tossed out, or even worse, it may be sent out the Internet link and be lost that way.
There are several ways around this problem, and any one may be better depending on the circumstances of a given case.
A static route could be entered into the gateway router that will redirect traffic destined for the far side of the tunnel to the pfSense firewall. Even with this route, additional complexities are introduced because this scenario results in asymmetric routing as covered in Bypass Firewall Rules for Traffic on Same Interface.
A static route could be added to the client systems individually so that they know to send that traffic directly to the pfSense firewall and not via their default gateway. Unless there are only a very small number of hosts that need to access the VPN, this is a management headache and should be avoided.
In some situations it may be easier to make the pfSense firewall the gateway and let it handle the Internet connection instead of the existing gateway.
pfSense-initiated Traffic and IPsec¶
To access the remote end of IPsec connections from the pfSense firewall itself, “fake” the system out by adding a static route pointing the remote network to the LAN IP address of the pfSense firewall. Note this example presumes the VPN is connecting the LAN interface on both sides. If the IPsec connection is connecting an OPT interface, replace Interface and IP address of the interface accordingly. Because of the way IPsec is tied into the FreeBSD kernel, without the static route the traffic will follow the system routing table, which will likely send this traffic out the WAN interface rather than over the IPsec tunnel. Take Figure Site-to-Site IPsec, for example.
A static route is required on each firewall, which is done by first adding a gateway pointing to the LAN IP address of the firewall (See Gateways), and then adding a static route using this gateway (See Static Routes). Figure Site A Static Route to Remote Subnet show the route to be added on each side.