IPsec VPN Overview

The following are some of the IPsec VPN topologies that Junos operating system (OS) supports:

• Site-to-site VPNs—Connects two sites in an organization together and allows secure communications between the sites.

• Hub-and-spoke VPNs—Connects branch offices to the corporate office in an enterprise network. You can also use this topology to connect spokes together by sending traffic through the hub.

• Remote access VPNs—Allows users working at home or traveling to connect to the corporate office and its resources. This topology is sometimes referred to as an end-to-site tunnel.

Table 2 summarizes the differences between policy-based VPNs and route-based VPNs.

An IPsec VPN tunnel consists of tunnel setup and applied security. During tunnel setup, the peers establish security associations (SAs), which define the parameters for securing traffic between themselves. (See IPsec Overview.) After the tunnel is established, IPsec protects the traffic sent between the two tunnel endpoints by applying the security parameters defined by the SAs during tunnel setup. Within the Junos OS implementation, IPsec is applied in tunnel mode, which supports the Encapsulating Security Payload (ESP) and Authentication Header (AH) protocols.

This topic includes the following sections:

Packet Processing in Tunnel Mode

IPsec operates in one of two modes—transport or tunnel. When both ends of the tunnel are hosts, you can use either mode. When at least one of the endpoints of a tunnel is a security gateway, such as a Junos OS router or firewall, you must use tunnel mode. Juniper Networks devices always operate in tunnel mode for IPsec tunnels.

In tunnel mode, the entire original IP packet—payload and header—is encapsulated within another IP payload, and a new header is appended to it, as shown in Figure 1. The entire original packet can be encrypted, authenticated, or both. With the Authentication Header (AH) protocol, the AH and new headers are also authenticated. With the Encapsulating Security Payload (ESP) protocol, the ESP header can also be authenticated.

Figure 1: Tunnel Mode

In a site-to-site VPN, the source and destination addresses used in the new header are the IP addresses of the outgoing interface. See Figure 2.

Figure 2: Site-to-Site VPN in Tunnel Mode

In a dial-up VPN, there is no tunnel gateway on the VPN dial-up client end of the tunnel; the tunnel extends directly to the client itself (see Figure 3). In this case, on packets sent from the dial-up client, both the new header and the encapsulated original header have the same IP address: that of the client’s computer.

Some VPN clients, such as the Netscreen-Remote, use a virtual inner IP address (also called a “sticky address”). Netscreen-Remote enables you to define the virtual IP address. In such cases, the virtual inner IP address is the source IP address in the original packet header of traffic originating from the client, and the IP address that the ISP dynamically assigns the dial-up client is the source IP address in the outer header.

Figure 3: Dial-Up VPN in Tunnel Mode

In the SRX5400, SRX5600, and SRX5800 devices, IKE provides tunnel management for IPsec and authenticates end entities. IKE performs a Diffie-Hellman (DH) key exchange to generate an IPsec tunnel between network devices. The IPsec tunnels generated by IKE are used to encrypt, decrypt, and authenticate user traffic between the network devices at the IP layer.

The VPN is created by distributing the IKE and IPsec workload among the multiple Services Processing Units (SPUs) of the platform. For site-to-site tunnels, the least-loaded SPU is chosen as the anchor SPU. If multiple SPUs have the same smallest load, any of them can be chosen as an anchor SPU. Here, load corresponds to the number of site-to-site gateways or manual VPN tunnels anchored on an SPU. For dynamic tunnels, the newly established dynamic tunnels employ a round-robin algorithm to select the SPU.

In IPsec, the workload is distributed by the same algorithm that distributes the IKE. The Phase 2 SA for a given VPN tunnel termination points pair is exclusively owned by a particular SPU, and all IPsec packets belonging to this Phase 2 SA are forwarded to the anchoring SPU of that SA for IPsec processing.

Multiple IPsec sessions (Phase 2 SA) can operate over one or more IKE sessions. The SPU that is selected for anchoring the IPsec session is based on the SPU that is anchoring the underlying IKE session. Therefore, all IPsec sessions that run over a single IKE gateway are serviced by the same SPU and are not load-balanced across several SPUs.

Table 3 shows an example of an SRX5000 line with three SPUs running seven IPsec tunnels over three IKE gateways.

The three SPUs have an equal load of one IKE gateway each. If a new IKE gateway is created, SPU0, SPU1, or SPU2 could be selected to anchor the IKE gateway and its IPsec sessions.

Setting up and tearing down existing IPsec tunnels does not affect the underlying IKE session or existing IPsec tunnels.

Use the following show command to view the current tunnel count per SPU: show security ike tunnel-map.

Use the summary option of the command to view the anchor points of each gateway: show security ike tunnel-map summary.

In an SRX5400, SRX5600, or SRX5800 chassis cluster, you can insert new SPCs on the devices without affecting or disrupting the traffic on the existing IKE or IPsec VPN tunnels. When you insert a new SPC in each chassis of the cluster, the existing tunnels are not affected and traffic continues to flow without disruption.

Starting in Junos OS Release 19.4R1, on all SRX5000 line chassis cluster, you can insert a new SRX5K-SPC3 (SPC3) or SRX5K-SPC-4-15-320 (SPC2) card to an existing chassis containing SPC3 card. You can only insert the cards in a higher slot than the existing SPC3 card on the chassis. You must reboot the node after the inserting SPC3 to activate the card. After the node reboot is complete, IPsec tunnels are distributed to the cards.

However, existing tunnels cannot use the processing power of the Service Processing Units (SPUs) in the new SPCs. A new SPU can anchor newly established site-to-site and dynamic tunnels. Newly configured tunnels are not, however, guaranteed to be anchored on a new SPU.

Site-to-site tunnels are anchored on different SPUs based on a load-balancing algorithm. The load-balancing algorithm is dependent on number flow threads each SPU is using. Tunnels belonging to the same local and remote gateway IP addresses are anchored on the same SPU on different flow RT threads used by the SPU. The SPU with the smallest load is chosen as the anchor SPU. Each SPU maintains number of flow RT threads that are hosted in that particular SPU. The number of flow RT threads hosted on each SPU vary based on the type of SPU.

Tunnel load factor = Number of tunnels anchored on the SPU / Total number of flow RT threads used by the SPU.

Dynamic tunnels are anchored on different SPUs based on a round-robin algorithm. Newly configured dynamic tunnels are not guaranteed to be anchored on the new SPC.

Starting in Junos OS Release 18.2R2 and 18.4R1, all the existing IPsec VPN features that are currently supported on SRX5K-SPC3 (SPC3) only will be supported on SRX5400, SRX5600, and SRX5800 devices when SRX5K-SPC-4-15-320 (SPC2) and SPC3 cards are installed and operating on the device in a chassis cluster mode or in a standalone mode.

When both SPC2 and SPC3 cards are installed, you can verify the tunnel mapping on different SPUs using the show security ipsec tunnel-distribution command.

Use the command show security ike tunnel-map to view the tunnel mapping on different SPUs with only SPC2 card inserted. The command show security ike tunnel-map is not valid in an environment where SPC2 and SPC3 cards are installed.

Inserting SPC3 Card: Guidelines and Limitations:

• In a chassis cluster, if one of the nodes has 1 SPC3 card and the other node has 2 SPC3 cards, the failover to the node that has 1 SPC3 card is not supported.

• You must insert the SPC3 or SPC2 in an existing chassis in a higher slot than a current SPC3 present in a lower slot.

• For SPC3 ISHU to work, you must insert the new SPC3 card into the higher slot number.

• On SRX5800 chassis cluster, you must not insert the SPC3 card in the highest slot (slot no. 11) due to the power and heat distribution limit.

• We do not support SPC3 hot removal.

Table 4 summarizes the SRX5000 line with SPC2 or SPC3 card that supports kmd or iked process:

SRX5000 line with SRX5K-SPC3 card (Services Processing Card), SRX mid-range platforms (SRX4100, SRX4200, SRX1500 and SRX4600 Series Firewalls) and vSRX Virtual Firewall requires junos-ike package as the control plane software to install and enable IPsec VPN features.

• On SRX5000 line with RE3, by default, junos-ike package is installed in Junos OS Releases 20.1R2, 20.2R2, 20.3R2, 20.4R1, and later. As a result, iked and ikemd process runs on the routing engine by default instead of IPsec key management daemon (kmd). The SRX5000 line with SRX5K-SPC3 offloads the cryptographic operations to the hardware cryptographic engine.

• The SRX mid-range platforms covering SRX1500, SRX4100, SRX4200 and SRX4600 Series Firewalls, offloads the DH, RSA and ECDSA cryptographic operations to the hardware cryptographic engine with devices running junos-ike software. This feature is available from Junos OS Release 23.2R1 with devices having junos-ike package installed. The devices that continue to run legacy iked software (kmd process) do not support this feature.

• On vSRX Virtual Firewall, the data plane CPU thread offloads the DH, RSA and ECDSA operations. Hardware acceleration is not available on these devices. This feature is available from Junos OS Release 23.2R1 with junos-ike package installed on the device.

The Table 5 describes the hardware acceleration support for various ciphers:

To install the Junos IKE package on your SRX Series Firewall, use the following command:

To use kmd process in order to enable IPsec VPN features on SRX5000 line without an SPC3 card, you must run the request system software delete junos-ike command. After running the command, reboot the device.

To check the installed junos-ike package, use the following command:

The two processes, iked and ikemd support IPsec VPN features on SRX Series Firewalls. A single instance of iked and ikemd run on the Routing Engine at a time.

By default, junos-ike package is installed in Junos OS Release 19.4R1 and later for SRX5K-SPC3 with RE3. Both the iked and ikemd processes running on the Routing Engine are available with this package. On SRX5K-SPC3 with RE2, this is an optional package and needs to be installed explicitly.

To restart ikemd process in the Routine Engine use the restart ike-config-management command.

To restart iked process in the Routing Engine use the restart ike-key-management command.

Table 6 shows the details of Junos OS releases where junos-ike package is introduced.

To operate IPsec VPN features using the legacy kmd process on SRX5000 line without an SRX5K-SPC3 card, you must run the request system software delete junos-ike command and reboot the device. The SRX1600, SRX2300, and SRX4300 Series Firewalls do not support kmd process.

On SRX Series Firewalls, anti-replay-window is enabled by default with a window size value of 64.

On the SRX Series 5000 line with SPC3 cards installed, you can configure the anti-replay-window size in the range of 64 to 8192 (power of 2). To configure the window size, use the new anti-replay-window-size option. An incoming packet is validated for replay attack based on the anti-replay-window-size that is configured.

You can configure replay-window-size at two different levels:

• Global level—Configured at the [edit security ipsec] hierarchy level.

  For example:

• VPN object—Configured at the [edit security ipsec vpn vpn-name ike] hierarchy level.

  For example:

If anti-replay is configured at both levels, the window size configured for a VPN object level takes precedence over the window size configured at the global level. If anti-replay is not configured, the window size is 64 by default.

To disable the anti-replay window option on a VPN object, use the set no-anti-replay command at the [edit security ipsec vpn vpn-name ike] hierarchy level. You cannot disable anti-replay at the global level.

You cannot configure both anti-replay-window-size and no-anti-replay on a VPN object.

If you create two VPN tunnels that terminate at a device, you can set up a pair of routes so that the device directs traffic exiting one tunnel to the other tunnel. You also need to create a policy to permit the traffic to pass from one tunnel to the other. Such an arrangement is known as hub-and-spoke VPN. (See Figure 4.)

You can also configure multiple VPNs and route traffic between any two tunnels.

SRX Series Firewalls support only the route-based hub-and-spoke feature.

Figure 4: Multiple Tunnels in a Hub-and-Spoke VPN Configuration