13.2. FB105 tunnels

13.2. FB105 tunnels
	Chapter 13. Tunnels

The FB105 tunnelling protocol is a FireBrick proprietary protocol that was first implemented in the FireBrick FB105 device, and is popular with FB105 users for setting up VPNs etc. It is 'lightweight' in as much as it is relatively simple, with low overhead and an easy set up procedure, but it does not currently offer encryption. Although encryption is not available, the protocol does digitally sign packets, so that tunnel end-points can be confident that the traffic originated from another 'trusted' end-point. Where it matters, encryption can be utilised via secure protocols such as HTTPS or SSH over the tunnel.

The protocol supports multiple simultaneous tunnels to/from an end-point device, and Local Tunnel ID values are used on an end-point device to identify each tunnel. The 'scope' of the Local ID is restricted to a single end-point device - as such, the tunnel itself does not possess a (single) ID value, and is instead identified by the Local IDs in use at both ends, which may well differ.

13.2.1. Tunnel wrapper packets

The protocol works by wrapping a complete IP packet in a UDP packet, with the destination port number of the UDP packet defaulting to 1, but which can be set to any other port number if required. These UDP packets are referred to as the 'tunnel wrappers', and include the digital signature. As with any other UDP traffic originating at the FB2500, the tunnel wrappers are then encapsulated in an IP packet and sent to the IP address of the far-end tunnel end-point. The IP packet that is contained in a tunnel wrapper packet is referred to as the 'tunnel payload', and IP addresses in the payload packet are not involved in any routing decisions until the payload is 'unwrapped' at the far-end.

Payload packet traffic is sent down a tunnel if the FB2500's routing logic determines that tunnel is the routing target for the traffic. Refer to Chapter 8 for discussion of the routing processes used in the FB2500. Often, a dynamic route is specified in the tunnel definition, such that traffic to a certain range of IP addresses (or possibly all IP addresses, for a default route) is routed down the tunnel when it is in the Up state - see Section 13.2.4 for details.

Tip

Payload IP addressing is not restricted to RFC1918 private IP address space, and so FB105 tunnels can be used to transport public IP address traffic too. This is ideal where you want to provide public IP addresses to a network, but it is either impossible to route the addresses directly to the network - e.g. it is behind a NAT'ing router, or is connected via networks (e.g. a 3rd party ISP) that you have no control over - or you wish to benefit from having 'portable' public IP addresses e.g. you can physically relocate a tunnel end-point FB2500 such that it is using different WAN connectivity, yet still have the same public IP address block routed to an attached network.

13.2.2. Setting up a tunnel

You define a tunnel by creating an fb105 top-level object. In the web User Interface, these objects are created and managed under the "Tunnels" category, in the section headed "FB105 tunnel settings".

The basic parameters for a tunnel are :-

name : name of the tunnel (OPTIONAL)
local-id : the Local ID to use for the tunnel (REQUIRED)
remote-id : the ID used at the far-end for this tunnel (this will be the Local ID used on the far-end for this tunnel) (REQUIRED)
secret : this is a pre-shared secret string that must be set to the same value in the tunnel definitions on both end-point devices
ip : the IP address of the far-end end-point device (OPTIONAL)

The far-end IP address is optional, and if omitted, tunnel wrapper packets will be sent to the IP address from which wrapper packets are being received (if any). As such, at least one of the two end-points involved must have a far-end IP address specified, but it is not necessary for both ends to specify the other. This allows you to set up a tunnel on an end-point without knowing (or caring) what the far-end IP address is ; this means you can handle cases such as one of end-points being behind a NAT router that has a dynamic WAN IP address, or can be used to simplify administration of end-points that are used to terminate a large number of tunnels, by omitting the far-end IP address in tunnel definitions on such 'shared' end-points. The latter case is typical where an ISP deploys a FireBrick device to provide a 'head-end' device for tunnel bonding.

If you wish to use a different UDP port number than the default of 1, specify the port number using the port attribute.

13.2.3. Viewing tunnel status

The status of all configured FB105 tunnels can be seen in the web User Interface by selecting "FB105" from the "Status" menu. The tunnels are listed in ascending Local ID order, showing the far-end IP in use, the tunnel name, and the state. The table row background colour is also used to indicate tunnel state, with green for Up and red for Down.

Note that there is a third state that a tunnel can be in, that is "Up/Down" - this indicates that tunnel wrapper packets are being received, but that they are informing this end-point that the far-end is not receiving tunnel wrapper packets. This means the tunnel is essentially only established unidirectionally, typically because of a firewalling, routing, NAT or similar issue that is preventing the correct bidirectional flow of tunnels wrapper packets between the tunnel end-points.

Tunnel status can also be seen using the show fb105 CLI command - see Appendix J.

13.2.4. Dynamic routes

Since a tunnel can only carry traffic properly when in the Up state, any traffic routed down a tunnel that is not Up will be discarded. The ability to dynamically create a route when the tunnel enters the Up state (and automatically delete the route when the tunnel leaves the Up state) allows the route to be present only when traffic can actually be routed down the tunnel. In combination with the use of route preference values, you can use this to implement fall-back to a less-preferred route if the tunnel goes down. Alternatively, you may want to intentionally use a different tunnel to carry traffic, and use profiles to enable/disable tunnel(s) - the dynamic route creation means that you do not need to manually change routing information to suit.

A dynamic route is defined by setting the routes attribute on the tunnel definition, specifying one or more routing destinations in CIDR format, as discussed in Section 8.1.

13.2.5. Tunnel bonding

Multiple FB105 tunnels can be bonded together to form a set, such that traffic routed down the bonded tunnel set is distributed across all the tunnels in the set. This distribution is done on a round-robin per-packet basis i.e. the first packet to be sent is routed down the first tunnel in the set, each subsequent packet is routed down the subsequent tunnel in the set, and the (N+1)'th packet (where N is the number of tunnels in the set) is again routed down the first tunnel. This provides the ability to obtain aggregated bandwidths when each tunnel is carried over a different physical link, for example, such as using multiple ADSL or VDSL (FTTC) connections.

Note

Using tunnel bonding to aggregate access-network connections such as ADSL or VDSL to provide a single 'fat pipe' to the Internet requires there to be another FB105 tunnel end-point device to terminate the tunnels. Ideally this 'head-end' device is owned and operated by your ISP, but it is also possible to use a head-end device hosted by a third party, or in a datacentre in which you already have equipment. ISPs that can offer tunnel-bonding for Internet access include Andrews & Arnold and Watchfront.

To form a bonded tunnel set, simply specify the set attribute of each tunnel in the set to be a value unique to that set. Although not required, you would typically use a set value of 1 for the first set you have defined. You can defined multiple bonded sets by using different values of the set attribute in each set.

13.2.6. Tunnels and NAT

If you are using NAT in your network, it may have implications for how to successfully use FB105 tunnelling. The issues depend on where (on what device) in your network NAT is being performed.

13.2.6.1. FB2500 doing NAT

If you have a bonded tunnel set implementing a single logical WAN connection, then the FB2500 will typically have multiple WAN-side IP addresses, one per physical WAN connection. If you are using the FB2500 to NAT traffic to the WAN, the real source IP address of the traffic will be translated by the NAT process to one of the IP addresses used by the FB2500.

When this NAT'd traffic is carried via a tunnel, it will be the source address of the tunnel payload packet that is modified.

Whatever address is used, reply traffic will come back to that address. In order to ensure this reply traffic is distributed across the tunnel set by the far-end tunnelling device, the address used needs to be an address that is routed down the tunnel set, rather than one associated with any particular WAN connection.

In order to handle this scenario, the internal-ip attribute can be used to define which IP address is used as the source IP address of the tunnel payload packets.

13.2.6.2. Another device doing NAT

If you are using another device that is performing NAT (for example, a NAT'ing ADSL router) and that device is on the route that tunnel wrapper packets will take , you may have to set up what is generally called port forwarding on your NAT'ing router.

If the FB2500 is behind a NAT router, it will not have a public IP address of its own which you can reference as the far-end IP address on the other end-point device. Instead, you will need to specify the WAN address of the NAT router for this far-end address. Whether you need to set up a port forwarding rule on your NAT router depends on whether the FB2500 behind the router has a far-end IP address specified in tunnel definition(s), as follows :-

If it does, then it will be sending tunnel wrapper packets via the NAT router such that a session will have been created in the NAT router by the session tracking functionality that is used to implement NAT (this assumes there is no outgoing 'firewall' rule on the NAT router that would prevent the wrapper packets from being forwarded). The established session will mean that UDP packets that arrive from the WAN side will be passed to the UDP port number that was the source port used in the outgoing wrapper packets.
If it does not, then you will have to manually set up a port-forwarding rule, since there will have been no outbound packets to initiate a session. The forwarding rule should specify the UDP port number that is being used by the tunnel wrapper packets (the port attribute value in the tunnel definintion, or the default of 1 if the port is not specified)