Although there are likely numerous ways in which you can construct workable rule-sets that implement firewalling in addition to any traffic-shaping or NAT etc., we recommend that you implement firewalling as follows :-

Alternatively, you could have a single firewalling rule-set without any entry-criteria and with no-match-action attribute set to either drop or reject - that way, all traffic, regardless of its origin, or its characteristics, will be subject to the 'default drop' policy. A disadvantage of this approach is that you will need to specify target interfaces in every rule in order to replicate the functionality of the method described previously.

In any case, you can verify that your rule-sets function the way you intended using the diagnostic facility described in Section 15.1.

The XML fragment below shows a small firewalling rule-set for an interface, with a 'default drop' policy :-

 <rule-set name="firewall_to_LAN"
           target-interface="LAN"       
           no-match-action="drop">   
  <rule name="web"   
        target-port="80"
        protocol="6"
        comment="WAN access to company web server"/>   
 
 </rule-set>

You may want to perform some outbound traffic filtering as well. This would normally want to work the other way around to inbound filtering. With inbound you want to block all but those listed hence using a no-match-action of drop. However, for outbound you will typically want to allow all but those listed. To this end, you could create a rule-set for traffic from inside interfaces, such as LAN and a no-match-action of continue. Then include specific rules for those things you wish to block with an action of reject.

Normally, a session table entry holds enough information to allow return traffic to reach its destination, without potentially being firewalled.

However, a session-rule can specify certain changes to be made to the outbound traffic in a session, and the session-table entry will hold additional information that allows the FB2500 to account for these changes when processing the return traffic.

For example, a session-rule can specify that the source IP address of the outbound packets be changed, such that they appear to be coming from a different address, typically one owned by the FB2500 itself. Return traffic will then be sent back to this modified address - assuming that the intention is that this traffic reach the original source IP address, the FB2500 will change the destination IP address in return traffic to be the original source IP address. It can do this because it has stored the original source IP address in the session table entry.

The set-source-ip, set-source-port, set-target-ip and set-target-port attributes request this kind of change to be made.

Quite separately to firewalling and session tracking, the FB2500 has to route traffic, and this is done using normal routing logic (see Chapter 8). The routing is done based on the destination IP address, as normal. However, it can be useful for session tracking rules to override the normal routing. The set-gateway allows a different IP address to be used for the routing decision, instead of the actual destination IP in the packets. Setting this causes all subsequent packets matching the session to use that gateway IP for routing decisions.

The obfuscation attribute causes all traffic in the session to be simply scrambled using a hexadecimal key of up to 64 bits (up to 16 hex digits). To receive the traffic, the far end must also be configured with the same obfuscation value.

As well as matching on interface and/or ip you may wish to limit this rule to specific protocols such as TCP (6) or UDP (17). This will ensure that, for example, ICMP messages can still be delivered. If an ICMP message quotes part of an obfuscated packet, we attempt to make sure that the original addresses and quoted headers are still correct, even though the quoted payload may already have been obfuscated.

By default, the protocol checksum will not be modified when obfuscating. For a valid packet, this means that the checksum will be correct before obfuscation and correct after de-obfuscation, but it will be incorrect in between. Usually, this does not matter, but there may be some cases (e.g. passing over carrier-grade NAT) where equipment will look at the checksum in transit and will require it to be correct.

The obf-checksum attribute provides a way of adjusting the protocol's checksum value. This is currently only supported for TCP and UDP packets.

The set-graph and set-reverse-graph attributes cause the session traffic to be graphed, and therefore possibly be subject to traffic shaping ; they perform the same function as the graph attribute that can be specified on many different objects, as described in Chapter 10.

Each direction of the final established session can have a graph set. The normal set-graph attribute sets the forward direction graph, and set-reverse-graph sets the reverse session graph (remember, sessions have two sides). Because of this, with set-graph, the graph "Tx" direction will be the direction in which the session was established, and the "Rx" direction is therefore the opposite direction. With set-reverse-graph, the "Tx"/"Rx" directions are swapped compared to using set-graph.

There is also an option for set-graph-dynamic which causes the session to set a (forward) graph that is based on the MAC address of the source packet, if from an Ethernet interface, or the source IP. The graph is created if it does not exist. If set-graph is also defined then each new session created also causes the speed settings and long term shaper parameters to be copied from the named graph (in set-graph) to the MAC named graph being used. This is aimed at management of open WiFi and the like, allowing a named shaper to be defined and a copy of its settings created for each client based on MAC address.

As discussed in Section 7.2.1, each session-table entry has a timer associated with it - this ensures that inactive sessions are removed from the session-table. Two time-out values are configurable :-

Session tracking rules can include an additional set of share records which define a choice of possible changes to make when setting up the session. This choice is normally random and based on a weighting for each choice.

This allows various forms of load balancing to be applied. E.g. you could port map to one of a set of web servers or mail servers.

Each share can also have a profile which can be used to exclude the option from the selection - this is typically tied to a ping or some other test to confirm the choice makes sense. In the example of web services being load balanced, a ping based profile could confirm each web server is actually up.

Normally the choice is random, but there is an option (hash) which can be set to make the choice determined based on a hash of the source and target IP address. This allows consistent mapping of sessions to the same server. As the choice depends on the set of servers which have an active profile, if the profiles change, sessions will get a new consistent mapping based on IP addresses.

It is possible for the creation of a session tracking entry to clash with an existing entry. This happens where the reverse session already exists. It cannot happen where the forward exists as in that case the traffic is mapped based on the existing session.

For example, if you had a session mapping traffic from an IP (A) port 5060 inside, to an IP (B) port 5060 outside, using NAT, so changing to be from the FireBrick's IP outside (C) random port to the target (B) port 5060, this creates two sides to the session, A->B one side, and B->C the other (reverse side), and allows traffic both ways.

Now, if you had a new session from outside (B) port 5060 to the FireBrick's IP (C) port 5060 which you want to map to (A) port 5060, you have a problem. The reverse (A) port 5060 to (B) port 5060 exists already, so the new session could not be created.

Normally, in the case of a clash with an existing session like this, the new session is not created. However, when the IPs are the same (in this example, A and C), and it is UDP, And the source port matches, then the original session is dropped and a new one created. This is primarily to assist NAT.

The other case where the existing is dropped and a new one created is were the existing clashing session is one sided, i.e. to drop or reject traffic.

The FireBrick supports protocols that allow devices on your LAN to request port mapping and firewall holes using RFC6886 (NAT-PMP) and RFC6887 (PCP). These are newer protocols than uPnP (universal plug and play) and used by many devices and applications. PCP is the later protocol and allows handling of IPv6 and also allows finer control such as opening a firewall hole for a specific block of external IPs only.

It is important to understand that there are two stages to the use of these protocols. Firstly a device on the local network will send a message to the FireBrick as the gateway device requesting a mapping or firewall hole. Secondly, a session may be created that matches that mapping or firewall hole, and needs to be allowed or not according to normal firewall rules.

The first stage is controlled by a setting on the subnet. By default, any subnet marked as nat="true" has pcp true as well, as does any subnet created using IPv6 Prefix Delegation. Settings on the interface and subnet can override these defaults. Only if a subnet has pcp set will a NAT-PMP or PCP packet be accepted and processed to create mappings.

Once created, the mappings can be viewed on the status page for sessions. When a new session starts it is checked against these mappings. The mappings may relate to incoming traffic or outgoing traffic. The mapping is applied to the new session, so, for example, incoming traffic to a specific mapped external port may be mapped to an internal device IP and port. This mapped traffic is then processed via the firewall rules as normal, but as it is already mapped it allows the firewall rules to consider the target (typically a private) IP address and port. This allows much finer control than would be possible otherwise, and one can, for example, easily allow all mapped traffic to specific internal devices such as gaming consoles.

In order to make the firewall rules easier to manage, any mapped traffic using a NAT-PMP/PCP rule will be flagged as pcp and so you can make rules relating specifically to these mapped sessions. As such a rule to allow all incoming mapped sessions is simple to create.

It is important to note that by default, on factory reset, the firewall rules block all incoming sessions to the LAN, and this will mean that even mapped sessions set up using NAT-PMP or PCP will also be blocked. An additional rule is necessary to allow such sessions. This rule can test for destination IPs even where these are private IP addresses on the LAN as the mappings are already applied before checking firewall rules. This means the FireBrick can be set to allow NAT-PMP and PCP sessions only for specific devices, ports and protocols by using firewall rules.