Ordering of NAT and Firewall Processing¶
Understanding the order in which firewalling and NAT occurs is important when configuring NAT and firewall rules. The basic logical order is illustrated by Figure Ordering of NAT and Firewall Processing. The figure also depicts where tcpdump ties in, since its use as a troubleshooting tool is described later in this book in Packet Capturing.
Each layer is not always hit in typical configurations, but the use of floating rules or manual outbound NAT or other more complicated configurations can hit each layer in both directions. The diagram only covers basic scenarios for inbound and outbound traffic.
Traffic from LAN to WAN is processed as described in the following more detailed list. If a type of rules do not exist or do not match, they are skipped.
Port forwards or 1:1 NAT on the LAN interface (e.g. proxy or DNS redirects)
Firewall rules for the LAN interface: Floating rules inbound on LAN, then rules for interface groups including the LAN interface, then LAN tab rules.
1:1 NAT or Outbound NAT rules on WAN
Floating rules that match outbound on WAN
In this case, port forwards on WAN and WAN tab firewall rules do not apply.
For traffic initiated on the WAN, the order is the same but direction is reversed:
Port forwards or 1:1 NAT on the WAN interface (e.g. public services)
Firewall rules for the WAN interface: Floating rules inbound on WAN, then rules for interface groups including the WAN interface, then WAN tab rules.
1:1 NAT or Outbound NAT rules on LAN
Floating rules that match outbound on LAN
tcpdump is always the first and last thing to see traffic, depending on the
direction. First, on the incoming interface before any NAT and firewall
processing, and last on the outbound interface. It shows what is on the wire.
(See Packet Capturing)
See Rule Processing Order for more information about the firewall rule processing order.
Extrapolating to additional interfaces¶
The previous diagram and lists only illustrate a basic two interface LAN and WAN deployment. When working with additional interfaces, the same rules apply. Traffic between two internal interfaces behaves the same as LAN to WAN traffic, though the default NAT rules will not translate traffic between internal interfaces so the NAT layer does not do anything in those cases. If Outbound NAT rules exist that match traffic between internal interfaces, it will apply as shown.
Rules for NAT¶
On the way into an interface, NAT applies before firewall rules, so if the destination is translated on the way in (e.g. port forwards or 1:1 NAT on WAN), then the firewall rules must match the translated destination. In the typical case of a port forward on WAN, this means the rule must match a destination of the target private IP address on LAN.
For example, with a port forward for TCP port 80 on WAN with an automatically added firewall rule, Figure Firewall Rule for Port Forward to LAN Host shows the resulting firewall rule on WAN. The internal IP address on the port forward is 10.3.0.15. Whether using port forwards or 1:1 NAT, firewall rules on all WAN interfaces must use the internal IP address as the destination.
On the way out of an interface, outbound NAT applies before firewall rules, so any floating rules matching outbound on an interface must match the source after it has been translated by outbound NAT or 1:1 NAT.