A connection starting in the LAN requesting a resource on the WAN would have the source as the LAN and the destination as the WAN, and so on.Ĭonfiguring DNAT for a Plex media server using Sophos XG The terminology is relative to the perimeter device and is determined by the direction of a connection. For example, Sophos XG uses ‘Translated Source’ instead of just calling it SNAT, which would both be more accurate and specific. Sometimes, firewalls use different terms in their interface to refer to these industry-standard terms. PAT can come in handy where you have multiple internal IPs providing a service on a common port number that you need to expose publicly on a single IP address. If the internal server is listening on TCP port 32400 and the traffic arrives at the public address on port 32400 then it is just a straightforward 1:1 mapping and no translation is required. Port address translation is not as frequently required. When traffic is received on the public IP address the destination IP address is replaced by the internal IP address. Typically, DNAT is required when the server is on an internal network and must be accessed through another external IP address on a perimeter device. DNAT – Destination Network Address Translation All of the source addresses are replaced with the public IP address. It does this by having a many-to-one relationship of many internal addresses all being translated to the same single external address. It allows for all of the internal addresses to be hidden from the external network. The most common form of a SNAT is a masquerade or MASQ rule. Common terminology SNAT – Source Network Address Translation This process is sometimes called port forwarding or Destination Network Address Translation (DNAT). Since a device on the internet cannot connect directly to a device on someone’s private network, they need to have the traffic forwarded from the perimeter device to the internal device. Once internal hosts can access the internet or WAN, the next test is to expose something on the internal network, such as a web server, and make it available from the internet.
As you don’t want to expose all private IP addresses on the public internet, Source Network Address Translation (SNAT) is required here. When configuring any firewall or perimeter device, one of the first steps is to make sure you can connect from your internal network to the internet or WAN for DNS, HTTPS traffic etc. When network traffic moves back and forth between an internal private IP address space (LAN) and a public IP address space (WAN), there needs to be some sort of network address translation (NAT) that occurs. This article helps you understand the types of NAT available and uses the example of exposing a Plex server on the public internet without the extra DNAT rules that are not needed. By knowing your environment, some basic theory, and what is and is not required, you can configure clean concise DNAT rules. However, this does generate a lot of configuration that is not strictly required.
Sophos XG makes it easy to expose internal services to the public internet using the Server Access Assistant (DNAT) wizard. Understanding and Optimizing Sophos XG’s DNAT Rules
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