Reflection and Hairpin NAT
Index
Networks used in this How-To section
Interface |
IPv4 Subnet |
Hosts |
Gateway |
|---|---|---|---|
WAN |
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DMZ |
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LAN |
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NAT - Quick Overview
Because there are not enough available IPv4 addresses, a workaround called NAT (Network Address Translation) was implemented into the IPv4 Standard. It basically enables a router like the OPNsense to translate IPv4 addresses to other IPv4 addresses. Most of the time it is used to translate the limited external IPv4 address space to the shared internal IPv4 address space (RFC 1918, 192.168.0.0/16 - 172.16.0.0/12 - 10.0.0.0/8) and vice versa.
Note
- SNAT - Source Network Address Translation
Changes the source IP of a packet
Firewall –> NAT –> Outbound using the option Translation / target in a rule
- DNAT - Destination Network Address Translation
Changes the destination IP of a packet
Firewall –> NAT –> Port Forward using the option Redirect target IP in a rule
- PAT - Port Address Translation
Changes the destination port of a packet
Firewall –> NAT –> Port Forward using the option Redirect target port in a rule
If you create a DNAT rule, you enable all clients in the WAN access to an internal IPv4 address. The OPNsense acts like a translator, translating IPv4 addresses between client and server. The OPNsense writes all translations into a file called the NAT table. It knows exactly how traffic should flow back and forth with the translations in place.
Warning
NAT is not a security feature. It only acts as a translator. If you want security, you need firewall rules in addition.
Introduction to Reflection and Hairpin NAT
For example, you have a Webserver example.com with the internal IP 172.16.1.1 in your DMZ. It has a public DNS Record of example.com in A 203.0.113.1.
Your internal client 192.168.1.1 can’t reach the Webserver if it resolves the DNS A-Record 203.0.113.1. When the OPNsense receives the packet from the client 192.168.1.1 with the destination IP 203.0.113.1, it chooses itself as the target, and not 172.16.1.1. That’s because the external IPv4 address 203.0.113.1 is mapped to the WAN interface of the OPNsense.
That’s where Reflection NAT comes into play. It creates NAT rules which help your internal client 192.168.1.1 to communicate with your webserver 203.0.113.1, by using the OPNsense as the “translator” to the actual destination 172.16.1.1.
Attention
You should choose your preferred Reflection NAT method from the three possible choices presented here. They’re exclusive to each other, picking one method and sticking to it will prevent mistakes.
Method 1 - Creating manual Port-Forward NAT (DNAT), manual Outbound NAT (SNAT), and automatic firewall rules
Method 2 - Creating automatic Port-Forward NAT (DNAT), manual Outbound NAT (SNAT), and manual firewall rules
Method 3 - Creating automatic Port-Forward NAT (DNAT), automatic Outbound NAT (SNAT), and manual firewall rules
Note
Reflection NAT: The client and the server are in different subnets (layer 2 broadcast domains) and the OPNsense routes traffic between them. They can’t communicate directly by resolving ARP requests. You only need DNAT.
Hairpin NAT: The client and the server are in the same subnet (layer 2 broadcast domain). They can communicate directly with each other by resolving ARP requests. You need SNAT and DNAT.
Note
When using IPsec, by default NAT only matches on policy based VPN. NAT on VTI (Virtual Tunnel Interfaces) won’t match unless some tunables are set. These tunables change the behavior of firewall filter and NAT on if_enc and if_ipsec interfaces. You can read more about the tunables in IPsec VTI - Route based setup
Best Practice
The best way to do Reflection NAT in the OPNsense is not to use the legacy Reflection options in (Advanced) Settings. Creating the NAT rules manually with Method 1 prevents unwanted traffic and makes auditing easy. There will be no hidden rules. All rules will be perfectly visible in the GUI and .xml config exports.
Start of the How-To Section:
The goal is to access the Webserver 172.16.1.1 on port 443 with it’s external IP 203.0.113.1 from a client in WAN, LAN and DMZ.
Method 1 - Creating manual Port-Forward NAT (DNAT), manual Outbound NAT (SNAT), and automatic firewall rules
- Go to
Disable Reflection for port forwards, Reflection for 1:1 and Automatic outbound NAT for Reflection
- Go to
Select + to create a new Port Forward rule.
Interface:
Select
WAN,DMZandLAN- Select all interfaces in which clients are that should access the webserver. This will create a linked Firewall rule in which allows the traffic.Protocol:
Select
TCPSource:
Select
AnySource port range:
Select
AnyDestination:
Input
203.0.113.1- It’s the external IPv4 address of the webserver.Destination port range:
Input
443- Or select the aliasHTTPSRedirect target IP:
Input
172.16.1.1- It’s the Webserver’s internal IPv4 address in the DMZ.Redirect target port:
Input
443- Or select the aliasHTTPSDescription:
Input
Reflection NAT Rule Webserver 443- Add a description because the linked Filter rule association will use that as its name and the rule will have it in the description.NAT reflection:
Use system default
Filter rule association:
Add associated filter rule
Tip
Reading the DNAT rule like a sentence makes it clearer:
If a packet is received by the OPNsense on any of the interfaces WAN, DMZ and LAN with protocol TCP from the source IP ANY and the source port range ANY to destination
IP 203.0.113.1 and destination port 443 –> rewrite the destination IP to 172.16.1.1 and the destination port to 443.
Note
Due to “Add associated filter rule”, the added linked firewall rule in will allow traffic to the destination IP 172.16.1.1 because NAT rules match before Firewall rules. That means the firewall receives the packet and the NAT rule converts the destination from 203.0.113.1 to 172.16.1.1 first, before passing the packet to the firewall filter. You could also set “Filter rule association: Pass”, but then the resulting firewall rule would be invisible.
Note
In some setups (e.g. an external IP address is bound on an additional VPN interface) you need to set “Filter rule association: None” and create your own Firewall rules. One of those firewall rules should match only on the VPN interface, and in “advanced features” of that rule “reply-to” should be your VPN interface. The other firewall rule (without “reply-to”) should match the remaining interfaces.
Attention
Now you have Reflection NAT. The traffic from the internal LAN client 192.168.1.1 and any WAN client reaches the Webserver.
But there is a caveat - any DMZ client and the Webserver itself are still unable reach the external IP 203.0.113.1. For that you need Hairpin NAT, which involves an additional SNAT rule.
- Go to
Select Hybrid outbound NAT rule generation and save. That way you can have manual outbound rules in conjunction with automatic IP-Masquerading rules. You could also choose Manual outbound NAT rule generation. Please make sure that you create your own IP-Masquerading rules with the manual outbound NAT enabled.
Select + to create a new Outbound NAT rule.
Interface:
Select
DMZ- It’s the interface of the subnet the Webserver is in.Protocol:
Select
TCPSource Address:
Select
DMZ net- It’s the alias for the DMZ Network172.16.1.0/24Source Port:
Select
AnyDestination Address:
Input
172.16.1.1- It’s the Webserver’s internal IPv4 address in the DMZ.Destination Port:
Input
443- Or select the aliasHTTPSTranslation/target:
Select
DMZ address- It’s the alias for the OPNsense Interface IPv4 address172.16.1.254in the DMZ Network.Description:
Input
Hairpin NAT Rule Webserver 443
Tip
Reading the SNAT rule like a sentence makes it clearer:
If a packet is received by the OPNsense on the interface DMZ with protocol TCP from the source net 172.16.1.0/24 and the source port ANY to destination IP 172.16.1.1 and destination port 443 –> rewrite the source ip to 172.16.1.254 and answer from the OPNsense DMZ interface.
Note
Now all DMZ clients (and the Webserver itself) can reach the Webserver with its external IP.
You need this additional SNAT rule to avoid asymmetrical traffic between clients and servers in the same layer 2 broadcast domain. TCP traffic won’t work otherwise.
Repeat Method 1 until all additional servers are reachable.
If you encounter any issues, check Troubleshooting NAT Rules for a few tips.
Warning
The following methods are not adviced, but are still explained in order to prevent misconfigurations. There is more information in (Advanced) Settings.
Method 2 - Creating Automatic Port-Forward NAT (DNAT), Manual Outbound NAT (SNAT), and Manual firewall rules
- Go to
Enable Reflection for port forwards to create automatic rules for all entries that have
WANas interface.
- Go to
Create the NAT rule as in Method 1 - Port Forward but change the following things:
Make sure that your Port Forwarding rule specifies only
WANas interface.
- Go to
Action:
Select
PassInterface:
Select
WAN,DMZandLAN- Select all interfaces in which clients are that should access the webserver.Protocol:
Select
TCPSource:
Select
AnyDestination:
Input
172.16.1.1- It’s the Webserver’s internal IPv4 address in the DMZ. NAT matches before firewall.Destination port range:
Input
443- Or select the aliasHTTPSDescription:
Input
Reflection NAT Rule Webserver 443- Go to
Create the NAT rule as in Method 1 - Outbound
Method 3 - Creating Automatic Port-Forward NAT (DNAT), Automatic Outbound NAT (SNAT), and Manual firewall rules
- Go to
Enable Reflection for port forwards to create automatic rules for all :menuselection: Firewall –> NAT –> Port Forward that have
WANas interface. Enable Automatic outbound NAT for Reflection to create automatic SNAT rules.- Go to
Create the NAT rule as in Method 2 - Port Forward
- Go to
Create the floating firewall rule as Method 2 - Floating
One-to-One NAT Reflection
When Reflection for 1:1 is activated, automatic Reflection NAT rules for all One-to-One NAT rules are generated.
If you want to create manual Reflection and Hairpin NAT rules, leave Reflection for 1:1 disabled and follow the steps in Method 1. The only change is not adding the WAN interface to the Port Forward rules you create. The resulting Port Forward and Outbound NAT rules are in addition to the existing One-to-One NAT rules.
If your Port Forward rule has 1 interface selected (e.g. LAN), the resulting Filter rule association: Add associated filter rule will appear in . If you have more than 1 interface selected, it will appear in Firewall –> Rules –> Floating.
Troubleshooting NAT Rules
Tip
Open SSH shell:
Display all loaded and active NAT rules:
pfctl -s nat“rdr” means rules.
“nat” means rules.
You can also check the rules in the GUI in
Tip
Displays all NAT rules in the OPNsense debug:
cat /tmp/rules.debug | grep -i natIf there are more rules here than in
pfctl -s nat, it means you forgot to hit apply somewhere.
Tip
Look at the default drops of the firewall live log in
Turn on logging of the NAT and Firewall rules you have created, and check if they match in . NAT rules have the label “NAT” or “RDR”. Firewall rules have their description as label.
In “ you can check if there is a session between your internal client and your internal server, and which rule matches to it.
Use tcpdump on the client, the opnsense and the server, and test if the traffic goes back and forth between the devices without any mistakes. Look for TCP SYN and SYN ACK. If there are only SYN then the connection isn’t established and there are mistakes in your rules.