One to one NAT on OVH vRack with OPNSense

I have a virtual OPNSense router as the gateway to a production network based on Proxmox servers (more about this another day). All VMs and containers get an unrouted IP address in the 10.x.x.x range and I have a couple of IP ranges for public access, generally but not always behind Haproxy. This doesn\t seem to be documented properly elsewhere.
## A quick summary:
1. Assign IP range(s) as Virtual IPs
2. Create a One to One NAT between an external IP from a virtual IP range and an internal address
3. Create a WAN rule for the ports you want open to the internal address.
### 1. Assign IP ranges as Virtual IPs
Our servers are with OVH and connected by their vRack service, which is remarkably flexible - connected servers can use an unrouted IP network, which is shared between them automatically, and for routed IP addresses, you rent a range (minimum is a /30, for reasons I\ll explain shortly) and allocate it to the vRack, and in the simplest usage they are allocated to a physical interface on a server and routed to the ranges\ gateway, which is, somewhat counter-intuitively, the second to last address in the range, which is why the smallest range possible is a /30:
| Range | CIDR | Gateway | Usable addresses |
|-------|------|---------|------------------|
| 192.168.10.20 | /30 | 192.168.1.22 | 1 |
| 192.168.10.16 | /29 | 192.168.1.22 | 5 |
| 192.168.10.16 | /28 | 192.168.1.29 | 12 |
For the purposes of this article, I\ll add a /29 as I have a routed one that\s about to be reallocated from my account.
a) Go to Interfaces > Virtual IPs > Settings
b) Press \+\
c) For the mode in this context, you will use Proxy ARP as IP Alias is for a single IP - the \Single IP\ option is disabled.
d) Select WAN for the interface
e) Select \Network\ for Type
f) Enter the network range in the Address field without the CIDR range and select that from the dropdown. You don\t define a gateway at this point, which also seems a bit counterintuitive but that comes at allocation time.
g) Set a description and save.
h) Apply changes on the main page.
### 2. Enable Reflection in the Firewall settings
UPDATE 19/12/2022: I\ve been trying to work out why I couldn\t search other ActivityPub services from my Mastodon instance and found that on my local network at least, I couldn\t find the address of my blog, so this is the fix for being able to access services internally.
a) Go to Firewall > Settings > Advanced
b) In the Network Address Translation section, check the Reflection options that you use. I went with all three just to be sure.
c) Save.
### 2. Add a One to One NAT
a) Go to Firewall > NAT > One-to-One
b) Press \+\
c) For \Interface\ select \WAN\
d) For Type selected \BINAT\ so outbound traffic is rewritten as well
e) For \External network\ select an IP from your allocation and give it the CIDR range /32
f) For \Source\ select \Single host or Network\ and enter the IP address of the destination server/VM/container, again with a CIDR range of /32. This is also somewhat non-intuitive at first as the pfSense instructions say the CIDR range should be the same for both addresses and in this case we have a /29 for external traffic and a /22 for internal traffic.
g) Set \Destination\ to \any\
h) Add a description
i) Set NAT reflection to \Use system default\ or as required. The default is set in Firewall:Settings:Advanced and is Disabled.
j) Save
k) Apply changes on the main page
### 3. Create firewall rules
a) Go to Firewall > Rules > WAN
b) Press \+\
c) Action is \Pass\
d) Interface is WAN
e) Direction is in
f) TCP/IP version is generally TCP
g) Source is \any\
h) Destination port range will usually a single port for a protocol such as HTTPS
i) Destination is \Single host or Network\ and the value is the IP address of the backend device with the correct CIDR range for your network, so in this case it\s 10.10.x.x/22
j) Optionally set a description, but it\s advisable as it\s useful for documentation.
k) Save
l) Apply changes on the main page.
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