So, I’m experimenting with running a Mailu instance on my home server but proxying all of the relevant traffic through a WireGuard tunnel to my VPS. I’m currently using NGINX Proxy Manager streams to redirect the traffic and it all seems to be working.

The only problem is that, all connections appear to come from the VPS. It’s really screwing with the spam filter. I’m trying to figure out if there’s a way to retain the source IP while still tunneling the traffic.

The only idea I have, and I don’t know if it’s a bad one, is to us iptables to NAT the ports inbound on the VPS and on my home router (opnsense) route all outbound traffic from that IP back through the VPS instead of the default gateway. This way I shouldn’t need to rewrite the destination port on the VPS side.

It sound a bit hacky tho, and I’m open to better suggestions.

Thanks

Edit: I think I need to clarify my post as there’s some confusion in the comments. I would like the VPS to masquerade/nat for my mailu system accessible over a WG tunnel so that inbound traffic to the SMTP reports it’s actual public IP instead of the IP of the VPS host that’s currently proxying.

After giving that some thought I think the only way this could work would be if I treated the VPS as the upstream gateway for all traffic. My current setup is below:

[VPS] <-- wg --> [opnsense] <–eth–>[mailu]

I can source route all traffic from mailu to the VPS, via wg, but I don’t know how to properly configure iptables to do the masquerading as I’d only want to masquerade that one IP. I’m not concerned about mailu not having internet access when wg is down, and frankly, I think I’d prefer it didn’t.

Edit 2: I got the basic masquerading working. Can ping public IPs and traceroute verifies it’s taking the correct path.

iptables -A FORWARD -i wg0 -s <mailu-ip> -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -s <mailu-ip> -j MASQUERADE

I think I got the port forwarding working.

iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25 -j DNAT --to-destination <mailu-ip>
iptables -A FORWARD -p tcp -d <mailu-ip> --dport 25 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

• tcpdump on the VPS eth0 shows traffic in.
• tcpdump on the VPS wg0 shows the natted traffic.
• tcpdump on mailu shows both inbound and outbound traffic.
• tcpdump on opnsense shows 2 way traffic on the vlan interface mailu is on.
• tcpdump on opnsense only shows inbound, but not outbound traffic on the wg interface.

I think the problem is now in opnsense but I’m trying to suss out why. If I initiate traffic on mailu (i.e. a ping or a web request) I see it traversing the opnsense wg interface, but I do not see any of the return SMTP traffic.

Edit 3:

I found the missing packets. They’re going out the WAN interface on the router, I do not know why. Traffic I initiate from the mailu box gets routed through the WG tunnel as expected but replies to traffic sourced from the internet and routed over the WG tunnel, are going out the WAN.

The opnsense rule is pretty basic. Source: <mailu>, Dest: any, gateway: wg.

Edit 4:

I ran out of patience trying to figure out what was going on in opnsense and configured a direct tunnel between the mailu vm and the VPS. That immediately solved my problems although it’s not the solution I was striving for.

It was pointed out to me in the comments that my source routing rule likely wasn’t configured properly. I’ll need to revisit that later. If I was misconfiguring it I’d like to know that.