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.

  • RedFoxEnglish
    1•1 year ago

    I don’t know what open sense calls this, but you want a route map in Cisco world.

    A route map overrides the routing table based on ACL.

    Your local router thinks unless there is a moreb specific route, then use default out WAN. Route map says ACL if source is mailu, then next hop will be VPS over tunnel. I did this with Cisco DMVPN between Germany and states.

    Sorry I don’t know terms of open sense, but the concept is the same.

      • RedFoxEnglish
        1•1 year ago

        When you did your dump of the tunneled traffic, was the VPS preserving the public IPs?

        I was only assuming since you mentioned seeing the traffic at each segment.

        • 𝓢𝓮𝓮𝓙𝓪𝔂𝓔𝓶𝓶OPEnglish
          2•1 year ago

          Once I got masquerading configured it was preserving the public IP. I tcpdumped every interface in the path and watched the traffic. When it hit opnsense instead of respecting the policy based routing it was routing the traffic out the WAN.

          What baffles me is if I initiated traffic from the mailu server (ping, wget, etc…) I could see that opnsense was routing all traffic in that conversation out the WG interface, none of it hitting the way.

          I need to update the post because after fighting with it all day, I realized I was being stubborn (I have a need to solve the problem). I configured a direct WG tunnel between the VPS and the mailu VM and routed the traffic that way. It’s all working exactly as I need it to now.

          I’d still like to know if opn has a bug or if I was missing some setting as I’d rather not be littering my network with tunnels when I shouldn’t need to and I can leverage some smarts in opn (i.e. if the tunnel is down, the gateway would get marked down in opn and it would ignore the policy route).

          • RedFoxEnglish
            2•1 year ago

            Yeah, the tunnel would annoy me unless it was open and I could scan the traffic as it went through my proxy. My OCD kicks in when stuff doesn’t do what it’s supposed to

            You could always swap out that OPNS for Cisco, NBD. /Sarcasm