Simple question, difficult solution. I can’t work it out. I have a server at home with a site-to-site VPN to a server in the cloud. The server in the cloud has a public IP.
I want people to access server in the cloud and it should forward traffic through the VPN. I have tried this and it works. I’ve tried with nginx streams, frp and also HAProxy. They all work, but, in the server at home logs I can only see that people are connecting from the site-to-site VPN, not their actual source IP.
Is there any solution (program/Docker image) that will take a port, forward it to another host (or maybe another program listening on the host) that then modifies the traffic to contain the real source IP. The whole idea is that in the server logs I want to see people’s real IP addresses, not the server in the cloud private VPN IP.
Short answer: Don’t bother, it’s too complex to setup (unless your app is HTTP or supports the PROXY protocol). You better read your proxy logs instead.
Long answer: What you want is called “IP transparency” and require your proxy to “spoof” the IP address of the client when forwarding packets to the remote server. Some proxies do it (Nginx plus, Avi Vantage, Fortinet) but are paid services. I don’t know for free solutions as I only ever implemented it with those listed above.
This require a fairly complex setup though:
0. IP address spoofing
The proxy must rewrite all downstream request to spoof the client IP address, making it look like the traffic originates from the client at the TCP layer.
1. Backend server routing
As the packet will most likely originate from random IP on the internet, your backend server must have a way to route back the traffic to the proxy, instead of it’s default gateway. Otherwise you’d implement what is called "Direct Server Return*, which won’t work in your case (packet will be dropped by the client as originating from your backend server directly, and not from the proxy).
You have two solutions here:
- set your default gateway to the proxy over its VPN interface (don’t do that unless you truly understand all the implications of such a setup)
- use packet tagging and VRF on the backend server to route back all traffic coming from the VPN, back to the VPN interface (I’m not even sure this would work with an IPsec VPN though because of ACL…)
3. Intercept and route back return traffic
The proxy must be aware that it must intercept this traffic targeted at the destination IP of the client as part of a proxied request. This require a proxy that can bind on an IP that is not configured on the system.
So yeah, don’t do that unless you NEED to do that (trust me as I had to do it, and hated setting it up).
Edit: apparently haproxy supports this feature, which they call transparent mode
I think this is right but to make it work you’d need to do one of two things to pull it off. First off, if you’re doing it just for Web the nginx proxy putting original ip in the header and unpacking on the other side is the smart move. Otherwise.
1: route all your traffic on your side via the vpn, and have the routing on the vpn side forward the packets to the intranet ip on your side not do dnat on it.
2: if you want to route normal traffic over your normal link then you could do it with source routing on the router. You would need two subnets, one for your normal Internet and one for the vpn traffic. Setup source routing to route packets with the vpn ip addresses go via vpn and the rest nat the normal way then the same as before, vpn on cloud forwards not nat to your side of the vpn.
In both cases snat should be done on the cloud side.
It’s a fiddly setup just to get the ip addresses though.
I think you meant to reply to another comment. I never talked about setting up NAT rules, neither source, nor destination.
The proxy is responsible for responding with the correct IP address as it terminates the connection. Setting up NAT rules is not needed.
Well, I was replying to OP through your reply since it was pretty much spot on. Except I was giving some idea of other ways to bring the original IP through a VPN using the linux ip stack features. Whatever way they go about it, it’s a lot of effort for not that much upside though.
Is there any solution (program/Docker image) that will take a port, forward it to another host (or maybe another program listening on the host) that then modifies the traffic to contain the real source IP. The whole idea is that in the server logs I want to see people’s real IP addresses, not the server in the cloud private VPN IP.
Not that I’m aware of. Most methods require some kind of out-of-band way to send the client’s real IP to the server. e.g. X-Forwarded-For headers, Proxy Protocol, etc.
If your backend app supports proxy protocol, you may be able to use HAProxy in front on the VPS and use proxy protocol from there to the backend. Nginx may also support this for streams (I don’t recall if it does or not since I mainly use HAProxy for that).
Barring that, there is one more way, but it’s less clean.
You can use iptables on the VPS to do a prerouting DNAT port forward. The only catch to this is that the VPN endpoint that hosts the service must have its default gateway set to the VPN IP of the VPS, and you have to have a MASQUERADE rule so traffic from the VPN can route out of the VPS. I run two services in this configuration, and it works well.
iptables -t nat -A PREROUTING -d {VPS_PUBLIC_IP}/32 -p tcp -m tcp --dport {PORT} -j DNAT --to-destination {VPN_CLIENT_ADDRESS}
iptables -t nat -A POSTROUTING -s {VPN_SUBNET}/24 -o eth0 -j MASQUERADE
Where eth0 is the internet-facing interface of your VPS.
Edit: One more catch to the port forward method. This forward happens before the traffic hits your firewall chain on the VPS, so you’d need to implement any firewalls on the backend server.
Setting the default gateway to the VPN has many implications that you must take into account before doing it:
- you need to allow ALL traffic through the VPN ACL, which nullify the concept of ACL as a security measure.
- it breaks the VPN as the encapsulated packets cannot reach the other site. You need a /32 route to the other site to keep the VPN up.
- it will route ALL the internet traffic from this host through the VPN, and the internet access of the other site.
- it could break access to LAN of the server, so you might need to set your local routes manually.
- it can let your server access the LAN of the remote server, this leaking local networks.
A better option would be to use VRFs to route back traffic coming through the VPN back to it.
Thank you so much for the quick and detailed reply, appreciate it!
Done all of the iptables stuff, just trying to change the default gateway on the server at home now:
network:
version: 2
renderer: networkd
ethernets:
eth0:
dhcp4: true
routes:
- to: 0.0.0.0/0
via: <vps public ip>
Does the above netplan yaml look right? When it’s applied, I can’t access the internet or even the VPS public IP.
Forgot to ask: Is your server a VPN client to the VPS or a VPN server with the VPS as a client? In my config, the VPS is the VPN server.
Not sure about the netplan config (all my stuff is debian and uses oldschool /etc/network/interfaces), but you’d need logic like this:
Server is VPN client of the VPS:
routes:
# Ensure your VPS is reachable via your default gateway
- to: <vps public ip>
via: <your local gateway>
# Route all other traffic via the VPS's VPN IP
- to: 0.0.0.0/0
via: <vps vpn ip>
You may also need to explicitly add a route to your local subnet via your eth0 IP/dev. If the VPS is a client to the server at home, then I’m not sure if this would work or not.
Sorry this is so vague. I have this setup for 2 services, and they’re both inside Docker with their own networks and routing tables; I don’t have to make any accommodations on the host.
Everything I use is in Docker too, I’d much rather use Docker than mess around with host files, but to try it out I don’t mind. If you have an image you could share, I’d appreciate it.
Anyway, neither are clients or servers as I just used ZeroTier as a quick setup. On my other infra I use wireguard with the VPS being the server (that setup works well but I only reverse proxy HTTP stuff so X-Forwarded-For works well).
Do I need to specify to forward VPN traffic through my router and then traffic to 0.0.0.0/0 through the VPN?
Short answer no, but you can add the source IP as part of the http header https://www.nginx.com/resources/wiki/start/topics/examples/forwarded/ then you have to log that bit of the header at the app level.
There can be ways of your are using ipv6, basically turning your cloud host into a router, but but ipv4 you would have to have a 1:1 mapping and setup the routing carefully to make it work.
Isn’t that what the logs on the proxy are for?
I realised I forgot to update this. Thank you to everyone that contributed, I appreciate it. This was a weird use case and barely anyone online has documented it, only a handful of places. Nevertheless, I figured it out.
So basically, you run HAProxy with the send-proxy-v2 protocol. Let’s say I’m forwarding SSH from VPS to home, I’d have the VPS running HAProxy listening on port 22. Then I’d have it forward to home on port 220. Then, on the home server, you run this amazing piece of software called go-mmproxy. Configure that to listen on port 220 and forward to localhost 22. And there you have it.
HAProxy passes the real source IP to go-mmproxy with the proxy protocol, go-mmproxy takes the proxy header and strips it from the request, spoofs the source IP address from localhost to the real source IP contained in the proxy header then makes the request to localhost. And then you also have to configure traffic to go back through localhost so go-mmproxy can pick it up and add the proxy header back to the request, to be sent back to the source.
