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How can I restrict visiting a service through a domain to VPN-connected devices? (lemmy.world)

submitted 1 year ago by [email protected] to c/[email protected]

19 comments fedilink hide all child comments

Say I have a public server with a service (email, web server, etc) that's accessible through https://myservice.example.com, and I would like to restrict that service with a VPN. How do I do that?

I know how to setup a VPN. I know how to use some of the services through that VPN. But see, if I want to use that VPN, I connect my client to that VPN, then I get the subnet of that VPN, say 10.10.100.0, through which I can access the devices by address.

But I see some services offer things like https://myservice.example.com, and they only work when that VPN is connected. How does that work? Is it just some DNS setting at the domain level or there's more to it?

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[–] [email protected] 2 points 1 year ago (1 children)

Same as the public one, just with an internal address.

Have this on my domain, public domain with a subdomain server behind VPN and 1 host that points to an internal address.

Anyone tries to reach from outside just times out or something.

DNS is just a lookup of names to numbers, that's all it is, the numbers can be anything, I can point my domain to Google if I want.

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago) (3 children)

Please elaborate a little more. So assuming the server where the service lies has IP address 1.2.3.4, and some VPN that I can connect to with 1.2.3.4:1194. If my DNS server points to 1.2.3.4, and say there's an http server there that's normally accessible with 1.2.3.4:80, how will we enforce that working only through VPN?

[–] [email protected] 1 points 1 year ago (1 children)

Whatever you do with IPs, you can do with ports too.

"Allow this port, but not that one".

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago) (1 children)

Please see this comment to understand my frustrations with the answers in this thread (copy/pasted from another comment):

I’ve been managing servers for over 10 years, and I never have felt stupider, and I still don’t understand how to do this. Everyone is making a comment that I don’t understand.

Let’s talk internet 101, and please tell me where I’m wrong.

You make a request to https://myservice.example.com. The DNS responds to a query giving you an IP address, say 1.2.3.4. Now the client software makes another request to 1.2.3.4:433 (say if we’re attempting to access an https server, binding the SNI address to the SSL/TLS header). The request will be sent to that server, and the server will respond. In what part of all this process can the VPN can do anything?

Normally if you want to access a device through VPN, you make a request to a WHOLE other ip address in another subnet on another (virtual) device locally. It has absolutely nothing to do with 1.2.3.4. It’s something like 10.10.100.X… or similar. How will my domain, myservice.example.com, route to that address, 10.10.100.X? Is it as dumb and simple as routing there? Or is there more to it? It doesn’t sound right to make the DNS server record point to 10.10.100.X.

[–] [email protected] 1 points 1 year ago

I'm sorry. I don't know enough about the subject. I'm sure there are firewall daemons, or IPTABLES configuration that can do what you want, but that's beyond my area of expertise. Good luck.

[–] [email protected] 1 points 1 year ago (1 children)

About the DNS, you can use an internal DNS and find some way for your VPN to configure it in all the machines connected to it.
DNS usually has a fall back if the name is not found, so you can always have your custom DNS on and it'll first check its own records then check for some level up (I'm basing this off of my experience with with pihole https://docs.pi-hole.net/ftldns/ )

About your ports question: you just need to change the ip to the VPN one.
For example, I have a VPS which has a public IP and I have tailscale installed.
If I were to make my service listen to all interfaces I could use 1.2.3.4:1194 or 100.100.100.100:1194 (this being the tailscale ip)
But I usually only configure them to listen to tailscale0, so I can no longer reach them with 1.2.3.4:1194, only with the tailscale ip.
In your DNS you need to configure this new IP to be served.

I'm guessing you can also do some configuration with a firewall.
Probably ufw add allow from 10.0.0.0/8 could work if this was the IP range of your VPN, then any one can still use your public IP and only your VPN will be able to connect (But don't quote me on this, I haven't done it).
(Just be sure to check the configuration of your service, docker can bypass ufw :/ )

[–] [email protected] 2 points 1 year ago

Thank you, but my question was specifically about DNS. Another person pointed out that setting the DNS record to the VPN destination is the right answer. I appreciate the details you wrote and I'll look into them.

[–] [email protected] 1 points 1 year ago

There's an internal ip address for the VPN server, say 4.3.2.1, you point the http dns record to that address.

The VPN server has 2 addresses by definition, an internal address and an external, public one that you connect the VPN to. Make sure the webserver only exposes itself on the private address, either by configuration (nginx/apache listen address) or by firewall (iptables -A input -j DROP)