How to SSH to device which is connected via VPN to server?

Question

I have a laptop, a server (static IP) and a PC (dynamic IP).

The PC is connected to the server via OpenVPN service. I want to use this to SSH from my laptop to this PC via server. I assume this is the easiest way to SSH to this computer given it uses a dynamic IP address.

Is it possible? If yes, how can it be done?

Solution

Since the PC has a dynamic address, it must be assigned a hostname.

1. If the dynamic IP is internal to your network and you control the DHCP/DNS server such as Dnsmasq, you can make it assign a hostname.
2. If the dynamic IP is ISP-assigned, use a DDNS service such as DynDNS to assign your PC a hostname.

Once your PC has a proper hostname, you can setup your server as the SSH jumphost:

1. Install your laptop's public SSH key in the server and the PC.

2. In your laptop's ~/.ssh/config, add the following

   Host <PC_hostname>
   ProxyJump <server_hostname>
   

There are 2 alternatives that don't use DDNS.

Make the PC dump its IP in the server:

1. PC dumps its IP in user@server:~/pc-ip-addr.txt at 1-minute intervals.
2. In the laptop, read the file and SSH to the PC's IP.

Make PC maintain a permanent SSH connection to server:

1. In the PC, ssh user@server -R 2222:localhost:2222
2. In the laptop, ssh user@server "ssh user@localhost -p2222"

Comments

[Артур Мурадянц]

I heard that connecting via VPN is similar to connecting device to your local network. So I thought I can use VPN connection instead of dynamic dns. Is it possible? Is it possible to do with out services like dynnds?

[Eugene Chow]

It's true that a VPN connection makes your device "local". Since your IP is dynamic, your your laptop needs a way to identify the PC. I've edited the answer above to include 2 DDNS-free methods. Tell me which you prefer and I'll provide more details.