You could use openvpn with dynamic DNS for the "server" side (which is the side that is at a "known" address so the other side can find it to start a connection)

https://openvpn.net/index.php/open-source/documentation/howto.html#dynamic

Feel free to ask here for tips if you have questions setting it up; Linux security tools (such as openvpn) happen to be one of my specialties.

@Speedster: thanks for tip. Will it work in case I am behind NAT? I mean, I own no server to which I can connect to. I looked on it a little, seems quite difficult, but i will give it a chance. I heard about services, which use some public server (like jabber) just to connect 2 PCs. Sorry if I said some nonsense, networking (and my english :-)) are my greatest weakness.

I also looked at some other apps:

1) Hamachi.
Working great (even on Linux), but it is closed source (dont trust it). Only take it as last option.

2) n2n "http://www.ntop.org/products/n2n/"
Sounds great, opensource and quite simple. Unfortunatelly, i cant find any supernode server, VPNHosting.cz is offline since april :-(

3) Wippien "http://www.wippien.com/"
It is also open source (it consists of 2 files, one is easily compiled, but second require some files, which are missing... but on web is also binary file). It is working great on Windows (just need some xmpp account to connect), but on Linux it is not working at all (this project is dead for few years).

4) SocialVPN. I did not try it yet, seems cool, but needs Mono. I will try it later, maybe.

?) Do you know anything else?

openvpn is the "home level" solution with which I'm familiar -- as opposed to even more complicated VPNs like ipsec more commonly used by businesses. Plus I did use hamachi for a while when I had to, because a client was using it, but I agree with your sentiments about it.

For openvpn, the "server" side which is registered with dynamic DNS also needs to have one port-forwarding rule (normally for port 1194, but can be changed) set up on the NAT box. By "NAT box" I refer to whatever is doing the outgoing NAT for you: a combination firewall/cable-or-dsl-modem that your internet provider gave you, or perhaps a wirelesss+wired router separate from the modem (I've got an openwrt router + DSL modem myself).

If you can get that one NAT rule configured, then the only other outside dependency is on the dynamic DNS server that will publish changes in the temporary outside IP behind which your traffic is being NAT-ed.

Thanks a lot, Speedster, for OpenVPN tip! Yesterday I found out that our school has OpenVPN server. So OpenVPN is great solution for me (for few next years when I will be in that school).

Awesome, glad I could help!

I use my proper VPN with OVH server

Hey guys,

I am back with new info. I finally managed to start p2p connection with my friend using Wippien. It is not fully open source software (VPN part is closed), but it is still better than hamachi, isn't it?

Wippien on Linux consist of two parts:

1) server, which is used to create new network device. This software has to be run with root privileges. Fortunately, this part can be compiled from source code - if you are running 64 bit machine, you have to
apply this little patch (or you can use precompilled 32 bit binary - if you trust it)

2) client, which will create VPN session on network device created by server. Unfortunately, this part depends on third party VPN library, so it can not be recompilled from source code. You have to download 32bit binary file.
This software depends on little bit obsolete crypto library (libcrypto.so.0.9.8), so if you don't have it in your distribution, download it here. If you have 64 distro, use config like this "setarch i386 ./config shared threads -m32". Then add this library to some place for LD_LIBRARY_PATH.
Another bad thing about client is that it is compilled with old XMPP library - so you have to use some XMPP server with old/none crypto (like [url=im.apinc.org]this[/url])
Last bad thing is when your friend disconnect from wippien, your client will fail on Sigsegv

Final step is run server with root privileges and client as regular user. We have tested it with my Windows friend on Arma 2 game (Wine Gallium Nine vs Windows 10) - works fine!

I'm using NeoRouter Free (link: http://www.neorouter.com/downloads), it's easy to setup, you host and manage your own server and create user accounts. Didn't test it much, only for Torchlight 2 with my friend on Windows 7, very stable connection and no lag whatsoever.