vpn
I know there are many tutorials on the net for setting up VPN client and server.
I you find one that is simple/short, I am interested only in the server part.
For client, I will use OS X so I prefer a solution that uses L2TP over IPsec or PPTP because I suppose Cisco IPsec would cost something.
I don't want to spent too many hours setting it up. You know, maybe someone already made a setup script 🙂
Note: I am using an EC2 micro instance running Ubuntu 12.04 on it.
Try:
conn tab-vpn
...
rightid=192.168.122.2
in /etc/ipsec.conf
The rightid parameter can be used to specify the IP address that the server identifies itself as (ie. it's LAN IP address rather than the public IP address of the router).
If the expected IP address and the IP address that the server identifies as don't match then ipsec abandons the connection attempt.
(I did not manage to test this yet, as of lack of terminal skills)
somebody made a little docker image with an vpn client with preshared-key support. The motivation behind:
Does your office or a client have a VPN server already setup and you just need to connect to it? Do you use Linux and are jealous that the one thing a MAC can do better is quickly setup this kind of VPN?
https://github.com/ubergarm/l2tp-ipsec-vpn-client
(also check out the fork - there is atm just one real which extends it with a run.sh script)
In a best case scenario you need to:
deb [arch=amd64] https://download.docker.com/linux/ubuntu xenial stable 
I hope this helps a little
Best Answer
UPDATE: As of Sierra, macOS no longer supports PPTP vpn. This answer is not valid for macOS clients Sierra and beyond.
PPTP via PoPToP is easy
apt-get install pptpd/etc/pptpd.confand set the remoteip option to a range on your network that is NOT served by your DHCP server./etc/ppp/chap-secretsand add a username and passworde.g.
This is all it takes to setup pptp. Now test it with your OS X client.