Prerequisites

I use libreswan on my Debian box. Install required packages:

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apt install -y xl2tpd libreswan

Example Configurations

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conn someconnnamehere
    auto=add
    authby=secret
    type=transport
    # ... or 17/1701 for L2TP
    leftprotoport=gre
    rightprotoport=gre
    # set right to the ip address of your vpn server
    right=1.1.1.1
    left=192.168.1.1

… and edit /etc/ipsec.secrets, for a line of your PSK for the connection:

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192.168.1.1 1.1.1.1 : PSK "your_pre_shared_key"

Then service ipsec restart to reload confs, and enable the connection by ipsec auto --add someconnnamehere.