How to configure StrongSwan IKEv2 VPN with PSK (pre-shared key)?

Assuming that you want to setup your right side with psk. This is fairly easy.

1. remove eap_identity and rightsendcert fields. 2. set rightauth=secret

Now edit /etc/ipsec.secrets file:

1. remove "your_username %any% : EAP "your_password"" line. 2. add ": PSK <your_password>"

Then reread the secrets and restart the service.

$sudo ipsec rereadsecrets $sudo ipsec reload $sudo ipsec restart

All set. Follow "Connecting from iOS" and create a new ikev2 vpn connection. In authentication settings select none and put the shared secret key. Hopefully you connect.

Edit:

Based on the comments, configuration changes required to switch to pre-shared key authentication:

config setup
    charondebug="ike 1, knl 1, cfg 0"
    uniqueids=no

conn ikev2-vpn
    auto=add
    compress=no
    type=tunnel
    keyexchange=ikev2
    fragmentation=yes
    forceencaps=yes
    ike=aes256-sha1-modp1024,3des-sha1-modp1024!
    esp=aes256-sha1,3des-sha1!
    dpdaction=clear
    dpddelay=300s
    rekey=no
    left=%any
    leftid=@server_name_or_ip
    leftsubnet=0.0.0.0/0
    right=%any
    rightid=%any
    rightdns=8.8.8.8,8.8.4.4
    rightsourceip=10.10.10.0/24
    authby=secret

Remove the following line from ipsec.secrets:

server_name_or_ip : RSA "/etc/ipsec.d/private/vpn-server-key.pem

Then reread the secrets and restart the service.

Based on my tinkering and @ChandanK answer, I've made two scripts to deploy a StrongSwan VPN server on a fresh Ubuntu 16.04 install here: https://github.com/truemetal/ikev2_vpn