letsencrypt_strongswan_guide

##Requirement

A domain name: for example xyz.wuruxu.cn and resolve to VPS public IP
VPS: Linode VPS
Ports 4500/UDP, 500/UDP, 51/UDP and 50/UDP opened in the firewall

1. Setup VPN Server

1.1 Build strongswan

openssl >= 1.0.2 is required for enable ECP521/ECP384/ECP256 & ECDSA support

# apt-get install libssl-dev

#./configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var --enable-af-alg --enable-ccm --enable-chapoly --enable-ctr --enable-gcm --enable-newhope --enable-openssl --enable-aesni --enable-sqlite --enable-dhcp --enable-eap-identity --enable-eap-tls --enable-eap-ttls --enable-eap-mschapv2 --enable-systemd CFLAGS=-O2 --disable-ikev1

1.2 Issue free certificate (RSA or ECDSA)

1.2.1 clone acme.sh to issue certificate

# git clone https://github.com/Neilpang/acme.sh

ECDSA certiifcate - recommend

# ./acme.sh --issue --standalone -d xyz.wuruxu.cn --keylength ec-384 --server letsencrypt

RSA certificate

# ./acme.sh --issue --standalone -d xyz.wuruxu.cn --keylength 4096 --server letsencrypt

1.2.2 install certificates in strongswan

# cp ca.cer /etc/ipsec.d/cacerts/
# cp fullchain.cer  /etc/ipsec.d/certs/acme_xyz_server.cert.pem
# cp xyz.wuruxu.cn.key  /etc/ipsec.d/private/acme_xyz_ecc.pem

###1.3 Configure strongswan server in ipsec.conf, left as 'local', right as 'remote'

1.3.1 /etc/ipsec.conf

# ipsec.conf - strongSwan IPsec configuration file

config setup
  uniqueids=never

conn %default
  keyexchange=ikev2
  left=%defaultroute
  leftauth=pubkey
  leftfirewall=yes
  right=%any
  mobike=yes
  compress=no
  ike=chacha20poly1305-sha512-newhope128,chacha20poly1305-sha512-x25519,aes256-sha512-modp2048,aes128-sha512-modp2048,aes256ccm96-sha384-modp2048,aes256-sha256-modp2048,aes128-sha256-modp2048,aes128-sha1-modp2048!
  esp=chacha20poly1305,aes256gcm128,aes128gcm128,aes256ccm128,aes256
  
  
 # IPv4 connection server config
 conn ec4
  leftsendcert=always
  leftcert=acme_xyz_server.cert.pem
  leftid=xyz.wuruxu.cn //same as certificate domain name
  leftsubnet=0.0.0.0/0
  rightauth=eap-mschapv2
  rightsourceip=10.18.0.0/24
  rightsendcert=never
  eap_identity=%any
  auto=add
 
# IPv6 connection server config
conn ec6
  leftsendcert=always
  leftcert=nginx.ssl.ipv6.wuruxu.ecc.cer
  leftid=@ipv6.wuruxu.cn
  leftsubnet=0.0.0.0/0,::/0
  rightauth=eap-mschapv2
  rightsourceip=2001:0909:1818:2d18:1::/80,10.128.0.0/24
  rightdns=2001:4860:4860::8888,1.1.1.1
  rightsendcert=never
  eap_identity=%identity
  auto=add

1.3.2 /etc/ipsec.secrets

: ECDSA acme_xyz_ecc.pem
user : EAP "userpasswd"

1.3.3 /etc/sysctl.conf

net.ipv4.ip_forward = 1  
net.ipv4.conf.all.accept_redirects = 0  
net.ipv4.conf.all.send_redirects = 0

enable sysctl rules

#  sysctl -p

###1.4 Start strongswan

1.4.1 apply iptables rule

# iptables -A INPUT -p udp --dport 500 --j ACCEPT
# iptables -A INPUT -p udp --dport 4500 --j ACCEPT
# iptables -A INPUT -p esp -j ACCEPT
# iptables -t nat -A POSTROUTING -s 10.18.0.0/24 -o eth0 -j MASQUERADE

1.4.2 start ipsec daemon

# ipsec start

2. Strongswan Client Configuration

additional NOTES for android app to handle multi SSL certificate on one server:

Linux

1. install strongswan on linux platform(same options as above)

./configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var --enable-af-alg --enable-ccm --enable-chapoly --enable-ctr --enable-gcm --enable-newhope --enable-openssl --enable-aesni --enable-sqlite --enable-dhcp --enable-eap-identity --enable-eap-tls --enable-eap-ttls --enable-eap-mschapv2 --enable-systemd CFLAGS=-O2 --disable-ikev1

install DST Root CA

$ cp /etc/ssl/certs/DST_Root_CA_X3.pem /etc/ipsec.d/cacerts/

2. config strongswan

2.1 /etc/ipsec.secrets

# /etc/ipsec.secrets - strongSwan IPsec secrets file

user : EAP "userpasswd"

2.2 /etc/ipsec.conf

# ipsec.conf - strongSwan IPsec configuration file

# basic configuration

config setup
  strictcrlpolicy=no
  uniqueids = never

conn %default
  ikelifetime=3h
  keylife=60m
  rekeymargin=9m
  keyingtries=3
  keyexchange=ikev2
  ike=chacha20poly1305-sha512-x25519,aes256-sha512-modp4096,aes128-sha512-modp4096,aes256ccm96-sha384-modp2048,aes256-sha256-modp2048,aes128-sha256-modp2048,aes128-sha1-modp2048!
  esp=aes256gcm128,aes128gcm128,aes256ccm128,aes256

conn ec4
  left=%any
  leftfirewall=yes
  leftauth=eap-mschapv2
  leftid=debian # specifiy the client ID
  leftsourceip=%config4
  leftikeport = 4500
  eap_identity=wuruxu  # user in /etc/ipsec.secrets
  right=xyz.wuruxu.cn
  rightauth=pubkey
  rightid=xyz.wuruxu.cn
  rightsubnet=0.0.0.0/0
  auto=add
  
conn net0
  leftsubnet=192.168.0.0/16
  rightsubnet=192.168.0.0/16
  authby=never
  type=pass
  auto=route

conn net1
  leftsubnet=10.0.0.0/8
  rightsubnet=10.0.0.0/8
  authby=never
  type=pass
  auto=route

2.3 start up connection ec3

# ipsec start
# ipsec up ec4

NOTES: mtu size issue on linux platform

edit /etc/strongswan.d/charon/kernel-netlink.conf to change mtu size from 0 to 1280

mtu = 1280