Repost - Building IPSec VPN with Strongswan

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This article is mainly a backup of an article that has been stored on my note-taking app, Wiz Note. I do not know who the original author is, but every time I install Strongswan, I refer to this article. It is very detailed and easy to follow. If the author sees this, please comment below and I will add the repost link. This article will now be based on CentOS instead of Ubuntu, while retaining the original Ubuntu style for the convenience of the original author’s reference. It will also include examples based on the actual operations of Linode Japan2.(English version Translated by GPT-3.5, 返回中文)

There was a very good tutorial on building an IKEv2 VPN with Strongswan on nsshell.com. However, it seems that nsshell.com is down (I can’t access it). So, I decided to repost the tutorial here. There were one or two omissions in the original article, and I will add them.

Software

Strongswan 5.7.2 MD5: d449aa1936218a42e34c32494947308b Official download RuterFu My Storage

Prerequisites:

VPS or server with sufficient RAM (96MB RAM is enough, 64MB RAM not tested)
Can be OpenVZ, but pay attention to the special steps for OpenVZ mentioned in the tutorial.

This article uses Linode Japan2 server as the installation environment, CentOS 7.6 system

p.s. Linode costs just a few cents for installation, even though it costs more than 20 dollars…

isLinode

The ping is as unstable as ever…

LindoePing

System version attached

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[root@li****-18 ~]# cat /etc/centos-release
CentOS Linux release 7.6.1810 (Core) 
[root@li****-18 ~]#

Process

  1. Preparation

    Please install CentOS operating system on the virtual machine or server, both 32-bit and 64-bit are fine. This article uses version 7.6. Execute the following command to install the PAM library and SSL library on the system.

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    yum install pam-devel openssl-devel make gcc wget -y

  2. Download the latest Strongswan source code

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    [root@li****-18 ~]# wget -c https://download.strongswan.org/strongswan-5.7.2.tar.gz
    [root@li****-18 ~]# tar -zxvf strongswan-5.7.2.tar.gz
    [root@li****-18 ~]# cd strongswan-5.7.2

  3. Compilation

    Execute this if using OpenVZ

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    ./configure  --enable-eap-identity --enable-eap-md5 --enable-eap-mschapv2 \
    --enable-eap-tls --enable-eap-ttls --enable-eap-peap --enable-eap-tnc \
    --enable-eap-dynamic --enable-eap-radius --enable-xauth-eap \
    --enable-xauth-pam  --enable-dhcp  --enable-openssl  --enable-addrblock \
    --enable-unity --enable-certexpire --enable-radattr --enable-tools \
    --enable-openssl --disable-gmp --enable-kernel-libipsec

    Execute this for other servers (executed in this article)

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    ./configure  --enable-eap-identity --enable-eap-md5 --enable-eap-mschapv2 \
    --enable-eap-tls --enable-eap-ttls --enable-eap-peap --enable-eap-tnc \
    --enable-eap-dynamic --enable-eap-radius --enable-xauth-eap --enable-xauth-pam \
    --enable-dhcp  --enable-openssl --enable-addrblock --enable-unity \
    --enable-certexpire --enable-radattr --enable-openssl --disable-gmp \
    --prefix=/usr/local/strongswan

    If the output is as follows, it means successful

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    ..........
    config.status: creating config.h
    config.status: executing depfiles commands
    config.status: executing libtool commands

     strongSwan will be built with the following plugins
    -----------------------------------------------------
    libstrongswan: aes des rc2 sha2 sha1 md5 random nonce x509 revocation constraints pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp dnskey sshkey pem openssl fips-prf curve25519 xcbc cmac hmac
    libcharon:     attr kernel-netlink resolve socket-default stroke vici updown eap-identity eap-md5 eap-mschapv2 eap-dynamic eap-radius eap-tls eap-ttls eap-peap eap-tnc xauth-generic xauth-eap xauth-pam dhcp certexpire radattr addrblock unity counters
    libtnccs:      tnc-tnccs
    libtpmtss:    

    [root@li****-18 strongswan-5.7.2]#

    Wait for the compilation process to finish and then execute the following command (the make command will compile the source code, and it may take some time depending on the host performance):

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    make && make install

    Wait patiently for the compilation, the time required for compilation varies depending on performance.

    Create environment variables

    1. Go to the /usr/local/strongswan/sbin/ directory

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      [root@li****-18 strongswan-5.7.2]# cd /usr/local/strongswan/sbin/

    2. Make a note of this directory and then edit /etc/profile, add export PATH=/usr/local/strongswan/sbin:$PATH

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      vi /etc/profile
      追加
      export PATH=/usr/local/strongswan/sbin:/usr/local/strongswan/bin:$PATH
      按:wq保存并退出

    3. Activate the environment variables source /etc/profile

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      [root@li****-18 sbin]# source /etc/profile
      [root@li****-18 sbin]#

      这时候运行ipsec version就会出现以下内容
      [root@li****-18 sbin]# ipsec version
      Linux strongSwan U5.7.2/K3.10.0-957.el7.x86_64
      University of Applied Sciences Rapperswil, Switzerland
      See 'ipsec --copyright' for copyright information.
      [root@li****-18 sbin]#

  4. Configure Strongswan and certificates

    1. Generate CA certificate, generate private key

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      在这里之前建议到一个新的目录, 我这里选择/home/ssl目录(自己创建), 然后接着以下命令
      ipsec pki --gen --outform pem > ca.pem

    2. Sign the CA certificate with the private key

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      ipsec pki --self --in ca.pem --dn "C=com, O=myvpn, CN=VPN CA" --ca --outform pem >ca.cert.pem

    3. Server certificate, generate private key

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      ipsec pki --gen --outform pem > server.pem

    4. Issue the server certificate with the CA certificate
      First, confirm the IP address or domain name used to access the server. Only use this address when connecting, do not use any other address. Replace 123.123.123.123 in the following command with the IP address or domain name of your own server. You need to replace it in two places.

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      ipsec pki --pub --in server.pem | ipsec pki --issue --cacert ca.cert.pem \
      --cakey ca.pem --dn "C=com, O=myvpn, CN=123.123.123.123" \
      --san="123.123.123.123" --flag serverAuth --flag ikeIntermediate \
      --outform pem > server.cert.pem

    5. Client certificate, generate private key

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      ipsec pki --gen --outform pem > client.pem

    6. Sign the client certificate with the CA

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      ipsec pki --pub --in client.pem | ipsec pki --issue --cacert ca.cert.pem --cakey ca.pem --dn "C=com, O=myvpn, CN=VPN Client" --outform pem > client.cert.pem

    7. Generate the pkcs12 certificate

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      openssl pkcs12 -export -inkey client.pem -in client.cert.pem -name "client" -certfile ca.cert.pem -caname "VPN CA"  -out client.cert.p12
      这里需要输入密码, 随便输入一个即可, 注意2次密码要一致, 至此, 运行的命令如下

      [root@li****-18 sbin]# cd /home
      [root@li****-18 home]# mkdir ssl
      [root@li****-18 home]# cd ssl
      [root@li****-18 ssl]# ipsec pki --gen --outform pem > ca.pem
      [root@li****-18 ssl]# ipsec pki --self --in ca.pem --dn "C=com, O=myvpn, CN=VPN CA" --ca --outform pem >ca.cert.pem
      [root@li****-18 ssl]# ipsec pki --gen --outform pem > server.pem
      [root@li****-18 ssl]# ipsec pki --pub --in server.pem | ipsec pki --issue --cacert ca.cert.pem \
      > --cakey ca.pem --dn "C=com, O=myvpn, CN=139.***.***.18" \
      > --san="139.***.***.18" --flag serverAuth --flag ikeIntermediate \
      > --outform pem > server.cert.pem
      [root@li****-18 ssl]# ipsec pki --gen --outform pem > client.pem
      [root@li****-18 ssl]# ipsec pki --pub --in client.pem | ipsec pki --issue --cacert ca.cert.pem --cakey ca.pem --dn "C=com, O=myvpn, CN=VPN Client" --outform pem > client.cert.pem
      [root@li****-18 ssl]# openssl pkcs12 -export -inkey client.pem -in client.cert.pem -name "client" -certfile ca.cert.pem -caname "VPN CA"  -out client.cert.p12
      Enter Export Password:
      Verifying - Enter Export Password:
      [root@li****-18 ssl]#

    8. Install the certificate

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      cp -r ca.cert.pem /usr/local/strongswan/etc/ipsec.d/cacerts/
      cp -r server.cert.pem /usr/local/strongswan/etc/ipsec.d/certs/
      cp -r server.pem /usr/local/strongswan/etc/ipsec.d/private/
      cp -r client.cert.pem /usr/local/strongswan/etc/ipsec.d/certs/
      cp -r client.pem  /usr/local/strongswan/etc/ipsec.d/private/

    9. Certificate installation is complete, next configure Strongswan, edit /usr/local/strongswan/etc/ipsec.conf

      It is recommended to backup the original file before performing the following operations

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      config setup
      	uniqueids=never

      conn iOS_cert
      	keyexchange=ikev1
      	# strongswan version >= 5.0.2, compatible with iOS 6.0,6.0.1
      	fragmentation=yes
      	left=%defaultroute
      	leftauth=pubkey
      	leftsubnet=0.0.0.0/0
      	leftcert=server.cert.pem
      	right=%any
      	rightauth=pubkey
      	rightauth2=xauth
      	rightsourceip=10.31.2.0/24
      	rightcert=client.cert.pem
      	auto=add

      conn android_xauth_psk
      	keyexchange=ikev1
      	left=%defaultroute
      	leftauth=psk
      	leftsubnet=0.0.0.0/0
      	right=%any
      	rightauth=psk
      	rightauth2=xauth
      	rightsourceip=10.31.2.0/24
      	auto=add

      conn networkmanager-strongswan
      	keyexchange=ikev2
      	left=%defaultroute
      	leftauth=pubkey
      	leftsubnet=0.0.0.0/0
      	leftcert=server.cert.pem
      	right=%any
      	rightauth=pubkey
      	rightsourceip=10.31.2.0/24
      	rightcert=client.cert.pem
      	auto=add

      conn windows7
      	keyexchange=ikev2
      	ike=aes256-sha1-modp1024! 
      	rekey=no
      	left=%defaultroute
      	leftauth=pubkey
      	leftsubnet=0.0.0.0/0
      	leftcert=server.cert.pem
      	right=%any
      	rightauth=eap-mschapv2
      	rightsourceip=10.31.2.0/24
      	rightsendcert=never
      	eap_identity=%any
      	auto=add

    10. Edit /usr/local/strongswan/etc/strongswan.conf

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      charon {
      	load_modular = yes
      	compress = yes
      	plugins {
      		include strongswan.d/charon/*.conf
      	}
      	dns1 = 8.8.8.8
      	dns2 = 8.8.4.4
      	nbns1 = 8.8.8.8
      	nbns2 = 8.8.4.4
      }
      include strongswan.d/*.conf

    11. Edit the username and password in /usr/local/strongswan/etc/ipsec.secrets

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      : RSA server.pem
      : PSK "[预共享密钥]"
      : XAUTH "[预共享密钥]"
      [用户名] %any : EAP "[密码]"

      Note that you need to edit the “Pre-shared key” to a unique and confidential string in both the PSK and XAUTH fields. Replace [username] with the desired username and [password] with the desired password (remove the square brackets). You can add multiple lines to have multiple users.

  5. Modify system forwarding and firewall configuration

    1. Edit /etc/sysctl.conf, remove the # in front of net.ipv4.ip_forward=1, save it and execute sysctl -p, the output should be as follows

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      [root@li****-18 etc]# vi /etc/sysctl.conf 
      [root@li****-18 etc]# sysctl -p
      net.ipv4.ip_forward = 1
      [root@li****-18 etc]#

    2. Next, modify iptables.

      Execute this if using OpenVZ

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      iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
      iptables -A FORWARD -s 10.31.2.0/24  -j ACCEPT
      iptables -A INPUT -i venet0 -p esp -j ACCEPT
      iptables -A INPUT -i venet0 -p udp --dport 500 -j ACCEPT
      iptables -A INPUT -i venet0 -p tcp --dport 500 -j ACCEPT
      iptables -A INPUT -i venet0 -p udp --dport 4500 -j ACCEPT
      iptables -A INPUT -i venet0 -p udp --dport 1701 -j ACCEPT
      iptables -A INPUT -i venet0 -p tcp --dport 1723 -j ACCEPT
      iptables -A FORWARD -j REJECT
      iptables -t nat -A POSTROUTING -s 10.31.2.0/24 -o venet0 -j MASQUERADE

      Execute this for other servers

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      iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
      iptables -A FORWARD -s 10.31.2.0/24  -j ACCEPT
      iptables -A INPUT -i eth0 -p esp -j ACCEPT
      iptables -A INPUT -i eth0 -p udp --dport 500 -j ACCEPT
      iptables -A INPUT -i eth0 -p tcp --dport 500 -j ACCEPT
      iptables -A INPUT -i eth0 -p udp --dport 4500 -j ACCEPT
      iptables -A INPUT -i eth0 -p udp --dport 1701 -j ACCEPT
      iptables -A INPUT -i eth0 -p tcp --dport 1723 -j ACCEPT
      iptables -A FORWARD -j REJECT
      iptables -t nat -A POSTROUTING -s 10.31.2.0/24 -o eth0 -j MASQUERADE

      Since CentOS 7’s firewall is handled by firewall-cmd, use firewall-cmd for configuration here.

      Reference article: IPSEC VPN on Centos 7 with StrongSwan

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      firewall-cmd --zone=dmz --permanent --add-rich-rule='rule protocol value="esp" accept'
      firewall-cmd --zone=dmz --permanent --add-rich-rule='rule protocol value="ah" accept'
      firewall-cmd --zone=dmz --permanent --add-port=500/udp
      firewall-cmd --zone=dmz --permanent --add-port=4500/udp
      firewall-cmd --permanent --add-service="ipsec"
      firewall-cmd --zone=dmz --permanent --add-masquerade
      firewall-cmd --set-default-zone=dmz
      firewall-cmd --reload

    3. Next (common part), save the iptables configuration and configure it to load automatically on startup. If you are using firewall-cmd configuration, skip this step.

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      iptables-save > /etc/iptables.rules
      将以下内容保存到/etc/rc.local

      iptables-restore < /etc/iptables.rules

      chmod +x /etc/rc.d/rc.local

      以上命令如下
      [root@li****-18 etc]# iptables-save > /etc/iptables.rules
      [root@li****-18 etc]# vi /etc/rc.local 
      保存上面 iptables-restore这一段, :wq保存并退出
      [root@li****-18 etc]# chmod +x /etc/rc.d/rc.local
      [root@li****-18 etc]#

  6. Start Strongswan

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    [root@li****-18 etc]# ipsec start
    Starting strongSwan 5.7.2 IPsec [starter]...
    [root@li****-18 etc]#

  7. Try connecting

    Windows Phone: I don’t have the device, so I won’t provide an example.

    For Windows Phone 8.1, install ca.cert.pem, go to Settings -> VPN -> Add IKEv2 Connection, the address should be the same as the one in the certificate. Connect using username and password.

    Windows

    The connection process for Windows is the same, but note that the certificate should be imported into the local computer’s “Trusted Root Certification Authorities” instead of the current user’s “Trusted Root Certification Authorities”.

    1. Safely copy the server’s ca.cert.pem to a local directory

    2. Open Run with Win + R, enter mmc
      openMMC

    3. File -> Add/Remove Snap-in

    ![openMMC](https://oss.ruterfu.com/blog/2024/05/16/lZztBy6Cxi6aKKiE3g7tZv5WivGAoUIpR5d.webp)

    1. Add Certificates -> Computer Account -> Local Computer (the computer running this console) -> OK
      addCert

    2. Open Certificates -> Trusted Root Certification Authorities -> Right-click on the certificate -> All Tasks -> Import
      importCert

    3. Import ca.cert.pem, File -> Import all certificates into the Trusted Root Certification Authorities -> Finish
      import ca.cert.pem

    4. Open Settings -> Network & Internet -> VPN -> Add a VPN connection
      1. VPN Provider - Windows (built-in)
      2. Connection Name - Any name
      3. Server Name or Address - Enter the server’s IP address
      4. VPN Type - IKEv2
      5. Type of Sign-in info - Username and password (do not select Certificate)
      6. Username, Password - Enter the server’s username and password

    5. Click Connect, the connection should be successful, but internet access may not work yet

      At this point, Windows is not yet using the VPN. After checking, it is determined that Windows 10 will default to enabling Split-Tunneling, causing traffic to not go through the VPN. The solution is as follows:
      Reference official documentation: Introduction to strongSwan: Forwarding and Split-Tunneling
      Reference article: WIN10能连上VPN，却无法访问网页

    1. Open PowerShell and enter the command: `Set-VpnConnection "Connection Name from Step 7" -SplitTunneling 0`
2. Follow the steps in [WIN10能连上VPN，却无法访问网页](https://blog.csdn.net/gaojinshan/article/details/51249147) to solve the problem
    1. After the problem is resolved, connect again to complete the setup.
    ![windowsSuccess](https://oss.ruterfu.com/blog/2024/05/16/TulEsIQQwPHHY7uUyqfSY8x7N3JB2oBI5YX.webp)

    iPhone

    For iOS/Android/Mac OS X devices, add a Cisco IPSec PSK connection. The pre-shared key is the string after PSK in /usr/local/strongswan/etc/ipsec.secrets (without quotes). Use any domain name or IP to connect, no certificates are required.

    iPhoneConnection

    Mac Connection

    macConnect